Cover Sheet
Research Proposal
Sensitivity of Regional Climate to the Great Salt Lake
by
University of Utah
Principal Investigator:
John D. Horel
Professor
(801) 581-7091
(801) 581-4362 fax
jhorel@met.utah.edu
Department of Meteorology
135 S. 1460 E
Salt Lake City, UT 84112-0110
Official Signing for University:
, Ph.D., Manager
Requested funding: FY05 K,
Total K
Use of human subjects: No
FY06 K,
FY07 K,
FY08 K
Use of vertebrate animals: No
PI Signatures: John D. Horel: _________________ Date: _________________
University Official Signature: Whoever: __________________
Date:______________TABLE OF CONTENTS
Project Summary
Project Description
1 Research Objective .......................................................................................................................4
2 Relevance and Significance of the Research ................................................................................5
3 Background ..................................................................................................................................6
3.1 Introduction ..........................................................................................................................8
3.2 Previous Research on Climate Variability of the Great Salt Lake.......................................9
3.2.1 Colorado Plateaus Basin .............................................................................................9
3.2.2 Columbia Basin .........................................................................................................10
3.2.3 Salt Lake Basin .........................................................................................................11
3.2.4 Small Basins in the Eastern Alps ..............................................................................12
3.2.5 Summary of Previous Research ................................................................................14
4. Research Design and Methods ...................................................................................................15
4.1 Analysis of Existing Data ..................................................................................................16
4.1.1 SLB Cold Pool Analysis ...........................................................................................16
4.1.2 Climatology of Cold Pools in the SLB .....................................................................17
4.1.3 Austrian Basin Cold Pools ........................................................................................18
4.2 Collection and Analysis of New Winter Data in the SLB .................................................19
4.3 Numerical Models and Comprehensive Simulations of Cold Pool Episodes ....................20
4.3.1 RAMS .......................................................................................................................21
4.3.2 HYPACT...................................................................................................................22
4.4 Examination of Cold Pool Processes .................................................................................23
4.4.1 Differential Temperature Advection .........................................................................23
4.4.2 Turbulent Erosion .....................................................................................................24
4.4.3 Stratiform Clouds within the Cold Pool ...................................................................25
4.5 Summary ............................................................................................................................26
5. Broader Impacts .........................................................................................................................27
6. Collaborations ............................................................................................................................27
7. References Cited ........................................................................................................................29
8. Biographical Sketches ................................................................................................................40
9. Budget
................................................................................................................................35
10. Current and Pending Support ...................................................................................................38
11. Facilities, Equipment and Other Resources .............................................................................39
PROJECT SMMARY
A four-year research project is proposed to investigate the processes leading to the
formation, maintenance and destruction of multi-day, mid-winter cold air pools in the
Salt Lake basin (SLB). These pools, which occur frequently in the western US and
throughout the world are one of the most difficult wintertime meteorological events to
forecast. They can produce severe, long-lasting pollution episodes by trapping pollutants
within the basin and releasing them vertically during the cold pool destruction period,
thus impacting regional air quality.
The proposed research include observations, analysis, and modeling. Analyses will be
focused initially on a cold pool event in the SLB that lasted from 25 December 2000 to 9
January 2001 when special observations were available in the basin. These analyses will
be run in parallel with a climatological study of cold pool events in the SLB using 20
years of rawinsonde data and model reanalysis data. The climatological study and
analyses of prior events will determine the synoptic drivers of the cold pool events and
the individual mechanisms leading to cold pool formation and destruction.
A multi-investigator cold pool field experiment will be conducted in the SLB in the
winter of 2005-2006 to obtain detailed meteorological data for cold pool events suitable
for model simulations and analysis.
A mesoscale meteorological model will be used to interpret the cold pool
observations, verify hypotheses, provide further insight into physical mechanisms, and
evaluate the air pollution implications of the cold pool processes. The simulations and
their comparison with observations will be focused on 1) identifying the meteorological
processes that lead to cold pool buildup, maintenance and breakup, 2) determining the
consequences of these processes on air pollution transport and diffusion in urban basins,
and 3) determining how models can be improved to provide more accurate simulations of
cold pools. The simulations will take account of the often-observed fog and stratiform
cloudiness that forms and dissipates in the SLB.PROJECT DESCRIPTION
1 Research Objective
We propose a comprehensive program of field study, analysis and modeling to
determine the processes responsible for the formation, maintenance and destruction of
multi-day cold air pools that form in the Salt Lake Basin in winter, to evaluate their
influence on pollutant transport and mixing, and to determine how models can be
improved to make more accurate simulations of cold air pools.
This objective will be accomplished in a four-year program of research including a
winter field study and data analyses. The field data to be used in the research will come
primarily from the Salt Lake Basin (SLB, Fig. 1), the site of extensive meteorological
investigations conducted in October 2000 (Doran et al. 2002) and the location where we
will collect new data in the winter of 2005-2006. Data sets and models from the PI’s
earlier work in other valleys and basins will supplement data from the topographically
complicated SLB, leading to a more comprehensive understanding of cold pool
evolution. The primary model to be used for the cold pool simulations is the mesoscale
numerical model RAMS. Selected simulations will be supplemented with a Lagrangian
particle dispersion model, HYPACT, that runs in conjunction with RAMS.
island
Great
Salt
La
Salt
Salt
Lake
City
slope
flow
Lake
experiment
Big
Ctnw
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Little
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copper
mine
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Figure 1. SLB topography, showing the locations of the Great Salt Lake, the Utah Lake,
Salt Lake City, the Traverse Range which divides the northward-draining Jordan or Salt
Lake Valley into an upper (Utah Lake Basin) and lower basin (SLB), and the major basin
tributaries from the Wasatch Range east of the valley. The Oquirrh Mountains are west of
the valley. Drainage flow experiments conducted by the PI during VTMX 2000 were
southwest of Salt Lake City (rectangular box). The proposed data logger lines are
indicated by the three bold lines.
2 Relevance and Significance of the Research
A cold air pool is an accumulation of air within a confined basin or valley that is
colder than the air above. Cold pools are prevalent in winter in basins or valleys,
especially in those with poorly developed along-valley wind systems. Major urban areas
in the Western US and the Appalachians that experience cold pools include Salt Lake
City, Los Angeles, Phoenix, Tucson, and Las Vegas. Smaller cities with frequent cold
pools include Reno, Nevada, Medford, Oregon, and Roanoke, Virginia. The Salt Lake
City cold pools are familiar to wintertime travelers landing at the Salt Lake City airport
because pollution trapped in the cold pool is clearly visible. The pollutants come mainly
from the Salt Lake City urban area and the chain of cities along the Wasatch Front.
Cold pools can trap air for many days in winter, allowing pollutants, clouds and
moisture to build up in valleys and basins (Fig. 2). Forecasting the buildup and removal
of cold pools is presently very difficult because the mechanisms leading to wintertime
cold pool development, maintenance and removal are not yet fully understood and are not
well handled by forecast models. The need for new observational and numerical
modeling research to address forecasting and modeling deficiencies is widely recognized
(Smith et al. 1997).
Air pollution can reach unacceptably high levels in cold pools (Reddy et al. 1995;
Brock LeBaron, 2004, pers. comm., Utah Division of Air Quality) and the breakup of the
cold pool can carry pollution vertically into regional scale flows, resulting in regional
scale air pollution and climate impacts. Because of the poor mixing in cold pools,
moisture sometimes builds up in the pools causing fog and stratus to form. Cloudy
conditions can lead to hazardous episodes of persistent freezing rain, drizzle or fog (Larry
Dunn, pers. comm., NWSFO, Salt Lake City, 2004; Petkovsek 1974) and can extend the
duration of the cold pool. Further, in cold climates, cold air pools delay thawing of
wintertime snow cover, interfering with ground transportation (Smith et al. 1997).
Because the buildup and breakup mechanisms affect the vertical transport and mixing
of air pollution within the pools and the timing and strength of pollutant releases from the
pools, research into the evolution of persistent wintertime cold air pools could advance
the understanding of air pollution transport and diffusion in basins and valleys and
improve both local scale and regional scale weather forecasting. Our research goals are
to:




Identify the fundamental processes that lead to persistent cold air pools in the Salt
Lake basin
Make measurements to identify and quantify these processes
Improve numerical simulations and forecasts of cold pool evolution
Determine the nature of (and, where possible, quantify) the interaction of synoptic
and terrain-induced flows with cold air pools in basins, and assess how such flows
affect the formation and erosion of those pools and the dispersion of pollutants in
them
Photo by Dan Judd Communications.
From www.webeyes.at/panoramafotos/Steiermark/2002-02.2-0202
From www.webeyes.at/panoramafotos/noe/200112021258_
Figure. 2. Panoramic photos of polluted and/or cloudy wintertime cold air pools.
Pollution or moisture are often trapped in the pools allowing them to be easily seen. a)
View of the Salt Lake Basin from Mt. Ogden at 1pm on 24 December 2001. b) View
from 2500 m MSL from a hot air balloon between Trieben and Admont, Austria on 2
February 2002. c) View from Klaustalberg near Türnitz, Austria on 2 December 2001.
3. Background
3.1 Introduction
A significant body of scientific literature now exists on diurnal inversion buildup
and breakup in narrow valleys (reviewed by Whiteman 1990). The present proposal is
focused on the complete evolutionary cycle of wintertime multi-day cold air pools in
basins, a topic that has, to date, received relatively little research attention. The buildup
and breakup of multi-day cold pools, governed by atmospheric processes covering a
broad range of scales (including synoptic-scale), differ from diurnal cold pools in that
they are not destroyed daily by the diurnal cycle of heating and cooling.
In valleys, along-valley circulations tend to equilibrate valley temperatures with
those of the nearby plain, advecting cold air into the valley during daytime when the
valley is warming, and advecting warm air into the valley (via sinking motions) during
nighttime when the valley is cooling and draining (Whiteman 2000). Basins, on the other
hand, experience enhanced cooling during nighttime and enhanced heating during
daytime because along-valley flows are weak or absent. Nighttime cooling is
pronounced, as evidenced by the many minimum temperature records that have been set
in basins. Record low temperatures have been recorded, for example, at Tincup in
Colorado’s Taylor Park (Basin), in Utah's Logan or Peter Sinks Basins and in Montana's
Bighole Basin. The lowest wintertime minimum temperatures in Europe are often found
in Austria's Gstettneralm (or Gruenloch) Basin (Sauberer and Dirmhirn 1954, 1956). The
enhancement of daytime heating is less pronounced because heat escapes through the top
of the basin atmosphere if the CBL grows beyond ridgetop height.
A number of meteorological studies have been conducted in basins. Air pollution
problems in the basins of Slovenia and Croatia have been widely recognized and studied
(Petkovsek 1978, 1980, 1992; Vrhovec 1991). In wintertime, the basins are often capped
by low clouds that decrease insolation and cause cold air pools to persist for long periods
(Petkovsek 1974). Investigations in Japanese basins have focused on basin surface and
atmospheric energy budgets (Kondo et al. 1989; Kondo and Okusa 1990; Ishikawa 1977;
Kuwagata et al. 1990a, 1990b; Kuwagata and Sumioka 1990). In addition, numerical
simulations of thermally-driven circulations have been performed with the goal of
understanding pollutant transport from Tokyo and other industrial areas into nearby
basins (Kimura and Kuwagata 1993). A meteorological investigation was recently
conducted in a basin on the east side of the New Zealand Alps (Kossmann et al. 2002;
Sturman et al. 2003). In the United States, meteorological research has been conducted in
basins ranging in size from 102 to 105 km2, including Utah’s Peter Sinks Basin (Clements
et al. 2003), Colorado’s Sinbad (Fast et al. 1996; Whiteman et al. 1996) and South Park
(Banta 1984) basins, Virginia’s Roanoke Basin (Allwine et al. 1992), the Columbia Basin
(Staley 1959; Doran and Zhong 1994), the Los Angeles Basin (Smith et al. 1980;
Glendening 1990; Ulrickson and Mass 1990; and Lu and Turco 1995, 1996; Lu et al.
1997) and the Great Basin (Wolyn and McKee 1989; Mayr and McKee 1995; Savoie and
McKee 1995). Most of these studies were conducted in summer, fall or spring rather than
in winter.
3.2 Previous Research on Wintertime Cold Pools
The mechanisms leading to multi-day, wintertime cold pools in wider valleys or
basins like the SLB have received very little research attention despite being widely
recognized as factors in persistent air pollution episodes. The serious logistical problems
encountered in running wintertime field studies, the difficulty and expense of making
frequent vertical soundings through the cold pool with the requisite resolution of
temperature and wind structure near the top of the pool, and limitations on modeling
imposed by the need for high vertical resolution of the slope flows and processes taking
place in the upper cold pool have all contributed to this lack of attention. It is only
recently that suitable remote sensing instruments have become available and computer
power and model formulations have been improved so that the multi-day cold pool
problem can be studied in depth.
Much of the early work on cold air pools focused on the heat input necessary to break
up the cold pool (Whiteman and McKee 1982; Petkovsek 1985; Savoie and McKee
1995). The literature also includes several theoretical studies of the role of turbulent
erosion in the breakup of cold air pools (Petkovsek 1992, Bian et al. 2000; Rakovec et al.
2002), although none of these studies included observations of a turbulent erosion
episode. Recent research by Zhong et al. (2003) raises doubt that this is one of the key
mechanisms in deep basins or valleys because the intense stability that forms at the top of
the cold pool during such an episode acts as a brake on the erosion. Zängl (2004) has
recently performed model simulations of the effects of the geostrophic pressure gradient
and drainage flows on topographically idealized basins and valleys. He found that the
main effect of the large scale pressure gradient is to induce a circulation within the cold
pool until the upper boundary of the cold pool is inclined to compensate for the ambient
pressure gradient. Drainage flow out of a valley is important because it allows the cold
air to escape from the valley and for warmer air to intrude. Blocking is important in
producing a deep cold pool which is much more difficult to destroy than a shallow cold
pool. Other research relevant to persistent wintertime cold air pools is summarized below
for each of the locations where such research has been performed.
3.2.1 Colorado Plateaus Basin
Whiteman et al. (1999b) used twice-daily rawinsondes from January-March 1990 to
determine how the atmospheric temperature and wind structure changed during multi-day
cold pool events in the Colorado Plateaus Basin (CPB), the large basin upstream from the
Grand Canyon of the Colorado River. This research showed that a very broad range of
physical processes affected cold pool formation, maintenance and destruction. Cold pools
tended to form when cold air was advected into the basin (often after cold frontal
passages) or when cold air drained off snow-covered mountains on the basin periphery
during the long winter nights. Convective boundary layers (CBLs) that developed during
daytime on the floor and sidewalls of the basin were, in mid-winter, insufficient to break
through the cold pool, but became more effective as spring approached. Mid-winter cold
pool breakup nearly always occurred during daytime, when sensible heat flux was able to
remove the shallow cold pool remnant that often persisted in the bottom of the basin
during nighttime. The strength and depth of the cold pool, after its initial formation,
underwent oscillations that seemed to depend primarily on cold or warm air advection
above the pool as synoptic scale troughs and ridges passed over the CPB. The cold pool
strengthened when warm air was advected over the pool and weakened when cold air was
advected over the pool.
The changing strength of basin cold pools during the entire cycle of cold pool
formation, maintenance and destruction was evaluated by performing a volumetric
integration of basin temperature profiles to compute the heat deficit required to break the
basin inversion (Whiteman et al. 1999b). The heat deficit was normalized by dividing by
the drainage area of the basin at the cold pool top, and the 3-month series of these
Normalized Basin Heat Deficits (NBHD) was used to characterize the changing strength
and depth of the basin cold pool. The method worked well, but the twice-daily soundings
lacked the time resolution to fully track inversion buildup and breakup processes that
occurred on shorter time scales.
3.2.2 Columbia Basin
The CPB cold pool research was extended into the Columbia Basin in the winter of
1998/1999 (Whiteman et al. 2001; Zhong et al. 2001). A radar profiler (RP)/Radio
Acoustic Sounding System (RASS) was operated in the Lower Columbia Basin at the
same time that a line of surface-based temperature data loggers was operated on a
mountainside at the edge of the basin. These data were used to investigate two cold pool
episodes in the winter of 1998/1999 that were accompanied by fog and stratus. The cold
pools were formed and destroyed in very different ways. The December 1998 episode
was initiated by an arctic outbreak that was followed by warm air advection aloft. This
cold pool was destroyed by a downslope windstorm that occurred at the south edge of the
pool. The cold air there was eroded away by strong shear at the top of the pool (i.e.,
turbulent erosion) and was progressively pushed northward across the basin, emulating a
warm front. The second and, we believe, more typical cold pool in January 1999 began as
a nocturnal temperature inversion on a clear night and strengthened as warm air was
advected aloft from the west. This 6-day cold pool was nearly destroyed one afternoon by
strong cold air advection aloft. It re-strengthened during the night and was finally
destroyed the next day when the stratus broke up and a CBL grew upwards through the
remaining shallow cold pool.
The January cold pool episode in the Columbia Basin was simulated with a mesoscale
flow model - to our knowledge, the first such simulation of a multi-day cold pool (Zhong
et al. 2001). Cold pools in the Columbia Basin are affected by processes that cover a
range of scales from the synoptic scale (e.g., advection) to the mesoscale (downslope
warming in the lee of the Cascades) to the microscale (cloud microphysics). Despite these
challenges, the RAMS model performed well, providing useful information on the roles
of surface radiative heating and cooling, large-scale subsidence, temperature advection,
Chinook-induced subsidence warming and low-level cloudiness. The model showed that
turbulent erosion had little effect on the inversion in this case, and that incoming solar
radiation would have been insufficient to destroy the pool when it was fully developed
even if stratus clouds had not been present. Application of a Lagrangian particle
dispersion model to the cold pool episode showed better pollution dispersion in the pool
than we anticipated, with a substantial fraction of particles dispersing out the top of the
pool into the regional scale flows. The dispersion was enhanced by cold pool
destabilization when stratiform clouds formed at the top of the pool. Chinook warming
played a role in increasing the strength of the inversion at the top of the pool. In contrast
to the good model results for the January cold pool, we could not get acceptable results
for the December cold pool, leaving uncertainty about the conditions under which
mesoscale models will prove useful for simulating cold pool events.
3.2.3 Salt Lake Basin
Following the finding in the Columbia Basin that temperature observations on the
sidewalls of basins could be used to track cold pool evolution within the basin, a line of
eleven temperature dataloggers were installed and operated in the SLB on a line running
from the Jordan River southwestward up the Kennecott Utah Copper, Inc. waste pile in
the Oquirrh Mountains at 100-m vertical spacing, collected temperature data every 5
minutes for 15 months spanning two winter seasons. This network observed a persistent
cold pool that formed in the SLB on 25 December 2000 and lasted until 9 January 2001,
the longest such episode in the SLB in recent years. The Deseret News and the Salt Lake
Tribune reported on the effects of the cold pool, including fog and haze, landing
restrictions at the SLC airport, grounding of medical helicopters, auto accidents,
increased air pollution, and increased frequency of post-cold-pool snow avalanches.
The persistent cold pool formed when a synoptic-scale blocking pattern caused a 500
hPa ridge to persist over the western United States, with a surface high pressure center
over the Intermountain Basin. We produced a CD-ROM containing a data set for this
event, including surface and upper air charts, local air pollution data
(www.eq.state.ut.us/EQAIR/aq_home.htm), data from the temperature datalogger
line, the MesoWest network(Horel 2002; http://www.met.utah.edu/jhorel/html/mesonet/),
SLC rawinsondes, photographs (e.g., see Fig. 3), and articles from the local newspapers.
Figure 3. Photos of downtown Salt Lake City during the December 2000-January 2001
cold pool episode. The left photo was taken on 26 December looking across the valley
from above the University toward the Oquirrh Mountains. The right photo was taken of
the downtown area on the same date (photos provided by Robert Grandy, Utah Air
Quality Division).
One of the goals of the present project is to conduct a thorough analysis of this event,
using the data set and numerical model simulations. We have had a first look at the
rawinsonde winds for this episode (Fig. 4) and have produced a Quicktime animation of
the hourly vertical temperature structure during this event. The temperature structure was
similar to that observed in the Columbia Basin (Whiteman et al. 2001; Zhong et al. 2001).
When no clouds were present, a strong inversion extended through the surface-based cold
pool. When stratiform clouds formed at the top of the pool, the temperature structure
below the clouds tended to become near-moist-adiabatic, but with a strong inversion cap
at the top of the pool that varied in depth and strength during the multi-day episode. The
temperature structure in the pool appears to be modified in cloudy episodes by radiative
cooling at the cloud top and convective overturning forced by sinking of the cold air.
Further study is needed to verify or reject this hypothesis. The destabilization of the
temperature structure in the cold pool below the clouds will have important consequences
for pollution dispersion in the pool.
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Figure 4. Time-height cross section of vector winds from rawinsonde ascents at the Salt
Lake City airport during the December 2001-January 2002 cold pool episode. The shaded
area marks winds less than or equal to 5 m/s.
3.2.4 Small Basins in the Eastern Alps
A wintertime field experiment was conducted in a set of small basins on Austria’s
Hetzkogel Plateau in the winter of 2001-2002 (Steinacker et al. 2002; Whiteman et al.
2004a, 2004b, 2004c). In these experiments, a set of 58 temperature data loggers and
three automatic weather stations were deployed in October 2001 in five high-altitude
basins ranging in size from 0.3 to 2.1 km2. The sinkholes, of various sizes and shapes,
were fully enclosed by surrounding topography. Because stable air formed readily in the
enclosed basins, there was little effect on the cold pools by advection from their
surroundings, providing near-laboratory conditions for the local development of cold
pools. The data loggers were operated through the winter and into the early summer,
recording data at 5-min intervals. The largest quantity of data came from three lines of
loggers that ran up the northwest, southwest and southeast sidewalls of the largest basin,
the Gruenloch, at vertical intervals of 5 to 20 meters. An intensive meteorological
investigation was run in the Gruenloch in early June 2002 with two tethersondes and
additional radiometers.
Cold pool events occurred frequently in these small, snow-covered basins throughout
the winter. Several multi-day events were recorded, but most cold pools were destroyed
on a daily basis, despite the short days, low sun angles, and high surface albedos
(Whiteman et al. 2004?). The multi-day events occurred after fresh falls of new snow.
Lines of dataloggers running up three sidewalls of the largest basin provided pseudovertical temperature profiles that compared well with free-air soundings from the center
of the basin under stable conditions (Whiteman et al. 2004?). During nighttime, cold air
continuously drained out of the basin through the lowest pass (Pospichal et al. 2002). A
classification scheme was developed to categorize the cold pool events (Eisenbach et al.
2004). Four turbulent erosion events occurred during the winter in which the inversions
were destroyed by strong winds that removed the cold pool from above. These events
motivated us to develop a numerical model of turbulent erosion (Bian et al. 2000; Zhong
et al. 2003) that has, to date, not been tested with actual data because wind data were not
available at the inversion top. The model predicts that a shallow inversion cap that
develops at the inversion top strengthens as the turbulent erosion proceeds, that winds
above the cold pool must not only be of moderate strength, but also accelerating (see,
also, Petkovsek, 19xx), and that the time required to break the inversion can be quite long
(sometimes tens of hours).
Zängl (2004) has recently submitted a paper showing that a modified version of the
MM5 mesoscale numerical model can provide accurate simulations of cold pool episodes
in idealized topography that is similar to the Gruenloch (but 3 times its width and depth).
3.2.5 Summary of Previous Research
Previous and ongoing research show a variety of competing and, often, superimposed
processes that affect cold pool evolution. Only a few cold pool episodes have been
investigated to date. Nonetheless, the analyses show why the forecasting of cold pools,
especially their onset and cessation, is such a difficult proposition. The number of
processes that affect the buildup and breakdown of the pools is quite varied, involving a
range of atmospheric scales. Potentially important processes include cold air drainage
into the basin, removal of air mass from the basin by growing convective boundary layers
and upslope flows, differential temperature advection, turbulent erosion at the cold pool
top, development and dissipation of clouds within the basin, radiative and latent heating
effects of clouds in the pool, radiative effects of clouds above the pool, radiative effects
of pollution in the pool, effects of precipitation falling through the pool, synoptic-scale
blocking and unblocking on the upwind side of mountain barriers, synoptic or mesoscale
vertical motion fields, and the mechanical effects on the pool of background flows
impacting on upwind topography. In addition to the existing uncertainty about the
relative roles of these various processes there is also uncertainty about the ability of
mesoscale models to accurately simulate them, although the simulation of the January
episode in the Columbia Basin shows promise.
Two results come clearly from the analyses to date. First, the seasonal cycle of
incoming solar insolation is quite important in determining the seasonal distribution of
multi-day cold pools. The weak insolation of winter is often ineffective in breaking up
strong or deep pools, while the stronger insolation of spring, fall and summer causes
rapid CBL growth and vigorous upslope flows that destroy cold pools on a daily basis.
Second, cold pool breakup tends to occur much more quickly than buildup. The breakup
cycle is often incomplete, however, resulting in a shallow cold pool remnant that remains
in the lower elevations of basins. The cold pool remnant may be protected from the
usually stronger winds above the pool by the surrounding terrain or the remnant may
simply be in shadow.
4. Research Design and Methods
Our research will investigate the life cycle of persistent wintertime cold air pools that
form in the SLB using existing meteorological and air pollution data and new data to be
collected in the basin in a mid-winter experiment that we will conduct in the winter of
2005-2006. Our proposal includes collection and analysis of these data (and analysis of
existing data from other basins), and simulations of the observed cold pool episodes with
models. Modeling will focus on entire cold pool episodes, with the intent to compare
these simulations extensively with concurrent meteorological data to identify the
important processes and to determine what model improvements are needed to provide
better forecasts of these events. Additionally, we will focus on evaluating three processes
that we think are likely to be important cold pool evolution mechanisms: 1) differential
temperature advection, 2) turbulent erosion, and 3) stratiform cloudiness.
The RAMS full-physics mesoscale numerical model will be the primary modeling
tool used to simulate the full cycle of cold pool evolution in the SLB. The air pollution
transport and mixing consequences of the cold pool evolution will be simulated by
coupling RAMS to the HYPACT (Hybrid Particle and Concentration Transport)
Lagrangian particle dispersion model. A new turbulent erosion model that we have
developed recently (Zhong et al. 2003) will be used to supplement the RAMS/HYPACT
simulations of turbulent erosion, allowing us to focus more clearly on turbulent erosion
without considering the complications of interacting processes.
4.1 Analysis of Existing Data
The primary analysis task for existing data will be a detailed analysis of the 25
December 2000 - 9 January 2001 cold pool event in the SLB to determine the processes
that affected its formation, maintenance and destruction. Additionally, we will develop a
climatology of multi-day cold pools in the SLB to provide a climatological context for
the specific cold pool events being investigated.
4.1.1 SLB cold pool analysis
In our previous studies of cold pools in the Colorado Plateaus Basin and the
Columbia Basin we formed some initial hypotheses concerning mechanisms leading to
the formation and destruction of cold air pools. These studies, however, were based on
analyses of a limited number of cold pools in basins other than the SLB. While we have
developed some initial hypotheses, we did not have sufficient data to test them
adequately and we were unable to observe enough events to be confident that we
identified the key mechanisms that affect cold pool development. The research proposed
for the SLB brings some exciting new opportunities to make progress on this important
scientific and practical issue, as we will have the computational tools and the data
necessary to make critical progress on identifying cold pool mechanisms in a region that
has a great deal of supporting data and a suite of instruments designed specifically to
address the cold pool problem.
The analyses of data from the 25 December 2000 - 9 January 2001 cold pool will be
performed with assistance from Dr. John Horel. These analyses will follow the approach
that we developed for the CPB, in which meteorograms (i.e., multiple time series of
meteorological variables) are compared to a time series of NBHD. Computations of
NBHD will be made using data from the twice-daily rawinsondes and the hourly pseudovertical temperature profiles from a line of data loggers on the west sidewall of the basin
that we operated from December 1999 through March 2001 which have not yet been
analyzed. Surface and upper air charts will then be reviewed to relate the time series to
the movements of fronts, troughs and ridges. Rawinsonde observations at the SLC airport
will be used to make time-height cross sections depicting the evolution of the vertical
structure of winds, potential temperatures and mixing ratios within the basin. Hourly
cloud observations from SLC will be used to relate cold pool changes to cloud types and
amounts. Using software already available at the University of Utah, we will create and
view animations of the MesoWest data to determine how the surface wind and
temperature patterns evolved within the basin during the event. Finally, we will analyze
our hourly pseudo-vertical temperature profile animations for this event, producing better
time resolution (5-minute) profiles when appropriate. Analyses will focus on the physical
processes leading to inversion buildup and inversion destruction, relating observed
changes in NBHD to above-basin temperature advection, wind shear at the cold pool top,
a measure of cloudiness and a measure of downslope flow strength.
Additional analyses will be performed on the SLB data to compare the pseudovertical temperature profiles with concurrent rawinsonde temperature profiles to
determine under what conditions the pseudo-vertical profiles are a good proxy for
rawinsondes. The data logger line was nearly 25 km south from the Salt Lake City airport
where the rawinsondes were launched. Similar comparisons in the Columbia Basin and,
more recently, the Gruenloch showed that pseudo-vertical profiles on well-exposed
mountain slopes are good proxies for free air soundings under stable nighttime and
wintertime cold pool conditions on scales ranging from 1 to 25 km. If this is the case in
the SLB the pseudo-vertical soundings will provide a far more detailed look at processes
involved in cold pool buildup and breakup than afforded by the twice-daily rawinsonde
data.
4.1.2 Climatology of Cold Pools in the SLB
A climatology of cold pools in the SLB will be developed using a 50-year CD ROM
record that we have on hand of twice-daily rawinsonde data from the Salt Lake City
airport. The climatology will determine the distribution of multi-day cold pools
throughout the winter half-year, the strength of the pools and their persistence. The
climatology will be based on temperature and wind data extracted from the rawinsonde
records from two levels - the surface and an altitude near the top of typical cold pools.
As an initial approach, cold pools will be considered to be present in the SLB when the
temperature excess at the cold pool top relative to the surface exceeds some threshold
value (to be chosen) and the surface winds are below some chosen threshold value. These
two criteria define the essence of a cold pool since, when the criteria are met, an
inversion is clearly present in the basin and the air in the basin is quiescent. The analysis
will then focus on the determination of the length of the multi-day episodes when the
criteria are continuously met and their monthly and yearly distribution over the period of
record. The thresholds will be chosen by reference to known cold air pools in the period
of record, including the 25 December 2000 to 9 January 2001 pool, for example. The
sensitivity of the climatology to changes in the threshold values will also be determined.
These analyses will provide a basic climatological overview of the cold pool problem in
the SLB, which we consider to be necessary background information for studies of
individual cold pool events.
4.1.3 Austrian Basin Cold Pools
Through collaborations with Austrian investigators, we will compare the cold pools in
the SLB with persistent cold air pools that form in the much more moist climate regime
of the Austrian Alps. There, our Austrian collaborators will analyze data from two
sources.
Dr. Thomas Haiden of the Austrian Weather Service (Central Institute for Meteorology
and Geodynamics) will be analyzing meteorological data collected routinely by the
Austrian weather service from the Lungau and Klagenfurt basins. Through a loan of
equipment, we will provide him with temperature data loggers that will be installed in
these basins to gain better vertical resolution of the Austrian cold pools.
Dr. Reinhold Steinacker and his colleagues and students at the University of Vienna have
instrumented the 1-km diameter Gruenloch sinkhole in the Eastern Alps. The Gruenloch
sinkhole holds the record for the lowest temperature minimum in central Europe. Our
previous collaborations with Dr. Steinacker have led us to identify persistent wintertime
cold pools in this sinkhole and to develop some initial hypotheses regarding physical
processes responsible for their formation and breakup. We have determined that
temperature inversion evolution in this sinkhole can be observed from lines of
temperature data loggers on the sinkhole sidewalls. Since this finding, Dr. Steinacker has
instrumented the sinkhole with temperature dataloggers and is using this site for student
practica. Because this basin differs in size and climate regime form the Salt Lake Basin,
we are confident that collaboration with Dr. Steinacker’s group will allow us to more
easily generalize from our findings in the SLB.
4.2 Collection and Analysis of New Winter Data in the SLB
We propose to instrument the SLB during the winter of 2005-2006 with a set of
continuously operating, unmanned, remote and in-situ meteorological instruments to
observe persistent wintertime cold pools. This set of instruments includes three lines of
temperature data loggers running up the basin sidewalls, a 915 MHz radar wind profiler
(RP), a Radio Acoustic Sounding System (RASS), and a Doppler sodar. Upward- and
downward-looking pyranometers, and upward- and downward-looking pyrradiometers
will monitor snow cover, clouds and insolation. Three towers with multiple threedimensional sonic anemometers at different heights will measure sensible heat flux and
mean and turbulent surface winds. This set of instruments is, in itself, sufficient to
monitor the evolution of mid-winter cold air pools, continuously measuring vertical
profiles of horizontal winds and virtual temperatures within and above the cold pool
along with components of the surface radiation and energy balances.
The three lines of temperature data loggers will be installed to determine whether
cold pool temperature structure varies with location in the basin. We will compare
pseudo-vertical temperature profiles from the logger lines with twice-daily rawinsondes
launched from the Salt Lake City International Airport and with the hourly RASS
temperature profiles, to determine whether the logger profiles are useful proxies for free
air soundings in the various parts of the basin. One line will be located on the west slope
of the Wasatch mountains north of SLC running down a ridge toward the oil refinery, one
at the north end of the Oquirrh Mountains and one running up the north side of the
Traverse Range south of SLC (see line locations in Fig. 1). The locations of the
individual data loggers on each line will be chosen to ensure their placement in locations
that are not subject to local microclimates. The loggers will be programmed to sample
and record temperatures at 15-min intervals with eleven instruments per line spaced at
100-m vertical height increments. This range of heights and data logger spacing will
provide profiles through typical cold pool depths with sufficient vertical resolution for
our planned analyses. Our previous experience with a logger line in the SLB gives us
confidence that we will be able to obtain good quality data.
The Radar Wind Profiler/RASS will be requested from NCAR’s Field Observing
Facility through a separate proposal. Dr. Eric Pardyjak at the Mechanical Engineering
Department at the University of Utah will investigate the fine scale atmospheric structure
within wintertime cold pools and on sequences of events that lead to changes in cold pool
structure. He will contribute a second Atmospheric Research RASS that works on the
Bragg Reinforcement search technique to obtain 10-15 min averages of virtual
temperature through a range of 50-700 m with 35 m range gates. he will also provide an
Aerovironment mini-SODAR that provides 10-min averages of winds over a range from
15-200 m with a resolution of 5 m, an energy budget station including a sonic
anemometer and hygrometer that will provide net radiation, ground, sensible and latent
heat fluxes at half-hourly intervals, and three towers containing three or more 3-d sonic
anemometers, humidity sensors, fine wire thermocouples and a fast response Krypton
hygrometer. During good cold air pool events a team of students will collect tethered
balloon profiles including CO concentration data. The students will keep this equipment
working and will ensure that snow is removed promptly from antennas.
The data collected in the winter of 2004-2005 will provide continuous measurements
of vertical temperature and wind profiles, sensible heat flux, and radiation. Analyses will
focus on wintertime cold pool events using these data and supporting data from the SLC
rawinsondes and cloud observations, weather maps, the MesoWest Network and satellite
photos. The NBHD/meteorogram approach will be used for the 2005-2006 data analysis
as it was for the cold pool in the CPB. The time resolution will be half-hourly rather than
twice-daily as in 2001. Gradient Richardson number profiles will be computed using the
wind and temperature profiles. The wintertime data will be prepared for comparison with
RAMS cold pool simulations.
4.3 Numerical Models and Comprehensive Simulations of Cold Pool Episodes
This section describes the models and the plans for using them to make
comprehensive simulations of entire cold pool events in the SLB. This will be followed
in Section 3.4 by a description of the additional simulations that will be made to
investigate the three meteorological processes that we believe may have important effects
on the evolution of multi-day cold pools.
4.3.1 RAMS
RAMS (Pielke et al. 1992) is a nonhydrostatic, mesoscale model that predicts wind,
potential temperature, perturbation pressure and mixing ratio in the atmosphere using the
primitive equations. Subgrid-scale turbulent diffusion is parameterized using a MellorYamada level 2.5 scheme with a prognostic turbulent kinetic energy equation (Mellor and
Yamada 1974, 1982; Helfand and Labraga 1988). Despite known deficiencies in
mesoscale models such as RAMS for stable boundary layer simulations (e.g., Derbyshire
1999), RAMS has been proven capable of simulating boundary-layer development and
terrain-induced flows in basins and valleys in previous studies of the Mexico City Basin
(Bossert 1997; Zhong et al. 1997, 1998b; Whiteman et al. 2000; Fast and Zhong 1998),
Colorado’s Sinbad Basin (Fast et al. 1996), Washington’s Columbia Basin (Doran and
Zhong 1994; Zhong et al. 2001), Switzerland’s Riviera Valley (DeWekker 2002) and in
idealized terrain (DeWekker et al. 1998). Three RAMS features are particularly attractive
for the current proposal. First, two-way interactive grid nesting in both horizontal and
vertical directions allows the model to describe local features such as upslope flows while
still capturing larger-scale forcing on a larger and coarser grid. Second, a fourdimensional data assimilation (FDDA) modification developed by Fast (1995) allows
RAMS to incorporate a variety of field measurements in addition to the large-scale
analysis fields assimilated by earlier versions of the model. Finally, a module is available
to evaluate tendency terms in the heat and momentum budget equations for any given
sub-domain and for any given time or time period. This module has been used in previous
complex terrain simulations by Fast (1995), Zhong and Whiteman (1995) and DeWekker
(2002) to determine the relative contributions of various processes to boundary layer
evolution.
RAMS will be used to simulate selected mid-winter cold pool episodes. Initial
simulations will be made for the cold pool event in the SLB that began on 25 December
2000, followed by simulations of other events that are captured in the 2005-2006 winter
experiments. Since cold pool formation and destruction occur over the basin as a whole,
we foresee using three to four nested grids, with the innermost grid slightly larger than
the immediate SLB at 0.5 to 2 km horizontal resolution and the outermost grid
encompassing the western half of the US at much coarser resolution to capture spatial and
temporal variations of synoptic forcing and to provide the correct large-scale temperature
advection over the basin. Because these simulations must resolve the vertical structure in
and above the basin, the vertical nesting capability of RAMS will be useful. Fourdimensional data assimilation will be used in the outer grids to reduce model errors, but
will not be used in the grid of interest so that the simulated boundary-layer structure and
evolution are determined primarily by the local physics in the innermost domain. Our
earlier work shows that mid-winter episodes may last for several days so that the RAMS
simulations will need to be run for extended periods. It may be advantageous, at times, to
break the long simulations down into specific phases of cold pool evolution (e.g., buildup
and breakup). The initialization and lateral boundary conditions will be based on threedimensional objective analyses performed at 12-h intervals on the outermost grid using
output from NCEP's (National Center for Environmental Prediction) Eta or Aviation
models and surface and upper air observations from the field experiment networks. The
inner domains will then be initialized through interpolation of coarse-grid analyses.
The simulated wind and temperature structures will be compared to the analyses of
data from our field instruments and from supplemental measurements in the basin as
detailed in Sections 3.1 and 3.2. The focus of the comparisons will be to identify
uncertainties and errors in the simulations. Different phases of the cold pool simulations
may then be rerun, turning on and off various RAMS options to try to further identify the
causes of the model deviations from observations.
4.3.2 HYPACT
HYPACT is a Lagrangian particle dispersion model which simulates atmospheric
transport and dispersion based on mean and turbulent velocities by tracking non-buoyant,
non-reactive particles released in the model domain. In HYPACT (Walko et al. 2001;
Tremback et al. 1994), the resolvable wind components are obtained by interpolations
from mean wind fields obtained from RAMS, while the turbulent wind components are
derived though a Markov process. A prognostic equation for turbulent kinetic energy
(TKE) is solved in RAMS and the parameters required for the Markov formulation are
derived from the TKE using analytical formulations developed by Mellor and Yamada
(1974, 1982). By tagging each particle by its release location and release time, the
transport history of a particle can be derived at any subsequent time. Lagrangian particle
dispersion models have been used to investigate boundary-layer transport and dispersion
in Colorado's Front Range (Fast 1995), in the Mexico City Basin (Zhong et al. 1998a;
Fast and Zhong 1998), in Switzerland’s Riviera Valley (DeWekker 2002) , in the Bernese
Oberland (DeWekker 2002), and off the coast of New England (Doran et al. 1996). Other
applications of the combined RAMS and HYPACT model include those of Lyons et al.
(1994) and Lagouvardos et al. (1996).
HYPACT will be used to simulate the transport and diffusion of pollutants during
cold pool episodes in the SLB. Because the HYPACT simulations use wind fields from
RAMS, we will pick periods for HYPACT simulations when RAMS agrees well with
observed meteorological data in the basin including vertical wind and temperature
profiles. The pollutant simulations will be focused on the three phases of cold pool
evolution, the buildup phase, the cold pool maintenance phase, and the breakup phase.
Particle releases during the buildup and breakup phases will evaluate how location within
the basin affects dispersion from ground and near-ground sources. In these simulations,
sequences of particles would be released at points that are widely separated within the
basin including points over the Great Salt Lake, the city, SLB tributaries, and in the
valley center. Additional points would be chosen on a line running up the center of the
Salt Lake Valley from the GSL into the Utah Lake Valley, and from different altitudes on
an orthogonal line running through the basin center and up the Oquirrh and Wasatch
Mountains. These particle simulations will allow us to examine the effects of the
diurnally reversing lake breezes, along- and cross-valley flows, and slope flows in the
basin and to determine whether recirculations will redistribute pollution in the basin over
the multi-day period. Simulations such as these have been shown to be reasonably
accurate in our previous research (Zhong et al. 2001) and can be tested in the SLB by
reference to the MesoWest network surface wind patterns. The ability of
RAMS/HYPACT to accurately simulate vertical dispersion in the basin under stable cold
pool conditions is less dependable because of known deficiencies in the treatment of
dispersion under conditions of light winds and intermittent turbulence in stable layers
(see, e.g., Derbyshire 1999). Simulations with HYPACT in the cold pool maintenance
phase will be focused on events where stratiform clouds are in the pool. These
simulations are described further in Section 3.4.3.
4.4 Examination of Cold Pool Processes
[SHARON, THIS SECTION SHOULD BE CONDENSED, AND REWRITTEN TO
LOOK AT A VARIETY OF PROCESSES RATHER THAN CHOOSING ONLY
THREE. SPECIFICALLY, IT SHOULD INVESTIGATE PROCESSES THAT COME
FROM OUR ANALYSES OF DATA. JIM SHOULD BE ABLE TO HELP US WITHT
HE SYNOPTIC ANALYSES]
4.4.1 Differential Temperature Advection
The comprehensive RAMS simulations of cold pool episodes described in Section
3.3.1 will provide high spatial and temporal resolution information on temperature
structure evolution in and above the SLB. The simulations will also provide information
on the warm and cold temperature advection above the pool during these episodes. We
will examine how the temperature structure inside the basin responds to changes in
temperatures aloft as a result of large-scale advection for complete cold pool episodes.
The analyses require a variable that quantifies temperature advection above the cold pool
and a variable that quantifies the cold pool heat deficit. Temperature advection will be
quantified using the ‘tendency code’ that is used with RAMS simulations to write out the
time derivatives of terms in the heat and momentum budget equations. The temperature
advection terms will be determined at a fixed height that is always above the basin cold
pool using a suitable horizontal spatial averaging domain (~1 km2 over the basin center)
and time averaging interval (~1 hour). In a second step, we will analyze the temporal
development of vertical profiles of the temperature advection tendencies to determine
how temperature advection height profiles change with time as an indicator of the descent
or ascent of the top of the cold pool. We will then attempt to determine cold pool depths
as a function of time using an automated procedure that will identify heights where the
TKE or wind speed gradients suddenly increase with height. (TKE normally increases
rather suddenly above the quiescent cold pool as the synoptic scale flows are
encountered.) The cold pool heat deficit or NBHD will then be computed (again over
appropriate space and time scales) up to that height. A joint time series of temperature
advection and cold pool heat deficit will then be constructed to examine the relationship
between the two variables. Adjustments to this procedure may be required. This analysis
method will also be used to examine other terms in the heat and momentum equations.
We will compare the differential temperature advection analyses from the RAMS
output with meteorological data, with the goal of determining whether differential
temperature advection is a key process affecting cold pool evolution and whether
differential temperature advection is accurately simulated by RAMS. Analysis methods
have been discussed previously in Sections 3.3.1, 3.1 and 3.2.
4.4.2 Turbulent Erosion
We will make detailed analyses of RAMS simulations for events where turbulent
erosion appears to play a significant role in the destruction of cold pools in the SLB. Our
emphasis in these simulations will be on the vertical structure and evolution of simulated
turbulent kinetic energy (TKE) in and immediately above the cold pool, and its
relationship to temperature structure and evolution. Detailed TKE budget analyses should
reveal how turbulence generated by wind shears at the top of the inversion penetrates
downward into the inversion, causing the top of the cold pool to descend. Because the
focus of the model analyses will be on TKE, FDDA will be applied to the innermost grid
as well as the outer grids, in contrast to the simulations of temperature advection where
FDDA will be applied to the outer grids only. Data from RP/RASSes, sodars,
tethersondes, and the MesoWest network will be assimilated to improve simulation
accuracy and to produce optimal results. Our previous work indicates that turbulent
erosion events are rarely ‘pure’ events and are often combined with other effects (wave
motions, advection, etc.). The numerical simulations are expected to help us investigate
this further. Tendency terms will be calculated for the thermal energy equation and
momentum equations over appropriate domains just above and below the cold pool top to
gain more insight into the turbulent erosion process and other mechanisms that may
complicate the analysis of the RAMS simulations. HYPACT particle simulations will be
made for these events using RAMS wind fields. These simulations will help us visualize
mixing and diffusion processes that occur near the top of the pool.
Our turbulent erosion model (Zhong et al. 2003) will be used initially to simulate
turbulent erosion events in both the Austrian Basins and the Salt Lake Basin. The
Austrian basin simulations will allow us to test the model before turbulent erosion data
sets are available from the SLB. The turbulent erosion model can be configured to have
good vertical resolution near the inversion top, and its simple form and low
computational requirements will allow us to make simulations in which model
parameterizations can be easily varied. The simulations with the turbulent erosion model
and comparisons of the model with data will support the development and testing of
hypotheses (e.g., does continuous turbulent erosion require continuously increasing wind
shears?) and may suggest needed improvements in mesoscale numerical models such as
RAMS for turbulent erosion modeling. It may also help answer questions regarding the
vertical resolution needed in observations at the cold pool top. After the Austrian
simulations are completed we will apply the model to the SLB turbulent erosion episodes
simulated with RAMS. Comparisons between the models and data from these events will
evaluate the different approaches in the two models to gain a better understanding of
these episodes and the reasons for any differences in model performance.
4.4.3 Stratiform Clouds within the Cold Pool
Clouds, often present in SLB cold pools, affect cold pool formation and dissipation in
three ways. First, sensible heat is released or expended in the basin as clouds form and
dissipate. Second, longwave radiative divergence at the cloud top affects the temperature
structure in the basin. Cold air generated at cloud top convects downwards into the cloud
and the sub-cloud layer below, destabilizing the sub-cloud temperature structure. Third,
clouds affect short wave radiative transfer. Their high albedo reflects incoming insolation
back to space, thus reducing insolation and sensible heat below the clouds on the basin
floor. Each of these effects are included in our RAMS simulations, which will be run
with moist microphysics when appropriate for the cold pool being modeled. Our previous
experience with cold pool simulations in the Columbia Basin shows that the role of
clouds in cold pool evolution can be determined from pairs of RAMS simulations where
the cloud microphysics scheme is turned on or off to produce or remove clouds from the
simulations (Zhong et al. 2001). Comparisons between the cloudy and clear simulations
then allows a determination of the effects of clouds on the cold pool structure.
Investigation of the tendency terms in the heat and momentum equations allows further
quantitative conclusions to be drawn. This is the approach that we will take, using as a
starting point a successful RAMS meteorological simulation of a cold pool maintenance
phase in which the wind and temperature structure predictions match observations well.
Simulations of the cloudy and clear events will be compared with observations.
Some of the properties of the stratiform clouds in the basin will be reasonably well
known. Rawinsonde observations at the Salt Lake City airport provide twice-daily
temperature and moisture profiles from which cloud depths can be estimated. The
heights of the cloud bases are measured continuously by a ceilometer and reported in
hourly airways observations, which also provide information on fractional sky coverage.
Six-hourly visual observations of the cloud types are also recorded at the airport. The
microphysical properties of these relatively long-lived clouds will be estimated from the
literature. We will collaborate with Dr. Sharon Zhong at the University of Houston on
these simulations. She will be submitting a separate proposal to VTMX for this research
and her experience with microphysical options and land-atmosphere exchange options in
RAMS will be very beneficial.
HYPACT simulations will also be run for the cloudy/clear simulations discussed
above. Because of the radiative and convective overturning effects of clouds, the cloudy
cold pool is expected to be only weakly stable compared to cold pools that form under
clear skies. RAMS/HYPACT turbulence and particle simulations are expected to be more
dependable under these conditions than for the stronger stability in clear-sky cold pools.
We will make HYPACT simulations of dispersion for both the clear and cloudy cold pool
cases (but with more confidence in the cloudy results than the clear sky results), with
particles released at different heights in the center of the basin. These simulations will be
run for multiple days to examine the vertical rates of dispersion within the pool and the
loss of particles out the top of the cold pool.
4.5 Summary
We have proposed a research program that uses observations, analysis and modeling:
to determine the processes responsible for the formation, maintenance and
destruction of multi-day cold air pools that form in urban basins in winter, to evaluate
their influence on pollutant transport and mixing, and to determine how models can be
improved to make more accurate simulations of cold air pools.
This research will be conducted using data from the Salt Lake Basin, one of many
urban basins in the United States that experience serious cold pool-related air pollution
episodes. Analysis and modeling will begin with a cold pool episode that formed in the
SLB on 25 December 2000 and lasted until 9 January 2001. These analyses will use the
NBHD method of quantifying and monitoring the heat deficit in the cold air pool that we
developed in a prior project in the Columbia Basin. Additionally, we have designed and
will lead a new data collection effort that will deploy a comprehensive set of remote
sensing and in situ instruments to observe cold pools in the SLB during the winter of
2004-2005. The instrument deployment will provide a first look at the full threedimensional structure of basin cold pools and its evolution with time.
The experiment will provide the continuous space- and time-resolved data to monitor
the evolution of cold air pools and the processes leading to their formation, maintenance
and destruction. The new data and model simulations of complete episodes are designed
to identify the key processes leading to cold pool evolution in the SLB. Additional
simulations have been designed to investigate three specific processes : differential
temperature advection, turbulent erosion, and the formation of stratiform clouds in the
pools. Detailed comparisons between the models and the meteorological data sets will
identify shortcomings in the models. Such shortcomings are clearly present in the
operational numerical models (and probably also in RAMS), as weather forecasters are
still experiencing difficulty in making accurate forecasts of cold pool onset and cessation
times. To our knowledge, the new wintertime data set will be the first to support
investigations of the turbulent erosion process. Simulations of the cold pool episodes with
RAMS will identify and determine the relative importance of the different processes
leading to the different phases of their life cycle. Coupled simulations with HYPACT will
identify the effects of these processes on the vertical and horizontal transport and mixing
of pollutants released in the basin. A turbulent erosion model and data from other basins
supplement the main modeling approaches and provide the means of testing hypotheses
using data from other basins.
5. Broader Impacts
a. Integration of research and teaching
The project will feature the direct infusion of results into undergraduate and graduate
courses at the University of Utah. This will be done in three ways. First, new findings
will be incorporated into traditional classroom lectures. Second, learning modules based
on the cases being examined will be built by the PIs and used for classroom instruction in
stable boundary layers. Finally, students will work with the PIs and their students in
performing the analyses. Regular discussions related to cold pool evolution will occur as
part of a weather discussion class that meets three times a week. The PIs have integrated
results from earlier NSF- and DOE-funded work into lectures and teaching modules that
have been used at other universities, at the Summer School on Mountain Meteorology in
Trento, Italy, and for National Weather Service, National Interagency Fire center and
American Meteorology Society training courses. Similar integration of research findings
into education and training efforts of this type will continue as part of the proposed
project.
b. Graduate student and post-doctoral mentoring
The project involves the support and mentoring of two graduate students and partial
support of one post-doctoral research associate.
c. Dissemination of project results
Based on past experience, we anticipate publishing two peer-reviewed manuscripts per
year in AMS journals (e.g., Monthly Weather Review). Project results will also be
presented at the AMS Mesoscale Processes, Mountain Meteorology, and Weather and
Forecasting conferences.
d. Potential benefits of proposed activity for society at large
Between the 1990 and 2000 national censuses, the five fastest growing states were
Nevada, Arizona, Colorado, Utah, and Idaho. Socioeconomic impacts of cold air pools in
this region continue to grow, as well as vulnerability to climate variability. Potential
benefits of the proposed activity for society at large include improved understanding,
analysis, and prediction of cold pool events over the western United States. This will help
mitigate the socioeconomic impacts of cold pools over the region and improve our
knowledge of western U.S. climate.
6. Collaborations
The research will be conducted in collaboration with co-Principal Investigators from
the University of Utah and the University of Houston, as well as with collaborators from
other institutions who will bring their own funding to this research. All investigators will
have full access to cold pool data and analyses and will be part of a team that will be
reporting the research results.
Dr. Sharon Zhong and her students from the University of Houston will perform
simulations of persistent cold pool events in the Salt Lake Basin using the RAMS and
HYPACT models. Dr. Zhong has extensive experience in this type of research. Her
contributions will be primarily in simulating cold pool events, but may extend to the
testing of sub-models of cold pool buildup and destruction processes, including turbulent
erosion and slope flows.
Co-PIs from the University of Utah include Drs. James Steenburgh and John Horel at
the Department of Meteorology, and Dr. Eric Pardyjak at the Mechanical Engineering
Department.
Dr. Jim Steenburgh and one of his students will assist with the climatological
identification and investigation of cold pool formation and destruction processes, using
historical and new case study data and model reanalyses. Dr. Steenburgh’s extensive
experience as a synoptic analyst and his years of experience in the Salt Lake Basin
uniquely qualify him for this role.
Dr. John Horel and one of his students will assist with the analysis of the persistent
cold pool event of 25 December 2000 - 9 January 2001. In addition, Dr. Horel will assist
in arranging student support for the 2005-2006 field experiment and in accessing
MesoWest data from the University of Utah archive that he maintains.
Dr. Pardyjak and one of his students will participate in 2004-2005 winter field
experiment by supplying data and analyses from three flux towers that he will be
operating in the SLB as part of the ongoing NSF-supported UTES experiment. His
analyses will focus on the fine-scale atmospheric structure and surface fluxes during cold
pool events.
Collaborators at other institutions will bring their own funding to the research project.
Dr. Thomas Haiden (ZAMG, Vienna, Austria) will investigate cold pool formation
and destruction processes in Austrian basins and apply the findings from the SLB and
Austrian basins to the improvement of cold pool forecasts in Austria. He will extend an
analytical model of basin cold pool evolution that he developed with the PI in a scientific
visit in 2003. Dr. Haiden’s research contributions will be funded through internal funding
at ZAMG, which seeks to improve forecasting tools in Austria’s Klagenfurt and Lungau
Basins. Some travel and living expenses for his annual visits to the University of Utah are
included in our budget and we will be loaning several temperature data loggers to him to
support his research.
Drs. Reinhold Steinacker and Manfred Dorninger and their students (University of
Vienna) will collaborate on analyzing wintertime cold pool data from a set of small
closed basins (sinkholes) on the Hetzkogel Plateau in the Eastern Alps southwest of
Vienna using an extensive data set. Additional data is still being collected routinely in
these basins. These data extend the SLB analyses into new climate settings and
topographies. Joint analyses to date have involved two Univ. of Vienna exchange
students, are leading to 3 Master’s theses and have produced new research ideas.
Dr. Elford Astling at the US Army’s Dugway Proving Ground (DPG) will participate
in the planning for the 2005-2006 experiment and may be able to provide data from a
radar profiler/RASS that is operated at DPG and equipment or field personnel for the
wintertime experiment.
Mr. Brock LeBaron from Utah’s Division of Air Quality will provide assistance in
securing air quality and meteorological data for cold pool events from their network. His
participation will bring an air pollution and regulatory agency perspective to the cold
pool problem.
Dr. Larry Dunn from the National Weather Service Forecast Office in Salt Lake City
will participate in the planning of the cold pool research as an observer interested in
application of the research findings to the forecaster community.
Letters from the non-Utah investigators, describing their contributions to the project
are provided in the supplementary documents section of this proposal.7. References
Cited [HAS NOT BEEN CHECKED YET FOR CORRESPONDENCE BETWEEN
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