Steven V. Vasiloff

IMPROVING QPE AND VERY SHORT TERM QPF An Initiative for a Community-Wide Integrated Approach

Accurate quantitative precipitation estimates (QPE) and very short term quantitative precipitation forecasts (VSTQPF) are critical to accurate monitoring and prediction of water-related hazards and water resources. While tremendous progress has been made in the last quarter-century in many areas of QPE and VSTQPF, significant gaps continue to exist in both knowledge and capabilities that are necessary to produce accurate high-resolution precipitation estimates at the national scale for a wide spectrum of users. Toward this goal, a national next-generation QPE and VSTQPF (Q2) workshop was held in Norman, Oklahoma, on 28–30 June 2005. Scientists, operational forecasters, water managers, and stakeholders from public and private sectors, including academia, presented and discussed a broad range of precipitation and forecasting topics and issues, and developed a list of science focus areas. To meet the nations needs for the precipitation information effectively, the authors herein propose a community-wide int...

Improving Tornado Warnings with the Federal Aviation Administration's Terminal Doppler Weather Radar

Abstract The potential role of the Federal Aviation Administrations Terminal Doppler Weather Radar (TDWR) to supplement the Weather Surveillance Radar–1988 Doppler (WSR–88D) for tornado detection is discussed. Compared to the WSR–88D, the TDWR has a narrower beam, lower scan angles, and faster update rates. The 11 August 1999 Salt Lake City, Utah, tornado is used as an illustration of the utility of the TDWR. The Salt Lake City TDWR was much closer to the tornado than the WSR–88D and the WSR–88D was 750 m higher than the TDWR. Because the tornado developed rapidly upward from a surface convergence line, the TDWR detected the formation earlier than the WSR–88D. Also, the vortex signatures associated with the tornado were much better defined by the TDWR. The enhanced spatial and temporal coverage provided by the TDWR network is shown. A significant improvement in tornado detection, as well as other low–altitude phenomena, would be gained. However, ground clutter and signal attenuation can degrade coverage....

An Investigation of the Transition from Multicell to Supercell Storms

Abstract Nearly 2½ hours of dual-Doppler radar data with high temporal and spatial resolution are used to examine the evolution and morphology of a thunderstorm that evolved from a complex of small cells into a supercell storm. Individual storm cells and updrafts moved east-northeastward, nearly with the mean wind, while the storm complex, which encompassed the individual cells, propagated toward the south–southeast. Cells were first detected at middle levels (5–10 km) on the storms right flank and dissipated on the left flank. Generally, the storm contained three cells—a forming cell, a mature cell, and a dissipating cell; life stages were apparently dictated by the source of updraft air. During the growth stage, cell inflow had a southerly component. As the cell moved through the storm complex, it started ingesting stable air from the north and soon dissipated. A storm-environment feedback mechanism of updraft–downdraft interactions, in conjunction with increasing environmental vertical wind shear and ...

Investigation of a Severe Downburst Storm near Phoenix, Arizona, as Seen by a Mobile Doppler Radar and the KIWA WSR-88D

Abstract A Shared Mobile Atmospheric Research and Teaching Radar (SMART-R) was deployed near Phoenix, Arizona, during the summer of 2004. The goal was to capture a severe microburst at close range to understand the low-altitude wind structure and evolution. During the evening of 27 July, a severe storm formed along the Estrella Mountains south of Phoenix and moved south of the SMART-R as well as the National Weather Service’s (NWS) Weather Surveillance Radar-1988 Doppler (WSR-88D) in Phoenix (KIWA). Several microburst–downburst pulses were observed by radar and a surface wind gust of 67 mi h−1 was reported. The radar data illustrate the finescale structure of the microburst pulses, with the SMART-R’s higher-resolution data showing Doppler velocities 3–4 m s−1 greater than the KIWA radar. SMART-R wind shear values were 2–3 times greater with the finer resolution of the SMART-R revealing smaller features in the surface outflow wind structure. Asymmetric outflow may have been a factor as well in the differen...

Difficulties with Correcting Radar Rainfall Estimates Based on Rain Gauge Data: A Case Study of Severe Weather in Montana on 16-17 June 2007

Abstract The principal source of information for operational flash flood monitoring and warning issuance is weather radar–based quantitative estimates of precipitation. Rain gauges are considered truth for the purposes of validating and calibrating real-time radar-derived precipitation data, both in a real-time sense and climatologically. This paper examines various uncertainties and challenges involved with using radar and rain gauge data in a severe local storm environment. A series of severe thunderstorm systems that occurred across northeastern Montana illustrates various problems with comparing radar precipitation estimates and real-time gauge data, including extreme wind effects, hail, missing gauge data, and radar quality control. Ten radar–gauge time series pairs were analyzed with most found to be not useful for real-time radar calibration. These issues must be carefully considered within the context of ongoing efforts to develop robust real-time tools for evaluating radar–gauge uncertainties. Re...