Resolution dependence of tropical cyclones simulated by a spectral cumulus parameterization

Abstract

The resolution dependence of tropical cyclones (TCs) simulated by a spectral cumulus parameterization (spectral scheme) was investigated using an atmospheric general circulation model with three ensemble members and atmosphere-only experiments. The focus of the analysis was the cause of resolution dependence of TCs which has not been clarified in previous studies. The results showed that underestimation of the TC track density and TC intensity was improved by an increase in the horizontal resolution as reported in previous studies. A mean state analysis revealed that the mean states were qualitatively similar regardless of model resolution although TC statistics were different; this implies that local differences in the atmospheric condition may have a strong impact on the TC statistics. Analyses of the horizontal structure indicated that the local TC structure better approximated observations as the resolution increased, but this was not due to enhanced atmospheric instability because the TC center location coincided with the highly unstable region only in the early stage of TC development. Influential differences for TC statistics were found in the vertical TC structure. A temperature anomaly with a large radial gradient induced by diabatic heating within the eyewall of a TC first impacted the vertical–radial circulation and then caused strong boundary layer inflow toward the eyewall. If a low resolution was adopted, the radial gradient of the temperature anomaly with strong low-level inflow was not well simulated, resulting in a weaker TC intensity and lower TC genesis frequency. Some TC property changes associated with resolution in the current spectral scheme were found to be similar to those of existing convection scheme, but there were some improvements that were specific to this spectral scheme. The cause of the improvements introduced by the present spectral scheme is briefly discussed.