Simulations of Top-Down and Bottom-Up tropical cyclogenesis

A cloud-resolving (2 km) WRF model is used to simulate an idealized tropical cyclone (TC) genesis event with precursor signals at either the middle or lower troposphere. It was found that the following three common development stages are found for both the top-down and bottom-up genesis scenarios: 1) a transition from non-organized cumulus-scale (~5km) convective cells or vortical hot towers (VHTs) to an organized vortex-scale (50-100km) system (OVS), 2) a stratiform-convective phase oscillation, and 3) the establishment of near-saturated vortex-scale air column and a rapid pressure drop. At stage 1, convective clouds are randomly excited due to an unstable atmospheric stratification. The non-organized cumulus clouds are merged and form an OVS through the vorticity segregation and Ekman-pumping processes. At stage 2, the OVS experiences a multiple convective-stratiform phase transition. As one convective burst diminishes, stratiform clouds form, leading to the enhancement of the mid-level vorticity maximum and "cold core". The mid-level dryness and cooling associated with downdrafts induces a mid-level ƒÆ„u minimum. This ƒÆ„u reduction, along with the recharge of the PBL moisture due to the surface evaporation, leads to the re-establishment of a convectively unstable stratification and triggers the subsequent new convective burst. Associated with this process, a near-saturated air column is established in the core region. At stage 3, the central minimum pressure drops rapidly while the low-level cyclonic vorticity penetrates into the upper troposphere and a warm core is established. The numerical experiments indicate that a pre-existing mid-level or low-level meso- (~500km) or large- (~5000km) scale cyclonic circulation is necessary for setting up the OVS and for the subsequent TC formation.