The Time Scales of Variability of Marine Low Clouds

Abstract

Multidecade global regressions of inversion strength, vertical velocity, and sea surface temperature (SST) on low cloud amount, from subdaily to multiyear time scales, refute the dominance of seasonal inversion strength on marine low cloud variability. Multiday low cloud variance averaged over the eastern Pacific and Atlantic stratocumulus regions [5 × 10−2 (cloud amount)2] is twice the subdaily variance and 5 times larger than the multimonth variance. The broad multiday band contains most (60%) of the variance, despite strong seasonal (annual) and diurnal spectral peaks. Multiday low cloud amount over the eastern tropical and midlatitude oceans is positively correlated to inversion strength, with a slope of 2%–5% K−1. Anecdotes show multiday low cloud and inversion strength anomalies propagate equatorward from midlatitudes. Previously shown correlations of low clouds to strong inversions and cool SST on monthly and longer time scales in the stratocumulus regions imply positive cloud-radiative feedbacks, with e-folding time scales of 300 days for SST and 14 days for atmospheric boundary layer temperature. On multimonth time scales, removing the effect of SST on low clouds reduces the low cloud amount explained by inversion strength by a factor of 3, but SST has a small effect at other time scales. Contrary to their positive correlation in the stratocumulus cloud decks, low clouds are anticorrelated to inversion strength over most of the tropics on daily and subdaily time scales.