Radar characteristics of convective clouds during squalls in the southeastern part of western Siberia

Abstract

The following radar characteristics of squall clouds in southeastern western Siberia are studied: the frequency of the maximum height of cumulonimbus clouds in squalls, the height of the 0ºC isotherm, radar reflectivity at three levels, maximum reflectivity, and a complex criterion of thunderstorm likelihood. The radar characteristics are determined for squalls in different synoptic situations. The results of this study can significantly improve the existing methods of squall forecasting. Squalls (wind increases of more than 25 m/s) cause cummulative losses comparable to droughts, spring floods, and forest fires [2], because squalls, like tornadoes, are difficult to predict. Satellite and meteorological radar data are used to improve the forecast skill of hazardous hydrometeorological events. It should be noted that radar characteristics of thunderstorm clouds, unlike those of squalls, are well studied [3,5,6,10,15,16] and are successfully used in practice. Radar characteristics of convective formations generating squalls have been rarely investigated in recent years [7,9], and such studies have not been carried out for western Siberia until present.The goal of the present work is to study radar characteristics of Cb clouds associated with squalls and to choose the most informative ones for squall forecasting near the airport of Tomsk.Since 1987, a synoptic-radar method has been used at the Tomsk aerological station for squall forecasting. The method is based on the data of the incoherent meteorological radar MRL-5 with a range of 55 dB. In [4,14], a method for squall diagnosis is proposed for Cb clouds with tops no less than 10 km.This study is based on radar characteristics derived from operational radar measurements at the Tomsk airport radar station during the summers from 1991 to 2004. According to [11,13], the wind speed in the squall exceeds 10 m/s, and the squall duration is a minute or more. The squall is considered as a severe weather event if the wind speed (including wind gusts) is equal to or greater than 25 m/s [12]. We took as a squall case each radar echo located in the near field (30´30 km) that detected the event, and the instantaneous maximum wind speed was considered. In all cases, the times of radar and visual observations coincided or differed by ±20 min. A total of 58 squall cases were observed, of which 26% were detected in May, 21% in June, and 27% in July. The following characteristics were studied: forecast weather events; the maximum height or the upper boundary of the radar echo (H max ); the 0ºC isotherm height (H 0 ); the -22ºC isotherm height (H -22 ); the logarithm of radar reflectivity at the level of precipitation (logZ 1 ), at the level corresponding to the crystallization initiation (logZ 2 ), and at the level of mass crystallization (logZ 3 ); the height of the third level (H Z log 3 ); maximum radar reflectivity (logZ max ); and a complex criterion of thunderstorm likelihood (Y cr ) determined according to [14] as a product of the logZ 3 minimum in a thunderstorm and H -22 ....

Atmospheric circulations associated to the interannual variability of cumulonimbus cloud frequency in the southern part of Romania

“…Radar echoes allow the detection of the thunder phenomena that occur as a result of the development of convective clouds (Francisco, 2001). Most methods for the identification of convective echoes as thunder clouds use as a criterion their height and intensity (Ananova et al , 2007). The process of forecasting the occurrence of convective clouds is not a simple one and the identification of echoes associated to potentially hazardous clouds is not certain (Krauss et al , 2007).…”

“…Most methods for the identification of convective echoes as thunder clouds use as a criterion their height and intensity (Ananova et al, 2007). The process of forecasting the occurrence of convective clouds is not a simple one and the identification of echoes associated to potentially hazardous clouds is not certain .…”