An Objective Climatology, Classification Scheme, and Assessment of Sensible Weather Impacts for Appalachian Cold-Air Damming

Christopher M. Bailey NOAA/NWS/HPC, Camp Springs, Maryland

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Gail Hartfield NOAA/NWS Forecast Office, Raleigh, North Carolina

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Gary M. Lackmann North Carolina State University, Raleigh, North Carolina

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Kermit Keeter NOAA/NWS Forecast Office, Raleigh, North Carolina

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Scott Sharp NOAA/NWS Forecast Office, Raleigh, North Carolina

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Abstract

The geostrophic adjustment process for a rotating, stratified atmosphere in the presence of an orographic barrier may be manifest as a phenomenon known as “cold-air damming” (CAD). The degree of blocking by an orographic barrier, and therefore CAD intensity, is related to the static stability of the upstream air mass. When precipitation falls into dry near-surface air, differential evaporational cooling can increase static stability, and strengthen or initiate CAD. The sheltering effect of clouds can also maintain surface-based stability. Therefore, the ability of numerical forecast models to accurately predict CAD requires adequate representation of cloud and precipitation processes. Operational forecasters in the Appalachian damming region have previously developed a subjective classification scheme that distinguishes those CAD events that are heavily influenced by diabatic processes from those that are dominated by synoptic-scale forcing. In this study the subjective scheme is formalized in order to elucidate distinct synoptic-scale patterns associated with different CAD types. Knowledge of CAD types will enable forecasters to interpret and adjust numerical model forecasts, and reinforce understanding of atmospheric processes during CAD. An objective CAD-identification algorithm, based on hourly surface reports in and around the Appalachian damming region, was used to construct a 12-yr climatology of CAD events of varying intensity. Between the years 1984 and 1995, 353 CAD events were identified. The annual frequency of strong CAD events is consistent with previous studies. However, the overall frequency of CAD reveals a large number of weak warm-season events, with maximum overall frequency in September. A CAD-classification algorithm, based on the aforementioned subjective operational scheme, was used to quantify differences in synoptic setting and sensible weather impacts between CAD types. Analysis of the climatological CAD sample reveals that despite similar patterns in the sea level isobars, some CAD events exert strong influences on sensible weather parameters while others do not. The climatological departure of the maximum temperature at Greensboro, North Carolina, was used to define “high impact” and “low impact” CAD events. Composites of high-impact cases reveal a coupled jet signature at the 250-mb level similar to that accompanying some East Coast cyclones. The low-impact composite exhibits much more pronounced ridging west of the damming region at the 500-mb level relative to the high-impact composite. These results support the interpretation that CAD is not a monolithic phenomenon.

Corresponding author address: Dr. Gary M. Lackmann, Dept. of Marine, Earth, and Atmospheric Sciences, North Carolina State University, 1125 Jordan Hall, Box 8208, Raleigh, NC 27695-8208. Email: gary@ncsu.edu

Abstract

The geostrophic adjustment process for a rotating, stratified atmosphere in the presence of an orographic barrier may be manifest as a phenomenon known as “cold-air damming” (CAD). The degree of blocking by an orographic barrier, and therefore CAD intensity, is related to the static stability of the upstream air mass. When precipitation falls into dry near-surface air, differential evaporational cooling can increase static stability, and strengthen or initiate CAD. The sheltering effect of clouds can also maintain surface-based stability. Therefore, the ability of numerical forecast models to accurately predict CAD requires adequate representation of cloud and precipitation processes. Operational forecasters in the Appalachian damming region have previously developed a subjective classification scheme that distinguishes those CAD events that are heavily influenced by diabatic processes from those that are dominated by synoptic-scale forcing. In this study the subjective scheme is formalized in order to elucidate distinct synoptic-scale patterns associated with different CAD types. Knowledge of CAD types will enable forecasters to interpret and adjust numerical model forecasts, and reinforce understanding of atmospheric processes during CAD. An objective CAD-identification algorithm, based on hourly surface reports in and around the Appalachian damming region, was used to construct a 12-yr climatology of CAD events of varying intensity. Between the years 1984 and 1995, 353 CAD events were identified. The annual frequency of strong CAD events is consistent with previous studies. However, the overall frequency of CAD reveals a large number of weak warm-season events, with maximum overall frequency in September. A CAD-classification algorithm, based on the aforementioned subjective operational scheme, was used to quantify differences in synoptic setting and sensible weather impacts between CAD types. Analysis of the climatological CAD sample reveals that despite similar patterns in the sea level isobars, some CAD events exert strong influences on sensible weather parameters while others do not. The climatological departure of the maximum temperature at Greensboro, North Carolina, was used to define “high impact” and “low impact” CAD events. Composites of high-impact cases reveal a coupled jet signature at the 250-mb level similar to that accompanying some East Coast cyclones. The low-impact composite exhibits much more pronounced ridging west of the damming region at the 500-mb level relative to the high-impact composite. These results support the interpretation that CAD is not a monolithic phenomenon.

Corresponding author address: Dr. Gary M. Lackmann, Dept. of Marine, Earth, and Atmospheric Sciences, North Carolina State University, 1125 Jordan Hall, Box 8208, Raleigh, NC 27695-8208. Email: gary@ncsu.edu

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