Assessing the Southeast Pacific Stratocumulus Cloud Fraction Transition in MERRA Using CloudSat/CALIPSO

Abstract:

Some reanalyses and global climate models have a difficult time capturing the change in cloud fraction across stratocumulus to cumulus transitions; however, this has not yet been assessed for NASA’s Modern-Era Retrospective Analysis for Research and Applications (MERRA). Here, the vertical profile of cloud fraction from MERRA is compared with ECMWF Reanalysis (ERA-Interim) and CloudSat/CALIPSO observations across the stratocumulus transition region in the southeast Pacific. A recent study indicates that cloud fraction transitions along the climatological wind trajectory are a good representation of stratocumulus transitions along individual Langrangian trajectories. MERRA, ERA-Interim, and CloudSat/CALIPSO cloud fractions are extracted along the climatological wind trajectory in the southeast Pacific for 2007-2010 and the climatological and seasonal transitions are analyzed. Preliminary results indicate that MERRA underestimates cloud fraction in this region, similar to previous studies comparing reanalysis and observations in the northeast Pacific transition. Environmental variables such as temperature, specific humidity, vertical pressure velocity, lower tropospheric stability, and planetary boundary layer height from the reanalyses are also compared to in situ observations to understand potential causes for the lack of clouds. MERRA produces lower temperatures throughout the atmosphere, a shallower boundary layer, and a weaker inversion compared to observations. MERRA also shows weaker subsidence compared to ERA-Interim. This study shows that more work is needed to improve the representation of the stratocumulus transitions in reanalyses.