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Kermit K. Keeter, Steven Businger, Laurence G. Lee, and Jeff S. Waldstreicher

Abstract

Winter weather in the Carolinas and Virginia is highly variable and influenced by the area's diverse topography and geography. The Gulf Stream, the highest mountains in the Appalachians, the largest coastal lagoonal system in the United States, and the region's southern latitude combine to produce an array of weather events, particularly during the winter season, that pose substantial challenges to forecasters. The influence of the region's topography upon the evolution of winter weather systems, such as cold-air damming and frontogenesis, is discussed. Conceptual models and specific case studies are examined to illustrate the region's vast assortment of winter weather hazards including prolonged heavy sleet, heavy snow, strong convection, and coastal flooding.

The weather associated with these topographic and meteorological features is often difficult for operational dynamical models to resolve. Forecasting precipitation type within the region can be especially difficult. An objective technique to forecast wintry precipitation across North Carolina is presented to illustrate a 1ocally developed forecast tool used operationally to supplement the centrally produced numerical guidance. The development of other forecast tools is being pursued through collaborative studies between the National Weather Service Forecast Office in Raleigh–Durham, North Carolina, and the Department of Marine, Earth and Atmospheric Sciences at North Carolina State University.

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Antti T. Pessi, Steven Businger, K. L. Cummins, N. W. S. Demetriades, M. Murphy, and B. Pifer

Abstract

The waveguide between the earth’s surface and the ionosphere allows very low-frequency (VLF) emissions generated by lightning, called sferics, to propagate over long distances. The new Pacific Lightning Detection Network (PacNet), as a part of a larger long-range lightning detection network (LLDN), utilizes this attribute to monitor lightning activity over the central North Pacific Ocean with a network of ground-based lightning detectors that have been installed on four widely spaced Pacific islands (400–3800 km). PacNet and LLDN sensors combine both magnetic direction finding (MDF) and time-of-arrival (TOA)-based technology to locate a strike with as few as two sensors. As a result, PacNet/LLDN is one of the few observing systems, outside of geostationary satellites, that provides continuous real-time data concerning convective storms throughout a synoptic-scale area over the open ocean.

The performance of the PacNet/LLDN was carefully assessed. Long-range lightning flash detection efficiency (DE) and location accuracy (LA) models were developed with reference to accurate data from the U.S. National Lightning Detection Network (NLDN). Model calibration procedures are detailed, and comparisons of model results with lightning observations from the PacNet/LLDN in correlation with NASA’s Lightning Imaging Sensor (LIS) are presented. The daytime and nighttime flash DE in the north-central Pacific is in the range of 17%–23% and 40%–61%, respectively. The median LA is in the range of 13–40 km. The results of this extensive analysis suggest that the DE and LA models are reasonably able to reproduce the observed performance of PacNet/LLDN.

The implications of this work are that the DE and LA model outputs can be used in quantitative applications of the PacNet/LLDN over the North Pacific Ocean and elsewhere. For example, by virtue of the relationship between lightning and rainfall rates, these data also hold promise as input for NWP models as a proxy for latent heat release in convection. Moreover, the PacNet/LLDN datastream is useful for investigations of storm morphology and cloud microphysics over the central North Pacific Ocean. Notably, the PacNet/LLDN lightning datastream has application for planning transpacific flights and nowcasting of squall lines and tropical storms.

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James J. Gurka, Eugene P. Auciello, Anthony F. Gigi, Jeff S. Waldstreicher, Kermit K. Keeter, Steven Businger, and Laurence G. Lee

Abstract

The complex combination of synoptic and mesoscale interactions, topographic influences, and large population densities poses a multitude of challenging problems to winter weather forecasters throughout the eastern United States. Over the years, much has been learned about the structure, evolution, and attendant precipitation within winter storms. As a result, numerous operational procedures, forecast applications, and objective techniques have been developed at National Weather Service offices to assess the potential for, and forecast, hazardous winter weather. A companion paper by Maglaras et al. provided an overview of the challenge of forecasting winter weather in the eastern United States.

This paper focuses on the problem of cyclogenesis from an operational perspective. Since pattern recognition is an important tool employed by field forecasters, a review of several conceptual models of cyclogenesis often observed in the east is presented. These include classical Miller type A and B cyclogenesis, zipper lows, 500-mb cutoff lows, and cold-air cyclogenesis. The ability of operational dynamical models to predict East Coast cyclones and, in particular, explosive cyclogenesis is explored. An operational checklist that utilizes information from the Nested Grid Model to forecast the potential for rapid cyclogenesis is also described. A review of signatures related to cyclogenesis in visible, infrared, and water vapor satellite imagery is presented. Finally, a study of water vapor imagery for 16 cases of explosive cyclogenesis between 1988 and 1990 indicates that an acceleration of a dry (dark) surge with speeds exceeding 25 m s−1, toward a baroclinic zone, is an excellent indicator of the imminent onset of rapid deepening.

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Steven P. Oncley, Carl A. Friehe, John C. Larue, Joost A. Businger, Eric C. Itsweire, and Sam S. Chang

Abstract

An atmospheric surface-layer experiment over a nearly uniform plowed field was performed to determine the constants in the flux-profile similarity formulas, particularly the von Kármán constant. New instruments were constructed to minimize flow distortion effects on the turbulence measurements and to provide high-resolution gradient measurements. In addition, a hot-wire anemometer directly measured the turbulent kinetic energy dissipation rate.

An average value of the von Kármán constant of 0.365 ± 0.015 was obtained from 91 runs (31 h) in near-neutral stability conditions. However, four near-neutral runs when snow covered the ground gave an average value of 0.42. This result suggests that the von Kármán constant depends on the roughness Reynolds number, which may resolve some of the differences in previous determinations over different surfaces. The one-dimensional Kolmogorov inertial subrange constant was found to have a value of 0.54 ± 0.03, slightly larger than previous results.

The flux-profile relations for momentum and temperature variance were evaluated, and humidity variance data behaved similarly to temperature. Dissipation of turbulent kinetic energy was found to be less than production under near-neutral conditions, which suggests that turbulent or pressure transport may be significant.

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J. A. Businger, W. F. Dabberdt, A. C. Delany, T. W. Horst, C. L. Martin, S. P. Oncley, and S. R. Semmer

The Atmosphere-Surface Turbulent Exchange Research (ASTER) facility developed at the National Center for Atmospheric Research (NCAR) will support observational research on the structure of the atmospheric surface layer. ASTER will provide state-of-the-art measurements of surface fluxes of momentum, sensible heat, and water vapor, and support measurements of surface fluxes of trace chemical species. The facility will be available to the scientific community in the spring of 1990. The motivation for the development of ASTER and the elements that constitute this new national facility are briefly discussed.

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Steven Businger, Steven R. Chiswell, Michael Bevis, Jingping Duan, Richard A. Anthes, Christian Rocken, Randolph H. Ware, Michael Exner, T. VanHove, and Fredrick S. Solheim

This paper provides an overview of applications of the Global Positioning System (GPS) for active measurement of the Earth's atmosphere. Microwave radio signals transmitted by GPS satellites are delayed (refracted) by the atmosphere as they propagate to Earth-based GPS receivers or GPS receivers carried on low Earth orbit satellites.

The delay in GPS signals reaching Earth-based receivers due to the presence of water vapor is nearly proportional to the quantity of water vapor integrated along the signal path. Measurement of atmospheric water vapor by Earth-based GPS receivers was demonstrated during the GPS/STORM field project to be comparable and in some respects superior to measurements by ground-based water vapor radiometers. Increased spatial and temporal resolution of the water vapor distribution provided by the GPS/STORM network proved useful in monitoring the moisture-flux convergence along a dryline and the decrease in integrated water vapor associated with the passage of a midtropospheric cold front, both of which triggered severe weather over the area during the course of the experiment.

Given the rapid growth in regional networks of continuously operating Earth-based GPS receivers currently being implemented, an opportunity exists to observe the distribution of water vapor with increased spatial and temporal coverage, which could prove valuable in a range of operational and research applications in the atmospheric sciences.

The first space-based GPS receiver designed for sensing the Earth's atmosphere was launched in April 1995. Phase measurements of GPS signals as they are occluded by the atmosphere provide refractivity profiles (see the companion article by Ware et al. on page 19 of this issue). Water vapor limits the accuracy of temperature recovery below the tropopause because of uncertainty in the water vapor distribution. The sensitivity of atmospheric refractivity to water vapor pressure, however, means that refractivity profiles can in principle yield information on the atmospheric humidity distribution given independent information on the temperature and pressure distribution from NWP models or independent observational data.

A discussion is provided of some of the research opportunities that exist to capitalize on the complementary nature of the methods of active atmospheric monitoring by GPS and other observation systems for use in weather and climate studies and in numerical weather prediction models.

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R. Ware, M. Exner, D. Feng, M. Gorbunov, K. Hardy, B. Herman, Y. Kuo, T. Meehan, W. Melbourne, C. Rocken, W. Schreiner, S. Sokolovskiy, F. Solheim, X. Zou, R. Anthes, S. Businger, and K. Trenberth

This paper provides an overview of the methodology of and describes preliminary results from an experiment called GPS/MET (Global Positioning System/Meteorology), in which temperature soundings are obtained from a low Earth-orbiting satellite using the radio occultation technique. Launched into a circular orbit of about 750-km altitude and 70° inclination on 3 April 1995, a small research satellite, MicroLab 1, carried a laptop-sized radio receiver. Each time this receiver rises and sets relative to the 24 operational GPS satellites, the GPS radio waves transect successive layers of the atmosphere and are bent (refracted) by the atmosphere before they reach the receiver, causing a delay in the dual-frequency carrier phase observations sensed by the receiver. During this occultation, GPS limb sounding measurements are obtained from which vertical profiles of atmospheric refractivity can be computed. The refractivity is a function of pressure, temperature, and water vapor and thus provides information on these variables that has the potential to be useful in weather prediction and weather and climate research.

Because of the dependence of refractivity on both temperature and water vapor, it is generally impossible to compute both variables from a refractivity sounding. However, if either temperature or water vapor is known from independent measurements or from model predictions, the other variable may be calculated. In portions of the atmosphere where moisture effects are negligible (typically above 5–7 km), temperature may be estimated directly from refractivity.

This paper compares a representative sample of 11 temperature profiles derived from GPS/MET soundings (assuming a dry atmosphere) with nearby radiosonde and high-resolution balloon soundings and the operational gridded analysis of the National Centers for Environmental Prediction (formerly the National Meteorological Center). One GPS/MET profile was obtained at a location where a temperature profile from the Halogen Occultation Experiment was available for comparison. These comparisons show that accurate vertical temperature profiles may be obtained using the GPS limb sounding technique from approximately 40 km to about 5–7 km in altitude where moisture effects are negligible. Temperatures in this region usually agree within 2°C with the independent sources of data. The GPS/MET temperature profiles show vertical resolution of about 1 km and resolve the location and minimum temperature of the tropopause very well. Theoretical temperature accuracy is better than 0.5°C at the tropopause, degrading to about 1°C at 40-km altitude.

Above 40 km and below 5 km, these preliminary temperature retrievals show difficulties. In the upper atmosphere, the errors result from initial temperature and pressure assumptions in this region and initial ionospheric refraction assumptions. In the lower troposphere, the errors appear to be associated with multipath effects caused by large gradients in refractivity primarily due to water vapor distribution.

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