Regional and Cloud-scale Modeling of Tropical Western Pacific Clouds Using WRF

Jimy Dudhia NCAR

Category: Modeling

Working Group: Cloud Life Cycle

In the previous year, we have produced an initial analysis of December 2007 using an Ensemble Kalman Filter data assimilation method including COSMIC GPS for thermodynamic profiles over this data-sparse region. The analysis has revealed several areas for improvement in WRF that are relevant to regional climate model parameterizations. In this poster we will demonstrate some solutions to the problems we have found, particularly in the radiation and moist physics parameterizations applied to mesoscale models, and this will be part of an improved analysis planned for this year. The first of the problems relates to how ice clouds, in particular, are interacting with the radiation schemes. It was found that the new RRTMG longwave radiation scheme produced too little cloud effect in outgoing longwave radiation (OLR), an important climate parameter. This was traced to the assumption that the main effect of ice clouds came from the microphysics ice array. A better assumption was to add the ice and snow arrays to use in the radiation scheme, and this is a modification that will be in the 3.2 release of WRF due in 2010. Different microphysics schemes have this problem to different degrees depending on their relative amounts of snow and ice that represent the tropical convective anvils. A clear-sky problem we are currently addressing relates to the use of lower model tops in regional/mesoscale models than in global/climate models. Typical tops are in the 100 hPa to 10 hPa range (16–50 km), often being restricted by the height of commonly available analyses that are used for lateral boundary conditions. It has been noticed, particularly in the long simulations used for regional climate or data assimilation, that the top level cools unrealistically, possibly having impacts at lower levels and making satellite data assimilation difficult. This was traced to the downwelling radiation at the model top being calculated with an assumption of an isothermal layer between model top and the top of atmosphere. Better assumptions using observation-based lapse rates in the stratosphere improve the radiation, and this will also be available in the next WRF release. We have also participated in the TWP-ICE limited-area-model (LAM) intercomparison. Two three-day simulations with nesting down to cloud-permitting 1-km grids were carried out, and selected results may also be shown on the poster.

This poster will be displayed at ASR Science Team Meeting.

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