ИСТИНА

The purpose of this report was a studying of non-monotonous changes of wind velocity V with a height in the atmospheric boundary layer in conditions of flat relief. Doppler ‘MODOS’ sodar produced by METEK firm (Germany) operates at Lomonosov Moscow State University since 2004. Its operation frequency is 2000 Hz, vertical range of sounding is from 40 to 500 m, spatial and temporal resolution is equal to 20 m and 10 min correspondingly. Profiles of wind velocity were analyzed by these data during eight years (2202 days for the period from 2004 to 2012). Automatic algorithm of the negative wind shear and low-level jet indication has been created and used. Accordingly to it any low-level jets (local maxima of V) were accepted as real phenomena at all cases of statistically significant inverse wind gradient ∂V/∂z<0 (i.e. when a value of V reducing just above jet axis was larger than measurement accuracy). Thus, unlike other similar studies we take into account all cases of V reduce with a height even when a local maximum existed during only 10 minutes and inside only 20 m air layer. As a result all separate low-level jets have been indicated and analyzed. Statistical distribution of their values (i.e. differences of V between jet axis level and a point above where wind velocity begins to increase with a height again) represents a sharp maximum close to the lowest limit of significant values (about 0.4 m/s) so that the most of jets are weak and short-living. However some of them have great values up to about 10-12 m/s. The diurnal course of their occurrence probability has a strong and statistically significant daily maximum in summer and in spring, weak daily maximum in autumn and is comparatively smoothed in winter. The low-level jets’ mean annual course is characterized by maximum in spring and in summer (from April to August) and minimum in autumn and in winter. Probable explanation of both effects is comparatively smoothed wind profiles (small values of ∂V/∂z) under unstable stratification which leads to more often cases of ∂V/∂z<0, i.e. of V decrease with a height at some levels. The probability of negative wind gradient cases increases with a height in the layer from 40 to 120 m and remains nearly the same above; probably it is a result of the V quick growth in the ground air layer. As it was found the larger is wind velocity the less low-level jet probability is. However at the same time the larger is V the larger low-level jet value is usually. Both relations are statistically significant. Spatial field of low-level jets has been studied precisely in autumn of 2008 by the data of local network which included two radiosonde stations in Moscow and Kaluga regions of Russia, high meteorological mast in Obninsk (Kaluga region) and sodars. It has been found that sometimes probably the same low-level jet existed simultaneously at a distance of nearly 180 km (at two radiosonde stations).