Time and Spatial Variations of Mesoscale Rainfalls and their Relation to the Large-Scale Field in the Western Tropical Pacific

Abstract

We examine the relationship between mesoscale rainfalls and large-scale atmospheric fields in the western Pacific using data obtained in the TOGA-COARE IOP (Tropical Ocean and Global Atmosphere-Coupled Ocean Atmosphere Research Experiment, Intensive Observation Phase). The data used for our analysis were a two-months period of successive x-band radar data taken at Manus island P. N. G. (2S, 147E), sounding data and GANAL (Global Analysis by Japan Meteorological Agency) data. Radar observation data were converted into rainfall rates as an index of local connective activity, and averaged into two horizontal areas: land area and sea area, and into three vertical layers to evaluate the island effect and the vertical distribution.
The rainfall rates of the land area were dominated by a strong diurnal variation. On the other hand, diurnal variation in the sea area was much weaker than the land area. In the 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 examine the relationship between the local rainfalls and large-scale atmospheric fields. The correlation between the rainfall rates and the CAPE showed that before a rainfall event the CAPE increased for two days, and then decreased after it. The correlation between the rainfall rates 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 rainfall event started. After a rainfall event, the mixing ratio at 500hPa increased. This relationship between the mixing ratio and the rainfall rates was also found in the sounding data.