Evaluation of a New Parameterization of Shallow Cumuli

Larry Berg Pacific Northwest National Laboratory
William Gustafson Pacific Northwest National Laboratory
Evgueni Kassianov Pacific Northwest National Laboratory

Category: Modeling

Working Group: Cloud Life Cycle

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Frequency of occurrence of observed (blue) and WRF-predicted cloud fraction using the KF scheme (black) and KF-CuP scheme (red) during the summer of 2004.

A new parameterization for shallow cumuli, called the Cumulus Potential (CuP) scheme, has been developed. This new scheme explicitly links boundary-layer turbulence with shallow clouds and has been coupled with the Kain Fritsch (KF) cumulus parameterization in the Weather Research and Forecasting (WRF) model. In its default configuration, the KF scheme uses an ad-hoc temperature perturbation as a trigger function to determine if convection occurs. In the coupled KF-CuP scheme, that temperature perturbation is been modified to more realistically account for both sub-grid temperature and humidity variations. In order to evaluate the performance of WRF, two specially constructed data sets have been completed. The first documents the macroscale properties of shallow cumuli over the ARM Southern Great Plains Central Facility, while the second investigates the surface radiative forcing due to shallow cumuli. Both data sets make use of the Active Remotely Sensed Cloud Locations (ARSCL) value-added product (VAP) to estimate cloud boundaries and to help identify days with single-layer shallow cumuli. The surface shortwave forcing due to shallow clouds is determined using data from the Shortwave Flux Analysis VAP. Simulations using both the default KF and KF-CuP scheme have been completed for the summer (May‚ÄďAugust) of 2004. Overall, the modified scheme does a much better job predicting the cloud properties than does the default scheme. As shown in the figure, the default scheme under-predicts the frequency of small amounts of cloud fraction and over-predicts the frequency of large amounts of cloud fraction. The cloud fraction predicted by the modified scheme is in much better agreement with the observations made at the Central Facility, leading to better estimates of the radiative impact of shallow clouds.

This poster will be displayed at ASR Science Team Meeting.