2011 Marine Layer Clouds

Are we going to have more or less than normal amounts of marine layer clouds this season?

Nobody knows for sure, but there are some indicators such as sea surface temperature (SST) that can give us some hints. Generally, cooler than normal SST off of the Southern California coast is associated with higher than normal amounts of marine layer clouds in San Diego and most of Southern California.

The water temperatures off the Western U.S. coast were generally cooler than normal in March and April 2011 as shown in the figures below.

The cool SST acts to increase the strength of the inversion layer making it more likely that marine layer clouds will form. Additionally, the cooler SSTs increase the coast-inland temperature contrast during the day which enhances the strength of the sea-breeze circulation. Near the surface, the sea-breeze is an onshore flow and can help transport marine layer clouds over the Southern California coastal land regions.

If these cooler than normal SSTs persist into May and June, we might expect more clouds than normal along the coast this spring.

Closer to home here in San Diego, the SST at Scripps Pier was below normal for much of the winter which is in agreement with the earlier figure. However, in early April the SST warmed a bit and has since remained pretty close to the climatological SST curve. SST at any given coastal location usually exhibits some short term variability, which is evident in the "wiggles" in the red curve.

The table below shows the percentage of time that low clouds were present during daylight hours at San Diego Lindbergh Field for the months May through August. Included in the table are values for the present year and climatological ("normal") values. Values from 2009 and 2010 are also shown for comparison.

Values for 2011 will be updated as data becomes available.

Monthly Daytime Low Cloud Cover at San Diego Lindbergh Field for 2009-2011

MONTH CLIMATOLOGY 2009 2010 2011
MAY 30% 45% 18% 25%
JUN 36% 26% 47% 39%
JUL 29% 19% 58% 24%
AUG 24% 22% 19% 29%

The plot below shows the daytime low cloud fraction (1.0=100%) at San Diego Lindbergh for each individual day.

Present Year (2011) Low Cloudiness at San Diego Lindbergh
Low Clouds Only (Cloud Base < 2500 ft)
Red Line = Long-term Climatology

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