The fraction of the grid cell area covered by rainfall, 1, is a very important parameter in the descriptions of land surface hydrology in climate models. A simple procedure is presented for estimating this fraction, based on extensive observations of storm areas and rainfall volumes. It is often observed that storm area and rainfall volume are linearly related. This relation is utilized in rainfall measurement to compute rainfall volume from radar observations of the storm area. The authors suggest that the same relation be used to compute the storm area from the volume of rainfall simulated by a climate model. A formula is developed for computing y, which describes the dependence of the fractional coverage of rainfall on the season of the year, the geographical region, rainfall volume, spatial resolution of the model, and the temporal resolution of the model.

The new formula is applied in computing y over the Amazon region. Significant temporal variability in the fractional coverage of rainfall is demonstrated. The implications of this variability for the modeling of land surface hydrology in climate models are discussed.