This study utilized the MM5 mesoscale model to simulate the landfalling process of Typhoon Talim.The simulated typhoon track,weather patterns,and rainfall process are consistent with the observation.Using the simulation results,the relation of the second type thermal helicity（H2） to rainfall caused by the landfalling typhoon Talim was analyzed.The results show that H2 could well indicate the heavy inland rainfall but it did not perform as well as the helicity in predicting rainfall during the beginning stage of the typhoon landfall.In particular,H2 was highly correlated with rainfall of Talim at 1-h lead time.For 1-5-h lead time,it also had a higher correlation with rainfall than the helicity did,and thus showing a better potential in forecasting rainfall intensification.Further analyses have shown that when Talim was in the beginning stage of landfall,1） the 850-200-hPa vertical wind shear around the Talim center was quite small（about 5 m s-1）;2） the highest rainfall was to the right of the Talim track and in the area with a 300-km radius around the Talim center,exhibiting no obvious relation to low-level temperature advection,low-level air convergence,and upper-level divergence;3） the low-level relative vorticity reflected the rainfall change quite well,which was the main reason why helicity had a better performance than H2 in this period.However,after Talim moved inland further,1） it weakened gradually and was increasingly affected by the northern trough;2） the vertical wind shear was enhanced as well;3） the left side of the down vertical wind shear lay in the Lushan and Dabieshan mountain area,which could have contributed to triggering a secondary vertical circulation,helping to produce the heavy rainfall over there;hence,H2 showed a better capacity to reflect the rainfall change during this stage.
The physical processes associated with changes in the convective structure of an idealized tropical cyclone （TC） during landfall on a beta-plane were studied using the fifth-generation Pennsylvania State University- National Center for Atmospheric Research Mesoscale Model, version 3 （MM5）. The simulation results suggested that the suppression of moisture supply and increased friction acted to enhance the convection from the left and front quadrants of the TC to the front and right of the TC during different periods of landfall. When surface moisture flux was turned off, convection in other parts of the quadrant was clearly suppressed and the total rainfall was reduced. When surface friction was increased, precipitation showed a marked increase after the TC made landfall. Wetter air at low and intermediate levels, and drier air at high levels around the onshore side of the coastline led to a high value of convective available potential energy （CAPE）. Consequently, convection was enhanced immediately downstream of this area when the surface moisture flux was cut off. When surface friction was increased, the physical process was similar prior to landfall. After landfall, increased convergence at the onshore side of the land resulted in enhanced convection in front of the TC. Consistent with previous findings, our results suggest that during landfall the TC structure changes from one of thermodynamic symmetry to asymmetry due to differential moisture flux between the land and sea surface. The asymmetry of the thermodynamic structure, which can be explained by the distribution of CAPE, causes an asymmetric rainfall structure.
An observational analysis of satellite blackbody temperature （TBB） data and radar images suggests that the mesoscale vortex generation and merging process appeared to be essential for a tropical-depression-related heavy rain event in Shanghai, China. A numerical simulation reproduced the observed mesoscale vortex generation and merging process and the corresponding rain pattern, and then the model outputs were used to study the related dynamics through diagnosing the potential vorticity （PV） equation. The tropical depression （TD） was found to weaken first at lower levels and then at upper levels due to negative horizontal PV advection and diabatic heating effects. The meso-vortices developed gradually, also from the lower to the upper levels, as a result of positive horizontal PV advection and diabatic heating effects in the downshear left quadrant of the TD. One of these newly-generated vortices, V1, replaced the TD ultimately, while the other two, V2 and V3, merged due to the horizontal PV advection process. Together with the redevelopment of V1, the merging of V2 and V3 triggered the very heavy rain in Shanghai.
