ITCZ breakdown in three dimensional flows

This research is divided into two parts, a modeling study simulating the process of Intertropical Convergence Zone (ITCZ) breakdown through the vortex roll-up mechanism in a three dimensional model, and a comprehensive observational study of ITCZ breakdown. Both parts are original research.; The vortex roll-up mechanism states that diabatic heating in the tropical atmosphere generates positive (negative) potential vorticity (PV) below (above) the maximum heating, which satisfies the necessary conditions for combined barotropic/baroclinic instability and therefore the flow may become unstable. In the modeling study, shallow heating (confined below 600 hPa) only generates a positive PV strip on lower levels. Slower evolution of the shallow ITCZ may be caused by the lack of the upper level PV strip. The shallow ITCZ is less influenced by the background flow which is generated by a deep thermal forcing and is maintained in steady state. For deep heating cases, when the background flow enforces the PV induced wind field, breakdown of the PV strip occurs more rapidly, whereas when the background flow is opposite to the PV induced flow, breakdown takes longer and the PV strip may dissipate before breakdown.; The goal of the observational study is to quantify the frequency of occurrence of ITCZ breakdown and understand its role in producing tropical disturbances. We have visually identified thirty-four cases of ITCZ breakdown (including both shallow and deep ITCZ breakdown) in three independent datasets in the past four active seasons (July--October, 2000--2003) over the tropical eastern Pacific. The vortex roll-up mechanism produced nearly the same number of ITCZ breakdown events as westward propagating disturbances (WPDs). WPDs were thought to be the dominant mechanism for triggering ITCZ breakdown. Our result shows that the vortex roll-up mechanism is indeed a robust mechanism in the tropical eastern Pacific, and it is even more important than WPDs in the central Pacific where the atmosphere is less affected by WPDs.; Two approaches of cluster analysis were employed to help detect ITCZ breakdown in QuikSCAT scatterometer winds. Statistical models combined with basic vorticity dynamics hold promise as a method to develop and apply for practical use in early detection of tropical cyclogenesis in the future.