Evaluation of deep convective cloud systems in the NASA MERRA reanalysis

The 1997-1998 El Niño was one of the most intense in the past century, and provides a rare opportunity to evaluate climate model response to forcing from a well-observed event. The large-scale circulation and associated cloud systems in the eastern and western Pacific exhibited a large response to changes in sea surface temperature. In contrast to observations, many models used in the IPCC 4th Assessment Report simulations archive do not show a credible, consistent atmospheric response when driven with observed sea surface temperatures. For example, observations reflect a complete collapse of the Walker Circulation, while many models fail to reproduce the observed changes in the tropical overturning circulation.

The geophysical signal of the 1997-1998 El Nino was well observed by instruments on the Tropical Rainfall Measuring Mission (TRMM) satellite, and can be detected in the cloud-object data set derived from CERES and VIRS. These cloud objects can be used in process-level diagnostics to investigate the mechanisms of the atmospheric response to the sea surface temperature. As such, we are currently using these observations to evaluate clouds and radiation in the NASA Modern Era Retrospective-analysis for Research and Applications (MERRA). Since the GCM used in the MERRA (Goddard Earth Observing System Model, Version 5; GEOS-5) was constrained by the data assimilation system to be consistent with observed temperature, moisture, and winds, this case serves as a robust baseline test of the model physical parameterizations and of the realism of the analyzed hydrologic cycle. These comparisons represent a first step toward evaluation of AMIP-style GCM simulations using the GEOS-5 at a range of resolutions.

In this presentation, we compare MERRA-simulated clouds with deep convective cloud objects in the equatorial Pacific Ocean during January – August 1998, which was marked by a transition from a strong El Nino to a moderate La Nina. In addition to evaluating MERRA representation of deep convective cloud properties, we also examine the fidelity of the representation of the complex connection between El Nino, deep convective clouds, and the large-scale overturning (Walker) circulation. Preliminary results from this investigation suggest that the MERRA dataset realistically represents the observed occurrence and properties of deep convection over the equatorial Pacific Ocean. Analysis of several cloud variables (including cloud top temperature and height as well as liquid and ice water path indicate MERRA has utility as a re-analysis specifically tailored to capture the details of the global hydrological cycle.