Dry patterns in fall and winter may signal dry and less stormy spring and summer will follow.
A Climatological Analysis of Drought and Tornadic Activity in the Southeastern United States
Theresa K. Andersen
Antecedent Drought and Tornadoes
Drought periods were calculated from the soil moisture content data using a modified z?score. The % of normal tornado days were found by averaging the tornado days (per season) over the 27?year period, then dividing each season value by the total average and multiplying by 100%. ArcMap GIS was used to examine the spatial occurrence of tornados during tornado seasons following drought years and non?drought years.
The IPCC has indicated the probability of an increase in extreme events and acceleration of the water
cycle due to climate change. The lack of sufficient antecedent soil moisture has been hypothesized to reduce convective development and tornadoes in the spring (Shepherd et al. 2009; Hanesiak et al. 2009), possibly through soil memory and subsequent atmospheric response. The SE US experienced deadly tornado outbreaks during drought conditions (e.g. the 14 March 2008 Atlanta tornado), and it is the intent of this study to investigate the relationship, if any, between Southeastern tornadoes and drought.
0 ?2 ?1.5 ?1 ?0.5 0 0.5 1 1.5 2
Soil Moisture Departure
Fig.3. (above)Ascatterplotof standardized soil moisture z? scores and % of normal tornado days reveals a trend for negative soil moisture departures to correlate to less than 121% of normal tornado days while positive soil moisture departures correlate up to 240% of normal tornado days.
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The spatial distribution of mean CIN during February resulted in similar findings. The drought years have more negatively buoyant energy which hinders convective development.
In both drought (Fig. 6A) and non?drought years (Fig. 6B), CIN becomes more negative towards the southwest of the study area.
Fig. 4. (below) Composite tornado track maps for drought (A) and non? drought (B) cases are shown. The number of “tornado season” tornadoes that occurred during antecedent drought conditions was 72, while tornadoes during antecedent high soil moisture conditions totaled 102.
Convective Available Potential Energy
Fig. 6. A?B
140 120 100
80 60 40 20
TThe CAPE and CIN data were normalized using the z?score method and then correlated to normalized soil moisture values.
Negative soil moisture departures correspond to almost always negative
CAPE departures (Fig. 7).
The CIN plot indicates positive soil moisture departures almost always correspond with positive CIN anomalies. Negative soil moisture departures correspond to positive CIN values slightly more often than negative values (Fig. 8).
Fig. 8. (below)
correlated to normalized
soil moisture (1980?2006).
Jan Feb Mar April May June July Aug Sept Oct Nov Dec
Fig. 1. USGS canopy map of study area with climate zones 1?3 in AL and GA.
FFig. 2. Total tornado occurrences by month, 1980?2006.
1. Correlate normalized soil moisture data and PDSI Z?Index over a sample of the study area (Fig. 1) in order to assess how well the soil moisture captures drought over 1, 2 and 6?month antecedent periods to the tornado season.
2. Climatologically analyze the tornado season (Fig. 2) and quantify the frequency of occurrence of tornado activity under antecedent drought.
3. Understand how drought affects atmospheric stability in terms of CAPE and CIN.
4. Understand the synoptic patterns occurring during drought and non?drought conditions.
5. Determine if occurrences of drought are related to the Bermuda High Index (BHI).
•Soil moisture content, CAPE, CIN, geopotential heights, and MSLP data were obtained from NARR. •Monthly PDSI Z?Index values for six climate division zones were obtained for the study period from the NCDC.
•Tornado data were obtained from the NOAA SPC historical database of severe thunderstorm and tornado occurrences.
CComposite maps of 1?month antecedent CAPE were analyzed for drought years (Fig.5A) and the six wettest years (Fig. 5B).
The drought composite shows CAPE is consistently lower than in the non?drought composite over the study area.
In both cases CAPE values decrease to the northeast. To better compare the two maps, a difference map was created by subtracting the drought years from the non?drought years (Fig. 5C). It shows the CAPE values over the entire study area are positive, or the non?drought composite has a higher mean CAPE value than the drought composite at every point. It can be concluded that 6?month antecedent drought conditions correspond to lower 1?month antecedent CAPE values.
3 2.5 2 1.5 1 0.5 0 ?0.5 ?1 ?1.5
?2 ?1.5 ?1 ?0.5 0 0.5 1 1.5 2 1.5
Soil Moisture Departure
1 0.5 0 ?0.5 ?1 ?1.5 ?2 ?2.5 ?3
FFig. 7. (above) Normalized CAPE correlated to normalized soil moisture (1980?2006).
??1 ?0.5 0 0.5 1 1.5 2
Soil Moisture Departure
FFig. 5 A?C
SSpatial analysis of geopotential heights and the Bermuda High Index will be calculated to assess the synoptic environment during antecedent drought conditions.
% Normal Tornado Days