SEARCH

SEARCH BY CITATION

Keywords:

  • tropical cyclogenesis;
  • atmospheric modelling;
  • vortex dynamics;
  • convection

Abstract

This paper evaluates the adequacy of a reduced (three-layer) model for understanding hurricane formation from turbulent initial conditions. The evaluation is based on a direct comparison to tropical cyclogenesis in a cloud-system-resolving (CSR) model that employs single-moment warm rain microphysics. The reduced model has three cumulus parametrizations, referred to as the convergence-based (CB), boundary layer quasi-equilibrium (BLQ), and selective boost (SB) options. Regardless of which one is activated, the reduced model produces hurricanes on the same time-scale as the CSR model. Generally speaking, the hurricanes emerge from turbulence through the coalescence and convective intensification of cyclonic vorticity. Moreover, in both the reduced and CSR models, the onset of ‘rapid intensification’ follows pronounced local growth of the η-variable of Ooyama (1969), which is a combined measure of deep convective instability and middle tropospheric moisture. Eliminating the surface flux of moist entropy or surface friction in either model prevents or severely inhibits hurricane formation; however, hurricanes eventually form without surface friction in the BLQ or SB versions of the reduced model.

Despite some measure of success, the reduced model has notable deficiencies that are apparent during the intermediate stage of genesis. Compared to the CSR model, rotational storms are less sporadic and their peak winds are less severe. In the intermediate mesoscale of the reduced model, the horizontal kinetic energy spectrum is relatively steep, and horizontal divergence is relatively weak. Furthermore, the Lagrangian autocorrelation time of vertical vorticity is relatively long. These discrepancies reflect a simplified (quasi-two-dimensional) form of rotational convective turbulence. The simplified turbulence has comparatively robust mesoscale vortices, and tends to produce more tropical cyclones than its counterpart generates in the CSR model. Copyright © 2011 Royal Meteorological Society