A Climatology of Frost Extremes Across the Southeast United States, 1950-1999
Darren B. Parnell
Advisor: Dr. Susan L. Cutter
This dissertation focuses on growing season characteristics during the spring season across the southeastern United States. This study focused on three research questions: 1) What is the spatial and temporal variability of last spring frost dates?; 2) What are the synoptic conditions associated with frost extremes?; and 3) What are the agricultural impacts produced by the extreme frost events at the beginning of the growing season?
The length of the study period is from 1950-1999 and is based on the completeness of record for the selected weather stations. Weather stations were included in the study if the average last spring frost date occurred between early February and late March, (Julian days 40 to 120), based on the time period 1950-1999. Ninety-two stations across the study area met these criteria and were used to define the study region. Twenty-five extreme frost events were identified using a frost event criteria based on the timing and geographic extent of the freezing temperatures.
Daily minimum temperature data were used to examine the variability and trends in last spring frost dates while gridded surface pressure data were used to produce composite anomaly and mean maps to investigate the atmospheric conditions associated with the extreme frost events. Six major newspaper sources across the study area were utilized to provide a measure of agricultural damage produced from the freezing temperatures.
Considerable year-to-year spatial variability exists in the timing of last spring frost dates across the southeastern United States with the highest variability occurring in the southern portion of the study area. No overall increasing or decreasing trends in frost dates across the study area were found although a clear trend towards earlier last spring frost dates has been established at many locations since the late 1980s.
Several common surface features were associated with the extreme frost events including strengthened North Pacific and North Atlantic subtropical high pressure systems, strong cold-core low pressure systems in the Bering Sea and northern Atlantic, and positive and negative surface pressure anomalies across North America. Temperatures were highly variable in advance of the extreme frost events. A synoptic climatology was completed for five frost events that were identified as having obvious early season warmth.
A content analysis and a historical assessment of agricultural impacts from the extreme frosts was completed using newspaper articles from the major newspaper sources. Two-thirds of the frost events with newspaper reports of agricultural damage produced light to moderate damage and in some cases was considered beneficial to farmers. The remaining frost events either produced considerable damage or no damage at all. The increased use of preventative measures and the decreasing acreage of farmland across the southeastern United States have reduced the agricultural losses of frost events in recent times.