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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The characteristics are presented for the visibility regime at 42 aerodromes in European Russia calculated from the data of aerodrome observations reported in METAR telegrams with 30 minute (more rarely, 1 hour) time intervals. The occurrence frequency distributions of horizontal visibility ≤ 300 m and ≤ 800 m are calculated and analyzed over the period of 2001-2019. The tables are presented and discussed for the annual cycles of the occurrence frequency, as well as for its distributions under different weather phenomena. The occurrence frequency distributions depending on cloud base height, relative humidity, speed and direction of surface wind are presented. The results are also presented for the duration of low visibility episodes: it is demonstrated that such episodes are short, as a rule (for example, the visibility ≤ 300 m is continually observed for not more than 2 hours in 65-85 % of cases). The results of quantification of the correspondence between the occurrence or absence of low visibility and other weather characteristics observed at the same aerodromes demonstrate certain perspectives for developing (at least for several aerodromes) statistical methods to forecast this rather rare phenomenon basing on outputs of numerical atmosphere models. Keywords: visibility, aerodrome observations, annual cycle of low visibility, relative humidity, cloud base height, weather phenomena

Abstract. Conditions for freezing precipitation (FP), including freezing rain (FR) and freezing drizzle (FZ) for 8 airports in Russia and 4 in the Ukraine are studied on the basis of 10 to 20-year series of surface observations, radiosonde and objective analysis data. Statistical characteristics are presented of the FP episode durations and of occurrence frequency dependences on surface air temperature, wind direction and speed and cloud base height. From the radiosonde data, it is found that the "classical mechanism" of FP generation (for which, stratification of "warm nose" type in the cloud layer is necessary) is not frequent: most of FP cases are associated with "all cold" conditions in the lower 3-km layer, that is, with negative temperatures in and below the clouds.