[A-AS03] Large-scale moisture and organized cloud systems

[AAS03-03] A mechanism of convective self-aggregation: Coupling between low-level circulation and free-tropospheric variability

*Tomoro Yanase^1,2, Seiya Nishizawa², Hiroaki Miura^3,2, Tetsuya Takemi⁴, Hirofumi Tomita^2,1 (1.RIKEN Cluster for Pioneering Research, 2.RIKEN Center for Computational Science, 3.Graduate School of Science, The University of Tokyo, 4.Disaster Prevention Research Institute, Kyoto University)

Keywords:Radiative-Convective Equilibrium, Convective Organization, SCALE-RM

Convective self-aggregation (CSA) is a spontaneous organization of moist convection into a humid and cloudy region surrounded by a dry and clear-sky region. This study investigated the spontaneous onset mechanism of CSA by analyzing scattered and aggregated convection simulated in cloud-resolving radiative-convective equilibrium experiments in small and large domains, respectively. We argue that both low-level circulation and free-tropospheric variability play essential roles in the CSA onset. At first, we proposed a new method of reconstructing a stream function in the moisture-height space without a horizontal sorting of atmospheric columns. Based on this method, we quantified the difference in the low-level circulation patterns between scattered and aggregated convection. For the development of the low-level circulation in the aggregated cases, the strong radiative cooling in the boundary layer of the dry region played an important role by producing the horizontal gradient of buoyancy. On the other hand, for the triggering of the low-level circulation, the vertical intrusion of free-tropospheric subsidence into the boundary layer in the dry region seemed to be a key. According to the diagnosis based on the weak temperature gradient balance, the weakening in the convective heating largely contributed to the strengthening in the free-tropospheric subsidence in the dry region associated with the increase in the domain size. Furthermore, the horizontal contrast of convective heating between the moist and dry regions is closely tied to that of water vapor amount. By using a newly defined "moisture distance" as the ratio between the area and the perimeter of a dry region surrounded by an isoline of the total column water vapor, we showed that the length scale of dry patches increased as the domain size increased. Since such a water vapor variability at large horizontal scales was restricted by the horizontal domain size, CSA occurred only when the domain was large. From these results, we provide a new picture of the onset mechanism of CSA unifying the low-level circulation and the free-tropospheric variabilities of moisture, radiation, and convection.