Simon Paul de Szoeke — Oregon State University
Sandra Yuter — North Carolina State University
David B. Mechem — University of Kansas
Category
Entrainment
Description
Covariance of ISCCP low cloud amount with estimated inversion strength (top). Covariance on daily (synoptic) time scales (middle), and seasonal time scales (bottom). All values are normalized by a standard anomaly of EIS.
Marine low cloud amount has previously been found to be correlated to lower tropospheric stability (700hPa-surface potential temperature) or estimated inversion strength (EIS) on seasonal and interannual time scales. A stronger inversion is presumed to limit entrainment of dry air into the boundary layer, aiding cloud formation and limiting cloud evaporation. We investigate the marine low cloud-EIS relation on seasonal, daily, and diurnal timescales with 26 years of ISCCP D1 adjusted low cloud fraction and EIS from NCEP reanalysis. Seasonal covariance (stable-cloudy) dominates only over the southeastern Pacific and Atlantic Oceans equatorward of 18° latitude. Synoptic variability is responsible for almost all EIS-low cloud covariance poleward of 18°. Cloud-EIS covariance is positive over 15-45° latitude and negative (unstable-cloudy) over 45-60°. This dipole in the covariance is explained by the tilt of southwest-northeast orientation of clouds to the northwest-southeast orientation of EIS in synoptic storms, and is influenced by masking of low clouds by higher clouds in the satellite data. Synoptic covariance of low-topped cloud with downward vertical velocity is found centered in midlatitude storm tracks (45° latitude).
Low clouds cool the climate by reflecting sunlight, shading the ocean surface, and emitting thermal radiation at a warm temperature. Surface climate warming is expected to reduce inversion strength and thus reduce cloud amount, so the effect of the inversion strength on marine low cloud represents a positive climate feedback. We find the seasonal cycle in the EIS-low cloud correlation (stable-cloudy) is important but its dominance is limited to the southeastern tropical Atlantic and Pacific Oceans. Since the seasonal covariance of inversion strength to low cloud is a small fraction of the covariance around the globe, the time scale and processes responsible for marine low cloud correlations should be considered carefully when extrapolating low cloud-EIS correlations to the formulation of low cloud parameterizations, to evaluation of marine low clouds in models, and to evaluation of climate feedbacks.
