A Study of the Probability of Clear Line of Sight through Single-Layer Cumulus Cloud Fields in the Tropical Western Pacific

Abstract

The plane-parallel horizontal (PPH) hypothesis used to approximate clouds in GCMs neglects three-dimensional cloud effects. Such effects can amount to as much as 20 W m?2 in longwave radiation. Several investigators have proposed accounting for longwave three-dimensional cloud effects by using information on the probability of clear line of sight (PCLoS) to modify the PPH approximation. This study investigates the PCLoS at the Atmosphere Radiation Measurement (ARM) Program?s Tropical Western Pacific (TWP) site. PCLoS is estimated for single-layer tropical marine cumulus cases for 2-h intervals using the Whole Sky Imager (WSI) observations at the Manus and Nauru sites. PCLoS estimates are compared to calculations from a set of simple PCLoS models using measured cloud field statistics as input. A summary of the PCLoS at the TWP site is presented in addition to a statistical summary of retrieved cloud field characteristics. The results are used to investigate the spatial variability of the PCLoS and to test the usefulness of the parameterization of effective cloud fraction.