Regional Climate Dynamics

Blocking in areas of complex topography, and its influence on rainfall distribution

Nearly all precipitation in Southern California falls during wintertime cyclones that move across the coast from the Pacific Ocean.  As a result, a large proportion of its rainfall is orographically forced.  The simplest way topography influences precipitation is through forced ascent: as air flows over mountains, it is forced to rise, and thus cools, condensing its water vapor and causing rain.  However, when the approaching air does not have enough kinetic energy to surmount the mountains in a stably stratified environment such as Southern California, the flow decelerates well-upwind of the barrier, causing a region of 'blocked' flow and enhancing precipitation upstream.  This study investigates the effect of orographic blocking on the precipitation climatology using the 6-km resolution regional climate simulation of Southern California described here.  

To diagnose whether blocking occurs, we categorize precipitating hours by a bulk Froude number. The precipitation distribution becomes much more spatially homogeneous as Froude number decreases, and an inspection of winds confirms that this is due to increasing prevalence of orographic blocking. We then quantify the relationship between bulk Froude number and the impact of blocking on precipitation distribution by comparing the dynamical model results with those of a linear model of orographic precipitation that excludes the effects of blocking: The agreement is nearly perfect for highest Froude number, unblocked 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, and the disagreement becomes striking for the lowest Froude number, blocked cases. 

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 is a hybrid of blocked and unblocked cases. These results suggest orographic blocking may substantially affect climatological precipitation distributions in similarly configured coastal areas.  Further, we  surmise that the addition of a term to reduce the effective slope of the topography during blocked hours would significantly improve the linear model, allowing it to be used to predict precipitation distribution for a much broader range of atmospheric conditions. 

Download the publication (Hughes et. al. 2009) describing these results in more detail.

Mimi Hughes, Alex Hall, and Rob Fovell make up the team that performed this research.