Study On Summerrainstorm Of Asymptotic Linear Frontogenesis Convergence Line In Fujian Coastal Area

Statistical analysis and circulation situation analysis of heavy rain(rainfall > 50mm/24h)in Fujian coastal area from August 2012 to August 2017 reveal that there is an asymptotic linear frontogenesis convergence line system,which extends several hundred kilometers horizontally and has a vertical thickness of less than 700 hPa.It can be divided into zonal and meridional types.The zonal airflow converges inland from sea and has a cold frontogenesis accompied;the meridional airflow converges from south to north.There is warm front frontogenesis accompanied.According to statistics on non-typhoon rainstorm over Fujian during the past six years,the asymptotic linear frontogenesis convergence line rainstorms reaching to 33.5% in total samples.Therefore,the asymptotic linear frontogenesis convergence line is a relatively important low-level key system of summer rainstorm in Fujian,which has been never discussed before.A objective method is built in the research,which can recognize objectively the orientation and location of the convergence line and the relative position of the rainstorm that usually occurs at cyclonic shear area of the left side of the axis of the asymptotic linear frontogenesis convergence line.The zonal type is formed by the convergence of Southeasterly and Easterly airflows.The convergence area is often located in the Minjiang River Valley in the middle of Fujian coastline.Meridian type: formed by the convergence of southwesterly and southerly airflow,the convergence area is often located in the Wuyishan area of Northwest Fujian.The locations of the two types of asymptotic linear frontogenesis convergence lines rainstorms are corresponded to two of the four torrential rain centers in Fujian for many years.Diagnostic analysis of two types of asymptotic linear frontogenesis convergence lines show that in the 850 hPa level and below,zonal type: mainly caused by easterly airflow at the south side of the subtropical high through around from both the north and south ends of Taiwan Island,then forms the convergence at west of the island and lands in the central coast of Fujian,the cold front zone from of the sea to the coast with the asymptotic linear convergence line landing,meanwhile the river valley at middle coast enhances furtherlythe convergence effect of flow and front zone.Meridional type: the southerly airflow from the South China Sea mainly convergces along the Fujian Longitudinal Mountains,and pushs the southern warm air mass northward,forming a warm front zone accompanied it.Both types of asymptotic linear frontogenesisconvergence lines cause the superposition of three effects: air flow convergence and riseup,thermal lift of the frontal zone and topograph forcing lifting.So that the kind of systems supportthe enhancement and maintenance of strong vertical upward motion,unstable stratification and convective unstable,and water vapor transport from sea,they become one kind of key systems of heavy rainfall in summer Fujian.WRF numerical experiments were carried out to study the mechanism of thermal and topographic effects on the asymptotic linear frontogenesis convergence line and its rainstorm.(1)The sensitivity test of non-adiabatic heating shows that if latent heating is ignored,the sensible heating caused by temperatureadvection will be more significant,and the horizontal gradient of frontal zone is enhanced.If sensible heat and latent heat are both ignored,the regional thermal instability will be obviousweakened.Therefore,the effect of latent heat non-adiabatic heating is to enhance the thermal instability of two types of asymptotic linear frontogenesis convergence lines,including to enhance the temperature stratification instability of zonal convergence lines,and to enhance the convective instability of meridional convergence lines,such instability will cause the enhancement of vertical motion and rainstorm.(2)Topographic sensitivity tests show that topographic forcing is an important mechanism for the formation of asymptotic linear frontogenesis convergence lines: for example,Taiwan Island separates upstream and converges downstream in the easterly on the southern side of the subtropical high,and furtherly the Minjiang River estuary at middle of the coast induces the convergence line and cold air mass from sea to go through the estuarythen forms the zonal asymptotic linear convergence lines;Secondly Fujian's longitudinal mountains,especially Wuyi mountains and its Eastern valleys,force to form the meridional asymptotic linear frontogenesis convergence lines.The mountain blockade enhances the cold front gradient and the warm front gradient,and enhances the thermal intensity of the asymptotic linear frontogenesis convergence line.In addition,mountain blockade also has uplifting effect,which forms a triple uplift superposition effect with convergence and uplift of asymptotic linear airflow and thermal uplift of frontal zone,which is beneficial to the intensification and maintenance of asymptotic linear frontogenesis convergence line rainstorm.