10A.3 Why do long term regional climate simulations have reduced atmospheric moisture and precipitation in the Great Plains?

Wednesday, 3 June 2009: 2:00 PM
Grand Ballroom East (DoubleTree Hotel & EMC - Downtown, Omaha)
James Correia Jr., PNNL, Richland, WA ; and L. R. Leung

Reduced water vapor mixing ratio in the Great Plains has been seen in two realizations of WRF regional climate simulations when compared to reanalysis and observations. We hypothesize that poor representation of the model LLJ ranging from poor positioning and orientation to winds that are too SW aloft, and late onset of the inertial oscillation, is to blame. Analysis indicates that strong LLJs are negligible at 0000 UTC while by 0600 UTC strong LLJs make up roughly half of all LLJs and have nearly the same spatial pattern. Further analysis will detail the LLJ characteristics such as orientation, persistence, onset and duration within the model. With insufficient moisture transport by the LLJ to the Great Plains, precipitation is shifted eastward out of the Plains, hence missing the precipitation over the Midwest and points further south and southwest.

Another plausible explanation for the moisture deficit in the Great Plains is that soil moisture is too dry and sensible heat fluxes enhanced, shifting the LLJ eastward in response to the low level heating. Thus, feedbacks between the land surface and atmosphere may contribute to the atmospheric moisture bias locally through changes in surface fluxes and atmospheric stability, or remotely through the LLJ bias and moisture transport. Preliminary analysis suggests that land surface forcing may play a secondary role since soil moisture bias is not significant or systematic in any summer month as the soil moisture in spring has a rather long memory.

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