Regional climate model simulation of a multi-week heavy precipitation episode in the central U.S

A regional climate model (RCM), based on MM5, was used to simulate a period of heavy precipitation in the central United States, occurring during September 12-October 4, 1986. Precipitation amounts exceeding 100 mm fell over virtually the entire upper Mississippi River basin and Great Lakes basin. More than 250 mm fell over much of Missouri, eastern Kansas, and eastern Oklahoma, with some locations receiving in excess of 500 mm. The fundamental cause of the heavy precipitation was a nearly stationary amplified wave pattern with a trough over the western U.S. and a ridge over the eastern U.S. Frequent mesoscale convective systems occurred over the central U.S. Several RCM simulations, using different domains and physical parameterizations, were run. These simulations differed markedly in their ability to reproduce the location and magnitude of the heavy precipitation. In particular, better reproduction of the observed precipitation was obtained by extending the domain longitudinally. This improved the simulation of the low level jet and moisture transport into the central U.S. Also, proper representation of the surface energy budget was critical. One set of physical parameterizations for precipitation, clouds, radiation processes resulted in surface sensible and latent heat fluxes that were too low. Precipitation magnitudes were also too low. A more realistic set of parameterizations resulted in much better representation of precipitation. This suggests that recycled moisture is important and must be properly parameterized.