Jessica Erlingis Lamers-August 11-Defense

Moisture Sources for Flash Floods in the United States

Hotspots of coupling between soil moisture and precipitation have been identified in several regions worldwide, including the Southern Great Plains region of the United States. 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 surface conditions.

The goal of the work presented in this dissertation is to diagnose the moisture sources for flash flood events in the conterminous United States and to assess the relative contributions of moisture advection and of interactions with the land surface. Another goal of this study is to assess the seasonality of moisture source regions and land surface contributions as a function of geographic region. Backward trajectories from 19,253 flash flood reports from 2007-2013 published by the National Weather Service were used to delineate moisture source regions for flash flood events. Parcels were released from flooded locations and traced backward in time for 120 hours using North American Regional Reanalysis data. For parcel moisture increases that occurred within the boundary layer, the land surface properties from an offline run of the Noah land surface model were recorded. These data were then used to assess the state of the land surface that contributes to parcel moisture for six regions previously identified to be especially “flashy” by Saharia et al. (2017) based on their streamflow records (the West Coast, Arizona, the Front Range, Flash Flood Alley, the Missouri Valley, and the Appalachians).

This methodology, uniquely applied here to the flash flood problem, elucidates that many well-known climatological mechanisms are key components for heavy rainfall in these regions. The land surface was found to have a positive contribution to the moisture budget for flash flood events for the United States, though the extent of this effect varies by region and season.