Coastal Stratocumulus Cloud Edge Forecast

Improved coastal Stratocumulus (Sc) cloud forecast are needed because traditional satellite cloud motion vectors (CMV) do not accurately predict how Sc clouds move or dissipate in time which often results in underprediction of irradiance in the morning hours. CMV forecasts assume clouds to move in the direction of the average wind field, which is not the case for Sc. Sc clouds over land form at night and reach maximum coverage before sunrise. During the day, heating from solar radiation at the surface and entrainment of dry and warm air from above causes Sc clouds to dissipate. A Sc cloud edge forecast using the Geostationary Operational Environmental Satellite is proposed to improve the solar forecasting. The inland edge of Sc clouds is tracked in time and extrapolated into the future. In coastal California, the Sc inland boundary is correlated to the land elevation. Dissipation after sunrise often follows land elevation as the amount of air required to be heated to become cloud-free decreases with increasing elevation. The correlation between land elevation and Sc eastern boundary is exploited by extrapolating the evolution of cloud edge elevation in time. This method is tested in central and northern California on 25 days and southern California on 19 days. The results show lower mean absolute error (MAE) and higher forecast skill than the CMV and persistent forecasts at 7 and 9 out of 11 stations or a reduction of 30 W m-2 and 107 W m-2 in hourly MAE of global horizontal irradiance for forecasts issued between 0700 to 1000 PST with forecast horizons of 5 hours.