PNNL

Abstract

We present an analysis of subtropical marine stratocumulus cloud fraction variability using a 30-min and 3° x 3° cloud fraction dataset from 2003-2010. Each of the three subtropical marine stratocumulus regions has distinct diurnal characteristics but the SE Pacific and SE Atlantic are more similar to each other than to the NE Pacific. The amplitude and season-to-season diurnal cycle variations are larger in the southern hemisphere regions than in the NE Pacific. Net overnight changes in cloud fraction on 3° x 3° scales are either positive or neutral more than 77% of the time in the NE Pacific and more than 88% of the time in the SE Pacific and SE Atlantic. Cloud fraction often increases to 100% by dawn when cloud fraction at dusk is greater than 30%. In the SE Pacific and SE Atlantic, a typical decrease in cloud area (median of -570,000 km2) during the day is equivalent to 25% or more of the annual mean cloud deck area. Time series for 3° x 3° areas where cloud fraction was = 90% sometime during the night and