ITCZ precipitating systems observed by shipborne Doppler radar

The intertropical convergence zone (ITCZ) is a characteristic convective area in the tropics where the precipitating systems plays "heat source" of the global circulation. However, few studies described the detailed structure of the ITCZ precipitating systems. In June of 2000, we deployed R/V Mirai and carried out stationary observation at (7N, 140E) continuously for 11-days, from June 20 to July 1. The observation included 10-minute cycle of shipborne C-band Doppler radar and 3-hour cycle of radiosonde launch, etc.

The GMS-IR images shows that two active cloud region passed over Mirai during the observation period. The radar also detected active and wide spread echoes in the beginning and the end of the observation period.

On the former case, we successfully observed four typical MCSs with convective leading edge and trailing stratiform region. The each passage of the leading edge over the vessel accompanied gusts with the temperature and humidity discontinuous. Corresponding them, the VAD analyses shows strong southeasterly below the melting layer, while the northeasterly above. In addition, the periodic changes of the wind, which have the period of one to a few hours and inclined spatial pattern, are also found in the stratiform regions above 2-km height. Such short period could be occurred by the convection itself, like gravity wave. In the pattern, strong northerly component are included in the rearmost part of the stratiform regions. SSM/I images show that the relatively dry region existed in the north of the systems with the horizontal scale of 1000 km or more. These suggests that the convection-origined wavy structure contributed to the effective intake of the dry air as the mid-level rear inflow.

On the larger scale variation, on the other hand, we found the gradually growing of the radar echo area, for 8 days from no-echo state just after the first active period to the other active period. The embedded diurnal variation is also found. The diurnal cycle have its peak at midnight to early morning, and the peak is earlier than that of GMS IR image for a few to several hours.