Joint Centre for Mesoscale Meteorology (JCMM)
JCMM INTERNAL REPORT NO.53
The albedo of broken cloud fields: calculations using a modified two stream radiation code
by Imtiaz Dharssi and Roy Kershaw
Cloud resolving models are a potentially useful tool for studying the cloud radiation interaction. However, such models still use plane parallel radiation transfer theory because more sophisticated methods require too much computer time. Results from Monte Carlo models show that the ration of effective to geometric fractional cloud cover (Ne/N) is determined by the interception of direct solar radiation by the cloud sides, leakage of diffuse solar radiation through the cloud sides and cloud-cloud interactions. This note shows how the interception of direct solar radiation by the cloud sides can be included in a two stream radiation transfer code, suitable for use in cloud resolving models which improves agreement with Monte Carlo models without requiring greater computing resources. Broken cloud fields are simulated and the ratio Ne/N calculated using the modified radiation code. At large solar zenith angles the interception of direct solar radiation by the cloud sides has the dominant effect on Ne/N and the calculated values are found to be in good quantitative agreement with the results of Kite (1987) who uses a Monte Carlo model.