Diurnal variation of three-dimensional radar echoes and their possible role of preconditioning the atmospheric humidity

Preconditioning moistened atmosphere is essential for the development of organized convective systems constituting the active part of Madden Julian Oscillation (MJO), and also it is one of the processes initiating MJOs. IORGC/JAMSTEC conducted intensive observation at Republic of Palau (7 N, 134 E) from December 2004 to January 2005 using triple Doppler radars, and R/V Mirai cruise (7 N, 140 E) during June 2000 by single C-band radar. Radar echo diameter, echo top heights, and rainrate observed in the western tropical Pacific are studied in conjunction with their role of preconditioning the atmospheric humidity preceding the appearance of organized convections.

The radar echoes corresponding to shallow and congestus clouds, the development of their height are prevented by relatively dry environment in boreal winter than wet environment in boreal summer over this region. During the transition from “convectively undisturbed” period to “convectively disturbed” period, rainrate/ lower tropospheric humidity/ number of radar echoes/ echo top height gradually increased with superposed clear diurnal variation of those variables. Especially the number of isolated convective cells shows distinct diurnal variation. Diurnal variation, on average, shows 03LT and 15LT maximum in both seasons. This diurnal variation is appeared to play significant role for preconditioning the atmospheric environment.