Study of the Multiscale Variability of Precipitation during Extreme Flood and Drought Events

Understanding the nature of spatial and temporal variability of precipitation is important to improve the predictability of climatic events such as floods and droughts. This study proposes to explore the multiscale nature of extreme precipitation events such as floods and droughts and their link with associated atmospheric circulation patterns.

To date, we have characterized the multiscale behavior of precipitation events over the United States using a 30-year, gridded hourly precipitation data set. Orthogonal wavelet transform has been applied to the time series at each grid box to capture the temporal scales of fluctuation at several different time scales. Over the Midwest region, both the subdiurnal and synoptic scales showed large fluctuations. To examine whether a flood or a drought event is predominantly influenced by the precipitation fluctuations on synoptic or subdiurnal time scale, an area averaged precipitation index over the U.S. Great Plains and the corresponding energy indices for each temporal scales for the summer season is used. It is found that all temporal scale precipitation fluctuations contribute positively toward the summer seasonal mean precipitation anomalies. For the drought events, the lack of precipitation activity on the intermediate scales, ranging from 16 hours to 2.7 days, contributes the most toward the negative seasonal mean precipitation. For the floods, however, the enhanced precipitation activity on synoptic scales, at 2.7 days and 5.4 days, tend to contribute more toward the above normal precipitation. Whether the degree of different scale precipitation activity is related to boundary forcing such as the sea surface temperature anomalies and soil moisture fluctuations or the moisture transport in the atmosphere, is being investigated. This research will serve as the basis for a comprehensive analysis of the relation between each precipitation scale and the associated atmospheric circulation as well as the forcing mechanisms.