Improved forecasts of cloud fraction due to shallow cumuli

With the expanded use of solar energy it is becoming increasingly important to provide more accurate predictions of cloud fraction. Although they are relatively small, shallow cumuli can have a profound effect on the amount of solar radiation that is available at the surface; a recent study has indicated that the decrease of average solar radiation at the surface due to shallow cumuli can be as large as -400 W m-2. However, because of their small size and short life times, current weather forecast models do a poor job of predicting shallow cumuli. In order to address this shortcoming, a new parameterization has been added to the Weather Research and Forecasting (WRF) model. This new scheme explicitly couples boundary-layer turbulence with shallow clouds. A set of simulations with the new parameterization has been conducted for an entire summer over the Southern Great Plains. The simulations made using the new scheme showed marked improvement in the prediction of cloud fraction due to shallow cumuli when compared to results using the standard model configuration. We will report on the results of these simulations and how they affect forecasts of insolation within the context of solar energy.