The Seasonal Evolution of Low Clouds and the Southern ITCZ over the East Pacific Ocean

Earth system models continue to have major sources of uncertainty due in part to precipitation biases in the intertropical convergence zone (ITCZ). One common problem in the tropical southeast Pacific Ocean is that models underproduce low clouds, which leads to excessive surface solar radiation, a warming of the ocean surface, and the production of deep convection in the southern hemisphere ITCZ. Despite a myriad of studies on southeast Pacific ITCZ development, the month-to-month evolution in cloud populations, from horizontally extensive non-precipitating low clouds (stratocumulus, Sc, and cumulus, Cu) to the development of the convection in the ITCZ has remained poorly understood and understudied in observations and reanalyses.

In this study, we compare the low cloud populations in the Cumulus and Stratocumulus Cloudsat-CAlipso Dataset (CASCCAD) with ECMWF’s ERA5 reanalysis and NASA’s MERRA-2 reanalysis. ERA5 more faithfully captures the seasonal evolution in southeast Pacific low clouds compared to MERRA2, with an increase in low clouds above 800 hPa and a decrease in low clouds below 800 hPa from EQ–10S from September to March. We hypothesize that this decrease and increase in low clouds is attributable to Sc and Cu clouds, respectively and MERRA-2 overestimates Cu clouds and underestimates Sc clouds. The reasons for the discrepancies in low clouds between ERA5 and MERRA-2 are further investigated by analyzing low- to mid-level level lapse rates, moisture, and vertical velocity in the southern hemisphere. A well-defined trade wind inversion (TWI) layer can be seen during boreal fall prior to southern ITCZ development in both reanalyses, with an erosion of the TWI from January through March. We are currently investigating other factors to better understand the discrepancies between low clouds in ERA5 and MERRA-2.