Blocking in Areas of Complex Topography, and Its Influence on Rainfall Distribution

Abstract

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 homogeneous 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.1

Multiscale Comparative Evaluation of the GPM IMERG v5 and TRMM 3B42 v7 Precipitation Products from 2015 to 2017 over a Climate Transition Area of China

“…It is widely acknowledged that topography has a strong effect on precipitation [44][45][46], and the relationship between altitude and precipitation varies in different regions, especially in the climatic transition region. During this study, the negative logarithmic relationship between elevation and mean daily rainfall was found when the rainfall rate was less than 3.0 mm/day, which could be due to the effect of terrain and latitude [47].…”

Why does the Antarctic Peninsula Warm in climate simulations?

“…The local maximum in LH trends may also be displaced to the Peninsula's west because of orographic blocking of atmospheric flows, forcing ascent and condensation in advance of the land mass itself. This effect has been shown to be important in a climatological sense in other coastal regions (Jiang 2003;Neiman et al 2004;Galewsky 2008;Hughes et al 2009). Since the ensemblemean warming in 29 hindcasts is also enhanced over the Antarctic Peninsula (Fig.…”

Homogenization and Assessment of Observed Near-Surface Wind Speed Trends over Spain and Portugal, 1961–2011*

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