Parastou Hooshialsadat 1,S.J. Burian 2, and J.M. Shepherd 3 Parastou Hooshialsadat 1, S.J. Burian 2, and J.M. Shepherd 3 1 University of Arkansas, 2 University of Utah, 3 NASA Goddard Space Flight Center Assessing Urbanization Impacts on Long- term Rainfall Trends in Houston

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: 1.Downscaling analysis using TRMM-PR and rain gauge data (Shepherd and Burian, 2003) 2.Quantification of alterations to storm event characteristics and diurnal rainfall pattern (Burian et al., 2004a, 2004b) 3.Trend analyses of long-term rainfall records 4.Linked meteorological-hydrological modeling Research Objective

Houston is the 4 th largest city in the U.S. (1.6 million) and covers an area of 937 km 2 ; 10 th largest CMSA (more than 4 million) Houston sits on the 5,000 km 2 Gulf Coastal Plain with a high elevation of 27 meters above sea level Houston’s climate is subtropical humid with very hot and humid summers and mild winters

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 km 2 )

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 Orange ellipse has a 100 km horizontal diameter and 50 km vertical diameter and is centered on 29.77, 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 UCR – Upwind Control Region UIR- Urban Impacted Region

**(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 ( ) and an urban time period ( ) Diurnal Rainfall Pattern AnnualWarm Season

Average annual diurnal rainfall distributions at gage 4311 (UA) for the urban ( ) and pre- urban ( ) time periods Average warm season diurnal rainfall distribution at gage 4311 for the urban ( ) and pre- urban ( ) time periods The peak fraction of daily rainfall is more pronounced for the and hr time increments for the urban time period compared to the pre-urban time period; The warm season experiences a greater diurnal modification

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

UA*UCR* % Change % Change Total 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

 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 ( ) and an urban time period ( ) Storm Event Characteristics

Average Warm Season Maximum 1-hr Rainfall Intensity Data Series Comparison Two Sample Test Mean Statistically Different (  = 0.05)? Pre-Urban UAR > UCR t Test No WilcoxonNo Post-Urban UAR > UCR t Test Yes WilcoxonYes UAR Post-Urban > Pre-Urban t Test Yes WilcoxonYes UCR Post-Urban > Pre-Urban t Test No WilcoxonNo Average maximum 1-hr rainfall intensity during the warm season has increased by 16% in the UAR compared to 4% in UCR Storm Event Characteristics

Average Warm Season Number of Heavy Rainfall Events (> 25 mm) Data Series Comparison Two Sample Test Mean Statistically Different (  = 0.05)? Pre-Urban UAR > UCR t Test No WilcoxonNo Post-Urban UAR > UCR t Test Yes WilcoxonYes UAR Post-Urban > Pre-Urban t Test Yes WilcoxonYes UCR Post-Urban > Pre-Urban t Test No WilcoxonNo Average number of “heavy” rainstorms (> 25mm) during the warm season increased by 35% in the UAR compared to a 3% decrease in the UCR Storm Event Characteristics

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 ( ) Trend Analysis

Annual Warm Season UAUIRUCRUAUIRUCR Mean (mm) St. Dev. (mm) CvCvCvCv Skew Kurtosis  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

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  =0.05.

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.

Trend Analysis (cont’d)...  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

Linear: increasing trend (slope > 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 Annual

Linear: increasing trend (slope > 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 Warm Season

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 variabilityComparison 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 areaUrbanization in Houston may be responsible for increased rainfall amounts during the mid- afternoon to late evening time periods in the urban area

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 UCRFor 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. LouisThis corroborates findings by Balling and Brazel (1987) for Phoenix and Huff and Vogel (1978) for St. Louis Conclusions

Conclusions  Statistical comparison of average storm event characteristics from a pre-urban period and an urban time period indicates: 1.Average maximum 1-hr rainfall intensity during the warm season has increased in the UAR, but not in the UCR 2.Average number of “heavy” rainstorms (> 25mm) during the warm season has increased in the UAR, but decreased in the UCR

Conclusions  Annual rainfall amounts have had a strong increasing trend from in the UA and UIR; and a weak trend in the UCR  Warm season rainfall amounts have had very weak increasing trends from

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

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 ) (Shepherd, Menglin, and Burian)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 ) (Shepherd, Menglin, and Burian)

Questions??? Steve Burian