Identification of zones of atmospheric fronts as a problem of postprocessing the results of numerical prediction

Abstract

Presented is a review of the results of the studies that have been carried out in recent 15 years at the Hydrometcenter of Russia and deal with the objective analysis of atmospheric fronts and with diagnostic computations on the base of the results obtained. A unique experiment on the quantitative estimation of the measure of subjectivity of the frontal analysis carried out by weather forecasters in operational mode was accomplished in the process of the method development. Differences turned out to be so significant that no concrete synoptic archive can be considered as a source of actual data on the position of fronts. At the same time, the degree of agreement between different forecasters concerning the position of primary cold and warm fronts remains practically significant. The statistical method of objective analysis of atmospheric frontal zones is worked out as a method of postprocessing the results of numerical forecast (objective analysis) of the fields of pressure (geopotential), temperature, and humidity. The proposed method was operationally tested and recommended for the operational use by the Roshydromet Central Methodological Committee for Hydrometeorological and Heliophysical Forecasts. The data on precipitation at the stations (a degree of precipitation intensity on the front characterizes its activity) are used as a predictant (criterion of the absence or presence of a front). The frontal parameter initially obtained as a probability of the fact that the forecaster draws the front through the given grid cell is (in its physical essence) a quantitative characteristic of baroclinity near the surface and in the layers of 850-500 and 925-700 hPa and of cyclonicity of the surface pressure field. This variable turned out to be an effective diagnostic characteristic of baroclinic forcing of vertical circulations and formation of clouds and precipitation in frontal zones. The spectra of the frequency of precipitation of various intensity for different seasons and regions of the European part of the former USSR are constructed using the long-term data on two main characteristics of precipitation forcing, namely, the frontal parameter and the height of the convection level. Given are other potential application areas of the frontal parameter including the verification of numerical models.