| Based on the statistical analysis of heavy precipitation in Fujian Province during winter,this paper focuses on the heavy precipitation in the coastal complex mountain environment under the invasion of cold and dry air from the north during winter monsoon.The daily precipitation of 50mm-200 mm can still occur in winter Fujian Province.The winter precipitation data of Fujian Province in December,January and February of 2011-2016 from ECMWF(the European Centre for Medium-Range Weather Forecasts)were classified by EOF,REOF and North test.According to the location of the rainstorm center issued by Fujian Meteorological Service Center,the first and fourth modes in Fujian inland were selected.Sample compositive analysis was carried out for the two modes based on time series respectively.The first mode synthetic precipitation center was located in the northwest of Fujian Province,that is,the boundary between Fujian Province and Jiangxi Province.The fourth mode precipitation center is located in the South and southeast of Fujian Province,this is basically consistent with the Fujian Rainstorm Centers by statistics over 30 years.Through the synthetic analysis of the circulation and temperature fields,it is revealed that at the 1000 hPa level above the rainstorm center of different modes,there are the asymptotic linear convergent airflows from north to south,and companied by the the temperature front zone.their ranges are over hundreds kilometers and get to synopitic zise.The heavy rainfall is located at the cyclonic wind shear on the left side of the convergence axis.This is one of key systems of winter heavy precipitation that is never studied before.We name it as asymptotic linear frontogenesis convergence line.The convergence lines of the two modes can be identified by objective method and they showed that it is a relatively shallow low-level precipitation key system.To establish the coordinate system for the asymptotic linear frontogenesis convergence line,then the thermodynamic and dynamic structures of these two asymptotic frontogenesis convergence lines can be analyzed.In the winter it is cold,dry and rainless,and the northerly wind prevails,the asymptotic linear convergence line forms a strong convergence rising zone in the lower layer of the rainy area,the temperature front zone crosses with the asymptotic linear convergence airflow,forming a frontogenesis environment,the superposition of the uplifts of the frontal thermal structure and the dynamic structure of asymptotic linear convergence wind increase the rainy area ascending motion.The convergence line also collects the water vapor from the adjacent Taiwan Strait and the East China Sea to the heavy precipitation area,and meantiome cooperates with the southerly of subtropical high edge at the mid-upper level forming a relatively deep water vapor transport layer.A deep thermal convection instability is formed by diabatic heating,and the wet dynamic instability is formed by the dry air mass wedgeing into the wet air mass following the convergence airflow.The Pseudo-equivalent potential temperature decreases with the height increases over the convergence line,forming a convective unstable layer with warm,wet at upper and cold-dry at lower level.Therefore,the asymptotic linear frontogenesis convergence line has become one of the important systems of winter heavy precipitation in Fujian Province,it can affect the intensity and location of rainfall.Because the system is located in the low level and shallow itself,it is vulnerable to the mountain topography in the province.Using the verification test and sensitivity test of WRF mode,the influence mechanisms of topography and thermal environment on the asymptotic linear convergence line and the accompanying process of strong precipitation occurrence are investigated.The results show that the latent heat of condensation can affect the convergence position and strength of the convergence line,the location and strength of the frontal zone,and thus affect the activity of the system.The middle layer is heated by latent heat to suppress the cold advection into the heat air mass,and maintain the thermal instability and precipitation intensity in the rainfall area.The asymptotic linear frontogenesis convergence line is conducive to the occurrence of local heavy precipitation in the large-scale precipitation in winter in Fujian,and the latent heat release of condensate provides an important contribution.The experimental simulation of the impact mechanism of the underlying mountains shows that,the height of the Wuyishan is reduced to 50 meters for the first asymptotic linear frontogenesis convergence line in the northern Wuyi Mountain,which affects the position and orientation of the asymptotic linear frontogenesis convergence line.The convergence line of the original northeast-southwest was turned into a northwest-southeast when the absence of Wuyishan,and the frontal zone is also weakened.For the second type of asymptotic linear frontogenetic convergence line located in southern Daimao Mountains,if the height of Daimao Mountains and Boping Mountains is reduced to 50 meters,the convergence line will turn its orientation and move to the coastline when the obstruction of Daimao Mountains and Boping Mountains are deleted.The same as the first mountain area,the front zone is weaker and the frontogenesis conditions disappear without the mountains.Therefore,the mountain plays an important role in the process of enhancing the frontogenesis conditions and the asymptotic linear convergence line system,furthermore which forms the superposition of terrain uplift,frontal elevation and convergence line dynamic uplift.Based on the above results,the asymptotic linear frontogenesis convergence line superimposes frontogenesis dynamic process,environmental thermal conditions,terrain triggering effect and system induction to water vapor transport,which effectively supports the occurrence and maintenance of heavy precipitation in Fujian Province in winter. |
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