The Numerical Simulation of Orographic Effect on the Rain and Structure of Typhoon Rananim During Landfall

Numerical simulation has been carried out on the track and precipitation of typhoon Rananim (0414) using the non-hydrostatic version 3.6 of the Pennsylvania State University/NCAR mesoscale numerical model (MM5). The model can well simulate the track and precipitation of typhoon Rananim though the precipitation is underestimated in comparison with observation. However, the model can basically capture the characteristics of intensity change of typhoon Rananim, especially its weakening during landfall. Other features like the heavy rain pattern and the mesoscale rain-band can also be well simulated by the model, though their locations are slightly deviated from observation. During landfall of typhoon Rananim, the base reflectivity of Doppler radar at Wenzhou indicates that the strong echo bands are located in the right front along the typhoon moving direction, with a lot of mesoscale convective cloud masses in the spiral echo bands, and the high value of vertically integrated liquid (VIL) in the northwest of the typhoon eye. These characters are closely related to the effect of orography. The sensitivity experiments of orographic effect on the changes of precipitation and structure of typhoon Rananim during its landfall by keeping/removing mountains in the eastern area of Zhejiang and Fujian provinces show that: 1) there exists significant increase of rainfall. The precipitation pattern is consistent with orientation of orography, i.e. the rainfall increases on the windward side, while deceases on the lee side. 2) In the northwest of the typhoon eye, there occur many significant convergence zones and mesoscale convective systems in the lower layer, and divergence ones in the upper layer. Mesoscale convective cloud masses are generated and resulted in heavy rain with its distribution highly asymmetric from northwest to southeast. These phenomena are consistent with observations by Doppler radar at Wenzhou. 3) The circulation of typhoon Rananim can be partly changed as the result of orographic forcing. The local circulation of typhoon will be enhanced obviously when mesoscale turbulences initiated by orographic forcing is in phase with the local circulation of typhoon, and vice versa. 4) The vertical motion on the side of the eye wall of the typhoon close to land will be increased, with significant enhancement of potential vorticity leading to strengthening of the typhoon vortex.