Mean Rainfall Patterns for the Three Phases of GATE

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  • 1 Center for Environmental Assessment Services, NOAA, Washington, DC 20235
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Abstract

As a part of the GARP Atlantic Tropical Experiment (GATE), quantitative precipitation measurements were made during the summer of 1974 with four C-band digital radars complemented by shipboard raingages. Isohyetal maps covering a 125 000 km2 array centered at 8°30′N, 23°30′W are presented for each of three, approximately 20-day observational phases of GATE. Large mean rain rates exist for all three phases, with the largest ones corresponding to accumulations exceeding 500 mm for some of the maximum isohyets during Phase I. The mean rainfall rate averaged over the B-scale array for all three phases, 11.3 mm day−1, is apparently not significantly different from pre-GATE rainfall climatology. Another striking characteristic of the phase-mean precipitation patterns is the large spatial gradients; e.g., gradients as large as 200 mm in 16 km are observed.

Latitude shifts in the zone of maximum confluence (intertropical convergence zone) and in the tracks of the synoptic disturbances are reflected by interphase changes in the precipitation patterns. Also presented is a time-latitude rain cross section constructed from hourly precipitation amounts, which shows that the significant precipitating convection occurred most frequently in the vicinity of the troughs of African wave disturbances during Phase III.

Abstract

As a part of the GARP Atlantic Tropical Experiment (GATE), quantitative precipitation measurements were made during the summer of 1974 with four C-band digital radars complemented by shipboard raingages. Isohyetal maps covering a 125 000 km2 array centered at 8°30′N, 23°30′W are presented for each of three, approximately 20-day observational phases of GATE. Large mean rain rates exist for all three phases, with the largest ones corresponding to accumulations exceeding 500 mm for some of the maximum isohyets during Phase I. The mean rainfall rate averaged over the B-scale array for all three phases, 11.3 mm day−1, is apparently not significantly different from pre-GATE rainfall climatology. Another striking characteristic of the phase-mean precipitation patterns is the large spatial gradients; e.g., gradients as large as 200 mm in 16 km are observed.

Latitude shifts in the zone of maximum confluence (intertropical convergence zone) and in the tracks of the synoptic disturbances are reflected by interphase changes in the precipitation patterns. Also presented is a time-latitude rain cross section constructed from hourly precipitation amounts, which shows that the significant precipitating convection occurred most frequently in the vicinity of the troughs of African wave disturbances during Phase III.

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