Case study on a tropical continental squall line observed at Darwin, Australia using by four Doppler radars

In observation program named Japan Australia Tropical Mesoscale Experiment (JATMEX; Iwanami et al., 1999), four Doppler radars observed a continental squall line (CSQL) on 21 Jan. 1999 at Darwin, Australia. By using four Doppler radars, wind field could be synthesize for 120km~120km in horizontal space and for whole life cycle of the squall line.

The CSQL was organized about 60 km southeast to Darwin, and moved to northwest at the speed of 7.5m/s. In this stage, the squall line did not have distinct convective line yet. It seemed that a convective line of the CSQL formed rapidly after a merger the squall line with some isolated convective cells at the front of it. The echo top in convective line rose and maximum reflectivity, moreover, was intensified at the same time.

In this study, we consider mechanisms to cause the rapid formation of convective line and changes in echo top and reflectivity. In order to understand that, wind field syntheses using four Doppler radars were carried out and data of automated weather stations (AWS), which measures surface meteorological elements, were also analyzed. The results of the wind field syntheses showed that a kinematical and microphysical interaction between the CSQL and the isolated convective cells played important role on the rapid formation. Moreover, it was shown that the rapid formation of convective line happened when the CSQL moved to where sea breeze converged from the analysis of the AWS. This result indicated that the rapid formation was caused by not only the interaction between the CSQL and the isolated convective cells but also low level horizontal convergence between sea breeze and outflow from the CSQL.