PNNL

Abstract

Accurate forecast of Global Horizontal solar Irradiance (GHI) and Direct Normal Irradiance (DNI) in cloudy conditions remains a major challenge in the solar energy industry. This study focuses on the impact of cloud microphysics on GHI and its partition into DNI and Diffuse Hori-zontal Irradiance (DHI) using the Weather Research and Forecasting model specifically designed for solar radiation applications (WRF-Solar) and seven microphysical schemes. 3 stratocumulus (Sc) and 5 shallow cumulus (Cu) cases are simulated and evaluated against measurements at the US Department of Energy’s Atmospheric Radiation Measurement (ARM) user facility Southern Great Plain (SGP) site. Results show that different microphysical schemes lead to spreads in sim-ulated solar irradiance components up to 75% and 350% from their ensemble means in the Cu and Sc cases respectively. The Cu cases have smaller microphysical sensitivity due to limited cloud fraction and smaller domain-averaged cloud water mixing ratio compared to Sc cases. Cloud properties also influence the partition of GHI into DNI and DHI and the model simulates better GHI than DNI and DHI due to non-physical error compensation between DNI and DHI. The microphysical schemes that produce more accurate liquid water paths and effective radii of cloud droplets have better overall performance.