THE RELATIONSHIP BETWEEN ORGANIZED CONVECTIVE SYSTEMS AND LARGE-SCALE FIELDS OBSERVED IN GATE

The objective of this work is to investigate the controlling effects of large-scale circulations on the formation and maintenance of organized convective systems observed over the GATE A/B ship array. Surface and upper-air sounding data from the A/B and B-scale ships are used to define the large-scale dynamic and thermodynamic fields.; The first part of the thesis deals with case studies for convective systems observed during the period from 3 September through 6 September 1974. The horizontal divergence was computed using an objective analysis scheme developed earlier by Ogura and Chen (1977). The vertical velocity was then computed kinematically from the mass conservation equation. The results of the analysis indicated that the low-level convergence and consequently upward motion was present or enhanced prior to the development of organized convective systems. A low-level inversion was absent in the area of subsequent convective development in contrast to other areas having no convective activity.; The second part of the thesis discusses the relationship between the precipitation field and large-scale circulation characterized by easterly waves, the surface confluence line and the cross-equatorial flow. The surface and upper air sounding data for all A/B and B-scale ships for each phase of GATE were stratified according to the wave category defined by Reed and Recker (1971) before applying the objective analysis scheme.; In Phase III the surface confluence line moved rapidly southward behind the 700 mb ridge and gradually moved northward during the passage of the 700 mb trough. Heavy precipitation occurred ahead of the trough just south of the surface confluence line. Secondary local maxima of precipitation and surface convergence were present in the cross-equatorial flow where the surface streamlines exhibited an anticyclonic curvature. A time lag of about 5-10 hours between the maximum of the surface convergence and precipitation rate was found. The airflow south of the surface confluence line was anticyclonic, indicating that the boundary layer pumping played very little role in the formation of cumulus convection.; In sharp contrast to Phase III, easterly waves did not modulate the position of the surface confluence line in Phase I. This occurred because the surface meridional wind was decoupled with the 700 mb wind fluctuations associated with waves. Nevertheless, heavy precipitation also occurred ahead of the 700 mb trough.; The necessary condition for barotropic instability (Kuo, 1949) is met in the low and middle levels around 8.5(DEGREES)N in Phase I and 10.5(DEGREES)N in Phases II and III. In the mid-troposphere, the latitudinal variation of the mean zonal current makes the main contribution to the vertical component of the vorticity in the mean state. In Phase III the large wave-related fluctuations of the easterly jet and the southwest-northeast orientation of the 700 mb wave are the two main factors which account for the cyclonic vorticity maximum at 10.5(DEGREES)N on the easterly trough axis.; In the mean state, a distinct water vapor maximum appears in the low level south of the surface confluence line in Phases I and II; whereas in Phase III the water vapor has no distinct maximum in the A/B area. The strong upward motion is concentrated between 6(DEGREES)-9(DEGREES)N in Phase I and occupies a wider latitudinal band between 6(DEGREES)-11(DEGREES)N in Phase II. In Phase III there are two regions with upward motion. One is located in the cross-equatorial flow. The other is to the north. These features are in accordance with the phase-mean rainfall patterns estimated by radar (Hudlow, 1979).