Precipitation rate spectra as dependent on dynamic forcing:application to probabilistic forecasting

Abstract

Occurrence frequencies, OF, of 12-h precipita-tion amounts, P, at stations in the former European USSRare displayed as dependent on dynamic forcing of verticalmotions. The dynamic forcing is described by a " frontalparameter", F (calculated in the points of objective analysisgrid), which depends on the surface pressure field curvatureand on the baroclinicity in the lower half of the troposphere.The precipitation rate spectra for 4 seasons, calculated froma large sample of data (7 years, about 650 000 values of P forone season), show a monotonous OF growth of all ranges ofP> 1 mm/12 h with F increase. The growth is especially sig-nificant for heavy precipitation. As a result, F is shown to bean informative predictor of P spectrum or of probability ofany given range of P. As a next step, two-dimensional spec-tra of precipitation occurrence frequency, as a function of Fand LNB, that is, OF (F, LNB), are calculated, LNB beingthe level of neutral buoyancy at the gridpoint, an estimate ofgrid-scale convective instability. On this basis, an approachto probabilistic forecasting is suggested.