TIME AND SPATIAL VARIATIONS OF MESOSCALE RAINFALLS AND THEIR RELATIONTO THE LARGE-SCALE FIELD IN THE WESTERN TROPICAL PACIFIC

We examine the relationship between mesoscale rainfalls and large-scal e atmospheric fields in the western Pacific using data obtained in the TOGA-COARE IOP (Tropical Ocean and Global Atmosphere-Coupled Ocean At mosphere Research Experiment, Intensive Observation Phase). The data u sed for our analysis were a two-months period of successive x-band rad ar data taken at Manus island P.N.G. (2S, 147E), sounding data and GAN AL (Global Analysis by Japan Meteorological Agency) data. Radar observ ation data were converted into rainfall rates as an index of local con vective activity, and averaged into two horizontal areas: land area an d sea area, and into three vertical layers to evaluate the island effe ct and the vertical distribution. The rainfall rates of the land area were dominated by a strong diurnal variation. On the other hand, diurn al variation in the sea area was much weaker than the land area. In th e spectrum of rainfall rates in the sea area, 1.5 to 2-days and 3 to 5 -days-period peaks were more significant than those of the land area. Lagged correlation analysis and cross spectral analysis was done to ex amine the relationship between the local rainfalls and large-scale atm ospheric fields. The correlation between the rainfall rates and the CA FE showed that before a rainfall event the CAFE increased for two days , and then decreased after it. The correlation between the rainfall ra tes and the zonal wind or mixing ratio showed an eastward propagation of the correlation that may correspond to a Madden-Julian oscillation. The phase relation of this correlation was such that, when the lower tropospheric wind changed its direction from easterly to westerly, a r ainfall event started. After a rainfall event, the mixing ratio at 500 hPa increased. This relationship between the mixing ratio and the rai nfall rates was also found in the sounding data.