The Relationship Between Lower Tropospheric Stability and Marine Stratiform Cloudiness From Seasonal to Daily Time Scales

Widespread and shallow marine stratiform clouds have an important radiative cooling effect on the climate. Empirical regressions of seasonal cloud variability are used to assess cloud parameterizations in general circulation models and estimate cloud feedbacks in a changed climate. Here the relation between cloud fraction and lower tropospheric stability (LTS) are analyzed from annual to daily time scales from 25-year satellite records in the northeast subtropical Atlantic (NEA) and southeastern tropical Pacific (SEP) Oceans. As expected from the work of Klein and Hartmann, cross-spectra of cloud fraction with lower tropospheric stability show cloud fraction and LTS are correlated in the annual cycle. Power spectra of cloud fraction, LTS, and 700-hPa vertical velocity show an order of magnitude less variability at daily time scales compared to the annual cycle. Except for the diurnal cycle, the cloud fraction-LTS correlation is insignificant at frequencies less than a month. The lack of correlation on all timescales except those imposed by external solar forcing calls into question the applicability of the seasonal regression to shorter time scales, and the mechanistic notion that a more stable boundary layer inversion causes cloud fraction to increase.