2:30 PM - 2:45 PM
[AAS05-04] The Dependence of Tropical Cyclone Seed Genesis on Convection Aggregation Stages
★Invited Papers
Keywords:tropical cyclone seed genesis, convection aggregation, cloud-resolving model
The upscale development of TC seeds from convective clusters is an important process to both weather forecast and the future response of tropical extreme rainfall to climate change, and yet the scientific understanding to it remains insufficient. This study proposes a novel idealized experimental design to investigate the process by which convection clusters transition into tropical cyclone seeds (TC seeds), with a particular focus on the role of convection-induced secondary circulation in determining TC seed intensity. We use convection self-aggregation simulations of the vector vorticity equation cloud-resolving model (VVM) at different developmental stages in a non-rotating environment to represent various initial states of convective cloud clusters, ranging from highly scattered to fairly aggregated. The simulation domain is 1152 km by 1152 km, with a horizontal resolution of 3 km. We then introduce a uniform Coriolis force to these initial states to simulate and examine the short-term variations in seed intensity under the same background vorticity. The centroid of positive vorticity is used to identify both the convergence center of non-rotating convection clusters and the cyclonic circulation center of the TC seed. Additionally, axisymmetric analysis is applied to explore differences in water vapor distribution, convective core cloud, and secondary circulation among different convection clusters, further assessing their impacts on TC seed intensification over a three-day period after applying the background vorticity.
Our results reveal that environmental humidity differences influence the degree of convective aggregation. In convective clusters without an established secondary circulation, a TC seed can still form, but its tangential wind remains weak and its structure loose. In contrast, when secondary circulation develops, the dry region gradually expands, allowing the convective system to concentrate within a smaller moist area and sustain the high axisymmetric and stronger low-level inflow. Convection along the edges of the moist region helps sustain low-level inflow into the cloud cluster. When Coriolis forcing is introduced, the spatial distribution of dry and moist regions is maintained, enabling convection at the edge of the moist area to enhance the secondary circulation. This facilitates the transformation of low-level inflow into tangential winds, ultimately forming a stronger TC seed. We identified a linear relationship between the tangential wind strength of the TC seed formed within three days and the low-level inflow strength of the convection cluster. Additionally, a higher degree of secondary circulation in the convective cluster leads to a more pronounced criticality in TC seed intensification.
Our results reveal that environmental humidity differences influence the degree of convective aggregation. In convective clusters without an established secondary circulation, a TC seed can still form, but its tangential wind remains weak and its structure loose. In contrast, when secondary circulation develops, the dry region gradually expands, allowing the convective system to concentrate within a smaller moist area and sustain the high axisymmetric and stronger low-level inflow. Convection along the edges of the moist region helps sustain low-level inflow into the cloud cluster. When Coriolis forcing is introduced, the spatial distribution of dry and moist regions is maintained, enabling convection at the edge of the moist area to enhance the secondary circulation. This facilitates the transformation of low-level inflow into tangential winds, ultimately forming a stronger TC seed. We identified a linear relationship between the tangential wind strength of the TC seed formed within three days and the low-level inflow strength of the convection cluster. Additionally, a higher degree of secondary circulation in the convective cluster leads to a more pronounced criticality in TC seed intensification.