Blocking in areas of complex topography and its influence on rainfall distribution
Mimi Hughes, NRC RAL Postdoctoral Fellow
Using a 6-km resolution regional climate simulation of Southern California, the effect of orographic blocking on the precipitation climatology is examined. To diagnose whether blocking occurs, precipitating hours are categorized by a bulk Froude number. The precipitation distribution becomes much more spatially ho- mogeneous as Froude number decreases, and an inspection of winds confirms that this is due to increasing prevalence of orographic blocking. Simulated precipitation distributions are compared to those predicted by a simple linear model that includes only rainfall arising from direct forced topographic ascent. The agreement is nearly perfect for high Froude cases but degrades dramatically as the index decreases; as blocking becomes more prevalent, the precipitation/slope relationship becomes continuously weaker than that predicted by the linear model. We therefore surmise the linear model would be significantly improved during low Froude hours by the addition of a term to reduce the effective slope of the topography. Low Froude, blocked cases account for a large fraction of climatological precipitation, particularly at the coastline where more than half is attributable to blocked cases. Thus the climatological precipitation/slope relationship seen in observations and in the simulation is a hybrid of blocked and unblocked cases. These results suggest orographic blocking may substantially affect climatological precipitation distributions in similarly configured coastal areas.
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