An evaluation of NEXRAD precipitation estimates in complex terrain

This is the published version. Copyright 1999 American Geophysical UnionNext 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

Towards better utilization of NEXRAD data in hydrology: An overview of hydro-NEXRAD

With a very modest investment in computer hardware and the open-source local data manger (LDM) software from UCAR\u27s Unidata Program Center, an individual researcher can receive a variety of NEXRAD Level III gridded rainfall products, and the unprocessed Level II data in real-time from most NEXRAD radars. Additionally, the National Climatic Data Center has vast archives of these products and Level II data. Still, significant obstacles remain in order to unlock the full potential of the data. One set of obstacles is related to effective management of multi-terabyte data sets: storing, compressing, and backing up. A second set of obstacles, for hydrologists and hydrometeorologists in particular, is that the NEXRAD Level III products are not well suited for application in hydrology. There is a strong need for the generation of high-quality products directly from the Level II data with well-documented steps that include quality control, removal of false echoes, rainfall estimation algorithms with variety of corrections, coordinate conversion and georeferencing, conversion to a convenient data format(s), and integration with GIS. For hydrologists it is imperative that these procedures are basin-centered as opposed to radar-centered. Thirdly, the amount of data present in a multi-year, multi-radar dataset is such that simple cataloging and indexing of the data is not sufficient. Rather, sophisticated metadata extraction and management techniques are required. The authors describe and discuss the Hydro-NEXRAD software system that addresses the above three challenges. With support from the National Science Foundation through its ITR program, the authors are developing a basin-centered framework for addressing all these issues in a comprehensive manner, tailored specifically for use of NEXRAD data in hydrology and hydrometeorology. Through a flexible web interface users can search a large metadata database base, managed by a relational database, for subsets of interest. Well-chosen and documented defaults are provided for the flow from unprocessed NEXRAD data to basin-centered rainfall estimates at a desired space-time resolution. In addition to the web interface, there are web services that provide access to scripts and compiled programs. © 2007 ASCE

Extreme rainfall from Hurricane Harvey (2017): Empirical intercomparisons of WRF simulations and polarimetric radar fields

We examine extreme rainfall from Hurricane Harvey (2017) based on empirical analyses from polarimetric radar observations as well as high-resolution model simulations using the Weather Research and Forecasting model (WRF). Spatial and temporal structures of extreme rainfall from Hurricane Harvey were characterized using a dense network of rain gauges and high-resolution radar rainfall fields. Numerical simulations using two different microphysical parameterizations, the WRF 6-class single-moment (WSM6) scheme and Morrison double-moment scheme, were employed, together with an additional simulation using the hail version of the Morrison microphysical scheme. Extreme rainfall from Hurricane Harvey is closely tied to the structure and evolution of outer rainbands. Intercomparisons of the simulated and observed polarimetric radar variables show contrasts and similarities of different microphysical schemes in representing critical microphysical processes for extreme rainfall. All three WRF simulations overestimate the frequency of larger rain drops, but exhibit comparable signatures of specific differential phase to observations. The WSM6 simulation shows strong convection that leads to the largest coverage of convective rainfall over outer rainbands of all three WRF simulations. We highlight the capabilities of atmospheric model simulations and improved quantitative rainfall estimates in characterizing key features of extreme rainfall from landfalling TCs as well as critical storm ingredients that produce them

The fractional coverage of rainfall over a grid: Analyses of NEXRAD data over the southern plains. Water Resour Res 32

Abstract. An important parameter in land surface hydrology is the fractional area of a model grid receiving rainfall when rain is observed. Using NEXRAD (Next Generation Weather Radar) hourly rainfall estimates for the southern Plains, we examined the temporal variability in this quantity and tested the important assumptions of the threshold method. The following conclusions are reached. First, seasonal/diurnal variations explain less than 18% of the variance at all spatial scales examined; an efficient method of computing fractional coverage must consider the large event-scale variability. Second, stationarity in conditional distributions of spatial rainfall, a key assumption in the threshold method, is not warranted at the event scale for small grids or at the seasonal/ diurnal scale for large grids. Third, biases are introduced by the high correlation between the parameter and the independent variable of the threshold method. Fourth, the spatial conditional mean rain rate is higher than its temporal counterpart for all cases examined; the ergodicity assumption overpredicts the fractional coverage. Two simple methods were proposed to solve the above problems. Results of an intercomparison showed that they can modestly improve the threshold method

Lagrangian Analyses of Rainfall Structure and Evolution for Organized Thunderstorm Systems in the Urban Corridor of the Northeastern United States

Abstract In this study, a climatology of the structure and evolution of rainfall for organized thunderstorm systems in the urban corridor of the northeastern United States is developed. These storm systems are major agents of flash flooding for urban regions of the northeastern United States and, more generally, for the United States east of the Rocky Mountains. The analyses are motivated by problems that center on characterizing flash flood hazards. The authors focus on spatial heterogeneities of rainfall associated with urbanization in a region of complex landscape including mountainous terrain and land–water boundaries along the geometrically complex coastline of the New York City–New Jersey metropolitan region. The sample of storms selected for investigation consists of the 50 days from April to September 2001–09 with the largest cloud-to-ground lightning flash density derived from National Lightning Detection Network (NLDN) observations over the study region. Storm-tracking analyses of 3D radar reflectivity fields are performed for the 50 storm days and used to develop a Lagrangian climatology of storm structure and evolution for the study region. Rainfall analyses for the 50 storm days are based on high-resolution (1 km, 15 min) bias-corrected radar rainfall fields developed from the Hydro-NEXRAD system. The analyses suggest that complex terrain and land–water boundaries have large impacts on Lagrangian storm properties. Areas of increased heavy rainfall and lightning flash density over New York City were identified. The authors found evidence for changing storm structure as thunderstorms pass over New York City, but little evidence that thunderstorms split as they approach New York City.</jats:p

Urbanization and Rainfall Variability in the Beijing Metropolitan Region

Abstract In this study, rainfall variability in the Beijing metropolitan region and its link to urbanization during the first 10 years of the twenty-first century (2000–09) was examined. Analyses are based on both observations and regional climate model simulations. The study was focused on August, one of the summer months that receive most of the warm season precipitation in Beijing. Observations from surface stations and weather radars in Beijing and its surrounding regions along with satellite observations from the Tropical Rainfall Measuring Mission (TRMM) are used to characterize the spatial and temporal variability of precipitation. It is found that the urban area has fewer rain days and higher rainfall intensity compared to its surrounding region. This suggests a possible impact of urbanization on the spatial variability of rainfall for the region. Regional climate model simulations with the Weather Research and Forecasting (WRF) Model are thus performed with two land use–land cover datasets that represent different stages of urbanization in the Beijing metropolitan region to investigate such an impact. The modeling study demonstrates how urbanization modifies the surface energy budget and the planetary boundary layer, which in turn affects the production of precipitation.</jats:p

An Early Performance Evaluation of the NEXRAD Dual-Polarization Radar Rainfall Estimates for Urban Flood Applications

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.</jats:p

Extreme Flood Response: The June 2008 Flooding in Iowa

Abstract The authors examine the hydroclimatology, hydrometeorology, and hydrology of extreme floods through analyses that center on the June 2008 flooding in Iowa. The most striking feature of the June 2008 flooding was the flood peak of the Cedar River at Cedar Rapids (3964 m3 s−1), which was almost twice the previous maximum from a record of 110 years. The spatial extent of extreme flooding was exceptional, with more U.S. Geological Survey stream gauging stations reporting record flood peaks than in any other year. The 2008 flooding was produced by a sequence of organized thunderstorm systems over a period of two weeks. The authors examine clustering and seasonality of flooding in the Iowa study region and link these properties to features of the June 2008 flood event. They examine the environment of heavy rainfall in Iowa during June 2008 through analyses of composite rainfall fields (15-min time interval and 1-km spatial resolution) developed with the Hydro-NEXRAD system and simulations using the Weather Research and Forecasting Model (WRF). Water balance analyses of extreme flood response, based on rainfall and discharge observations from basins with extreme flooding, suggest that antecedent soil moisture plays a diminishing role in flood response as the return interval increases. Rainfall structure and evolution play a critical and poorly understood role in determining the scaling of flood response. As in other extreme flood studies, analyses of the Iowa flood data suggest that measurement errors can be significant for record discharge estimates.</jats:p

Modeling Land Surface Processes and Heavy Rainfall in Urban Environments: Sensitivity to Urban Surface Representations

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.</jats:p