Abstract
This paper presents results of the statistical analysis of cumulus-scale to mesoscale radar echoes observed over the 1.3×105km2 area in the equatorial western Pacific for the period of 20 days in May and June 1979 during MONEX. In this period easterly flows in the middle and lower troposphere began to be enhanced over the experimental area. The experimental area was located to the north of the ITCZ in the Southern Hemisphere during May and between the ITCZ's in the Northern and Southern Hemispheres during June. The mean echo area and the echo height averaged during the entire period were 125km2 and 2.9km, respectively. In average, 5.0% of the experimental area was covered by radar echoes. We could express the relationship between echo area and the number of echoes by a simple empirical formula. The number of mesoscale echoes larger than 103km2 in area was only 1.6%, but the total area of these echoes amounted to 47% of the total echo area.
The analyses of the radar echo population and the position of the ITCZ showed that the convective activity around the experimental area was enhanced in June more than in May. The number of radar echoes fluctuated with period of 3.5 to 4 days. Upper air data showed that large-scale westward propagating waves existed in the easterly flows at the middle and lower troposphere over the experimental area in June. The number of mesoscale echoes increased ahead of the 500mb trough (behind the 500mb ridge) in the waves. The radar echoes whose area were smaller than 103km2 tended to appear in the daytime rather than at night.
