Effect of Viewing Aspect on Climatological Cloud Distribution

摘要

The manner in which a cloud scene is observed and meteorologically reported produces varying results due to the observing system or procedures employed. As a satellite passes over a scene, the amount of reported cloud cover will be less at its zenith angle than at its limb scan angle. Ground observers estimate the sky cover of this scene relative to an imaginary sky dome. Neither system gives an accurate estimate of earth cover, an important parameter needed to determine albedo for earth radiation balance and climate studies. New models have been developed to specify the cloud distributions that depend on the way the clouds have been observed or on the way the distributions are to be used. The Burger distribution has been found to fit well traditional sky cover, e.g. coverage of the hemispherical sky dome as viewed by a ground observer. This distribution is the result of a large number of realizations of a sawtooth weather generator which approximates a Gaussian field with a particular correlation function. The parameters of the distribution - mean sky cover and scale distance (a spatial correlation parameter) - have easily recognized climatic meaning. An atlas of these parameters, giving seasonal and diurnal worldwide coverage for cloud distributions, has been prepared. When clouds are viewed at a specific angle, i.e. from a satellite, the probability of coverage will vary. Building on previous cloud-free line-of-sight modeling we have further specified this angular dependence. Models developed by Gringorten show that coverage also depends on the size of the area viewed as well as the smallest resolvable feature - pixel size.