Search Results

You are looking at 1 - 5 of 5 items for :

  • Author or Editor: LAURENCE G. LEE x
  • Refine by Access: All Content x
Clear All Modify Search
Gary M. Lackmann, Kermit Keeter, Laurence G. Lee, and Michael B. Ek

Abstract

During episodes of sustained moderate or heavy precipitation in conjunction with near-freezing temperatures and weak horizontal temperature advection, the latent heat released (absorbed) by the freezing (melting) of falling precipitation may alter thermal profiles sufficiently to affect the type and amount of freezing or frozen precipitation observed at the surface. Representation of these processes by operational numerical weather prediction models is incomplete; forecaster knowledge of these model limitations can therefore be advantageous during winter weather forecasting. The Eta Model employs a sophisticated land surface model (LSM) to represent physical processes at the lower-atmospheric interface. When considering the thermodynamic effect of melting or freezing precipitation at the surface, it is shown that limitations in the current version of the Eta LSM can contribute to biases in lower-tropospheric temperature forecasts. The Eta LSM determines the precipitation type reaching the surface from the air temperature at the lowest model level; subfreezing (above freezing) temperatures are assumed to correspond to snow (rain) reaching the surface. There is currently no requirement for consistency between the LSM and the Eta grid-scale precipitation scheme. In freezing-rain situations, the lowest model air temperature is typically below freezing, and the Eta LSM will therefore determine that snow is falling. As a result, a cold bias develops that is partly caused by the neglected latent heat release accompanying the freezing of raindrops at the surface. In addition, alterations in surface characteristics caused by erroneous snowfall accumulation in the model may also contribute to temperature biases. In an analogous fashion, warm biases can develop in cases with melting snow and above-freezing air temperatures near the surface (the LSM assumes rain). An example case is presented in which model misrepresentation of freezing rain is hypothesized to have contributed to a lower-tropospheric cold bias. A simple temperature correction, based on the first law of thermodynamics, is applied to lower-tropospheric model temperature forecasts; the neglect of latent heat released by freezing rain in the model is shown to contribute substantially to a cold bias in near-surface temperature forecasts. The development of a spurious snow cover likely exacerbated the bias.

Full access
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.

Full access
FRANKLIN P. HALL JR., CLAUDE E. DUCHON, LAURENCE G. LEE, and RICHARD R. HAGAN

Abstract

An air pollution episode during August 1970 over the central United States is examined. By use of surface visibilities and an 850-mb wind trajectory analysis, we observed the pollution to advance as much as 700 mi from the central midwest (source region) into the upper midwest and Great Plains (impact area). A large, nearly stationary high-pressure system over the source region allowed the pollution to accumulate beneath a mid-level subsidence inversion located generally near 700 mb. Southeasterly flow around the backside of the High and the northeasterly flow around a weak Low to the south advected the pollution into the impact area. At times, surface visibilities in parts of the impact area were restricted by haze to as little as 4 mi. Although particulate count data were meager, several stations recorded their highest particulate count of the year during the episode.

Full access
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.

Full access
Steve Keighton, Douglas K. Miller, David Hotz, Patrick D. Moore, L. Baker Perry, Laurence G. Lee, and Daniel T. Martin

Abstract

In late October 2012, Hurricane Sandy tracked along the eastern U.S. coastline and made landfall over New Jersey after turning sharply northwest and becoming posttropical while interacting with a complex upper-level low pressure system that had brought cold air into the Appalachian region. The cold air, intensified by the extreme low pressure tracking just north of the region, combined with deep moisture and topographically enhanced ascent to produce an unusual and high-impact early season northwest flow snow (NWFS) that has no analog in recent history. This paper investigates the importance of the synoptic-scale pattern, forcing mechanisms, moisture characteristics (content, depth, and likely sources), and low-level winds, as well as the evolution of some of these features compared to more typical NWFS events in the southern Appalachian Mountains. Several other aspects of the Sandy snowfall event are investigated, including low-level stability and mountain wave formation as manifested in vertical profiles and radar observations. The importance to operational forecasters of recognizing and understanding these factors and differences from more common NWFS events is also discussed.

Full access