Polarimetric Observations of Sea Breeze Circulation and Thunderstorm Complex over Pingtung Plain during SOWMEX/TIMREX

On 20 June 2008, under the influence of Western Pacific subtropical high, a multi-cellular thunderstorm complex developed over Pingtung plain (SW Taiwan) and brought pronounced local heavy rainfall and lightning activity. In this study, sea breeze circulation were retrieved using data collected during intensive field observational program SoWMEX/TiMREX (2008). Three-hourly soundings were used to reveal the evolution of convective boundary layer over the Pingtung plain. The mesoscale structure and evolution of the sea breeze circulation was investigated and documented using high-temporal surface observations and polarimetric observables obtained by NCAR SPOL.

The location and timing of sea breeze front (SBF) in the Pingtung plain is identified by using high-temporal surface station observations. The leading edge of sea breeze circulation has shown pronounced frontal-like structure, including temperature drop, humidity and wind speed increase, and wind direction change. The corresponding SPOL observations are analyzed accordingly. It is demonstrated the clear-air echo signal of SPOL can be very useful to identify the location and movement of the sea breeze circulation with proper clutter filtering and clear-air echo analysis.

The averaged moving speed of SBF is 2.2m/s, the inflow from ocean can reach 4-5 m/s at height 500m and the return flow can reach 6 m/s at height 1.5km. It becomes difficult to identify SBF after the front moved into the mountain area. The wind field derived from the clear-air echo is consistent with the sounding observations and vertical structure of the sea breeze circulation is described. The flow structures at SBF, pre-SBF and post-SBF show significant differences due to different atmospheric stability conditions. At pre-SBF, the flow was turbulent with pronounced vertical mixing and at post-SBF, the flow revealed two-layer stratified fluid characteristics. The depth of SB circulation increased from 600m to 1000m and the return flow increased from 1-2km to 1.5-3km. In the presentation, pPolarimetric signatures associated with the thunderstorm complex will be also examined.