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Timothy D. Crum

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Timothy D. Crum and Roland B. Stull

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The structure of the atmospheric entrainment zone, an interfacial layer between the convective boundary layer and the stable air aloft, is studied using coincident high resolution aircraft and lidar observations obtained during Boundary Layer Experiment–1983 in Oklahoma. Humidity as measured by a fast-response Lyman alpha humidiometer is used as a tracer to estimate the amount of surface-layer origin air reaching various heights in the entrainment zone.

Two approaches are taken to describe the humidity structure of the entrainment zone. The first approach models the frequency distributions of the three types of air in the entrainment zone: unmixed free atmosphere (dry); unmixed surface layer air (moist); and a mixture of these two. The resultant modeled frequency distributions of specific humidity capture the following observed features: Low in the convective boundary layer, surface layer air is frequently observed with little mixture air and no free atmosphere air. Higher in the convective boundary layer, near the middle of the entrainment zone, the proportion of unmixed free atmosphere and mixture air increases while the proportion of unmixed surface layer air decreases. Approaching the top of the entrainment zone, unmixed surface layer and mixture air proportions decrease to zero leaving only unmixed free atmosphere air. These results are critical for the successful forecasts of fair-weather cumulus. The second approach uses the linear mixing character of air with different specific humidities and yields vertical profiles of the proportion of surface layer air present. These profiles are described well by the cumulative distribution function of asymmetric double exponential functions.

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Timothy D. Crum and John J. Cahir

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Experiments were made in prediction of the elevation of warm season shower-tops, both prevailing and highest, using a one-dimensional cloud model run on a real-time minicomputer system. A forecaster inter-actively altered the initial temperatures and/or mixing ratios taken from 1200 GMT radiosondes over the eastern two-thirds of the United States. Subjective methods and numerical guidance were used to estimate upper air changes from morning to afternoon, but observed afternoon surface dewpoints were employed in the developmental work. A forecast based on the unaltered initial sounding was run as a control. Observed tops were taken from radar reports within 2° latitude boxes, fine-tuned somewhat by enhanced infrared satellite imagery.

Development results show root-mean-square errors (RMSE) of less than 2.0 km can be achieved for both prevailing and highest tops if the surface dewpoint is specified accurately. Independent tests were consistent only for highest tops, and the RMSE increased to 2.54 km when forecasters had to predict the dewpoint. “Prevailing tops” are apparently difficult to distinguish from “highest tops” reliably in real-time conditions.

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Timothy D. Crum and Ron L. Alberty

The Weather Surveillance Radar—1988 Doppler (WSR-88D) System is the product of the Next Generation Weather Radar (NEXRAD) program, a joint effort of the U.S. Departments of Commerce, Defense, and Transportation. WSR-88D Systems meet the common needs of the three agencies and are being installed across the United States and at selected overseas sites. These systems provide Doppler capabilities, increased receiver sensitivity, and real-time display of base and derived products that will enable forecasters to improve the detection of and give greater advanced warning of severe weather events. Many nonsevere weather and hydrological applications are also expected.

WSR-88D Systems will be modified and enhanced during their operational life to meet changing requirements, technological advancements, and improved understanding of the application of these systems to real-time operations. The NEXRAD agencies established the Operational Support Facility (OSF) to provide centralized WSR-88D operator training and software, maintenance, and engineering support.

This paper provides an overview of the NEXRAD program, the WSR-88D System, and the role of the OSF in supporting the WSR-88D and its users. Examples of some of the products are also presented.

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Timothy D. Crum, Ron L. Alberty, and Donald W. Burgess

In order to support NEXRAD program requirements, WSR-88D systems have the capability to record data and products at four levels. Of these, level II (base data) and level III (products) will be most commonly available for various applications by a wide range of users. This paper overviews the data-recording capabilities of the WSR-88D system, plans for recording and archiving these data, and some uses for these data.

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Timothy D. Crum, Roland B. Stull, and Edwin W. Eloranta

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Coincident observations of the daytime convective boundary layer over Oklahoma were made with the NCAR Queen Air aircraft and the University of Wisconsin ground-based lidar. The two data sets have been merged to provide a unique visual representation of the temperature, moisture, vertical velocity, turbulent kinetic energy and the momentum fluxes in a field of thermals. These data show that horizontal moisture profiles observed in thermals penetrating the entrainment zone tend to exhibit more of a top-hat profile than the corresponding temperature or vertical velocity profiles. The specific humidities observed at various heights including cloud base 1) are frequently nearly constant along the horizontal tracks within each thermal; 2) show thermal-to-thermal variability; and 3) have values nearly the same as found in the surface layer. This paper also proposes the concept of an “intromission zone” describing the zone of lateral entrainment at the edges of active thermals. For the data studied here, a lateral entrainment velocity of 0.3 m s−1 was observed.

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Timothy D. Crum, Robert E. Saffle, and James W. Wilson

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The operational network of WSR-88D systems is in place. These radars provide a large increase in performance and coverage over the radars they replaced—and in some locations are the first operational weather radars. The early years of experience using these radars have shown that they have contributed to increased weather warning and forecast performance. These systems must be regularly upgraded, however, to maintain a viable network, meet new requirements, and take advantage of new science and technology. In addition to the NEXRAD program’s standard modification and retrofit process, the NEXRAD agencies (Department of Commerce/National Weather Service, Department of Defense, and Department of Transportation/Federal Aviation Administration) have established a NEXRAD product improvement program to initiate major hardware upgrades to the WSR-88D. This paper describes the status of the NEXRAD program, improvements to the WSR-88D that have taken place, improvements that are planned, and a discussion of possible future changes to the WSR-88D and application of radar data.

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REFRACTT 2006

Real-Time Retrieval of High-Resolution, Low-Level Moisture Fields from Operational NEXRAD and Research Radars

Rita D. Roberts, Frédéric Fabry, Patrick C. Kennedy, Eric Nelson, James W. Wilson, Nancy Rehak, Jason Fritz, V. Chandrasekar, John Braun, Juanzhen Sun, Scott Ellis, Steven Reising, Timothy Crum, Larry Mooney, Robert Palmer, Tammy Weckwerth, and Sharmila Padmanabhan

The Refractivity Experiment for H2O Research and Collaborative Operational Technology Transfer (REFRACTT), conducted in northeast Colorado during the summer of 2006, provided a unique opportunity to obtain high-resolution gridded moisture fields from the operational Denver Next Generation Weather Radar (NEXRAD) and three research radars using a radar-based index of refraction (refractivity) technique. Until now, it has not been possible to observe and monitor moisture variability in the near-surface boundary layer to such high spatial (4-km horizontal gridpoint spacing) and temporal (4–10-min update rates) resolutions using operational NEXRAD and provide these moisture fields to researchers and the National Weather Service (NWS) forecasters in real time. The overarching goals of REFRACTT were to 1) access and mosaic the refractivity data from the operational NEXRAD and research radars together over a large domain for use by NWS forecasters in real time for short-term forecasting, 2) improve our understanding of near-surface water vapor variability and the role it plays in the initiation of convection and thunderstorms, and 3) improve the accuracy of quantitative precipitation forecasts (QPF) through improved observations and assimilation of low-level moisture fields. This paper presents examples of refractivity-derived moisture fields from REFRACTT in 2006 and the moisture variability observed in the near-surface boundary layer, in association with thunderstorm initiation, and with a cold frontal passage.

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