Computation of thunderstorm rainfall

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Abstract
The rainfall of a “young” thunderstorm is shown to be expressible as the product of qG, the average surface specific humidity of the air entering the storm, and C, the “convergence factor,” which is a function of the cloud base dimensions and of the wind blowing into the region beneath the cloud. The equation may be applied to more involved storms, such as hurricanes or wave cyclones, as long as orography is not an important feature; in these cases the moisture factor is still easily determined, but the convergence factor becomes more difficult to evaluate.For purposes of estimating the maximum possible thunderstorm precipitation it is shown that qG and C may be maximized through statistical study of weather records. For the problem of quantitative precipitation forecasting two methods are suggested: (1) Substitution of forecasted values of moisture, wind, and cloud dimensions into the equation, and (2) forecasting of rainstorm types whose convergence factors are known, and forecasting of the surface specific humidity. The second method necessitates a rather lengthy investigation of the run‐of‐the‐mill variety rather than of the excessively heavy rainstorms customarily studied by hydrometeorologists.

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