Using satellite and in-situ observations to evaluate short-term forecasts of cloud-topped boundary layers with the Community Atmospheric Model

The South East Pacific (SEP) has the most persistent stratocumulus deck in the world. Until recently, only sparse observations of the SEP were available. In the last few years, new satellite observations, research cruise data and buoy measurements have become available and give new prospects to improve our understanding and modeling of boundary layer clouds over the SEP. Stratocumulus are among the worst-simulated tropical clouds in climate models. However, it is difficult to take advantage of in-situ observations to understand the causes of the stratocumulus bias in climate simulations because limited observation periods are difficult to compare with model climatological statistics. Applying climate model in short-term forecasts can be extremely valuable to evaluate parameterizations against a limited-period observation. If the model is started from realistic conditions, the systematic errors in short forecasts are predominantly due to parameterization errors. This is because the large-scale circulation stays close to the observed state in these short-range runs. Therefore, it is possible to gain insight into the parameterization deficiencies and to diagnose the processes behind the drift away from reality. Here we evaluate short-term forecasts of the SEP with the Community Atmospheric Model (CAM) against a set of satellite and in-situ observations. The forecasts are analyzed from ECWMF analysis. The simulated period is October 2006, which corresponds to the SEP stratocumulus maximum. The set of observations was gathered for the Pre-VOCALS Model assessment* and includes a week of in-situ data (sounding, ship remote sensing and aerosols) and satellite observations (GOES, MODIS, ISCCP, AMSR-E, SSMI, Quikscat, CloudSat, CALIPSO, AIRS and COSMIC/GPS) We use this testbed to examine the forecast errors in CAM3 and to assess new parameterizations for the next generation model, CAM4. In particular, we evaluate the performance of a new moist turbulence/shallow cumulus parameterization in simulations of the cloud-topped boundary layers. Preliminary results obtained for another period show that the planetary boundary layer (PBL) height and the liquid water path (LWP) is too low in CAM3 when compared to observations. They also suggest that the CAM-UW physics is superior to CAM for representing stratocumulus topped PBLs. *Pre-VOCALS Model assessment: http://www.atmos.washington.edu/~robwood/PreVOCA/index.html