ESRL/PSD Seminar Series

NOTE: Special Friday Morning Seminar

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.

PSD-South Conference Room (1D403)
Friday, 4 April 2008
11:00 am

SECURITY: If you are coming from outside the NOAA campus, please be advised that you will need an on-site sponsor. Please contact that person in advance of the seminar to be put on the list and allow 10 minutes extra on the day of the seminar. Please contact Joe Barsugli (303-497-6042) or Barbara Herrli (303-497-3876) at least a day before the seminar if you have any questions.