In connection with analysis of anomalous weather in summer of 2010, the state-of-the-art is reviewed of climatology of the blocking anticyclones, of mechanism for their onset and persistence, as well as possibilities of their forecasting. The main results are quoted on occurrence frequency, intensity, and duration of blockings and on their connection with features of general circulation of the atmosphere in the Northern and Southern hemispheres. As the primary cause of the blocking formation, nonlinear instability of the Rossby waves is considered with intense energy exchange with planetary waves and with synoptic-scale eddies. Mathematical means have been developed to investigate the energy exchange on the real data of objective analysis (reanalysis). This approach can be applied to analyze causes of anomalous duration of the summer blocking. From the point of view of forecasting, blockings do not differ much from other atmospheric pressure systems. Namely, within the framework of medium-range forecasting, the blocking onset, under condition of its absence in the initial fields, can be predicted with a projection of several days; also, the ensemble forecasts are generally more successful than those obtained with any single model. The blocking duration and decay can be predicted successfully within the medium-range forecasting.

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.

Results of study of conditions for freezing precipitation (FP) at the airport of Nizhni Novgorod based on 20-year series of surface observations are described. The cloud tops are estimated from radiosonde data. It is found that the monthly mean FP frequency does not exceed 0.44%; the phenomenon occurs from October to February. Over 20 years, a total of 113 FP episodes were observed, or less than six episodes annually. Freezing precipitation is more frequent at night and in the morning and very rare in the afternoon, at surface air temperatures not exceeding 0 o C and not below -10 o C; in half of the cases, the air temperature is within -0.1 to -2.0 o C. Surface wind is most frequently from south or southwest, while in the lower 4-km layer, according to the radiosonde data, wind direction mostly veers with height from south to west and north. In the boundary layer, FP is often associated with low-level jet streams, most frequently of southwest direction in the cloudy layers. The warm layers within and below the clouds occur in more than 20% of the cases. The most typical precipitation is FP from "all cold" clouds. Using objective criteria of the fronts, synoptic situations, advection, and baroclinicity, it is shown that almost all cases of freezing rain are observed in frontal zones, while freezing drizzle is as frequent at the fronts as under airmass conditions. Both types of FP are associated mainly with high baroclinicity and warm advection. The results can be used to develop an objective method of FP forecasting. 431FP occurrence frequency as dependent on (a) temperature of the cloud base and top and (b) surface air temperature and maximum temperature in the cloudy layer. (1) Freezing rain; (2) freezing drizzle.

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