| Severe convective weather always is an important and difficult research issue of mesoscale studies.Several convective rainbands developed a few hours after Typhoon Matmo(2014)made the landfall in Fujian Province,which led to tremendous economic losses,although the radar echoes at its north were weak at initial time.To reveal outbreak of severe convective rainbands at the north of the typhoon,we used NCEP analysis data,surface auto station data and merged precipitation data with auto station data and CMORPH,carried out synoptic diagnostic analysis and conducted sensitivity WRF experiments with assimilation to examine synoptic background,mesoscale characteristics and convective triggering factors of the three severe convective rainbands at the north of the typhoon in this study.The results show that the three convective rainbands were triggered by the strengthened convective lines near the surface with persistent water vapor flux convergence,convective instability and local weak conditional instability under high humidity at the north of the typhoon.The convective cells formed along the convergence line,were intimately associated with strong convergence centers on the convergence line,and low-level strong convergence forcing provides dynamic upward lifting for the production of torrential rainfall.The first convective rainband formed by topography and frontal forcing,then developed northeastward and triggered convection over the high-value area of speed and convergence generated by easterly winds of the typhoon circulation after the landfall.The formation and orientation of the convergence line 1 were associated with land-sea contrast and curvature of coastal line along Jiangsu and south Shandong.With the northward extension of the first convective rainband,the second convective rainband was developed at the south of the first convergence line.The formation of the convergence line 2 was related to cold outflow at the south of the first convective rainband as well as the easterly flows at the north of the typhoon.The increase in the speed of the cold outflows pushed southward movement of the convergence line 2,and a new convection was triggered over the mesoscale baroclinic frontal zone.After the northward movement of the second convective rainband,the convergence line 3 was developed at the south of the second convective rainband,which is similar to the development of the second convergence line 2.The three convergence lines developed over similar areas,implying the important impacts of land-sea contrast on distribution of the convergence lines.The analysis of sensitivity experiments reveals that the Dabie Mountain affected the location and intensity of initial convection.The coastal line impacts on the orientation of the convergence lines.The formation of second and third convective rainbands was intimately associated with the cold outflows of the previous convective rainband.The analysis of simulations of convective rainbands reveals that the three convective rainbands extended to the high altitudes.The strong convergence and upward motions led to the development of the convective rainbands.The convection dissipated in the low-levels as a result of the subsidence due to precipitation.The cloud microphysical processes between the three convective rainbands are different.Although the precipitation in the three convective rainbands was produced by the melting of graupel and snow in warm clouds,and the collection of cloud water by rain,the ice-crystal effects in cold cloud of the second and third convective rainbands provided important supports for the production of torrential rainfall.The torrential rainfall away from typhoon has the characteristics of cold type rainfall,while the torrential rainfall associated with the landfall of the typhoon may have the characteristics of both warm type rainfall and cold type rainfall,while the severe convective rainbands associated with the typhoon may have a high precipitation efficiency due to the ice-crystal effects. |
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