UC Davis

This study uses a coupled atmosphere-ocean model with different numerical settings to investigate the mean and eddy momentum transfer processes responsible for Typhoon Muifa's (2011) early rapid intensification (RI). Three experiments are conducted. Two use the coupled model with a horizontal resolution of either 1 km (HRL) or 3 km (LRL). The third (NoTCFB) is the same as LRL but excludes tropical cyclone (TC)-induced sea-surface temperature (SST) cooling. HRL reasonably reproduces Muifa's intensity during its rapid intensification and weakening periods. The azimuthal mean tangential and radial momentum budgets are analysed before the RI rates diverge between HRL and LRL. Results show that the dominant processes responsible for Muifa's intensification are different in HRL and LRL. For HRL, the net eddy effect intensifies the storm's circulation and contracts the eyewall during early RI, and it dominates the net mean-flow effect inside the radius of maximum wind (RMW), except near the surface and between 2 and 5 km close to the RMW. In contrast, the mean and eddy effects in LRL almost cancel inside the RMW, while the mean-flow effects dominate and intensify tangential winds outside. Without TC-induced SST cooling, Muifa in NoTCFB reaches a similar storm intensity as in HRL but its rapid weakening rate is substantially underestimated. The dominant mechanisms for tangential wind intensification in NoTCFB are similar to those in LRL, but their magnitudes are larger, implying a misrepresentation of the dominant momentum transfer processes in NoTCFB during RI. For the radial momentum budget analysis, the dominant processes are similar among the three experiments except for some differences in their locations and strengths.