II. Data and Methodology
2.1 Data Sources
Various data sources were obtained and employed to investigate large-scale
circulation features, mesoscale coastal front features, and precipitation structures within
landfalling and transitioning tropical cyclones. The NCEP/NCAR Reanalysis Dataset
(Kalnay et al. 1996; Kistler et al. 2001) was used to construct plots of standard synoptic
parameters in an attempt to diagnose the impact of large-scale circulation features.
Although the coarse resolution of this dataset (2.5o x 2.5o) likely leaves mesoscale
aspects of landfalling tropical cyclones unresolved, reasonably accurate synoptic-scale
analyses can be produced. This dataset offers the most comprehensive reservoir
(available from 1948–2004) of synoptic-scale parameters, and has proven to be an
invaluable tool when researching periods containing few observations.
The NCEP 24 h daily (1200–1200 UTC) Unified Precipitation Dataset (UPD)
(Higgins et al. 2000) was used to construct precipitation analyses for landfalling and
transitioning events of interest. The UPD is a gridded precipitation analysis (0.25o x
0.25o) incorporating National Climatic Data Center (NCDC) daily coop stations, the
Climate Prediction Center (CPC) precipitation dataset, and daily accumulations from an
hourly precipitation dataset, producing smoothed and quality controlled results. The
UPD offers the most extensive and complete precipitation analysis to date, with
observations available from 1948–2002.
Individual tropical cyclone tracks were defined using the National Hurricane
Center (NHC) best track dataset at 6 h intervals (0000, 0600, 1200 and 1800 UTC). This
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dataset contains track information for every tropical storm or hurricane occurring in the
Atlantic Ocean basin from 1851 through the present. The best track dataset exhibits
limitations when studying ET cases, particularly due to the fact that the NHC does not
maintain responsibility for tracking tropical cyclones after they undergo transition. For
this study, the exact point of transition defined by the NHC is not important, due to the
fact that the analysis is independent of the transition state. However, for case studies
that contain incomplete tracks over land, access to alternative surface datasets such as
those archived at NCDC can allow an approximate estimate of the location of the center
of circulation and cyclone central pressure as required.
Surface observations were obtained from land, ship and buoy platforms, in an
attempt to provide the most detailed and comprehensive analyses possible. Three
archived datasets obtained from NCAR were employed to accomplish this task: (1) the
United States Air Force DATSAV3, providing land observations from 1901 through
2003, (2) the NCEP ADP Global Surface Observations, providing land, ship, and buoy
data from 1975 through near present, and (3) the International Comprehensive Ocean
Atmosphere Data Set (ICOADS), providing global ship and buoy observations from
1784 through 2002.
Additional archived surface analyses were obtained through the acquisition of
previously plotted maps produced by Paul Kocin (1995) (referred to hereafter as PK
plots). These analyses were conducted on a small number of landfalling and
transitioning storms, and served as the basis for a rigorous surface investigation until a
more comprehensive analysis containing marine observations could be constructed.
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2.2 Methodology
2.2.1 Construction of a 38-Storm Tropical Cyclone Dataset
David Vallee of the National Weather Service Weather Forecast Office (NWS
WFO) Taunton, Massachusetts, and Kermit Keeter of the NWS WFO Raleigh, North
Carolina, constructed an initial dataset of 41 landfalling and transitioning tropical
cyclones producing at least 100 mm (4 in) of precipitation over the Northeast United
States during 1950 through 2001. Due to limitations imposed by a lack of UPD
availability past 1998 at the time this project was initiated, the loss of Hurricane Floyd
(1999), Tropical Storm Gordon (2000), and Tropical Storm Allison (2001) was
unavoidable, and resulted in a final dataset comprising 38 storms during the time period
1950 through 1998.
A more rigorous investigation of synoptic-scale circulation features and
precipitation distributions for each of the 38 cases was employed to verify close
agreement between date(s) of tropical cyclone impact and total precipitation
accumulations identified by Vallee and Keeter. A subjective analysis of each case was
performed to determine the most accurate timeframe for precipitation associated with the
tropical cyclone and its transition process. However, limitations in the data parameters
and frequency, and the subjective nature of the analysis, produced results that can be
interpreted slightly differently by independent investigators. Daily and storm total
composite precipitation analyses were produced using the UPD data to verify
distributions of precipitation associated with features identifiable in the reanalysis data.
If agreement of tropical cyclone impact date(s) or total precipitation was not achieved, a
subjective adjustment of the relevant parameter was performed. Twice-daily (0000 and
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1200 UTC) NCEP/NCAR reanalysis data and the General Meteorological Package
(GEMPAK) were utilized to produce synoptic-scale analyses. These analyses included:
(1) 850 hPa heights and temperatures, to determine low-level thermal advections, (2)
500 hPa heights and absolute vorticity, to determine flow characteristics and upward
vertical motion forced by cyclonic vorticity advection, and (3) 200 hPa heights and
isotachs, to determine upper-level jet structures and their associated ageostrophic
circulations.
2.2.2 Track versus Precipitation Plots
In order to obtain an accurate representation of precipitation with respect to the
track of a tropical cyclone, plots containing 6 h best track positions (NHC Best Track
Dataset) versus composite storm total precipitation analyses (derived from the UPD)
were constructed for all 38 cases using the timeframes previously determined. An
analysis of this nature was useful in determining whether a preferential right- or left-oftrack shift in precipitation was evident, and provided a first-order diagnosis to the
underlying dynamical mechanisms governing the observed distributions (Atallah 2003).
2.2.3 NWS Precipitation Analyses
The gridded nature of the UPD produces a reasonably accurate representation of
the spatial distribution of precipitation; however, limitations imposed from smoothing
processes lead to underestimation of maximum accumulations. To obtain more accurate
accumulations, a second precipitation analysis was conducted using NCDC surface
archives for the northeastern US, and Arcview GIS mapping software. Approximately
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3500 surface stations were identified within the northeastern domain, and were analyzed
manually for each storm period (currently: 1950–1991) by Ron Horwood of the NWS
Northeast River Forecast Center (NWS NRFC) Taunton, Massachusetts. Obvious
erroneous data were subjectively removed to obtain the most accurate analyses possible.
2.2.4 Precipitation versus Topography
Plots were constructed to examine the effects of terrain modification on
precipitation distributions. The surface heights of the 40 km NCEP RUC (Rapid Update
Cycle) model served as the basis for the contours of surface elevations. Daily and storm
total composite precipitation analyses derived from the UPD were overlaid on the
surface elevations to produce the final plot. Although the grid resolution of the RUC
model will likely leave mesoscale terrain features unresolved, this method of obtaining
digitized topography produces an acceptable representation.
2.2.5 Construction of a Nine-Storm Subset and Selection of Case Studies
The 38-storm dataset described previously was further subcategorized into a
nine-storm subset to obtain storms in which precipitation distributions showed possible
influence from coastal frontogenesis. The criteria to develop this subset were governed
by the availability of data, and led to the investigation of well-documented or famous
cases. PK surface plots served as an integral component of this effort, providing detailed
surface analyses during time periods normally devoid of data.
A further refined subset of storms presented as case studies in this thesis was
chosen solely on the availability and quality of detailed surface data. Comprehensive
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surface data incorporating marine observations was available from 1975 to the present,
using a combination of the three datasets described previously. This limited availability
resulted in the investigation of three storms that combined the effects of synoptic and
mesoscale processes on precipitation distributions. Storms chosen for detailed case
studies include: (1) Hurricane Bob (1991), (2) Hurricane Gloria (1985), and (3)
Hurricane Belle (1976). An historical perspective, containing a less detailed
investigation of the events surrounding Hurricanes Connie and Diane (1955), will also
be presented.
2.2.6 Case Study Methodology
Utilizing the four times daily NCEP/NCAR Reanalysis Dataset (0000, 0600,
1200 and 1800 UTC), a quasi-geostrophic and jet dynamic approach to forced vertical
motions was employed to understand dynamical mechanisms responsible for the
observed precipitation distributions (UPD and NWS precipitation analyses). This type
of approach describes upward vertical motions in terms of forcing from differential
cyclonic vorticity advection (CVA), warm-air advection (WAA) and jet-induced vertical
branches of ageostrophic circulations. An in-depth synoptic-scale investigation was
conducted for each of the three cases (Bob, Gloria and Belle) discussed above, and
involved the construction and analysis of: (1) 925 hPa heights, θe and winds, to examine
the low-level jet, as well as advections and regions of warm, moist high θe air, (2) 850
hPa heights, θ and winds, to examine the positions of baroclinic zones and WAA
patterns, (3) 500 hPa heights and absolute vorticity, to analyze cyclonic vorticity
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advections and general flow characteristics, and (4) 200 hPa heights, isotachs and
divergence, to diagnose upper-level jet streak evolution and ageostrophic circulations.
Once synoptic-scale parameters were investigated, the focus then shifted to
mesoscale surface conditions (e.g., coastal fronts and orographic enhancement) and their
impacts on the observed precipitation distributions. The amassment and processing of
the three previously described datasets served as the basis for all surface observations.
Even though numerous surface datasets were employed to obtain the most
comprehensive analyses possible, lack of data in certain areas at crucial times led to an
inherently subjective representation.
Mesoscale frontal signatures identified in standard surface plots were more
clearly defined using GEMPAK plotted calculations of interpolated θ and θe, as well as
the gradients of these quantities. Calculations of θ and θe produce an accurate
representation of the effects of temperature and moisture that are independent of the
height of the observation under adiabatic conditions. This attribute is deemed especially
useful in areas of widely varying terrain, such as the northeastern US. The gradients of
these quantities describe the horizontal differences (e.g., temperature and moisture) that
exist between air masses, thus truly defining the frontal boundary. Even though surface
mesoscale processes occur on relatively short horizontal scales, for the most part, the
density of the constructed observing network is high enough to resolve these features.
Hurricanes Connie and Diane are of interest because the passage of the two
systems through the Northeast occurred in a period of one week and produced prolific
flooding. Although comprehensive surface observations are not available for these two
storms, slightly downscaled case studies incorporating reanalysis data, UPD and NWS
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precipitation analyses, and PK surface plots, will attempt to define synoptic and
mesoscale features observed throughout these catastrophic events.
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