Southeastern USA Rainfall

My research has focused on summertime rainfall in the Atlanta, GA region of the southeastern United States.  This region has a humid-subtropical climate, and the very wet summers are caused by migratory extratropical cyclones (e.g., fronts) moving across the region.  I have discovered that urbanization has caused higher-than-expected storm intensity downwind of Atlanta in Gwinnett County.  I named this phenomenon the “Norcross Anomaly” in a 2008 paper in the International Journal of Climatology.  In addition, heavy air pollution from the 1940s through 1960s suppressed summertime storms over and downwind of Atlanta.  This research is described in a 2013 paper in Atmospheric Environment.  After the 1970 Clean Air Act was passed, the suppression disappeared and storms then became enhanced by urban effects.


Diem, J.E., 2013. Response to comments on “Influences of the Bermuda High and atmospheric moistening on changes in summer rainfall in the Atlanta, Georgia region, the United States.” International Journal of Climatology 33: 3086–3088.

The purpose of this paper is to determine the impact of reduced atmospheric particulate resulting from the Clean Air Act of 1970 on changes in summer rainfall in the Atlanta, Georgia USA region. In order to determine if rainfall at nine candidate stations in the metropolitan area was influenced by changes in particulate concentrations within the 1948–2009 period, predicted rainfall characteristics were derived from rainfall frequencies at nine reference stations located more than 80 km from downtown Atlanta. Both parametric and non-parametric tests were used to test for significant differences between observed values and predicted values within 34 overlapping 30-year periods. For the country as a whole, emissions of PM10 (i.e. particulates with a diameter less than or equal to 10 μm) decreased by approximately 40% from 1970 to 1975. The reduction in emissions caused a rapid rebound in summer rainfall in the Atlanta region. There was suppression of rainfall over and downwind of the Atlanta urbanized area during 30-yr periods that comprise all or portions of the decades of the 1950s, 1960s, and 1970s. This suppression occurred even while urban-related factors that promote rainfall enhancement were present. During the 1948–1977 suppression period, there was a decrease in rainfall of at least 40 mm at affected locales, which is substantial given that the mean seasonal rainfall was approximately 300 mm. The rainfall suppression involved a decrease of heavy-rainfall days. Atlanta is most likely not a unique case; therefore, particulate-induced rainfall suppression might have occurred over and downwind of other U.S. urban areas prior to the late 1970s.


Diem, J.E. 2013. Comments on “Changes to the North Atlantic Subtropical High and Its Role in the Intensification of Summer Rainfall Variability in the Southeastern United States.” Journal of Climate 26:679–682.

In a recent article, Li et al. examined changes in the summer-season location of the western ridge of the North Atlantic subtropical high from 1948 to 2007 because there has been an increase in interannual summer rainfall variability in the southeastern United States. The following major conclusions by Li et al. are incorrect: the western ridge has undergone a significant westward trend since the late 1970s; the western ridge had increased meridional movement during 1978–2007 compared to 1948–1977; and global warming appears to be contributing to the westward expansion of the western ridge. Results presented in this paper reveal that the western ridge has been moving eastward over the past three decades, there was no change in latitudinal variance, and a westward movement of the western ridge should not be linked to global warming.


Diem, J.E., 2013. Influences of the Bermuda High and atmospheric moistening on changes in summer rainfall in the Atlanta, Georgia USA region. International Journal of Climatology 33:160–172.

This paper assesses the variability and trends in summer-season rainfall from 1948 to 2009 for the Atlanta, Georgia region. The rainfall variables are total rainfall, frequency of rainfall days, and frequency of heavy-rainfall days. The main methods involve classifying daily 500-hPa geopotential height fields into synoptic types, determining the rainfall characteristics of the synoptic types, testing for significant temporal trends in rainfall, middle-troposphere circulation, lower-troposphere circulation, and atmospheric humidity, and using multiple linear regression to determine the impact of circulation and humidity variables on inter-annual variations in the rainfall variables. There were a total of eight synoptic types: the wet types involved troughing across or to the immediate west of the Atlanta region, while the dry types involved either an anticyclone across or to the immediate west of the region. The rainfall variables and two lower-troposphere circulation indices, the Bermuda High Index (BHI) and the Western Bermuda High Index (WBHI), had significant positive trends in variance over time. Among the three rainfall variables, only the frequency of rainfall days had a significant trend: the periods 1976–2009 and 1977–2009 had significant positive trends in rainfall days. The BHI had a significant positive trend from the 1970s to 2009, and the western ridge of the Bermuda High moved significantly southeastward from approximately the mid-1970s to 2009. Atmospheric humidity (i.e. 850-hPa specific humidity, 500-hPa specific humidity, and precipitable water) over the region had significant positive trends during most periods, with all humidity variables having significant increases from the 1970s to 2009. Increased interannual variability in the WBHI appears to be the cause of the increased variance in rainfall variables. An increase in atmospheric humidity, which is actually a global phenomenon, appears to be the principal cause of the increase in rainfall days during the past three decades.


Diem, J.E. 2006. Synoptic-scale controls of summer precipitation in the southeastern United States. Journal of Climate 19:613–621.

Past climatological research has not quantitatively defined the synoptic-scale circulation deviations responsible for anomalous summer-season precipitation totals in the southeastern United States. Therefore, the objectives of this research were to determine the synoptic-scale controls of wet and dry multiday periods during the summer within a portion of the southeastern United States as well as to assess the linkages between synoptic-scale circulation and multidecadal variations in precipitation characteristics for the study domain. Daily precipitation data from 30 stations for June, July, and August from 1953 to 2002 were converted into 13-day totals. Using standardized principal components analysis (PCA), the study domain was divided into three precipitation regions (i.e., South, Northwest, and Northeast). Wet and dry periods for each region were composed of the top 56 and bottom 56 thirteen-day periods. Composite circulation maps for 500 and 850 mb revealed the following: wet periods were generally associated with an upper-level trough over the interior southeastern United States coincident with strong lower-tropospheric flow into the Southeast from the Gulf of Mexico, and dry periods were characterized by ridges or anticyclones over the midwestern and southeastern United States coupled with weak lower-tropospheric flow. Many of the wet periods had surface fronts. Over the 50-yr period, increased precipitation was significantly correlated with increased occurrences of midtropospheric troughs over the study domain. Future research can benefit from the main finding of a strong impact of synoptic-scale circulation features on summer precipitation in the southeastern United States.


Diem, J.E., and T.L. Mote. 2005. Inter-epochal changes in summer precipitation in the southeastern United States: Evidence of possible urban effects near Atlanta, Georgia. Journal of Applied Meteorology 44:717–730.

Through modification of the planetary boundary layer, urbanization has the potential to have a significant impact on precipitation totals locally. Using daily summer-season precipitation data at 30 stations from 1953 to 2002, this study explores the possibility of urban effects as causes of spatial anomalies in precipitation in a zone within 180 km of Atlanta, Georgia. The time period is divided into consecutive epochs (e.g., 1953–77 and 1978–2002), and interepochal differences in precipitation totals, heavy-precipitation days, cumulative heavy precipitation, and atmospheric conditions are explored. The southern stations experienced significant decreases in precipitation, whereas significant precipitation increases occurred at central/west-central stations. The most striking increases occurred at Norcross, Georgia, which is ∼30 km northeast of downtown Atlanta; Norcross had the third smallest number of heavy-precipitation days during 1953–77, but, during 1978–2002, it had the most heavy-precipitation days. Not only did the amount of urban land cover upwind of Norcross increase substantially from the earlier to the later epochs, but regionwide dewpoint temperatures also increased significantly. Therefore, it is suspected that the increased precipitation at Norcross was caused by urban effects, and these effects may have been enhanced by increased atmospheric humidity.

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