| Using Shanghai observational network, the development of the “0731” localsevere convection case was analyzed and discussed, then conceptual model of theearly-afternoon summer convections in Shanghai was put forward. By rapidupdated cycling forecasting experiments, the rationality of the conceptual modelwas validated. The future studied the role of UHI, sea breeze in the initiation anddevelopment of the local severe convection case. By experiments, the effects of allkinds of observation data in experiments were verified. Finally, using the samemethods, the “0813”local severe convection case was studied, and the rationalityof the research results and technical route was validated. The main conclusions canbe drawn as follows:1) With the development of urbanization, the zone of UHI in Shanghai isexpanding. the original UHI is in the urban area, then expanded to the suburbancounty, formed a more large UHI. The frequency of UHI is highest in theafternoon(14Z), and the intensity of UHI is greatest, these may be the causes oflocal severe convection cases. The frequency of severe convection in Shanghaisummer was increasing, obvious increase was appeared after2001. The severeconvections mostly occurred in downtown area and regions along the rivers orlakes.2) The peculiar geography of Shanghai led to the break out of the sea breezeand the lake breeze. When the intensity is greater, the surface convergence line indowntown area was formed due to the sea breeze effects, which cause the localconvection initialization and development. While the lake breeze can cause thesevere convection under the background of the UHI circulation. 3) Rapid updated cycling forecasting system successfully reveals the severeconvection case, which occurred in Shanghai on July312011. Through thediagnostic analysis of rainfall, surface wind, vertical circulation and verticalvelocity field and so on, the results revealed the mechanisms of the triggering anddeveloping. It was also found that due to the thermal difference between landand sea, and the peculiar geography of Shanghai, two sea breeze from differentnorth and south directions met in shanghai area. At the same time low levelconvergence line formed, as a results, weak updraft in boundary layer appeared,combined with the local urban heat island effect, created favorable conditions fortriggering the server convections; and the high gradient of moisture between lowlevel and middle level and its unstable vertical structure provided favorableconditions for moisture vertical transport. The coupling of lake breeze on the westside and the environment west wind, caused strong low level convergence line andstrong upward motion, which result the heavy rainfall.4)The simulation of sensitivity experiments show that the sea breeze has impact on genesis and development of server convection weather. In underlyingsurface test which using water body as the surface in Shanghai, sea breezeextended to Shanghai area, as the result the low level convergence line and theupward motion is more westward, and so heavy rainfall occurred westly,. All ofthese can restrain the unstable energy, and impede severe convection development.In diurnal variation test, the warming of the land surface temperature issignificantly slower than CTRL test, the steadiness of the atmosphere is notbeneficial to accumulate energy. And so the turbulent motion and the severeconvection are not beneficial. In addition, the change of surface temperature leadto the change of the location of low level convergence and the vertical circulation,and result that the change of the intensity and location of rainfall obviously.5) The sensitivity experiment was performed to investigate the effects ofobservation data. There are some conclusions as below.The initial radar data assimilation is able to significantly improve theprecipitation location and intensity prediction. Cloud analysis scheme that utilizesradar observations adjusted the initial state of the model on water substance, suchas cloud water mixing ratio, cloud ice mixing ratio, rain water mixing ratio, icemixing ratio, snow mixing ratio, graupel mixing ratio, et al. And lower cloudamount increased, false high clouds were removed. Cloud ice over was orientalsports center removed to improve distribution of cloud. It is found that cloudanalysis adjusted structure and intensity of a mesoscale convection system directly and the precipitation was a close relation to distribution of cloud water substance.The results show that the simulation results whose height fields and windfields at500hPa,700hPa and850hPa assimilating the radiosonde data are moreclose to the environment than the ones which not. This, region and strength of adrop in temperature have changed. However, the radiosonde data used once insimulation, it affected forecasting.The simulation of assimilated AMDAR data show that, without assimilatingAMDAR data, the intrusion of north wind accompanied by cold advection morequicker than actual weather and rainfall center locating by north at16:00~17:00.Location and strength of upper trough at500hPa and850hPa assimilating theradiosonde data are more close to the environment than the ones which not. Mostimportantly, every time can get AMDAR data, it supply and improve soundingsdata.The simulation results of assimilated surface meteorological observation datashow that temperature fields, humidity fields and wind fields were adjusted atsurface boundary layer, and an obvious heat island and breeze were morereasonable. Moreover, precipitation location affected by surface wind convergenceline which influenced distribution of temperature gradient. Surface observationdata was indispensable and important data in rapid refresh system.6) Severe convection weather on August132013can reproduce by using thesame way. Compare actual weather, the simulation precipitation location andprecipitation time is close to the real case, started and stopped of precipitationadvance in time, and range of precipitation is more wider than actual. Moreover,spatial-temporal distribution of PBL (planetary boundary layer) mesoscaleconvergence line and breeze, variation characteristics of scale vertical circulationwere similar to spatial-temporal distribution of server convection weather. Againthis weather case proved that the occurrence and development of the severeconvection weather in Shanghai were closely related to urban heat island effectand breeze. The direct reason for formation and strengthen of breeze wasdifference in heating between the land and sea. The southeasterly andnortheasterly boundary layer jet stream brought abundant plentiful moisture, itstrengthened the mesoscale convergence near surface and the development of localconvection weather. |
PDF Full Text Request