Spring Soil Temperature Anomalies in the Western United States and Summer Droughts in the Southern Plains

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

This project seeks to understand the causes of precipitation variability in the US Southern Plains (roughly equivalent to Nebraska, Missouri, Oklahoma, Arkansas, Texas, and Louisiana). This area is among the most drought-prone in the US, and droughts in the region have substantial economic impacts. This project examines the hypothesis that Spring subsurface soil temperature anomalies in the Western US in produce rainfall changes in the Southern Plains in the subsequent summer. The hypothesized relationship is such that warm subsurface soil temperature in the West, through its manifestation in warm land surface temperature, generates a cyclonic circulation in the overlying atmosphere, which propagates eastward to produce enhanced June rainfall to the east and south of the soil temperature anomaly, with the opposite result (i.e. drought) for cold subsurface soil temperature. A further issue to be examined in the project is the effect of frozen soil on the precipitation response to soil temperature. The project tests the extent to which a better representation of frozen soil in regional climate models (RCMs) can lead to a stronger relationship between Western soil temperature and Southern Plains precipitation simulated by the model. Preliminary work for the proposal has shown evidence for the effect of subsurface soil temperature, and work performed under the project will follow up with a set of model simulation experiments. The experiments will be conducted with a model configuration in which a global atmospheric model (the NOAA/NCEP Global Forecast System model, or GFS) will be combined with an RCM known as the Weather Research and Forecasting (WRF) model, in which land surface processes are simulated by the Simplified Simple Biosphere (SSiB) model. Experiments typically use imposed sea surface temperatures and sea ice to produce a global simulation for a particular year using the GFS model, which is in turn used to specify lateral boundary conditions for WRF simulations. The experiments test the influence of all local and remote effects on Southern Plains precipitation, including soil conditions in the Western states and also sea surface temperature (SST) fluctuations. SSTs, including those associated with El Nino can affect Southern Plains precipitation both directly and indirectly, through their influence on soil temperature to the West. The work has broader impacts due to the economic and societal impact of drought in the Southern Plains and the desirability of better methods to predict the extent and duration of droughts. The project will also support a post-doctoral researcher, thereby supporting the next generation of scientists in this research area.