SIMULATIONS EXPLORING THE DEPENDENCE OF CLOUD-COVER FREQUENCY-DISTRIBUTION ON CLOUD SIZE AND IMAGE PIXEL RESOLUTION

The dependence of pixel-scale cloud-cover frequency distribution on cl oud size and pixel size is examined through Monte Carlo simulations. A shape parameter, which describes the shape of the frequency distribut ion, is found to be a simple function of the ratio of cloud size to pi xel size. The form of the frequency distribution changes from U shape to bell shape when the ratio decreases. It becomes uniform when the ra tio is about 0.8-0.9 for the cases where the regional-scale cloud cove r is 0.5. It is shown that on average the cloud cover in partially clo udy pixels increases with increasing regional-scale cloud cover when t he ratio of cloud size to pixel size is small. It becomes insensitive to regional-scale cloud cover when the ratio becomes large. It is also shown that, in comparison with the results from Monte Carlo simulatio ns, the grid-scale cloud-cover frequency distribution obtained using a threshold method tends to be more U shaped, and that obtained using a method that assigns 50% cloudiness to partially cloudy pixels tends t o be less U shaped, particularly for subgrid cloudiness. A possible wa y of retrieving cloud size is suggested. It is found that the differen ce between a simulated cloud field, where clouds are uniformly distrib uted, and a real cloud field, where clouds may not be uniformly distri buted, biases cloud size retrieval. Investigations on how clouds are d istributed in a real cloud field are recommended.