Synoptic Variability of Low-Cloud Properties and Meteorological Parameters in the Subtropical Trade Wind Boundary Layer

Synoptic variability of low-cloud properties, temperature advection, and thermodynamic soundings of the trade wind boundary layer are analyzed, using the long data record from ocean weather station November (308N, 1408W). The variations in low-cloud amount at this subtropical site are most strongly correlated with variations in temperature advection, the stability of the lower troposphere, and the relative humidity of the cloud layer. No single predictor is capable of explaining more than 13% of the variance in low-cloud amount. However, the amount of variance explained increases considerably when the data are averaged over several days. Four parameterizations for the amount of stratiform cloud under a subsidence inversion are tested against the observed amount of low clouds. The four parameterizations are based upon relative humidity, the inversion strength, a mixing line slope, and the amount of condensed water. All parameterizations are positively correlated with the observed cloud amounts, although the variance explained is less than 16%.