A Climatological Study of Wind Systems of the US Intermountain West
Jebb Q. Stewart1, C. David Whiteman2, W. James Steenburgh1, and Xindi Bian2
1
NOAA/Cooperative Institute for Regional Prediction and Department of Meteorology
University of Utah
Salt Lake City, Utah
2
Pacific Northwest National Laboratory
Richland, Washington
Submitted to Bulletin of the American Meteorological Society
04 May 2001
Corresponding Author:
Jebb Q. Stewart
University of Utah
Meteorology Department
480-3 INSCC
Salt Lake City, UT 84112-0110
e-mail: jstewart@met.utah.eduABSTRACT
The Intermountain West of the United States, because of its dry continental climate, is an
area where local thermally driven winds occur frequently. This paper documents the diurnal
evolution of thermally driven valley, slope, and lake winds for summer fair-weather conditions
in four regions of the Intermountain West where dense wind networks have been operated.
Because of the diverse topography in these regions, the results are expected to be broadly
representative of variations in thermally driven wind climates throughout the Intermountain
West. The regions include the Wasatch Front Valleys of northern Utah, the Snake River Plain of
Idaho, the Southern Nevada basin and range province, and central Arizona where the Salt and
Gila Valleys run out onto a plain. The Analysis focuses on the interplay of valley, slope, and lake
winds, as well as the frequency and regularity of the winds.
In general, on fair weather days winds in all four regions exhibit a consistent direction from day
to day at a given hour. A measure of this wind consistency is defined. The nighttime hours
exhibit a high consistency, the daytime hours a moderate consistency, and transition periods
between daytime and nighttime regimes a low consistency. Low consistency during the transition
periods reflects day-to-day variations in the timing of thermally driven wind system reversals.
Thermally driven circulations are similar in the four regions, but the Wasatch Front Valleys are
influenced by lake breezes from the adjacent Great Salt Lake, the Snake River Plain is influenced
by wind channeling from nearby mountains, Southern Nevada winds are influenced strongly by
the distinctive Basin and Range topography, and winds in Central Arizona and the Southern
Nevada basin and range province areas experience regional-scale circulations associated with
regional scale contrasts in elevation and/or surface heating.1. Introduction
The complex topography of the western United States produces a wide variety of thermally
and dynamically driven mesoscale wind systems. Historically, knowledge and understanding of
such systems has been limited by a variety of factors, including a lack of observational data. In
the Intermountain West (IW), the semiarid area between the Cascade-Sierra mountain chain and
the eastern Rocky Mountains, drier air and soil conditions promote intense daytime solar heating
and nighttime radiation loss resulting in large diurnal temperature changes that produce strong
thermally-driven winds (Carter 2000). Thermally driven wind systems in mountainous terrain
incorporate two basic wind circulations: the slope winds and valley winds. The superposition of
these wind systems produces clockwise and counterclockwise diurnal rotations on the left- and
right-hand sidewalls, respectively, when looking up a valley (Hawkes 1967, Whiteman 1990). In
some regions of the IW such as the Wasatch Front Valleys (WFV) of northern Utah where
mountains are located adjacent to lakes (Fig. 2a), a third diurnal wind system is apparent, the
lake-land breeze. In the southern Nevada basin range province and central Arizona, the major
thermal wind component is regional-scale circulations associated with regional scale contrasts in
elevation and/or surface heating. The interaction of these wind systems creates complex flow
patterns which are a part of the everyday winds in complex terrain. Understanding of these wind
systems is valuable to boundary layer and air pollution meteorologists, fire weather forecasters
and operational meteorologists.
Over the past few years high-density surface data from more than 50 independent
meteorological observing networks has been gathered as part of MesoWest, a collection of
cooperative mesonets in the western United States (mesowest; Horel et al. 2000). MesoWest is
managed jointly by the NOAA Cooperative Institute for Regional Prediction at the University of
Utah and the Salt Lake City National Weather Service Forecast Office and presently collects data
from over 2500 surface stations in the western US. In this paper, we illustrate the interaction of
complex terrain wind flows in the IW MesoWest data from 1997  1999.
2. Areas of study
The four study regions were chosen because they have high-density observations and
illustrate typical thermally driven wind systems of the IW (Fig. 1). Here we introduce the
regions, in turn, going counterclockwise around the IW.
The Wasatch Front Valleys, including the Salt Lake, Tooele, and Rush Valleys, are located
between three 3000 m above mean sea level (MSL) mountain rages, the Wasatch, Oquirrh, and
Stansbury Mountains [(MSL), Fig. 2a]1. The Tooele and Rush valley's are separated by South
Mountain reaching elevations of 1900 m. The Tooele Valley slopes downward to the north from
1500 m near South Mountain to 1300 m at the shoreline of the Great Salt Lake (GSL). The Rush
Valley is a high elevation basin with a minimum elevation of 1560 m. Several low passes are
found to the southeast of the Rush Valley, which is bounded to the south by the 3000 m high
East Tintic and Sheep Rock Mountains. The Salt Lake Valley slopes downward to a north-south
axis from the mountains and downslope towards the GSL. The south end of the Salt Lake Valley
is closed off by the Traverse Mountains (1800 m), with the exception of a gap through which the
Jordan River flows to the GSL. From the Wasatch mountains, several major canyons issue into
the Salt Lake Valley. The region is semi-arid, with vegetation consisting mainly of grasses and
low shrubs (mainly sagebrush). Substantial urban development is found over the eastern two
thirds of the Salt Lake Valley. The shallow nature of the GSL, which is only 4.8 m deep on
average, causes climatological lake temperatures to exhibit little lag relative to climatological air
temperatures (Steenburgh et al 2000).
The Snake River Plain (SRP) is a arc-shaped valley in southern Idaho that slopes downward
to the west from 1500 m to 900 m (Fig. 2b). To the north are the central Idaho Mountains, which
reach elevations over 3000 m L. To the south, several mountain ranges reach elevations of
around 2100 m. Several major canyons issue into the SRP from the surrounding mountains, with
three especially prominent canyons entering the east end of the SRP from the north. The SRP is
arid or semi-arid with vegetation consisting of short grasses and shrubs (mainly sagebrush) at the
lower elevations and low density coniferous forests at higher elevations.
Within the Southern Nevada basin and range province region, the general terrain slopes down
from the mountainous area to the north (1500 m) to the low plains to the southeast ( 570 m; Fig.
2c). This landscape is broken up by many meridionally oriented mountain ranges that reach
elevations around 3000 m. The lowest elevations are to the southwest in the Death Valley of
California (below sea level) and to the southeast along the Colorado River (360 m). The climate
is semi-arid except on the highest mountain ranges. Vegetation is sparse with short grasses and
shrubs at mid elevations and low density forests at highest elevations. Barren ground and arid
vegetation are found at lower elevations.
The central Arizona region lies to the west of the Sierra Ancha (Fig 2d). The general terrain
slopes from the Sierra Ancha (1900 m) southwesterly to the Yuma Desert (200 m). The
1Unless otherwise indicated, all elevations specified in the remainder of this paper are relative to mean sea-level.
Mazatzal Mountains are a 1500 m barrier between the Sierra Ancha and Phoenix (PHO). From
the Sierra Ancha and surrounding mountain ranges, several canyons issue into the lower Phoenix
basin, including the Black Canyon north of Phoenix through which Interstate 17 runs. Central
Arizona is arid with intense sunlight throughout much of the summer. Only sparse grass
vegetation survives the lack of precipitation leaving large areas of barren ground. Short shrubs
and grasses survive at mid elevations, and low density coniferous forests are found at the highest
elevations.
3. Data and methods
Surface observations were provided by MesoWest, a collection of more than 50
independently-operated mesonets across the western U.S. The combined resources of the
MesoWest results in high-density observations in regions not well sampled by the conventional
Federal Aviation Administration/National Weather Service/ Department of Defense Network.
Data were collected by phone modems, internet connections, or radio transmission and archived
at the University of Utah where a quality control processes remove erroneous values.
Several further processing steps were implemented to identify the thermally driven
circulations in each study region. First, data for the summer months June through August (jja)
were extracted from the MesoWest archive for the years 1997-1999. The summer period was
chosen because when synoptic flows are typically weak and valley flows are more pronounced.
Second, any sub-hourly-averaged data (e.g., 5 min, 15 min, or 30 min) were averaged to one
hour, using the convention that the time indicated represented the beginning of the hour of
interest. Third, time periods in which thermally driven flows would likely be pronounced (i. e.,
when wind speeds aloft were weak and skies were clear or partly cloudy were identified). Such
periods were identified in 12-hour blocks centered on the rawinsonde observations times (2200 0900 LST for the 400 LST sounding and 1000 - 2100 LST for the 1600 LST sounding2), and
featured a 700-hPa wind speed of  7 m s-1 and a total daily solar radiation of  65% of the
theoretical extraterrestrial solar radiation computed from a solar model (Whiteman and Allwine
1986). The solar radiation criteria was considered to apply to the 2200-2100 LST period that
corresponds to the rawinsonde time periods above. These criteria are similar to those used to
identify periods of thermally driven flows in previous research in the Intermountain West
(Whiteman et al. 1999). Locations for the 700 hPa wind and solar radiation data are listed in
Table 1. All observations meeting the criteria described above were grouped into hourly bins and
processed to determine a mean vector wind and a mean arithmetic wind speed for that hour of
the day. The mean hourly vector winds were then plotted on maps and a video loop was prepared
to animate the hourly maps to investigate the evolution of the hourly winds during a mean
thermally driven wind day. Then, a method was developed to determine the fair weather day-tofair weather day variance of wind direction for each hour of the day. This was accomplished by
defining wind consistency, defined as the ratio of the vector mean and arithmetic mean wind
speeds for each hour of the day. If the wind blows from the same direction at a given hour on all
the fair weather days, the consistency is one; if it is equally likely from all directions, or blows
half the time from one direction and half the time from the opposite, the consistency is 0. This
definition of wind consistency is similar to Panofsky and Brier's (1965) definition of wind
persistence, defined as the steadiness of the wind over a continuous time period. We prefer the
2Soundings are launched an hour before 0000 and 1200 UTC soundings. Using data from lower region of sounding
(below 700-hPa) is applied to hour of launch.
term wind consistency because we deal with a discontinuous data set (only fair weather days and
observations at a given hour of the day). A wind consistency of 1 denotes that vector winds are
identical at the hour of the day of interest during all days that pass the joint fair weather criteria
in the period of record.
4. Results
a. Wasatch Front Valleys
The Wasatch Front Valleys are located south of the GSL where Hawkes (1947), Smidy
(1979), Astling (1986) and Stone (1987) investigated the interactions between the diurnal
land/lake and mountain wind systems. These studies did not, however, attempt to isolate the
thermally driven component of the flow by considering only situations with clear skies and weak
synoptic-scale forcing.
Winds in the Wasatch Front Valleys are the result of lake/land breezes, slope flows, and
valley flows. During the night (0400 LST), downslope and down-valley winds are observed in
the Salt Lake and Tooele Valleys (Fig, 3a). The down-valley winds are likely reinforced by
offshore flow induced by the GSL. Since the Tooele Valley slopes little near the GSL and
steepens as one moves towards South Mountain, weaker winds (1-3 m s-1) are found near the
GSL, while stronger winds (3-4 m s-1) are found farther up valley. In the Rush Valley, an
elevated basin with few low gaps for winds to exit, the slope of the land is gentle except near the
mountains. This results in light (~1 m s-1) downslope winds over lower portions of the valley
that converge into the basin. At higher elevation sites within the Rush Valley, stronger
downslope winds (1.5-4 m s-1) are found. Along the the South Mountain divide, 4-5 m s-1
southerly winds illustrate the nocturnal flow of air across the divide from the Rush to the Tooele
Valley. The southwest wind observed along I-15 near the summit of the Traverse Mountains
illustrates a similar flow pattern between the Utah and Salt Lake Valleys (small-scale terrain
effects result in southwesterly rather than southerly flow at this site). These conditions persist
until the morning transition period.
The morning transition period starts at sunrise (~0600 LST) and continues through about
1100 LST. During this period, downslope and down-valley winds weaken and eventually
become upslope and up-valley, with the slope flow transition preceding the valley flow transition
by about 2 h. At 1000 LST, the mid-point of the morning transition period, winds over the Salt
Lake Valley and eastern Tooele Valley have developed an upslope component (Fig. 3b) while
the down-valley flow is beginning to weaken. A lake breeze is beginning to move into the
northern Tooele Valley. This circulation feature is first observed near the GSL shoreline at 0700
LST and advances southward through the Tooele Valley to South Mountain by 1100 LST (not
shown). The southward lake-breeze penetration is slower over the Salt Lake Valley (Fig. 3b).
During the afternoon regime, circulations within the Wasatch Front Valleys are the result of
of interactions between upslope flows, up-valley flows, and the GSL breeze. At 1600 UTC,
observations in the Tooele Valley show a coupling between upslope and up-valley flows, which
are in phase with the GSL breeze and produce a diffluent up-valley wind pattern (Fig. 3c). A
similar but weaker flow pattern in which upslope flows are more dominant is observed over the
Salt Lake Valley. The anomalous westerly flow at Magna (QMG) in the Salt Lake Valley is the
result of the interaction of the GSL breeze with the local terrain. In contrast to the Tooele and
Salt Lake Valleys, there is little evidence of upslope and up-valley winds within the Rush Valley.
Instead, the GSL-breeze crosses the South Mountain divide, producing strong northerly flow
throughout the Rush Valley (Fig. 3c) and providing an example of how an external thermally
driven wind system can overwhelm local slope and valley flows.
The evening transition period begins around sunset (~1900 LST). Within three hours of
sunset (2200 LST), downslope and down-valley flows are all ready well developed (Fig. 3d). In
fact, downslope and down-valley flows reach their strongest magnitude during this period. In
contrast to the morning transition which required 5-6 h to complete, the evening transition is
accomplished in only 2-3 h.
Time series from selected observing sites are presented in Figs. 3e-h. Stations on the west
side of the Tooele Valley exhibit a counterclockwise rotation of the wind with time (e.g., RES,
Fig. 3e), while stations on the east side of the valley feature a clockwise rotation (e.g., TOO, Fig.
3f). This is consistent with the conceptual model of the diurnal turning of the winds over valley
sidewalls presented by Whiteman (1990) since the thermally driven lake/land and up/down
valley circulations are in phase and are superimposed on the up/down slope circulations.
Stations in the middle of the Tooele Valley, such as Erda Airport (POR, Fig. 3g), are
influenced primarily by coupled lake/land and up/down valley flows. At POR the day-to-day
wind consistency at individual hours is above 90% during the nighttime hours when offshore
down-valley flow dominates, then drops to 20% at ~0900 LST due to day-to-day variations in
the time of the development of the lake breeze and up-valley circulation. Consistency increases
to near 75% late in the afternoon (1600 LST) when the lake breeze and up-valley circulation
typically are well developed.
High consistency in nighttime downslope flow over the Tooele Valley also is evident at
Grantsville Reservoir (RES). At ~2100 LST down-slope winds are developed and persist for 9-
10 hours with a consistency above 90%. The strongest down-slope winds (4-5 m s-1) occur
during for the first two hours of this period right after sunset (Figs. 3e,h).
Finally, there are several major canyons that issue into the Salt Lake Valley from the east.
Of these, Parley's Canyon is known to produce strong outflow into the Salt Lake Valley during
the night and early morning hours. The Parley's Canyon (UT5) time series shows that this
outflow develops typically around 1900 LST, reaches peak magnitude around 0200 LST, and
persists overnight until 0900 LST. Such outflow has a consistency of more than 90%.
b. Snake River Plain
A dense network of meteorological observing stations is found on the eastern Snake River
Plain (SRP) in Idaho at the Department of Energy's Idaho National Engineering Laboratory
(INEL). These stations have provided data for several climatological studies focused on transport
and diffusion (e.g., Clawson 1989, Clements 1979, Wilkens 1955, and Yanskey et al. 1966). The
Eastern SRP generally experiences plain-parallel flows from either the southwest or the
northeast. Southwesterly flows are caused by the channeling of the prevailing westerlies by the
surrounding mountains or by daytime, thermally driven, up-plain circulations. Northeasterly
flows are, likewise, caused by channeling of winds from easterly directions or by nocturnal
down-plain flows (Clawson 1981). The strong up-valley flow can persist for many hours after
sunset (Clawson 1981). While the eastern SRP wind flows are well documented, little
information exists for flow patterns over the central and western SRP and prior studies have not
attempted to isolate on the thermally driven flow component.
Although winds in the SRP are generally oriented along the plain's longitudinal axis, they are
also influenced by up- and down-slope flows from the adjacent sidewalls and outflow from
canyons that extend into surrounding mountain ranges. During the nocturnal regime (0300 LST;
Fig 4a), 2-4 m s-1 down-plain flow extends along much of the SRP. Downslope flow and canyon
outflow are observed at several locations near adjacent mountain ranges. In the eastern SRP,
winds in the central portion of the plain exhibit a northerly component, apparently due to
downslope flow from the high terrain in the northern half of the SRP.
The morning transition period begins with solar heating shortly after sunrise (~0600 LST).
By 0900 LST, several stations have developed up plain or upslope flow, although downslope and
down plain flow persists in the other locations (Fig. 4b). Low level divergence occurs west of
IDF as up-slope flows develop on both the north and south sidewalls of the valley (Fig. 4b).
The transition from down- to up-slope takes 3-4 h while the down- to up-plain transition takes 56 h. The slope flow transition is quicker in the SRP than in the Wasatch Front Valleys, while the
valley flow reversal is slower.
During the afternoon regime (1500 LST) the up-plain flow develops and strengthens to 3-4
m s-1 along the longitudinal axis and becomes the dominant flow pattern throughout the SRP
(Fig. 4c). Topographic channeling helps to enhance this effect. At the peak of the up-plain flow,
around 1700 LST, it diverges only weakly into the surrounding topography. Stations farther
away from the mountain ranges are the first to develop strong up-plain flow as illustrated by
timeseries from Howe (HOW) which is located near the Central Idaho Mountains, and Richfield
(RHF), which is closer to the mid-plain axis (Fig. 4e,f). At HOW the south-southeasterly upslope flow develops around 1000 LST and persists until around 1400 LST when the
southwesterly up-plain flow dominates (Fig. 4e). At RHF the up-slope flow also develops at
1000 LST, but is overtaken by the strong up-plain flow at 1200 LST, 2 h earlier than HOW.
Within an hour of sunset (2100 LST), the evening transition period begins. At 2100 LST,
down-slope flows and canyon outflow combine with the developing down-plain flow to produce
a low level convergence zone over the eastern SRP (Fig. 4d). The up/down-slope transition takes
1-2 hours, where as up to down-plain transition takes 5-6 hours, eventually resulting in the
reestablishment of the evening regime (Fig. 4a).
Time series from selected observing sites in the SRP are presented in Figs. 4e-g. Consistent
with the Whiteman (1990) conceptual model for the diurnal turning of the winds over valley
sidewalls, winds rotate clockwise with time at stations on the north (right relative to downslope)
side of the plain (RHF, Fig. 4f) and counterclockwise with time at stations on the south(left
relative to downslope) side of the plain (IDF, Fig. 4g).
At RHF and HOW consistency remains above 80% through out the night, like similar
stations in the WFR region(0000-0600 LST; Figs 3g,h; 4e, f). The transition period for slope
flow last 3 hours at HOW (Fig 4e) where as valley flow transition last for 6 hours (ex RHF; Fig.
4f), with consistency values near or below 40%. During the daytime heating hours (1200-1800
LST) consistency remains at moderate levels near 60% until the evening transition period (~2100
LST) when consistency values drop to near 30% at HOW (Fig. 4e) and near 50% at RHF (Fig.
4f). Once the nocturnal regime is developed, consistency values return to 80% completing the
cycle.
c. Southern Nevada basin and range province
Winds within the Southern Nevada region are influenced by local thermally driven flows, as
well as regional-scale circulation features. In the northern half of the study area, where the
Nevada National Oceanic and Atmospheric Adminstration Air Resources Laboratory
(NOAA/ARL) maintains a dense surface observing network at the Nevada Test Site, the
topography is characterized by a complex system of mountain ranges separated by broad lowland
valleys and basins (Fig. 2c). To the south, the topography includes the Sheep Range, which
reaches elevations of up to 3000 m, and the Spring Range, which includes 3630 m Mt.
Charleson. The Las Vegas Valley is located just northeast of the Spring Range and runs
southeastward to Las Vegas and Lake Mead along the Colorado River.
Winds during the nocturnal regime are dominated by locally forced slope and valley flows
(0300 LST, Fig. 5a). In the southern half of the study region, downslope flows from the
northeast and southwest converge into Las Vegas, while southwesterly downslope flow is
observed northeast of the Spring Range. Winds in the northern portion of the domain show little
mesoscale organization. Instead, a wide variety of wind directions and speeds are observed due
to the complexity slope and valley flows that are produced by the complex assortment of
topographic features found in this region.
During the morning transition period, widespread southeasterly-southerly flow develops at
most southern Nevada observing sites, with weak confluence evident over the Nevada Test Site
(0900 LST, Fig. 5b). Although these flows may be partially thermally-driven, the erosion of the
nocturnal boundary layer and growth of the daytime convective boundary layer results in
increased coupling between surface-layer winds and the prevailing, regional-scale, southwesterly
winds that are produced by the pressure contrast between high pressure over the eastern Pacific
Ocean and low pressure over the interior Intermountain West. As a result, 3-4 m s-1
southwesterly flow eventually dominates the flow pattern over the Nevada Test Site during the
afternoon regime (1500 LST, Fig. 5c). These southwesterly winds have a high consistency that
during midday exceeds 70% (e.g,. DRA, Fig. 5e). In contrast, southeasterly up-valley flow is
able to develop over the Las Vegas Valley, a deep and relatively large-scale lowland feature
surrounded by very high topography, particularly to the southwest. This results in the
development of an afternoon convergence zone in the lowland region north of the Spring
Mountains. The Nellis Range observing site (A36), which is located in the central Las Vegas
valley near the base of the lower slopes of the Spring Mountains, observes the development of
up-valley flow at 0900 LST, which reaches 3-4 m s-1 during the afternoon (Fig. 5f). The
consistency of this up-valley flow approaches 80% and, in general, afternoon wind consistencies
throughout southern Nevada are higher than those observed over the Wasatch Front Valleys and
Snake River Plain (cf. Figs. 3e-h, 4e-g, 5e-g ). In addition, some stations, such as Area 2 (A02)
in the Nevada Test Site, have very high consistencies through most of the diurnal cycle, with
only small reductions in consistencies during transition periods (Fig. 5g). This suggests that the
transition from daytime to nightime flow regimes at this station are regular and relatively rapid.
Entering the Evening transition (2100 LST), the southwesterly flow weakens and nocturnal
drainage flows begin to develop (Fig. 5d). Stations near Las Vegas and within the Las Vegas
Valley are the last to complete the transition, and at 2100 LST are still experiencing strong
southewesterly winds or southeasterly up-valley flow.
d. Central Arizona area
Thermal winds within the central Arizona region can be separated into two subcategories,
those protected, and those exposed to the up-valley flow from the Yuma desert. Protected
stations observe a daily diurnal cycle of wind consistency similar to areas in STRV and SRP
regions. Exposed stations show similarity to those in the Southern Nevada area with wind
consistency highest during the afternoon. The thermal wind cycle in this region beginning with
the Nocturnal Regime (0300 LST) (Fig. 6a) show winds at mountain sites have a high
consistency nocturnal drainage down-slope flow. Sites around Phoenix show drainage
convergence into lower area of Phoenix basin. Most drainage flows including down-valley flow
are 1-2 m s-1. During this time, the two subcategories show similar conditions. Within two
hours of sunrise(~0530) the morning transition begins with down-slope transition into up-slope
winds and down-valley transition to up-slope winds (Fig. 6b). Transition times for down-slope
to up-slope (1 h) are comparatively quicker the transition times for up- down-valley(3-4 h), with
both transitions begin around the same time.(~8 LST). Immediately after the transition winds
speeds pick up to 3-4 m s-1 around the Phoenix area closer to the Yuma desert while winds along
the Sierra Ancha remain at 1-2 m s-1. This is the beginning step of segregation of the two
subcategories.
Through out the afternoon regime (1500 LST) (Fig. 6c) up-slope and up-valley flows
strengthen near Phoenix and close to the west desert reaching 5 m s-1 and higher. Especially
slopes exposed to the southwest. Stations in the Sierra Ancha remain the same around 1-2 m s-1.
Looking at the Luke Air Force Base (LUF) time series you can see the higher wind consistency
(70-80%) in the afternoon as the southwesterlies strengthen to 4.5 m s-1 (Fig. 6e). During this
time period is when the two subcategories become apparent. Stations exposed to the up-valley
winds from the Yuma desert observe higher wind consistency in the afternoon then stations
within protected areas with thermal up-slope and up-valley flow (80% compared to 50-30%).
As soon as the sun sets (~ 1930 LST) the Evening transition period begins as up-slope winds
reverse to down-slope winds in the Sierra Ancha (Fig. 6d). An hour later sunset the down-valley
wind transition begins. The slope wind transition is about one hour where as valley wind
transition takes 3-4 hours. Northern stations are the first to stabilize after the transition begins
while stations near Phoenix observe westerly winds 1 hour longer then stabilize. After the
transition, drainage winds speeds across the area are 1-2 m s-1 with some mountain stations
winds strengthening to 3 m s-1 completing the diurnal cycle and the two categories observe
similar conditions.
Following the time series two different regions begin to appear. Stations close to the desert
exhibit different wind patterns than stations in the Sierra Ancha. At EEK in the Mountains the
down-slope winds have high wind consistency (90 %) and strengthen from 1 to 3 m s-1
throughout the nocturnal period (Fig. 6f). During the transition from down-slope to up-slope
wind consistency drops and stays below 50% during the afternoon regime, with light (1-2 m s-1)
up-slope flows. This is the same pattern for many stations along the Sierra Ancha. At STT close
to the desert, there is high wind consistency (~80%) overnight with winds strengthening from 3
to 4 m s-1 (Fig. 6g). During the transition times consistency drops below 50% and during the
afternoon regime wind consistency regains to 80% as up-slope winds strengthen to 4.5 m s-1.
Several other stations near the desert including LUF mentioned above show the high wind
consistency off the desert around hours 13-14 LST indicating its an important feature that should
not be ignored in the diurnal cycle.
Like the Southern Nevada area and unlike the WFV and SRP areas, stations in the southwest
Central Arizona area observe 80% wind consistency in the afternoon. The area is similar to
Southern Nevada with a desert residing southwest of the region. The thermal wind cycle is
heavily influenced by up-valley winds from the Yuma desert. Stations within the northeastern
region of the Central Arizona area experience the typical diurnal cycle of wind consistency seen
in the WFV and SRP regions.
5. Discussion
An interesting feature to note is that if coupled thermal winds are well developed you can
have high consistency in the middle of a broad valley or plain. As with POR in the WFV located
within the Tooele Valley 8 km from the eastern sidewall and 14 km from the western side wall
(Fig. 3e). Because the thermal wind system is greatly developed, winds flow only in two
directions with a 180 degree wind reversal. The consistency is high (~90 %) throughout the night
and is moderately high (~60%) throughout the day.
Complex terrain not only creates diurnal flows but influences their direction. Large mountain
chains can channel and strengthen thermally driven winds. Large valley complexes with a
restricted valley outlet can produce strong drainage winds because of convergence at the outlet .
Several stations in the WFV and SRP illustrate this (e.g., UT5 (Fig. 3h) in the WVF and HOW
(Fig. 4e) in the SRP).
Lack of data makes it difficult to determine whether the down-plain flow is continuous along
the central valley floor along the entire SRP. Horse Butte (HBT) never has an easterly wind
component. An easterly component is visible, however, at Boise (BOI) on the west end of the
plain and at RHF on the east end of the plain. Several stations near HBT that did not have
enough days to meet the compositing criteria exhibit similar results. HBT is on a hill 300 m
above the Snake River. East of HBT, the river turns to the north-northwest. Drainage flows from
the mountains to the south of HBT and from the buttes to the west flow to the east into the river
gorge, converging with drainage winds from the eastern SRP and flowing north around the river
meander, then continuing westward towards BOI.
Within the southwestern region of the IW, another important thermal wind is observed. UPvalley flow from the deserts to the southwest of the Southern Nevada area and Central Phoenix
regions are observed to reach over 5 m s-1 in the climatology during the afternoon hours (1300 1500 LST) with wind consistency around 80 %. The afternoon winds in these regions are about
20 % more consistent then the afternoon winds in the other two areas. Stations such as A02 in
the Southern Nevada region maintain wind consistency over 60% for the entire diurnal cycle
with the stable nocturnal down-slope flow and the southerly up-valley winds from the desert
during the day (Fig. 6g). Within the Central Arizona region, this segregated the stations into two
subcategories. Those affected by the up-valley winds from the Yuma desert and those that were
not. Wind protected from the flow off the Yuma desert observed thermal winds similar to the
WFV and SRP areas, where as the exposed stations exhibit higher wind consistency mentioned
above
6. Summary and future work
Diurnal wind patterns and day-to-day wind consistency were analyzed to provide an
improved understanding of the wind patterns and their evolution in the Salt Lake, Tooele and
Rush Valleys, the Snake River Plain, Southern Nevada, and Central Arizona. Archived
MesoWest data was used for this analysis, and criteria were created to select thermally driven
wind days that could be composited into a mean diurnal pattern. Day-to-day wind consistency
calculations and time series were used to investigate the winds further. The wind fields were
animated in video loops to better visualize the diurnal turning of the winds and the interactions
between the various thermally driven wind systems.
The timing of reversals is consistent from area to area with down-sloping winds changing to
up-sloping winds first shortly after sunrise followed by the transition from down-valley to upvalley. The other transition period starts within an hour after sunset in all areas. Down-slope
winds were strongest during the first few hours, and were generally stronger than the daytime upslope winds. In each of the regions investigated, except as mentioned above in discussion, wind
consistency was generally high (~90%) at night, moderate (~60%) during the day, and low
(~20%) during the transition periods. The drop in consistency during transition periods reflects
the variation in the timing of wind reversals from valley wind day to valley wind day. The
moderate consistency throughout the day is attributed to wind speed and direction variations
caused by convection. Diurnal wind rotations on valley sidewalls were observed in all areas,
supporting the conceptual model by Whiteman (1980). Stations with exposure to winds from
desert locations in Southern Nevada and Central Arizona experienced a 20% increase in
consistency during the day.
While there are marked similarities among the diurnal wind patterns and timing of wind
reversals in the areas investigated, each of the areas exhibited unique wind features. Within the
STRV, the land-lake breeze interacts with the other thermally driven wind systems. Within the
SRP there is a distinct channeling effect caused by the terrain. Both Southern Nevada and
Central Arizona thermal winds are strongly controlled in the afternoon with wind off the deserts
relative to there location.
These and future analyses will be of value to boundary-layer and air pollution meteorologists,
fire weather forecasters, operational meteorologists and others who are interested in
understanding wind flow patterns in complex terrain.
6. Acknowledgements
MesoWest data were collected and processed by John Horel, Mike Splitt, and Bryan White of
the University of Utah and Larry Dunn and David Zaff of the National Weather Service's Salt
Lake City Forecast Office. We gratefully acknowledge the contribution of organizations
participating in MesoWest. One author (JQS) wishes to acknowledge the support of the Global
Change Education Program for supporting his summer appointment at Pacific Northwest
National Laboratory as part of their Summer Undergraduate Research Education program. This
research was supported by the U.S. Department of Energy, Office of Biological and
Environmental Research, Environmental Sciences Division as part of their Environmental
Meteorology Program under Contract DE-AC06-76RLO 1830 at PNNL. The U. S. Department
of Energy's PNNL is operated by Battelle Memorial Institute. [YOUR SPONSERS, JIM!]
Figures
Figure 1. Topography of the western United States with study regions annotated. Elevation (m)
based on scale at upper right.
Figure 2. Places and names for each study region for reference. Terrain shading as in Fig. 1. (a)
Wasatch Front Valleys. (b) Snake River Plain. (c) Southern Nevada basin and range province.
(d) Central Arizona.
Figure 3. Mean summer season vector winds in the Wasatch Front Valleys. Full and half barb
denote 1 an 0.5 m s-1, respectively. Terrain shading as in Fig. 1. (a) 0400 LST. (b) 1000 LST. (c)
1600 LST. (d) 2200 LST (e) RES time series. (f) TOO time series. (g) POR time series. (h) UT5
time series.
Figure 4. Mean summer season vector winds in the Snake River Plain. Full and half barb denote
1 an 0.5 m s-1, respectively. Terrain shading as in Fig. 1. (a) 0300 LST. (b) 0900 LST. (c) 1500
LST. (d) 2100 LST (e) IDF time series. (f) HOW time series. (g) RHF time series.
Figure 5. Mean summer season vector winds in the Southern Nevada basin and Range Province.
Full and half barb denote 1 an 0.5 m s-1, respectively. Terrain shading as in Fig. 1. (a) 0300 LST.
(b) 0900 LST. (c) 1500 LST. (d) 2100 LST (e) DRA time series. (f) A36 time series. (g) A02
time series.
Figure 6. Mean summer season vector winds in Central Arizona. Full and half barb denote 1 an
0.5 m s-1, respectively. Terrain shading as in Fig. 1. (a) 0300 LST. (b) 0900 LST. (c) 1500 LST.
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TABLES
1. Table listing locations where criteria was gathered.
Area
Tooele
Solar Observation Site(lat/lon)
WBB ( -111.85, 40.77)
700 hPa Wind Observation(lat/lon)
SLC ( -111.97, 40.78)
Area
Snake River Plain
Southern Nevada
Central Arizona
Solar Observation Site(lat/lon)
RID ( -112.94, 43.59)
700 hPa Wind Observation(lat/lon)
BOI ( -116.22, 43.57)
DRA (-116.02, 36.62)
TUS (-110.93, 32.12)