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Rainfall frequency analysis methods are developed and implemented based on high‐resolution radar rainfall data sets, with the Baltimore metropolitan area serving as the principal study region. Analyses focus on spatial heterogeneities and time trends in sub‐daily rainfall extremes. The 22‐year radar rainfall data set for the Baltimore study region combines reflectivity‐based rainfall fields during the period from 2000 to 2011 and polarimetric rainfall fields for the period from 2012 to 2021. Rainfall frequency analyses are based on non‐stationary formulations of peaks‐over‐threshold and annual peak methods. Increasing trends in short‐duration rainfall extremes are inferred from both peaks‐over‐threshold and annual peak analyses for the period from 2000 to 2021. There are pronounced spatial gradients in short‐duration rainfall extremes over the study region, with peak values of rainfall between Baltimore City and Chesapeake Bay. Spatial gradients in 100‐year, 1 hr rainfall over 20 km length scale are comparable to time trends over 20 years. Rainfall analyses address the broad challenge of assessing changing properties of short‐duration rainfall in urban regions. Analyses of high‐resolution rainfall fields show that sub‐daily rainfall extremes are only weakly related to daily extremes, pointing to difficulties in inferring climatological properties of sub‐daily rainfall from daily rainfall analyses. Changing measurement properties are a key challenge for application of radar rainfall data sets to detection of time trends. Mean field bias correction of radar rainfall fields using rain gauge observations is an important tool for improving radar rainfall fields and provides a useful tool for addressing problems associated with changing radar measurement properties. Rainfall frequency analysis tools based on long‐term, high‐resolution radar rainfall fields are developedSub‐daily rainfall extremes for the Baltimore study region exhibit increasing trends over 22‐year period of recordSub‐daily rainfall extremes exhibited pronounced spatial heterogeneities over the Baltimore study region Rainfall frequency analysis tools based on long‐term, high‐resolution radar rainfall fields are developed Sub‐daily rainfall extremes for the Baltimore study region exhibit increasing trends over 22‐year period of record Sub‐daily rainfall extremes exhibited pronounced spatial heterogeneities over the Baltimore study region

The 14 September 2015 Hildale, Utah, storm resulted in 20 flash flood fatalities, making it the most deadly natural disaster in Utah history; it is the quintessential example of the “paroxysmal precipitation of the desert”. The measured peak discharge from Maxwell Canyon at a drainage area of 5.3 km2was 266 m3/s, a value that exceeds envelope curve peaks for Utah. The 14 September 2015 flash flood reflects features common to other major flash flood events in the region, as well as unique features. The flood was produced by a hailstorm that was moving rapidly from southwest to northeast and intensified as it interacted with complex terrain. Polarimetric radar observations show that the storm exhibited striking temporal variability, with the Maxwell Canyon tributary of Short Creek and a small portion of the East Fork Virgin River basin experiencing extreme precipitation. Periods of extreme rainfall rates for the 14 September 2015 storm are characterized by KDPsignatures of extreme rainfall in polarimetric radar measurements. Similar KDPsignatures characterized multiple storms that have produced record and near‐record flood peaks in Colorado Plateau watersheds. The climatology of monsoon thunderstorms that produce flash floods exhibits striking spatial heterogeneities in storm occurrence and motion. The hydroclimatology of flash flooding in arid/semiarid watersheds of the southwestern United States exhibits relatively weak dependence on drainage basin area. Large flood peaks over a broad range of basin scales can be produced by small thunderstorms like the 14 September 2015 Hildale Storm, which pass close to the outlet. The 14 September 2015 Hildale storm was a hailstorm that produced catastrophic flooding in southern UtahThe climatology of monsoon thunderstorms that produce flash floods in the southwestern United States exhibits large spatial heterogeneityRecord flood peaks over a wide range of basin scales are produced by “small,” monsoon storms that pass close to the basin outlet

Rapid urbanization over Phoenix has resulted in increased water consumption for maintenance of green spaces and heat stress mitigation. The hydrometeorological impact of urban irrigation is not well understood and is the principal objective of this study. Our results are based on high‐resolution numerical experiments using the Weather Research and Forecasting model. A simple irrigation scheme is implemented into the Weather Research and Forecasting model to represent changes in soil moisture due to irrigation. A monthlong simulation using the Weather Research and Forecasting model with irrigation shows improved model performance in representing the regional water and energy cycle. Comparisons of model simulations with and without irrigation highlight the regional impacts of urban irrigation on surface heat fluxes and rainfall variability over Phoenix. There is a strong modulation of irrigation on surface energy partitioning over both irrigated and nonirrigated areas. Irrigation increases (decreases) surface latent (sensible) heat fluxes by enhanced evapotranspiration over irrigated areas, with opposing changes presented over nonirrigated areas. Irrigation contributes little to the domain‐average rainfall accumulation, but can noticeably modify its spatial distribution, with increased rainfall over the downwind mountainous regions and decreased rainfall over the irrigated areas. Irrigation‐induced rainfall anomalies can be tied to strengthened thermal gradients and induced changes in surface pressure fields, which lead to perturbations on large‐scale flow and its interactions with complex terrain. Our results shed light on the hydrometeorological impacts of increasing anthropogenic water use driven by urbanization and highlight the importance of accurate representations of land surface processes in better characterizing land‐atmosphere interactions in arid/semiarid regions. Hydrometeorological impacts due to urban irrigation are examined based on WRF model simulations with a simple irrigation schemeThe impacts of urban irrigation on surface heat fluxes and rainfall extend beyond local scales through perturbations to atmospheric forcingsAccurate representations of human water consumption are important in characterizing land‐atmosphere interactions in arid/semiarid regions

Urban development, topographic relief, and coastal boundaries can all exert influences on storm hydroclimatology, making rainfall and flood frequency analysis a major challenge. This study explores heterogeneity in extreme rainfall in the Baltimore Metropolitan region at small spatial scales using hydrometeorological analyses of major storm events in combination with hydroclimatological analyses based on storm catalogsdeveloped using a 16‐year record of high‐resolution bias‐corrected radar rainfall fields. Our analyses demonstrate the potential for rainfall frequency methods using storm catalogs combined with stochastic storm transposition (SST); procedures are implemented for Dead Run, a small (14.3 km2) urban watershed located within the Baltimore Metropolitan area. The results point to the pronounced impact of complex terrain (including the Chesapeake Bay to the east, mountainous terrain to the west and urbanization in the region) on the regional rainfall climatology. Warm‐season thunderstorm systems are shown to be the dominant mechanism for generating extreme, short‐duration rainfall that leads to flash flooding. The SST approach is extended through the implementation of a multiplier fieldthat accounts for spatial heterogeneities in extreme rainfall magnitude. SST‐based analyses demonstrate the need to consider rainfall heterogeneity at multiple scales when estimating the rainfall intensity‐duration‐frequency relationships. There is significant spatial heterogeneity in extreme rainfall across the Baltimore area, which is closely linked to the complex terrainStochastic storm transposition‐based analysis highlights the necessity of considering spatial heterogeneity in rainfall estimation

We examine the upper tail of flood peak distributions through analyses of annual peak observations from more than 8,000 U.S. Geological Survey (USGS) stream gaging stations and through hydrometeorological analyses of the storms that produce the most extreme floods. We focus on the distribution of the upper tail ratio, which is defined as the peak discharge for the flood of record at a stream gaging station divided by the sample 10‐year flood magnitude. The 14 June 1903 Heppner storm, which produced an upper tail ratio of 200, was the product of a hailstorm that formed along the Blue Mountains in eastern Oregon, a region dominated by snowmelt flooding. A striking contrast between record flood peaks and the larger distribution of annual flood peaks in the United States is in the seasonality of flood occurrence, with record floods reflecting a much stronger contribution from warm season thunderstorm systems. Mountainous terrain and intense convective rainfall are important elements of the geography and hydrometeorology of extreme upper tail ratio flood peaks. The distribution of upper tail ratio values for USGS stream gaging stations does not depend on basin area, a result which is consistent with scaling results based on extreme value theory. Downscaling simulations with the Weather Research and Forecasting model are used to examine the storm environment of the 1903 Heppner storm, along with two other record flood peaks near the Blue Mountains of eastern Oregon from the USGS miscellaneousflood record, the July 1956 Meyers Canyon flood and the July 1965 Lane Canyon flood. The upper tail of flood peaks in the United States is examined through analyses of the upper tail ratio for annual flood peak observationsTwo important aspects of strange floods in the United States are location near mountainous terrain and intense thunderstorms as causative agentsA fundamental distinction between record floods in the United States and the broader population of annual flood peaks is their seasonal distribution

We examine urban flood response through data‐driven analyses for a diverse sample of “small” watersheds (basin scale ranging from 7.0 to 111.1 km2) in the Charlotte Metropolitan region. These watersheds have experienced extensive urbanization and suburban development since the 1960s. The objective of this study is to develop a broad characterization of land surface and hydrometeorological controls of urban flood hydrology. Our analyses are based on peaks‐over‐threshold flood data developed from USGS streamflow observations and are motivated by problems of flood hazard characterization for urban regions. We examine flood‐producing rainfall using high‐resolution (1 km2spatial resolution and 15 min time resolution), bias‐corrected radar rainfall fields that are developed through the Hydro‐NEXRAD system. The analyses focus on the 2001–2015 period. The results highlight the complexities of urban flood response. There are striking spatial heterogeneities in flood peak magnitudes, response times, and runoff ratios across the study region. These spatial heterogeneities are mainly linked to watershed scale, the distribution of impervious cover, and storm water management. Contrasting land surface properties also determine the mixture of flood‐generating mechanisms for a particular watershed. Warm‐season thunderstorm systems and tropical cyclones are main flood agents in Charlotte, with winter/spring storms playing a role in less‐urbanized watersheds. The mixture of flood agents exerts a strong impact on the upper tail of flood frequency distributions. Antecedent watershed wetness plays a minor role in urban flood response, compared with less‐urbanized watersheds. Implications for flood hazard characterization in urban watersheds and for advances in flood science are discussed. We examine urban flood response through for a diverse sample of “small” watersheds (basin scale ranging from 7.0 to 111.1 km2in the Charlotte Metropolitan region. These watersheds have experienced extensive urbanization and suburban development since the 1960s. Our analyses are based on flood data developed from USGS stream gaging stations and are motivated by problems of flood hazard characterization for urban regions. We examine flood‐producing rainfall using high‐resolution radar rainfall fields. The analyses focus on the 2001–2015 period. The results highlight the complexities of urban flood response. The heterogeneities in flood response are mainly linked to watershed scale, the distribution of impervious cover, and storm water management. Warm‐season thunderstorm systems and tropical cyclones are main flood agents in Charlotte, with winter/spring storms playing a role in less‐urbanized watersheds. Antecedent watershed wetness plays a minor role in urban flood response, compared with less‐urbanized watersheds. The results provide implications for flood hazard characterization in urban watersheds and for advances in flood science. Simple approaches to regional flood frequency analysis, based on covariates like basin area and imperviousness, do not capture key elements of urban flood responseMixtures of warm‐season thunderstorm and tropical cyclone food agents exert a strong impact on the upper tail of flood frequency distributions in CharlotteEmpirical analyses of observations from the dense network of gaged watersheds provide a deeper understanding of urban flood hydrology

On 1 September 2021, the remnants of Hurricane Ida transformed into a lethal variant of tropical cyclone in which unprecedented short‐duration rainfall from clusters of supercells produced catastrophic flooding in watersheds of the Northeastern US. Short‐duration rainfall extremes from Ida are examined through analyses of polarimetric radar fields and rain gauge observations. Rainfall estimates are constructed from a polarimetric rainfall algorithm that is grounded in specific differential phase shift (KDP) fields. Rainfall accumulations at multiple locations exceed 1000‐year values for 1–3 hr time scales. Radar observations show that supercells are the principal agents of rainfall extremes. Record flood peaks occurred throughout the eastern Pennsylvania—New Jersey region; the peak discharge of the Elizabeth River is one of the most extreme in the eastern US, based on the ratio of the peak discharge to the sample 10‐year flood at the gaging station. As with other tropical cyclones that have produced record flooding in the Northeastern US, Extratropical Transition was a key element of extreme rainfall and flooding from Ida. Tropical and extratropical elements of the storm system contributed to extremes of atmospheric water balance variables and Convective Available Potential Energy, providing the environment for extreme short‐duration rainfall from supercells. Remnants of Hurricane Ida produced unprecedented short‐duration rainfall extremes at 1–3 hr time scaleSupercells were the principal agents of extreme rainfall and floodingExtratropical Transition of Ida contributed to the environment of record short‐duration rainfall Remnants of Hurricane Ida produced unprecedented short‐duration rainfall extremes at 1–3 hr time scale Supercells were the principal agents of extreme rainfall and flooding Extratropical Transition of Ida contributed to the environment of record short‐duration rainfall

The partitioning of evapotranspiration (ET) between plant transpiration (Et) and direct evaporation (Ed) presents one of the most important and challenging problems for characterizing ecohydrological processes. The exchange of water vapor (q) and CO2(c) are closely coupled in ecosystem processes and knowledge of their controls can be gained through joint investigation of qand c. In this study we examine the correlation of water vapor and CO2(Rqc) through analyses of high-frequency time series derived from eddy covariance measurements collected over a suburban grass field in Princeton, NJ during a 2 year period (2011–2013). Rqcat the study site exhibits pronounced seasonal and diurnal cycles, with maximum anticorrelation in June and maximum decorrelation in January. The diurnal cycle of Rqcvaries seasonally and is characterized by a near-symmetric shape with peak anticorrelation around local noon. Wavelet and spectral analyses suggest that qand care jointly transported for most eddy scales (1–200 m), which is important for ETpartitioning methods based on flux variance similarity. The diurnal cycle of the transpiration fraction (ratio of Etto total ET) exhibits an asymmetric diurnal cycle, especially during the warm season, with peak values occurring in the afternoon. These ETpartitioning results give similar diurnal and seasonal patterns compared with numerical simulations from the Noah Land Surface Model using the Jarvis canopy resistance formulation. The correlation of water vapor and CO2exhibits pronounced diurnal and seasonal cyclesWavelet and spectral analysis show that q and c are jointly transported for most eddy scalesET partitioning results based on the q–c correlation analysis are comparable with the Noah LSM

With a very modest investment in computer hardware and the open-source local data manager (LDM) software from University Corporation for Atmospheric Research (UCAR) Unidata Program Center, a researcher can receive a variety of NEXRAD Level III rainfall products and the unprocessed Level II data in real-time from most NEXRAD radars in the USA. Alternatively, one can receive such data from the National Climatic Data Center in Ashville, NC. 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 datasets. A second set of obstacles, for hydrologists and hydrometeorologists in particular, is that the NEXRAD Level III products are not well suited for applications 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, coordinate conversion, georeferencing and integration with GIS. For hydrologists it is imperative that these procedures are basin-centered as opposed to radar-centered. The authors describe the Hydro-NEXRAD system that addresses the above challenges. With support from the National Science Foundation through its ITR program, the authors have developed a basin-centered framework for addressing all these issues in a comprehensive manner, tailored specifically for use of NEXRAD data in hydrology and hydrometeorology. [ABSTRACT FROM AUTHOR]

Stormwater detention ponds have become ubiquitous in urbanized areas and have been suggested as potential hotspots of N transformation within urban watersheds. As a result, there is a great deal of interest in their use as structural best management practices to reduce the excessive N export from these watersheds. We conducted continuous monitoring of the influent and effluent N loads of a stormwater detention pond located on the Princeton University campus in Princeton, New Jersey. Our monitoring was conducted during four 21‐d periods representing the four seasons of the northeastern United States. Water quality samples were collected and analyzed for nitrate (NO3−) during all four monitoring periods. During two of these periods, loads of ammonium (NH4+), dissolved organic N, and particulate N (PN) were measured. Our results show that NO3−dominated the influent N load, particularly in dry weather inflows to the detention pond. However, PN, which is often neglected in stormwater quality monitoring, made up as much as 30% of the total load and an even greater fraction during storm events. The results of our monitoring suggest that seasonal variation may play an important role in N retention within the detention pond. Although retention of NO3−, the most dominant fraction of N in the influent stormwater, was observed during the summer sampling period, no significant NO3−retention was observed during the spring or the two cold‐weather sampling periods.

Gaming‐simulation methods have been successfully applied to problems of water resources management. In addition, computer‐aided negotiation exercises are increasingly being used to help address the complex political and institutional issues that surround the allocation of scarce water during crises. Methods used by the authors and their experiences are discussed.

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.

The goals of the Global Energy and Water Cycle Experiment Continental‐Scale International Project (GCIP) point to the need for high‐resolution data sets on all elements of the land surface and atmospheric hydrologic cycle. A high‐resolution precipitation data set has been derived from radar reflectivity observations taken from the National Weather Service WSR‐88D radars in the continental U.S. The data set is available for a continuous five‐year period (1996–2000) at an hourly, 4 × 4 km2resolution for the Mississippi River Basin. Development of the data set involved data management and quality control of input radar‐reflectivity, parameter estimation for radar‐reflectivity transformation, and product accumulation and quality control of the precipitation product. Quality control algorithms for the input radar‐reflectivity included procedures to deal with radar calibration differences, an especially important problem in developing a long‐term, continental‐scale data set for diverse hydroclimatological applications. Rainfall estimation was based on a Z‐R conversion algorithm that involved an optimization technique to determine the parameters for the transformation of radar‐reflectivity to rainfall. Rainfall accumulation involved integrating to hourly, 4 × 4 km2resolution and then visually inspecting the final product. Some limitations of the algorithm are presented and suggestions are proposed for improving the development of a long‐term, large‐scale precipitation product. Initial comparisons of the radar‐based product with a rain gauge based product after a quality analysis of both products show good agreement in the Mississippi River Basin.