Characterizing Some of the Influences of the General Circulation on Subtropical Marine Boundary Layer Clouds

Abstract

The seasonal and intraseasonal variability of boundary layer cloud in the subtropical eastern oceans is studied using combined data from the International Satellite Cloud Climatology Project and the European Centre for Medium-Range Weather Forecasts reanalysis. Spectral analysis reveals that most of the time variability of cloud properties occurs on seasonal to annual timescales. The variance decreases by one to two orders of magnitude for each decade of timescale decrease, indicating that daily to monthly timescales and their spatial extent have smaller, although nonnegligible, variability. The length of these dominant timescales suggests that the majority of the variability is influenced by the general circulation and its interaction with boundary layer turbulence, rather than being a product of local boundary layer turbulence alone. Although the dominance of seasonal to annual periods in the temporal power spectra of low-cloud fraction?TAU and CTP?justifies the previous focus of effort on seasonal variability, intraseasonal data can be better used to examine the cloud formation/dissipation processes as revealed in relationships between synoptic meteorology and cloud properties. Previous datasets have lacked the necessary combination of resolution and scope in either time or space coverage to properly characterize variability on synoptic and larger scales; this is remedied by using global satellite-retrieved cloud properties. The intraseasonal subtropical cloud variability in both hemispheres and in different seasons are characterized. In addition to cloud fraction, variability of cloud optical thickness and cloud-top pressure frequency distributions are examined. The intraseasonal variability is divided into three types. The first type, found in the Californian local summer and Southern Hemisphere regions year round, is characterized by lower-altitude, greater optical thickness, stationary clouds. The second type is found in the Canarian local summer and has more instances of smaller cloud-top pressures and a westward propagation direction. The third type, found in Northern Hemisphere regions during winter, is similar to the second type, but shows an eastward propagation direction. This study focuses on the third type more closely and finds it to be associated with the lower sea level pressure, upward vertical velocity phase of synoptic waves.