Dependence of subtropical low cloud diagnosed from AIRS, MODIS, and ECMWF-interim reanalysis and inferred changes in low cloud fraction in a warming climate

Abstract:

Decreases in subtropical low cloud cover (LCC) occur in climate model simulations of global warming. In this study the observational data records from MODIS and AIRS spanning 2002-2014 are combined with ECMWF-interim reanalysis to compare the observed variability of low cloud to the predicted response to warming. Large-scale thermodynamic and dynamical predictors of LCC selected based on insight from large-eddy simulations and observational analysis. Observations show that increasing inversion strength is associated with increased cloud cover. Drying of the free-troposphere is associated with decreased cloud cover, as entrainment of dry air thins the cloud. Decreased cloud cover accompanies subsidence. Finally, it is found that increasing SST at fixed wind speed leads to a robust decrease in cloud cover. Idealized estimates of the response of EIS, free tropospheric relative humidity, and subsidence to a uniform SST warming are used to calculate the change in LCC. Decreases in LCC due to enhanced SST dominate the response and are only slightly offset by increases in EIS. In combination, these effects infer an overall decrease in low cloud cover in the subtropics of 1-3% for a 1K SST increase.