A five-year climatology of precipitation organization in the southeastern U.S: seasonal cycle and extreme events

This paper presents analysis of a five-year climatology (2009-2013) of precipitation system organization in the southeastern U.S, including extreme events. The premise of this NSF-funded study is that a climatology of precipitation system organization (structural, temporal and water phase characteristics of a precipitation system) will lead to improved characterization of regional hydrology and will provide an unique tool for the process-based downscaling of climate simulations. Examples of precipitation organization that occur in the southeastern U.S. include short duration and spatially heterogeneous convective cells, large mesoscale convective systems, widespread long-lasting frontal precipitation, tropical cyclones, and winter precipitation. The value-added information beyond simple rainrate is that each type of precipitation organization may produce similar time-averaged precipitation totals, but have very different hydrological impacts due to different spatial and temporal distributions of precipitation. This is particularly important given that the southeastern U.S. has a wide variety of precipitation types year-round, and the relative contribution of different types of precipitation organization to annual precipitation is not well quantified.

The climatology is comprised of a pixel-based and spatial-average analysis of precipitation: frequency of occurrence, ice/liquid phase, organization, and precipitation event duration. The analysis is based on a portion of the National Mosaic and Multi-sensor Quantitative Precipitation Estimation (NMQ) three-dimensional radar reflectivity mosaic and QPE dataset. Every 15 minutes, all near-surface rainfall features > 0.5 mm/hr are identified and segregated into size categories, which determine whether or not each is associated with a mesoscale convective system (MCS) or group of isolated convective cells. Within these features, convective vs. stratiform, frozen vs. liquid precipitation, and shallow (warm) rain is identified. The duration and frequency distributions of each precipitation type at each pixel is tracked and integrated over daily, weekly, and seasonal time periods over the five-year period.

We present monthly precipitation maps that reveal the seasonal evolution of the spatial pattern and relative amounts of MCS versus isolated convection. For several extreme precipitation events (two standard deviations above the annual mean precipitation), centered on North Carolina, the time-space evolution and the relative amounts of MCS and isolated convection are examined. Results quantify the relative importance of MCS versus isolated convection for each season.