Impact of ice-phase microphysics on inner-core processes in simulated extremely intense tropical cyclones

Best track data have indicated that most extremely intense tropical cyclones (TCs) such as Categories 4 and 5 based on the Saffir-Simpson Hurricane Scale (http://www.nhc.noaa.gov/aboutsshws.php) underwent rapid intensification (RI) (Kaplan and DeMaria, 2003). RI is often accompanied by secondary eyewall formation (SEF) and eyewall replacement cycles (ERC), which is closely related to the inner-core processes. However, the physical mechanisms associated with RI, SEF and ERC and relevant inner-core processes have not been fully understood. To understand RI, SEF and ERC processes, we conducted sensitivity numerical experiments using two kinds of 2-km mesh non-hydrostatic models (NHM2), the JMANHM based on the Japan Metrological Agency operational mesoscale model (Saito et al. 2007) and the Cloud Resolving Storm Simulator (CReSS: Tsuboki and Sakakibara 2002) developed in HyARC, Nagoya University, respectively. Initial and boundary conditions for an extremely intense TC calculated by the two models were provided from results of future-climate numerical experiments previously performed by a 20-km mesh atmospheric general circulation model. Experimental configurations in both NHN and CReSS included 1-moment and 2-moment bulk-type microphysics with an ice-phase along with a second-order turbulence closure scheme (the MYNN scheme) and a 1.5-order turbulence closure scheme. More information on NHM2 was described in Kanada et al. (2012 and 2013).

The results indicated that all experiments conducted by NHM2 exhibited RI, defined as a decrease in central pressures of 42 hPa during less than 24 hours. Shallow inflow boundary layer accompanied by intense near-surface inflows led to an intense and tall eyewall updraft during the RI phase. TCs calculated by CReSS had larger eyes and smaller scales of horizontal expansion with a great amount of graupel, while TCs by JMANHM had smaller eyes and larger scales of the expansion with a great amount of snow. The strength of near-surface inflows was also related to the scale of horizontal expansion. When near-surface inflow experiment was relatively weak, the scale of horizontal expansion was small. Most experiments exhibited SEF and ERC processes. Only the exception was an experiment that snow was less calculated (CReSS with a 1-moment microphysics).