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[1] We used a three-year dataset from an impact disdrometer to study the diurnal to interannual variation of the raindrop size distribution (DSD) over Palau in the Western Pacific warm pool. The DSD variability was primarily seasonal, with a secondary contribution due to the El Nino Southern Oscillation (ENSO) cycle. We did not find evidence of a diurnal cycle. In the seasonal cycle, mean drop diameters tended to increase during the westerly monsoon period. Within this period, drop diameters generally increased during the El Nino year and decreased during the La Nina year. However, within the easterly monsoon period, they generally increased in the winter following La Nina, and decreased in the winter following El Nino. The amplitude of the seasonal variation in mean drop diameter was approximately the same as that in Singapore and Sumatra, and about one third of that in India.

[1] We examined the sea surface temperature (SST) distribution in the western North Pacific warm pool when a typhoon is generated in northern summer. Analyzing satellite observation data revealed higher (lower) SST north (south) of the typhoon. As a result, the meridional SST gradient became especially steep, and the SST was not homogeneously high, when a typhoon is generated. In-situ observation data in the oceanic surface layer suggested that the low SST was caused by vertical mixing related to the strong southwesterly monsoon. In contrast, the hot spots north of the typhoon were associated with weak sea surface wind and vertical mixing. According to satellite observation, the maxima of surface heat flux and water vapor shifted northward corresponding to the high SST. They may produce an asymmetry of the thermodynamic structure of a typhoon.

[1] We examined the convective activity over the tropical Indian Ocean in boreal autumn using satellite-observation data. In November and December, we detected an intertropical convergence zone-like (ITCZ-like) precipitation zone between Maldives and Somalia that is unobserved in the other periods of the year. It accompanied a high sea-surface temperature (SST) north of the equator, and a cold tongue near the equator, similar to the ITCZs over the Pacific and Atlantic Oceans. However, the formation mechanism of the SST anomalies over the Indian Ocean is not the same as that in the Pacific and Atlantic Oceans. The high-SST region forms in early November corresponding to weak sea-surface wind during the monsoon transition. The low SST along the equator appears, associated with the upwelling of the subsurface water.

[1] Satellite imagery and C-band radar on the R/V Mirai detected an isolated convective cloud band over the western tropical Pacific, and dropsondes observed the surrounding atmospheric environment. The reasonable development mechanism of this unusual cloud band is investigated using observational and numerical simulation data. Although theoretically, the surrounding atmospheric profile supported convective rolls, only a single cloud band was developed. Numerical simulations revealed a single cloud band and roll structure without condensate in the boundary layer. A westerly jet just above the cloud band influenced the environment with strong vertical shear supporting convective rolls; this was maintained by a typhoon at a distance of 1500 km away. A large-scale convergence in the meridional wind was also accompanied with this westerly jet. This special environment made by a typhoon could develop the unusual single cloud band.

[1] We performed synergy observation of subvisual cirrus clouds (SVC) with 1064-nm lidar, 95-GHz cloud radar and radiosondes launched every 3 hours at 2°N, 138°E for one month from 9 November to 9 December 2001. The estimated effective radius reff of SVC is 9–16 μm (5–9 μm in mode radius). This assumes that the shape of cloud particles is spherical and the number concentration of the particles is 105 m−3. SVC disappear when temperature anomalies exceed +1.5°C. The fall velocity of SVC is same as the downward velocity of the phase propagation of the Kelvin wave, and the terminal velocity of hexagonal particles. Thus sedimentation and phase propagation are important for dissipation of SVC. In addition, it was noted that SVC tended to disappear at midday during the observational period except on 27 November 2001.