20 TO 30-DAY AND 30 TO 60-DAY OSCILLATIONS IN ASSIMILATED GLOBAL DATASETS USING TRMM RAINFALL OBSERVATIONS

The influences of Tropical Rainfall Measuring Mission （TRMM） precipitation products on the structure and underlying physics of intraseasonal oscillation （ISO） are investigated with the U.S. National Aeronautics and Space Administration Goddard Earth Observing System model version 3 （GEOS-3） data assimilation system （DAS）. The strong ISO phase in the 1998 summer is apparently located in the Asian monsoon region and the east equatorial Pacific region. The eastward propagation is a dominant feature for the tropical ISO at 20 to 30-day oscillation while the northeastward propagation is the salient ISO at 30 to 60-day oscillation over the 10~N to 25~N belt region. It appears that the Kelvin wave structure is for the tropical 20 to 30-day oscillation. The tropical 30 to 60-day oscillation has the characteristics of the Kelvin-Rossby wave. The impact of satellite-derived precipitation （and its associated latent heating） on the ISO intensity is limited in the GEOS-3 assimilation system. However, its impact on the ISO spatial structures is obvious. Overall, the results demonstrate a better eastward propagation and a northward propagation of ISO with the TRMM precipitation simulation, indicating that latent heating is very important in exciting the equatorial ISO. Key words： 20 to 30-day oscillation; 30 to 60-day oscillation; GEOS data assimilation system; Kelvin wave; TRMM precipitation

Diurnal Variability of Precipitation Depth Over the Tibetan Plateau and Its Surrounding Regions

The diurnal variability of precipitation depth over the Tibetan Plateau and its surrounding regions is investigated using nine years of Tropical Rainfall Measuring Mission （TRMM） precipitation radar （PR） measurements. The Tibetan Plateau, the plains area, and the East China Sea are selected as the focus regions in this study. The average precipitation depths （PD） are about 4.6 km, 5.8 km, and 5.6 km, while convective （stratiform） PDs are about 6.6 （4.5） km, 7.5 （5.7） km, and 6.0 （5.6） km over the plateau, the plains, and the ocean region, respectively. Results demonstrate a prominent PD diurnal cycle, and its diurnal phase is generally a few hours behind the surface precipitation. The spatial variation of the PD diurnal magnitude is weaker near the coastal areas than that of surface precipitation. The height of the PD diurnal peak is around 6 7 km for convective systems and 5-6 km for stratifrom systems. The dominant afternoon diurnal peak for convective PD and the flat diurnal peak for stratiform PD over the Tibetan Plateau indicate that solar diurnal forcing is the key mechanism of the PD diurnal cycle over land. In addition, the diurnal variation is obvious for shallow and deep convective systems, but not for shallow and deep stratiform systems.