Stratiform rainfall rates from the water flux balance equation and cloud model

Abstract

The area-averaged stratiform precipitation rate can be estimated by equating the downward flux of precipitation with the upward flux of vapour. This study evaluates the usefulness of this approach by comparing the estimated rainfall rate with that computed using a time-dependent non-hydrostatic cloud model. Comparison with the cloud model results reveals how sensitive the water flux balance rainfall rates are to the magnitude and depth of the low-level convergence. It is found that the numerical cloud model overestimated rainfall observations for a case study of central Alberta for which the initial and boundary conditions of the convergence field were adopted from Doppler radar observations. The water flux balance equation and the cloud simulations agreed in that the rainfall rate became stronger when either the magnitude of the surface convergence or the convergence depth was increased. However, the water flux balance equation consistently underestimated the model rainfall rates. Also, rainfall estimates from water flux balance equation were not sensitive to the divergence aloft, whereas the model rainfall was dependent on the distribution of upper-level divergence. The basic conclusion to be drawn is that the water flux balance equation is easy to use, but can provide only a crude estimate of rainfall with a bias to underestimate.