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 e¤ects 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.

Investigations of boundary layer structure, cloud characteristics and vertical mixing of aerosols at Barbados with large eddy simulations

“…Other influence factors like terrain height, wind speed and their interactions have multiple impacts on flow regimes, turbulence, cloud trail lengths etc. Another study on island effects with similar topographical heights compared to Barbados was done by Minda et al (2010). They investigated the evolution of the convective boundary layer (CBL) above Okinawa Island, Japan.…”

Doppler lidar studies of heat island effects on vertical mixing of aerosols during SAMUM–2

“…It is well known that flat heat islands in the stable maritime environment can cause strong convection and vertical mixing of aerosols, trace gases and momentum (Carbone et al, 2000; Savijärvi and Matthews, 2004; Minda et al, 2010). The islands of Cape Verde with their pronounced orography with mountains of 1000–3000 m height may further disturb the long‐range transport of aerosols advected from west Africa because they act as mechanical obstacles (e.g.…”

“…However, exclusive simulations are not sufficient to understand the heat island effects. For example, Minda et al, (2010) describe the evolution of the island BL and the initiation of clouds over Okinawa Island, Japan, during conditions with weak synoptic forcing. It was shown that only the combination of radar observations and cloud‐resolved modelling enabled the authors to understand the island BL evolution.…”

A Numerical Study on a Mesoscale Convective System over a Subtropical Island with 4D-Var Assimilation of GPS Slant Total Delays

“…For example, Chang and Yoshizaki (1991) simulated an MCS observed over Okinawa with a two-dimensional numerical model and concluded that the initiation and movement of the MCS were forced by interactions between the mountains on the island and a cold pool created by the MCS. Minda et al (2010) also investigated the initiation process of MCSs over Okinawa by using Doppler radar observations and numerical simulations with a 250-m grid spacing. They conducted impact tests with and without incorporating mountain effects and reported that similar MCSs were reproduced in both simulations; however, the sea-breeze circulation was somewhat stronger when the mountain effects were included.…”

“…Although these various numerical simulations reproduced their target phenomena (MCS or convective system), Chang and Yoshizaki (1991) used a twodimensional model, whereas Minda et al (2010) and Mikami et al (2011) used high-performance numerical models but downscaled (so-called "cold start") initial conditions in their studies. From the viewpoint of weather forecasting, predicting the timing and location of an MCS or convective system is particularly important.…”