Assessment of parameters of mesoscale convective systems based on satellite and upper-air sounding database (West Siberia)

Using by ground-based and satellite tools, an analysis of mesoscale convective systems (MCSs) has been carried out. In the course of the study, we assessed a repeatability for the warm season (April to September) for 2008-2018. Within warm season of 2015-2018, the synoptic situations favorable to formation of MCSs which produced hazardous phenomena has been determined. To estimate of atmospheric stability we applied K-index and Lifted-index. On the basis of MatLab calculation method the linear dimensions of MCSs has been defined. The method used is visual interpretation of clouds to assess the repeatability and parameters of MCSs in the southeastern part of Western Siberia based on RGB composite imagery (0.65; 0.45; 0.35 μm) using MODIS data. The study area is limited to 50 and 60 °N and 70 and 90 °E, which includes five administrative units geographically located in the West Siberia: Tomsk Oblast, Novosibirsk Oblast, Kemerovo Oblast, The Altai Republic and Altai Krai. The object of study is considered the mesoscale convective system (MCS), in the paper is classified as a complex of Cumulonimbus clouds with a diameter of 50 km and more. It is important to note that the study area is not predisposed to the MCS formation, and even such a small cloud cluster is capable of generating hazardous phenomena such as thunderstorms, hail, severe and prolonged downpours. A specific feature of the MCSs is a long period of existence (about 10 hours), but its effect on weather conditions track up to 24-36 hours. To characterize the atmospheric stability on days with MCSs we used measurements taken by five upper-air stations (Aleksandrovskoe, Barabinsk, Barnaul, Kolpashevo, Novosibirsk) at 0000 and 1200 UTC from 2015 to 2018. The results of upper-air sounding accommodate discrete information about the values of air temperature and humidity and wind characteristics within the entire thickness of the atmosphere, in which convection over Western Siberia may develops, namely up to 10-15 km. As convection parameters, we applied a stability indices that are widely utilize to forecast the thun-derstorm/hail/squall development etc. Thus the Lifted-index describes the latent instability of the atmosphere whereas a calculation of K-index based on the vertical temperature lapse rate, and the amount and vertical extent of low-level moisture in the atmosphere. During warm season the total number of days with MCSs equals 230 days for the entire period 2008-2018. The peak frequency of days with MCSs (31) was observed in 2016. This is due to the 2016 anomaly in many meteorological parameters. The minimum number of days with MCSs (12) was registered in 2010. The evaluation of atmospheric stability on the days with MCSs showed the following: the Lifted-index classifies the state of the atmosphere as slightly unstable (53% of all cases) in accordance with the generally accepted classification and only showers are observed throughout the territory. Using the K-index, in 51% of cases the state was assessed as unstable. The MCSs are mainly formed when the atmosphere is moderate and slightly unstable and their evolution accompanied by the presence of showers and isolated thunderstorms across the territory. Comparing the synoptic situations related with MCSs generation, it was found that the share of MCSs formed on the cold front equal a third of all events, and this ratio decreases markedly while activation of strong meridional circulation. The total share of contribution of occlusion processes to the MCSs frequency largely depends on the anomalies and by the end of the study period has the predominant part of the processes associated with the occluded front. Unlike the frontal cases, the processes formed at the stage of development of a cloud vortex, cold air developed system or mesovortex, have been found to be much more dangerous due to less possibility of prediction.

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