Variability of warm-season cloud episodes over east Asia based on GMS infrared brightness temperature observations

The present study has used the Geostationary Meteorological Satellite (GMS) IR brightness temperature observations to investigate the regional and intraseasonal variability of east Asian warm-season cloud/ precipitation episodes (in distance-time space) due to land-sea contrast and latitudinal effects. The data period was May-August 1998-2001, and harmonic analysis was employed as the major tool for analysis. The full domain of study (20°-40°N, 95°-145°E) was divided into northern and southern zones, and into eastern and western sectors, and statistics of episodes in each subregion were derived and compared. For latitudinal effects, episodes were found to be significantly larger in span and duration in northern (30°-40°N) than in southern (20°-30°N) zones. In the northern zone, the propagation characteristics were also stronger and remain evident even in midsummer, while episodes south of 30°N reversed in direction and traveled westward in July and August. For land-sea contrast, the May-August transition over land (western sector, 95°-120°E) was mainly characterized by an increase in diurnal activities, while that over ocean (eastern sector, 120°-145°E) was characterized by decreased overall activities instead. Over the land itself, significant regional variability also existed, with strongest diurnal signals over the eastern Tibetan Plateau near 100°E, and increased diurnal activities over mountain areas in southeastern China since June. Between the two bands, near 107°E, semidiurnal signals were relatively strong and became dominant in June. This double-peaked structure in the diurnal cycle resulted from overlying signals of convection propagating eastward off the plateau with those induced locally in late afternoon, and the phenomenon was more evident in May-June. Over the ocean, on the other hand, both diurnal and semidiurnal waves had small amplitudes, and the regional variability was much weaker. For intraseasonal transition, the number of large episodes was reduced from May through July, as was mean propagation speed. In August, however, some larger events started to reappear over east Asia.