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William L. Woodley, Cecilia G. Griffith, Joseph S. Griffin, and Scott C. Stromatt


Quantitative precipitation estimates have been made for the GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment (GATE) from geosynchronous, infrared satellite imagery and a computer-automated technique that is described in this paper. Volumetric rain estimates were made for the GATE A scale (1.43 × 107 km2) and for a 3° square (1.10 × 105 km2) that enclosed the B scale for time frames ranging from all of GATE (27 June—20 September 1974) down to 6 h segments. The estimates for the square are compared with independent rain measurements made by four C-band digital radars that were complemented by shipboard raingages. The A-scale estimates are compared to rainfall estimates generated by NASA using Nimbus 5 microwave imagery. Other analyses presented include: 1) comparisons of the satellite rain estimates over Africa with raingage measurements, 2) maps of satellite-inferred locations and frequencies of new cumulonimbus cloud formation, mergers and dissipations, 3) latitudinal precipitation cross sections along several longitudes and 4) diurnal rainfall patterns.

The satellite-generated B-scale rainfall patterning is similar to, and the rain volumes are within a factor of 1.10, of those provided by radar for phases 1 and 3. The isohyetal patterns are similar in phase 2, but the satellite estimates are low, relative to the radar, by a factor of 1.73. The B-scale disparity in phase 2 is probably due to the existence of rather shallow but rain-productive convective clouds in the B scale. This disparity apparently does not carry over to the A scale in phase 2. Comparison of NASA Electronically Scanning Microwave Radiometer (ESMR) rain estimates with ours for several areas within the A scale for all GATE suggests that the former is low relative to the latter by a factor of 1.50. The satellite estimates of rainfall in Africa are similar to measurements by raingages in all phases of GATE up to 11°N and progressively greater than the gage measurements north of this latitude toward the Sahara desert.

The diurnal rainfall studies suggest a midday (about 1200 GMT) maximum of rainfall over the water areas and a late evening maximum (about 0000 GMT) over Africa and the northern part of South America. The latitudinal cross sections along several longitudes of phase rainfall clearly show the west-southwest/east-northeast orientation of the Intertropical Convergence Zone (ITCZ), the diminution of the rainfall west-southwestward from Africa into the Atlantic, and the northward progression of the ITCZ from phase 1 into phases 2 and 3. The center of action for cloud formation, merger and dissipation, and the area of maximum rainfall (>1600 mm for all of GATE) occur along the southwest African coast near 11°N. This agrees with past climatologies for this region. Superposition of the satellite-generated rainfall maps and sea surface temperature maps by phase suggests a strong relationship between the two. Almost all of the rainfall occurs within 26°C sea surface temperature envelope. The mean daily coverage of rainfall and the mean rainfall in the raining areas for the A scale for all GATE are 20% and 14.1 mm day−1, respectively. These and other results are discussed.

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