Mary Lynn Baeck

An Intercomparison Study of NEXRAD Precipitation Estimates

Systematic biases in WSR-88D (Weather Surveillance Radar–1988 Doppler) hourly precipitation accumulation estimates are characterized from analyses of more than 1 year of WSR-88D data and rain gage data from the southern plains. Biases are examined in three contexts: (1) biases that arise from the range-dependent sampling of the WSR-88D, (2) systematic differences in radar rainfall estimates from two radars observing the same area, and (3) systematic differences between radar and rain gage estimates of rainfall. Range-dependent biases affect hourly rainfall accumulations products over much of the area covered by the WSR-88D. Significant underestimation of rainfall occurs within 40 km range of the radar due to bias in reflectivity observations at the higher elevation angles used for rainfall estimation close to the radar. Bright band and anomalous propagation (AP) lead to systematic overestimation of rainfall at intermediate range. Beyond 150 km in spring-summer and beyond 100 km in winter-fall, underestimation of precipitation is pronounced due to incomplete beam filling and overshooting of precipitation. Radar-radar intercomparison studies suggest that radar calibration is a significant problem at some sites. Anomalous propagation during clear-air conditions, a major problem with previous National Weather Service network radars, has been largely eliminated by the WSR-88D processing. AP remains a problem for cases in which AP returns are embedded in rain. Radar–rain gage intercomparison analyses indicate systematic underestimation by the WSR-88D relative to rain gages for paired gage-radar rainfall estimates. Analyses of spatial coverage of heavy rainfall, however, illustrate fundamental advantages of radar over rain gage networks for rainfall estimation.

Rainfall Estimation by the WSR-88D for Heavy Rainfall Events

Abstract Storms that produce extreme flooding present a special challenge for the WSR-88D rainfall algorithms. The authors assess the utility of weather radar in the investigation of extreme rain-producing storms through both climatological analyses of long-term radar datasets and case studies of storm events. Climatological analyses are presented for long records of WSR-88D volume scan reflectivity observations, for hourly radar rainfall accumulations products (WSR-88D and WSR-57D), and for radar–rain gauge intercomparisons. These analyses provide a context for interpreting case study assessments of WSR-88D rainfall estimates. Case studies are presented of five storms that produced extreme floods in the United States. Events include 1) the orographically enhanced Rapidan storm in the Blue Ridge region of Virginia, which resulted in more than 600 mm of rain during a 6-h period on 27 June 1995; 2) the southeast Texas storms of 16–17 October 1994 in which approximately 750 mm of rain fell during a 6-h time ...

The Regional Hydrology of Extreme Floods in an Urbanizing Drainage Basin

Abstract The Charlotte, North Carolina, metropolitan area has experienced extensive urban and suburban growth since 1960. Five of the largest flood peaks in the 74-yr discharge record of Little Sugar Creek, which drains the central urban corridor of Charlotte, have occurred since August of 1995. A central objective of this study is to explain how these two observations are linked. To achieve this goal, a series of hypotheses of broad importance to the hydrology and hydrometeorology behavior of extreme floods will be examined. These hypotheses concern the roles of 1) space–time variability of rainfall, 2) antecedent soil moisture, 3) expansion of impervious area, and 4) alterations of the drainage network for extreme floods in urbanizing drainage basins. The methodology used to examine these hypotheses centers on diagnostic studies of flood response for the five major flood events that have occurred since August of 1995. Diagnostic studies exploit the diverse range of extreme precipitation forcing for the ...

An evaluation of NEXRAD precipitation estimates in complex terrain

Next Generation Weather Radar (NEXRAD) precipitation estimates are used for hydrological, meteorological, and climatological studies at a wide range of spatial and temporal scales. The utility of radar-based precipitation estimates in such applications hinges on an understanding of the sources and magnitude of estimation error. This study examines precipitation estimation in the complex mountainous terrain of the northern Appalachian Mountains. Hourly digital precipitation (HDP) products for two WSR-88D radars in New York state are evaluated for a 2-year period. This analysis includes evaluation of range dependence and spatial distribution of estimates, radar intercomparisons for the overlap region, and radar-gage comparisons. The results indicate that there are unique challenges for radar-rainfall estimation in mountainous terrain. Beam blockage is a serious problem that is not corrected by existing NEXRAD algorithms. Underestimation and nondetection of precipitation are also significant concerns. Improved algorithms are needed for merging estimates from multiple radars with spatially variable biases.

Catastrophic Rainfall and Flooding in Texas

Abstract Heavy rainfall and flooding occurred on the Gulf Coastal Plain physiographic province of southeastern Texas in October 1994 and caused 22 deaths and more than

Tropical storms and the flood hydrology of the central Appalachians

1 billion in damages. Record flooding occurred in the 1085 km2 Spring Creek catchment, which received peak rainfall accumulations of more than 500 mm during a 12-h period. Rainfall and flooding of greater magnitude occurred 100 km northeast of Spring Creek over Kickapoo Creek. The peak discharge of 2400 m3 s−1 in Kickapoo Creek at a drainage area of 148 km2 places the event just below the Texas and United States envelope curve of peak discharge. Peak rainfall accumulations in Kickapoo Creek at time intervals from 15 min to 24 h approached Texas and United States record values. The storms that resulted in flooding in Spring Creek and Kickapoo Creek were two components of one mesoscale convective system. The Spring Creek and Kickapoo Creek storms exhibited contrasts in storm structure, evolution, and motion that are of fundamental importance...

Extraordinary Flood Response of a Small Urban Watershed to Short-Duration Convective Rainfall

Flooding from Hurricane Fran is examined as a prototype for central Appalachian flood events that dominate the upper tail of flood peak distributions at basin scales between 100 and 10,000 km 2. Hurricane Fran, which resulted in 34 deaths and more than

Radar Rainfall Estimation for Ground Validation Studies of the Tropical Rainfall Measuring Mission

3.2 billion in damages, made land fall on the North Carolina coast at 0000 UTC, September 6, 1996. By 1200 UTC on September 6, Fran had weakened to a tropical storm, and the center of circulation was located at the North Carolina-Virginia border. Rain bands surrounding the tropical depression produced extreme rainfall and flooding in Virginia and West Virginia, with the most intense rainfall concentrated near ridge tops in the Blue Ridge and Valley and Ridge physiographic provinces. The most severe flooding occurred in the Shenandoah River watershed of Virginia, where peak discharges exceeded the 100-year magnitude at 11 of 19 U.S. Geological Survey stream-gaging stations. The availability of high-resolution discharge and rainfall data sets provides the opportunity to study the hydrologic and hydrometeorological mechanisms associated with extreme floods produced by tropical storms. Analyses indicate that orographic enhancement of tropical storm precipitation plays a central role in the hydrology of extreme floods in the central Appalachian region. The relationships between drainage network structure and storm motion also play a major role in Appalachian flood hydrology. Runoff processes for Hurricane Fran reflected a mixture of saturation excess and infiltration excess mechanisms. Antecedent soil moisture played a significant role in the hydrology of extreme flooding from Hurricane Fran. Land use, in particular, the presence of forest cover, was of secondary importance to the terrain-based distribution of precipitation in determining extreme flood response.

Mixture Distributions and the Hydroclimatology of Extreme Rainfall and Flooding in the Eastern United States

Abstract The 9.1 km2 Moores Run watershed in Baltimore, Maryland, experiences floods with unit discharge peaks exceeding 1 m3 s−1 km−2 12 times yr−1, on average. Few, if any, drainage basins in the continental United States have a higher frequency. A thunderstorm system on 13 June 2003 produced the record flood peak (13.2 m3 s−1 km−2) during the 6-yr stream gauging record of Moores Run. In this paper, the hydrometeorology, hydrology, and hydraulics of extreme floods in Moores Run are examined through analyses of the 13 June 2003 storm and flood, as well as other major storm and flood events during the 2000–03 time period. The 13 June 2003 flood, like most floods in Moores Run, was produced by an organized system of thunderstorms. Analyses of the 13 June 2003 storm, which are based on volume scan reflectivity observations from the Sterling, Virginia, WSR-88D radar, are used to characterize the spatial and temporal variability of flash flood producing rainfall. Hydrology of flood response in Moores Run is c...

Urbanization and Climate Change: An Examination of Nonstationarities in Urban Flooding

Abstract This study presents a multicomponent rainfall estimation algorithm, based on weather radar and rain gauge network, that can be used as a ground-based reference in the satellite Tropical Rainfall Measuring Mission (TRMM). The essential steps are constructing a radar observable, its nonlinear transformation to rainfall, interpolation to rectangular grid, constructing several timescale accumulations, bias adjustment, and merging of the radar rainfall estimates and rain gauge data. Observations from a C-band radar in Darwin, Australia, and a local network of 54 rain gauges were used to calibrate and test the algorithm. A period of 25 days was selected, and the rain gauges were split into two subsamples to apply cross-validation techniques. A Z–R relationship with continuous range dependence and a temporal interpolation scheme that accounts for the advection effects is applied. An innovative methodology was used to estimate the algorithm controlling parameters. The model was globally optimized by usin...