Parastou Hooshialsadat1, S.J. Burian2, and J.M. Shepherd3

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Assessing Urbanization Impacts on Long-term
Rainfall Trends in Houston
Parastou Hooshialsadat1, S.J. Burian2, and J.M.
Shepherd3 1University of Arkansas, 2University of
Utah, 3NASA Goddard Space Flight Center
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Research Objective

• To determine the effect of urbanization of the
  Houston metropolitan area on precipitation
  variability within the city compared to regions
  seasonally upwind and downwind.
• Analysis Components
• Downscaling analysis using TRMM-PR and rain gauge
  data (Shepherd and Burian, 2003)
• Quantification of alterations to storm event
  characteristics and diurnal rainfall pattern
  (Burian et al., 2004a, 2004b)
• Trend analyses of long-term rainfall records
• Linked meteorological-hydrological modeling

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• Houston is the 4th largest city in the U.S. (1.6
  million) and covers an area of 937 km2 10th
  largest CMSA (more than 4 million)
• Houston sits on the 5,000 km2 Gulf Coastal Plain
  with a high elevation of 27 meters above sea level

Houstons climate is subtropical humid with very
hot and humid summers and mild winters
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Houston Urbanization
Urban growth characterized using a combination of
multi-temporal population, multi-spectral, and
cadastral data
Average of 43 population increase in Houston
Metro per decade since 1900
Approximately 40 increase of urban surfaces in
Houston Metro between 1978 and 2000 (current city
limit is 1000 km2)
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Theoretical Coordinate System Defining Upwind and
Downwind Regions Based on Mean Annual 700 hPa
Steering Flow from 1979 to 1998 (following
Shepherd et al. 2002)
The wind rose below indicates that the prevailing
near-surface flow is predominately southeasterly
(e.g. sea breeze driven), however, for steering
flow and upwind-downwind delineation, the 700 hPa
surface is most critical.
Abcissa is aligned along the 230º
(south-southwest) 700 hPa mean vector
UCR Upwind Control Region UIR- Urban Impacted
Region
Orange ellipse has a 100 km horizontal diameter
and 50 km vertical diameter and is centered on
29.77,95.38. The westernmost boundary of the UCR
is 125 km from the orange ellipse and the
easternmost boundary of the UIR is 100 km from
the orange ellipse
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Diurnal Rainfall Pattern
Annual
Warm Season
(blue gages have both urban and pre-urban data)

• This component of the study focused on the
  analysis of rain gage records with the necessary
  temporal resolution (hourly or less increments)
  for a pre-urban time period (1940-1958) and an
  urban time period (1984-1999)

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Average annual diurnal rainfall distributions at
gage 4311 (UA) for the urban (1984-1999) and
pre-urban (1940-1958) time periods
Average warm season diurnal rainfall distribution
at gage 4311 for the urban (1984-1999) and
pre-urban (1940-1958) time periods
The peak fraction of daily rainfall is more
pronounced for the 12-16 and 16-20 4-hr time
increments for the urban time period compared to
the pre-urban time period The warm season
experiences a greater diurnal modification
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Average annual diurnal rainfall distribution for
the average of UCR gages 1671, 5193, 569, and 9364
Average warm season diurnal rainfall distribution
for the average of UCR gages 9364, 1671, and 3430
The change in diurnal rainfall distribution is
visibly less in the UCR compared to the UA The
warm season has also experiences a greater
diurnal modification
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Average warm season rainfall amounts (mm) in each
time increment
UA is the average of 4311 and 4309 UIR gages
had insufficient data for warm season analysis
UCR is the average of 1671, 3430, 9364
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Storm Event Characteristics

• This component of the study focused on the
  analysis of rain gage records with the necessary
  temporal resolution (hourly or less increments)
  for a pre-urban time period (1940-1958) and an
  urban time period (1984-1999)

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Storm Event Characteristics
Average maximum 1-hr rainfall intensity during
the warm season has increased by 16 in the UAR
compared to 4 in UCR
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Storm Event Characteristics
Average number of heavy rainstorms (gt 25mm)
during the warm season increased by 35 in the
UAR compared to a 3 decrease in the UCR
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Trend Analysis
The trend analysis used 10 rain gauges from the
UA, and 20 each from the UIR and UCR. The gauges
selected had the longest record lengths and the
highest data coverage for the 50-year study
period (1950-2000)
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• Average annual rainfall amount is greater in the
  UA and UIR than the UCR at the 0.95 confidence
  level
• Average warm season rainfall amount is greater in
  the UA than the UCR and UIR at the 0.95
  confidence level
• There is no statistical difference between
  average annual rainfall in UA and UIR at the 0.95
  level
• Average warm season rainfall amount is greater in
  the UIR than the UCR at the 0.95 confidence level

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• Average annual trends
• Linear no trend exhibited (slope not
  significantly different from 0) for UCR
  increasing trend for UA and UIR at 0.95 level
• Mann-Kendall Annual rainfall is significantly
  increasing with time (90 confident) in each
  region. For UA and UIR, results are significant
  even for a0.05.

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• Avg warm season trends
• Linear no trend exhibited (slope not
  significantly different from 0) at 0.95 level
• Mann-Kendall there is no evidence to conclude
  that the amount of warm season rainfall is
  increasing with time.

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Trend Analysis (contd)...

• The same battery of trend assessment tests were
  conducted for a difference statistic that
  represents the difference in average rainfall
  amount in a given year or warm season between the
  UA and UCR (?R UA-UCR), UIR and UCR (?R UIR-UCR),
  and the UA and UIR (?R UA-UIR)
• Objective Isolate the trend of differences
  between the three regions

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Annual
Linear increasing trend (slope gt 0 at the 0.95
level) for UA-UCR only Mann-Kendall no
significant trends found down to the 0.90
confidence level for all combinations UA-UIR and
UA-UCR differences increasing at ?0.20
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Warm Season
Linear increasing trend (slope gt 0 at the 0.95
level) for UA-UCR only Mann-Kendall no
significant trends found down to the 0.80
confidence level for all combinations
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Conclusions

• Comparison of pre-urban and urban time periods
  suggests the diurnal rainfall distribution has
  been modified in urban areas beyond that
  responsible from natural background climate
  variability
• Urbanization in Houston may be responsible for
  increased rainfall amounts during the
  mid-afternoon to late evening time periods in the
  urban area

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Conclusions

• For recent period annual and warm season diurnal
  rainfall patterns in the Houston UA and UIR
  display greater late afternoon and early evening
  rainfall amounts and occurrences compared to the
  UCR
• This corroborates findings by Balling and Brazel
  (1987) for Phoenix and Huff and Vogel (1978) for
  St. Louis

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Conclusions

• Statistical comparison of average storm event
  characteristics from a pre-urban period and an
  urban time period indicates
• Average maximum 1-hr rainfall intensity during
  the warm season has increased in the UAR, but not
  in the UCR
• Average number of heavy rainstorms (gt 25mm)
  during the warm season has increased in the UAR,
  but decreased in the UCR

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Conclusions

• Annual rainfall amounts have had a strong
  increasing trend from 1950-2000 in the UA and
  UIR and a weak trend in the UCR
• Warm season rainfall amounts have had very weak
  increasing trends from 1950-2000

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Conclusions

• An increasing trend of ?R UA-UCR versus time and
  population is observed for annual and warm season
  rainfall in Houston
• No trend is observed for ?R UIR-UCR and ?R UA-UIR
  versus time and population

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Acknowledgements

• This work has been supported by a NASA/ASEE
  Summer Faculty Fellowship (Burian), a NASA New
  Investigator Program (NIP) Grant (Shepherd), and
  a NASA Precipitation Measurement Mission award
  (PMM-0022-0069) (Shepherd, Menglin, and Burian)

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Questions???
Steve Burian burian_at_eng.utah.edu