Polarimetric Radar Observations of Sea Breeze Front during SoWMEX/TiMREX

Storm initiation has been a focus in many field experiments. The boundary layer processes play important roles on storm initiation. In Florida, many storm initiation cases are resulted from boundaries intersection which helped enlarge the vorticity and vertical velocity. Among these, sea breeze front plays a major role here.

On 20th June during SoWMEX/TiMREX, NCAR S-POL radar recorded the clear echo signals with the passage of the sea breeze front in SW Taiwan. On the morning, subtropical high dominated and prevailing wind was southeasterly. 11am sounding showed mixed layer reached 1km height and an inversion layer existed at about 2km height. Surface station data showed significant moisture and wind speed increase when the sea breeze front passed.

Low-elevation area-averaged polarimetric radar parameters observed by S-POL are calculated to show the differences before and after the front passage. The area is selected in plain area of northeast of S-POL. Mean reflectivity increased before the sea breeze front passage, from 4.5 to 6.5 dBZ and then decreased to 5.5 dBZ after the front passage. Mean radial velocity increased from -1 to 2 m/s, indicating the reversal of the wind direction. Differential reflectivity (ZDR) increased from 3.5 to 6 dB and depolarization ratio (LDR) is from -10 to -11 dB. Correlation coefficient (RHOHV) increased from 0.69 to 0.73. Changes of ZDR, LDR and RHOHV along the passage of sea breeze front collocated well with the changes of the radial velocity. The result shows that when sea breeze came into the area, there was an increase of wind speed. The particles (or insects) became more flat (less width-length ratio) and more regular indicating the consequence of stronger mixing and convergence.

Intensive RHI data are used to show the vertical structure of echoes during the passage of the sea breeze front. In the mixed layer (~1km height), the reflectivity was from -5 to 20 dBZ, ZDR was from 2 to 12 dB, LDR was from -22 to -2dB. Near inversion layer (~2km height), reflectivity was weaker (-10 to 10 dBZ) with ZDR from -1 to 2 dB indicating the presence of cloud droplets. The layer between the mixed layer and the inversion layer, the reflectivity was smaller than 5 dBZ and ZDR close to 0, and this is referred to the result from Bragg scattering. This feature becomes more obvious after sea breeze came inland. The result suggests the moisture transfer caused by the thermal bubbles was prevailed in this period.