Prognostic significance of dynamic factors of precipitation generation

Abstract

On the basis of objective analysis of meteorological fields and 12-h precipitation amounts observed at the stations in the former European USSR, statistical relationships are studied between the diagnostic characteristics of dynamic forcing of vertical motion and occurrence frequency of precipitation of different intensity ranges over the whole area and each of six separate regions. The following diagnostics are considered: frontal parameter as a measure of baroclinicity and of pressure field curvature; neutral buoyancy level as a measure of grid-scale convective instability; scalar frontogenetic function at the 850-hPa level and dynamic tropopause height as measures of intensity of vertical transverse circulations in unstable baroclinic zones. Informativity estimates are presented of each diagnostic and their pair combinations as precipitation predictors. The frontal parameter (in all seasons) and the neutral buoyancy level (in all seasons but winter) are significant as single predictors of precipitation. Among the pair combinations of the diagnostics, those which include the frontal parameter are more informative, and the combination of the latter and the neutral buoyancy level is the most informative one. A comparable informativity, under certain conditions, is demonstrated by the combinations of the frontal parameter with the frontogenetic function and with the tropopause height (in particular, in winter and fall for heavy and very heavy precipitation). In all the cases, the values of the informativity criterion (Pierce's index) are higher for more intense precipitation as a predictand.

Verification of the vertical motion generation mechanisms in global models and their initial fields in connection with precipitation numerical forecasting

“…If the latter are underestimated by the model, then, even with perfect schemes of cloud and precipitation generation, the model precipitation will be underestimated. As it has been shown in [13,14], there is a close statistical relationship between occurrence and intensity of the observed precipitation, on the one hand, and diagnostic characteristics of vertical motion forcing, on the other hand. This relationship is so close, that the mentioned characteristics become significant predictors of precipitation, especially of heavy one.…”

“…In particular, the frontal parameter F, proposed in [10][11][12], depends on humidity only through the equivalent additive in the formula, which is used for computation of baroclinicity characteristics as functions of horizontal gradient of the average equivalent temperature of the air layer. As it is shown on a large amount of real data [13], using the F parameter and the tropopause height or frontogenetic function (both these quantities do not depend on humidity) as predictors, it is possible to predict precipitation whose intensity exceeds 1 mm/12 h with virtually significant success rate (Pierce's index PI » 0.40), comparable with accuracy of numerical forecasting [1,2].…”

“…Results of studies presented in [10][11][12][13][14] show that dynamics is a dominating factor in precipitation generation in the atmosphere. This means not only dynamics of the lower troposphere (that is, of the planetary boundary layer), or that of the lower half of the troposphere, but dynamics of a thick layer embracing the tropopause and the lower stratosphere.…”