On the Numerical Prediction of Precipitation

With the three-dimensional field of velocity predicted by numerical methods it is possible to predict the moisture distribution and hence the occurrence of large-scale saturation. A three-parameter model was used to predict the 12-hour precipitation for the early stages of the storms of November 24, 1950 and November 5, 1953, neglecting cloud storage, supersaturation, a possible lack of condensation nuclei, evaporation from falling droplets, and moisture sources. Large-scale orographic influences were taken into account. A quantitative comparison of the predicted rainfall with the correspondingly large-scale smoothed observed precipitation indicates a skill comparable to that of the predicted flow. An examination of the small-scale observed rainfall indicates that in these cases convective instabilfty resulted in large standard deviations from the large-scale average. Numerical prediction of regions of convective instability, which is also shown, could for the time being be utilized for subjective interpretation.