Computation of Domain-Averaged Irradiance with a Simple Two-Stream Radiative Transfer Model Including Vertical Cloud Property Correlations

Recent development of remote sensing instruments by Atmospheric Radiation Measurement (ARM?) Program provides information of spatial and temporal variability of cloud structures. However it is not clear what cloud properties are required to express complicated cloud fields in a realistic way and how to use them in a relatively simple one-dimensional (1D) radiative transfer model to compute the domain averaged irradiance. To address this issue, a simple shortwave radiative transfer model that can treat the vertical cloud optical property correlation is developed. The model is based on the gamma-weighted two-stream approximation (Barker 1996). It separates the domain into clear and cloudy columns and computes radiation in the both columns (Kato 2003). The model can treat horizontally inhomogeneous overcast clouds as well as broken cloud fields with realistic cloud overlap feature. Inputs to the model are the vertical profile of cloud fraction, cumulative cloud fraction computed from the top of the atmosphere, and vertical profile of the conditional probability of cloud occurrence, which is the probability of the cloud occurrence in other computational layers provided that clouds are present at a given layer. The model is tested against a Monte Carlo model. A comparison shows that the model is capable to compute domain averaged irradiance of complicated cloud fields that are generated cloud resolving models.