On the Formation of Organized Convective Systems Observed over the Eastern Atlantic

Y. Ogura Laboratory for Atmospheric Research, University of Illinois, Urbana 61801

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Y-L. Chen Laboratory for Atmospheric Research, University of Illinois, Urbana 61801

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J. Russell Laboratory for Atmospheric Research, University of Illinois, Urbana 61801

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S-T. Soong Laboratory for Atmospheric Research, University of Illinois, Urbana 61801

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Abstract

Surface and upper air data gathered from a dense network of ship observations during the GARP Atlantic Tropical Experiment (GATE) were analyzed to correlate the upward motion with organized cloud convection. Two periods were. selected for analysis, 11–12 August and 3–6 September, 1974. The first period covers a case in which a spectacular development of an ITCZ rainband was observed. During the second period six cloud clusters were identified from radar and satellite photographs which either formed or intensified. within the data area under consideration.

The key variable in the analysis was the horizontal velocity divergence which was calculated using an objective analysis scheme. The, vertical velocity was then computed kinematically. The analysis results reveal good correlation between the vertical velocity fields and the development of organized convective systems. In all cases considered, the low-level convergence and, consequently, upward motion was present or enhanced prior to the development of organized convective systems. In some cases weak descending motion was observed in the upper troposphere. A low-level inversion was absent in the area of subsequent convective development in contrast to other areas having no organized convective activity. As organized convective systems developed, the upward motion intensified and extended up to the tropopause. The horizontal distributions of precipitation rates estimated from the moisture budget were compared to those calculated by Hudlow (1977) from radar measurements with fair agreement. A comparison was also made between the point value of the vertical velocity calculated by the analysis scheme and the vertical velocity averaged over the outer ship army or A/B-scale area.

Abstract

Surface and upper air data gathered from a dense network of ship observations during the GARP Atlantic Tropical Experiment (GATE) were analyzed to correlate the upward motion with organized cloud convection. Two periods were. selected for analysis, 11–12 August and 3–6 September, 1974. The first period covers a case in which a spectacular development of an ITCZ rainband was observed. During the second period six cloud clusters were identified from radar and satellite photographs which either formed or intensified. within the data area under consideration.

The key variable in the analysis was the horizontal velocity divergence which was calculated using an objective analysis scheme. The, vertical velocity was then computed kinematically. The analysis results reveal good correlation between the vertical velocity fields and the development of organized convective systems. In all cases considered, the low-level convergence and, consequently, upward motion was present or enhanced prior to the development of organized convective systems. In some cases weak descending motion was observed in the upper troposphere. A low-level inversion was absent in the area of subsequent convective development in contrast to other areas having no organized convective activity. As organized convective systems developed, the upward motion intensified and extended up to the tropopause. The horizontal distributions of precipitation rates estimated from the moisture budget were compared to those calculated by Hudlow (1977) from radar measurements with fair agreement. A comparison was also made between the point value of the vertical velocity calculated by the analysis scheme and the vertical velocity averaged over the outer ship army or A/B-scale area.

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