A Moisture Source Climatology for Flash Flood Events in the United States

Hotspots of coupling between soil moisture and precipitation have been identified in several regions worldwide, in the United States, most notably, the Southern Great Plains. This coupling can take the form of small-scale differences in heat and moisture near contrasts in land surface properties (mature cropland vs. a harvested field, for example) to large-scale feedbacks due to soil moisture availability at the seasonal scale. These land-atmosphere linkages, however, are often examined in a point-to-point context, neglecting the impact of atmospheric transport of air modified by heterogeneous surface conditions. Using a database of flash flood reports from the National Weather Service from 2007-2013, the moisture sources for flash flood events for the conterminous United States are diagnosed. Backward trajectories are computed from North American Regional Reanalysis data, and the moisture and energy budgets along the flow are presented, along with the relative contributions of advection and the terrestrial contribution via evapotranspiration. Results are also presented with an emphasis on seasonal antecedent conditions of the land surface.