Meso-γ-scale convective systems observed by a 443-MHz wind-profiling radar with RASS in the Okinawa subtropical region

We observed a meso-γ-scale convective system in July 2007 using a 443-MHz wind profiler radar (WPR) with a radio acoustic sounding system (RASS) at the NICT Ogimi observatory in Okinawa, Japan. We analyzed the virtual temperature, Tv, the Brunt–Vaisala frequency squared, N2, and three components of wind velocity profiles from the WPR-RASS data. We also employed a non-hydrostatic meso-scale (NHM) numerical model. Although the island of Okinawa was covered with a Pacific high-pressure system from 21–26 July, the atmospheric condition was convectively unstable below about 5 km. A number of convective clouds generally appeared from 11:00–18:00 in local time (i.e., Japan Standard Time; JST) with a typical horizontal scale of 10 km and temporal scale of 40–60 min. We focused on the convective system that passed over the Ogimi radar site on 23rd and 25th July. Just before rain occurred on these days, a low N2 region extended upward to 2.0 km, and this characteristics is also commonly seen around a convective cloud in the NHM model. The cloud water content from the NHM model indicated that the cloud top height correlates with the low N2 structure. Before the convective system was generated, N2 decreased below an altitude of about 1 km, because air with low Tv intruded at 1–3 km, and the surface temperature increased due to solar radiation. The sea-breeze from both the east and west coasts of Okinawa collided to force the convergence below 1 km. Thus, the synergetic effects of the low static stability and convergence seemed to trigger the generation of a convective system, which eventually grew to 11 km over the radar site.