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Mary Lynn Baeck
and
James A. Smith

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 period; 3) the Dallas, Texas, hailstorm of 5 May 1995, which resulted in 16 flash flood deaths during a period of several hours and property damage exceeding $1 billion; 4) the Chicago, Illinois, storms of 17 July 1996 during which the 24-h rainfall record for Illinois was shattered; and 5) Hurricane Fran, which resulted in unprecedented flooding in North Carolina and Virginia during September of 1996. For each event, analyses revolve around volume scan WSR-88D reflectivity observations. The climatological analysis presented, in conjunction with the case studies analyzed, illustrate the significance of 1) brightband contamination, 2) tilt selection, 3) hail, 4) radar calibration, and 5) ZR relationships for quantitative rainfall estimates by the WSR-88D.

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Brianne K. Smith
,
James Smith
, and
Mary Lynn Baeck

Abstract

The structure and evolution of flash flood–producing storms over a small urban watershed in the mid-Atlantic United States with a prototypical flash flood response is examined. Lagrangian storm properties are investigated through analyses of the 32 storms that produced the largest peak discharges in Moores Run between January 2000 and May 2014. The Thunderstorm Identification, Tracking, Analysis, and Nowcasting (TITAN) algorithm is used to track storm characteristics over their life cycle with a focus on storm size, movement, intensity, and location. First, the 13 June 2003 and 1 June 2006 storms, which produced the two largest peak discharges for the study period, are analyzed. Heavy rainfall for the 13 June 2003 and 1 June 2006 storms were caused by a collapsing thunderstorm cell and a slow-moving, low-echo centroid storm. Analyses of the 32 storms show that collapsing storm cells play an important role in peak rainfall rate production and flash flooding. Storm motion is predominantly southwest-to-northeast, and approximately half of the storms exhibited some linear organization. Mean storm total rainfall for the 32 storms displayed an asymmetric distribution around Moores Run, with sharply decreasing gradients southwest of the watershed (upwind and into the city) and increased rainfall to the northeast (downwind and away from the city). Results indicate urban modification of rainfall in flash flood–producing storms. There was no evidence that the storms split around Baltimore. Flood-producing rainfall was highly concentrated in time; on average, approximately 21% of the storm total rainfall fell within 15 min.

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Long Yang
,
James Smith
,
Mary Lynn Baeck
, and
Efrat Morin

Abstract

Flash flooding in the arid/semiarid southwestern United States is frequently associated with convective rainfall during the North American monsoon. In this study, we examine flood-producing storms in central Arizona based on analyses of dense rain gauge observations and stream gauging records as well as North American Regional Reanalysis fields. Our storm catalog consists of 102 storm events during the period of 1988–2014. Synoptic conditions for flood-producing storms are characterized based on principal component analyses. Four dominant synoptic modes are identified, with the first two modes explaining approximately 50% of the variance of the 500-hPa geopotential height. The transitional synoptic pattern from the North American monsoon regime to midlatitude systems is a critical large-scale feature for extreme rainfall and flooding in central Arizona. Contrasting spatial rainfall organizations and storm environment under the four synoptic modes highlights the role of interactions among synoptic conditions, mesoscale processes, and complex terrains in determining space–time variability of convective activities and flash flood hazards in central Arizona. We characterize structure and evolution properties of flood-producing storms based on storm tracking algorithms and 3D radar reflectivity. Fast-moving storm elements can be important ingredients for flash floods in the arid/semiarid southwestern United States. Contrasting storm properties for cloudburst storms highlight the wide spectrum of convective intensities for extreme rain rates in the arid/semiarid southwestern United States and exhibit comparable vertical structures to their counterparts in the eastern United States.

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Bettina Bauer-Messmer
,
James A. Smith
,
Mary Lynn Baeck
, and
Wenjie Zhao

Abstract

Measurement and forecasting of heavy rainfall requires interpretation of the small differences in the storm environment that distinguish a major flood-producing rainfall event from a relatively harmless storm system. This case study will examine some of the small differences in the storm environment that lead to a heavy rainfall event. On 8 July 1994 two storm systems developed in close proximity to each other in central Oklahoma. One of the storms developed into a squall line and produced low storm total precipitation accumulations. The other was a slow-moving multicellular storm that produced storm total precipitation of more than 130 mm and small stream flooding. The storms exhibited contrasting measurement errors in the operational WSR-88D rainfall products, with underestimation for the heavy rain event and overestimation for the squall line. The interactions of synoptic, mesoscale, and storm-scale processes for the 8 July storms are examined through analyses of WSR- 88D reflectivity and Doppler velocity observations, surface and upper-air observations from the GEWEX–GCIP Integrated Systems Test experiment, and GOES observations from visible, IR, and water vapor channels. This case study gives a unique opportunity to analyze the differences and similarities of the prestorm environment that lead to different storm structures and rainfall accumulations. Analyses also illustrate storm-scale and mesoscale processes that play a major role in determining the accuracy of WSR-88D rainfall estimates.

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James A. Smith
,
Gabriele Villarini
, and
Mary Lynn Baeck

Abstract

Flooding in the eastern United States reflects a mixture of flood-generating mechanisms, with landfalling tropical cyclones and extratropical systems playing central roles. The authors examine the climatology of heavy rainfall and flood magnitudes for the eastern United States through analyses of long-duration records of flood peaks and maximum daily rainfall series. Spatial heterogeneities in flood peak distributions due to orographic precipitation mechanisms in mountainous terrain, coastal circulations near land–ocean boundaries, and urbanization impacts on regional climate are central elements of flood peak distributions. Lagrangian analyses of rainfall distribution and storm evolution are presented for flood events in the eastern United States and used to motivate new directions for stochastic modeling of rainfall. Tropical cyclones are an important element of the upper tail of flood peak distributions throughout the eastern United States, but their relative importance varies widely, and abruptly, in space over the region. Nonstationarities and long-term persistence of flood peak and rainfall distributions are examined from the perspective of the impacts of human-induced climate change on flood-generating mechanisms. Analyses of flood frequency for the eastern United States, which are based on observations from a dense network of U.S. Geological Survey (USGS) stream gauging stations, provide insights into emerging problems in flood science.

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Molly Margaret Chaney
,
James A Smith
, and
Mary Lynn Baeck

Abstract

We examine polarimetric rainfall estimates of extreme rainfall through intercomparisons of radar rainfall estimates with rainfall observations from a dense network of rain gauges in Kansas City. The setting provides unique capabilities for examining range dependence in polarimetric rainfall estimates due to the overlapping coverage of the Kansas City, Missouri, and Topeka, Kansas, WSR-88D radars. We focus on polarimetric measurements of specific differential phase shift, K DP, for estimating extreme rainfall. Gauge–radar intercomparisons from the “close-range” Kansas City radar and from the “far-range” Topeka radar show that K DP can provide major improvements in estimating extreme rainfall, but the advantages of K DP rainfall estimates diminish with range. Storm-to-storm variability of multiplicative bias remains an important issue for polarimetric rainfall estimates; variability in bias is comparable at both close and far range from the radar. “Conditional bias,” in which peak radar rainfall estimates are lower than rain gauge observations, is a systematic feature of polarimetric rainfall estimates, but is more severe at far range. The Kansas City region has experienced record flooding in urban watersheds since the polarimetric upgrade of the Kansas City and Topeka radars in 2012. Polarimetric rainfall estimates from the far-range Topeka radar provide useful quantitative information on basin-average rainfall, but the ability to resolve spatial variation of the most extreme rain rates diminishes significantly with range from the radar.

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Luciana K. Cunha
,
James A. Smith
,
Mary Lynn Baeck
, and
Witold F. Krajewski

Abstract

Dual-polarization radars are expected to provide better rainfall estimates than single-polarization radars because of their ability to characterize hydrometeor type. The goal of this study is to evaluate single- and dual-polarization radar rainfall fields based on two overlapping radars (Kansas City, Missouri, and Topeka, Kansas) and a dense rain gauge network in Kansas City. The study area is located at different distances from the two radars (23–72 km for Kansas City and 104–157 km for Topeka), allowing for the investigation of radar range effects. The temporal and spatial scales of radar rainfall uncertainty based on three significant rainfall events are also examined. It is concluded that the improvements in rainfall estimation achieved by polarimetric radars are not consistent for all events or radars. The nature of the improvement depends fundamentally on range-dependent sampling of the vertical structure of the storms and hydrometeor types. While polarimetric algorithms reduce range effects, they are not able to completely resolve issues associated with range-dependent sampling. Radar rainfall error is demonstrated to decrease as temporal and spatial scales increase. However, errors in the estimation of total storm accumulations based on polarimetric radars remain significant (up to 25%) for scales of approximately 650 km2.

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James A. Smith
,
Mary Lynn Baeck
,
Julia E. Morrison
, and
Paula Sturdevant-Rees

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 $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 for extreme flood response of drainage basins. The Spring Creek storm produced near-record spatial cloud-to-ground lightning frequencies over Spring Creek and large, cold cloud tops. The Kickapoo Creek storm had a relatively low lightning frequency and a low echo centroid structure in radar reflectivity profiles. The envelope curve of flood peaks in Texas is defined by floods from the Edwards Plateau, which is separated from the Coastal Plain by the Balcones Escarpment. On 22 June 1997, flooding in Seco Creek, which is located in the Edwards Plateau 450 km southwest of Kickapoo Creek, resulted in a flood peak with a unit discharge comparable to that of the October 1994 Kickapoo Creek flood. The Seco Creek and Kickapoo Creek storms provide two contrasting examples of how storm structure, evolution, and motion can maximize flood peaks in a drainage basin. The storm properties that maximize flood peaks in Seco Creek and Kickapoo Creek are linked to drainage properties of the Edwards Plateau and Coastal Plain.

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James A. Smith
,
Mary Lynn Baeck
,
Gabriele Villarini
, and
Witold F. Krajewski

Abstract

Extreme floods in the Delaware River basin are examined through analyses of a sequence of record and near-record floods during September 2004, April 2005, and June 2006. The three flood episodes reflect three principal flood-generating mechanisms in the eastern United States: tropical cyclones (September 2004); late winter–early spring extratropical systems (April 2005); and warm-season convective systems (June 2006). Extreme flooding in the Delaware River basin is the product of heavy rainfall and runoff from high-gradient portions of the watershed. Orographic precipitation mechanisms play a central role in the extreme flood climatology of the Delaware River basin and, more generally, for the eastern United States. Extreme flooding for the 2004–06 events was produced in large measure from forested portions of the watershed. Analyses of flood frequency based on annual flood peak observations from U.S. Geological Survey (USGS) stream gauging stations with “long” records illustrate the striking heterogeneity of flood response over the region, the important role of landfalling tropical cyclones for the upper tail of flood peak distributions, and the prevalence of nonstationarities in flood peak records. Analyses show that changepoints are a more common source of nonstationarity than linear time trends. Regulation by dams and reservoirs plays an important role in determining changepoints, but the downstream effects of reservoirs on flood distributions are limited.

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Dan Li
,
Elie Bou-Zeid
,
Mary Lynn Baeck
,
Stephen Jessup
, and
James A. Smith

Abstract

High-resolution simulations with the Weather Research and Forecasting Model (WRF) are used in conjunction with observational analyses to investigate land surface processes and heavy rainfall over the Baltimore–Washington metropolitan area. Analyses focus on a 6-day period, 21–26 July 2008, which includes a major convective rain event (23–24 July), a prestorm period (21–22 July), and a dry-down period (25–26 July). The performance of WRF in capturing land–atmosphere interactions, the bulk structure of the atmospheric boundary layer, and the rainfall pattern in urban environments is explored. Results indicate that WRF captures the incoming radiative fluxes and surface meteorological conditions. Mean profiles of potential temperature and humidity in the atmosphere are also relatively well reproduced, both preceding and following the heavy rainfall period. However, wind features in the lower atmosphere, including low-level jets, are not accurately reproduced by WRF. The biases in the wind fields play a central role in determining errors in WRF-simulated rainfall fields. The study also investigates the sensitivity of WRF simulations to different urban surface representations. It is found that urban surface representations have a significant impact on the surface energy balance and the rainfall distribution. As the impervious fraction increases, the sensible heat flux and the ground heat flux increase, while the latent heat flux decreases. The impact of urban surface representations on precipitation is as significant as that of microphysical parameterizations. The fact that changing urban surface representations can significantly alter the rainfall field suggests that urbanization plays an important role in modifying the regional precipitation pattern.

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