Cold Air Damming Erosion and Associated Precipitation in the Southeastern United States

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Title: Cold Air Damming Erosion and Associated Precipitation in the Southeastern United States
Author: Green, Thomas A., Jr.
Advisors: Dr. Gary Lackmann, Committee Member
Dr. Allen Riordan, Committee Chair
Dr. Lian Xie, Committee Member
Abstract: Appalachian cold air damming (CAD) is often associated with significant sensible weather effects in Virginia and the Carolinas. Impacts include below-normal temperatures, reduced visibility, and the potential for freezing or frozen precipitation. Current operational numerical weather prediction (NWP) models have difficulty in predicting the end of CAD events, often prematurely eroding the cold dome. The poor model performance increases the difficulty of correctly forecasting the end of an event. In addition, NWP models are known for having low skill scores with quantitative precipitation forecasting (QPF). While precipitation can occur during CAD events without the presence of a cyclone, the synoptic system eroding the cold dome may be responsible for different precipitation patterns over the damming region. The combination of poor model performance on CAD erosion and QPF creates difficulty for the human forecaster in developing a precipitation forecast. The objectives of this study are to (i) develop high-resolution maps of synoptic and mesoscale patterns associated with CAD; (ii) determine whether particular erosion scenarios have different precipitation signatures, and if so, identify these signatures; (iii) identify physical processes associated with specific precipitation signatures; and (iv) establish parameters to forecast whether a given CAD event will yield above or below average precipitation amounts in the Carolinas. High-resolution maps of synoptic patterns associated with cold air damming were developed using Rapid Update Cycle (RUC) model data for five erosion scenarios. The scenarios include Coastal Lows, Northwestern (NW) Lows, NW Lows with Cold Front, Residual Cold Pool, and Southwestern (SW) Lows. From the five erosion scenarios, three (Coastal, NW, and SW) were chosen for further study of the spatial and temporal distribution of precipitation. Precipitation data from the North American Regional Reanalysis (NARR) was used to label cases as 'wet' or 'dry' in an individual erosion scenario, using the mean normalized precipitation value for each storm over the damming region. Differences were noted in the synoptic distribution of precipitation according to the location of the cyclone that was eroding the cold dome. Further investigation of composites from the wettest and driest cases in each scenario revealed differences between composites of sea-level pressure and 500 hPa geopotential heights. Greater differences were seen between erosion scenarios when examining precipitation coverage both spatially and temporally. NW Lows produced the smallest precipitation totals and had the smallest spatial coverage through the center of the damming region. Coastal Lows had moderate amounts of precipitation, with spatial coverage that peaked approximately nine hours before CAD demise, with decreasing coverage after that time. SW Lows produced the greatest precipitation totals and spatial coverage, with a peak in spatial coverage approximately three hours after CAD demise. Several variables were examined at multiple levels to determine what quantities and physical processes were important for precipitation in the three erosion scenarios. For NW Lows, dynamic and moisture variables such as relative humidity and mixing ratio had the highest correlation with precipitation amounts, suggesting that moisture and synoptic-scale lifting are limiting factors for large amounts of precipitation to occur in this scenario. The low center is well removed from the damming region; therefore, maximum amounts of moisture are necessary because of the weak dynamics. Advective variables such as temperature advection, vorticity advection, and moisture transport yielded the highest correlations with precipitation for SW Lows. For that scenario, moisture is not a limiting factor, and the location of dynamical support with the cyclone in the damming region is more important in determining precipitation amounts. No category of high correlating variables existed for Coastal Lows, suggesting that the track of the cyclone may be most important in determining precipitation amounts. Between all scenarios, relative humidity at 500 hPa, moisture transport at 700 and 850 hPa, and temperature advection at 700 and 850 hPa have the highest correlations with precipitation totals. Several findings from this study can be directly used by operational forecasters. Differences in spatial and temporal coverage of precipitation were found between erosion scenarios that can be used for forecasting or nowcasting. Some differences were further explained by the correlations between particular variables and precipitation totals. Focus can also be placed on whether adequate moisture and lift is present in NW Lows, whether dynamical support is strong enough to produce large amounts of precipitation in SW Lows, and if the cyclone track in Coastal Lows is close enough to the coastline for heavy precipitation to move inland.
Date: 2006-01-18
Degree: MS
Discipline: Marine, Earth and Atmospheric Sciences
URI: http://www.lib.ncsu.edu/resolver/1840.16/1451


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