Convective Boundary Layer above a Subtropical Island Observed by C-band Radar and Interpretation using a Cloud Resolving Model

Abstract

This paper describes the evolution of the convective boundary layer (CBL) and the initiation of clouds above Okinawa Island, Japan, during summer conditions with weak synoptic forcing and clear skies. An intensive observational campaign using a C-band precipitation radar (COBRA) and radio sonde was conducted on the island in early July 2005. These soundings exhibited diurnal variations in the potential temperature and water vapor in the atmospheric boundary layer. COBRA observed clear-air radar echoes (CAEs) within a 10-km radar range, and the CAE height showed a diurnal variation as well. To investigate the nature of the CAEs and to describe the dynamics of the CBL structure, a high-resolution cloud resolving model simulation was performed for two cases; a cumulus street and a distinct roll cloud. Flat and undulated surface models were used to understand the orographic effects on cloud initiation. On the basis of these investigations, we found strong consistency within the simulated radar reflectivity and observed CAE distributions. COBRA regularly observes turbulent moisture motions at cloud edges and occasionally detects moisture convergence in the CBL. The model CBL evolutions coincide well with the diurnal variations of CAE height, thereby showing that the island presents a good case for studying CBL evolution and that COBRA is useful to find the CBL depth here. The model results show that the warmed island induces horizontal convective rolls and a convergent zone above the island. A merging of the thermals, the rolls, and the sea-breeze reinforces the upward energy transfer, which generates a cumulus street and a distinct roll cloud being supported by orographic uplift. Wind direction characterizes a moist CBL formation and the cloud initiations above Okinawa Island.