The Role of Boundary Layer Convergence Line in Initiation of Severe Weather Events

CINRAD/SA Doppler Weather Radar can always detect weak and narrow echoes. Generally, those echoes can be closely associated with boundary layer convergence line, the feature and evolution of which may indicate the initiation of severe weather events. The data detected by Doppler radar from June to September of 2008 and 2009 in Tianjin areas, and the data on the events of sea breeze fronts from 2002 to 2007 are statistically analyzed. It shows that radar echo of boundary layer convergence line is also weak and thin, the intensity of echoes is 15—25 dBZ, and the width of echoes is 3—10 km. Boundary layer convergence line may be divided into three types, including sea breeze front, gust front and thin line due to special weather conditions. Single boundary layer convergence line may form isolated thunderstorm with favorable situation with moderate impact. The collision between boundary layer convergence lines can trigger thunderstorms or enhance the existing thunderstorms to be more severe.56 and 44 sea breeze front cases are observed by radar in 2008 and 2009 respectively. The total number of 25 events of the initiation of severe weather events is closely associated with sea breeze front, including 17 events in 2008 and 8 events in 2009. The formation times of sea breeze front are different, the earliest time is 09:30, while the latest is 16:00. The sea breeze front's maintenance time is also of different length, the longest maintenance time is 6.5 hours, and the shortest is only 1 hour. The general distance that sea breeze front extends to the interior is 70—80 km, and the farthest distance reaches 120 km, with the height of 1.5 km in general. Those provide important information on the initiation of thunderstorms for nowcasts.11 and 9 gust front cases are observed by radar during the observation period in 2008 and 2009 respectively. The maintenance time of gust fronts are inhomogeneous, with the longest maintenance time of 4.0 hours. Collision between sea breeze front and gust front can trigger thunderstorms. The boundary layer convergence lines due to local conditions will be discussed in future research. Analysis on the AWS data in Tianjin area combined with the data of Beijing sounding above 850 hPa indicates that thunderstorms are more likely to occur in the region with large temperature gradient, the temperature difference between 850 hPa and 500 hPa may reach 28℃.