Merger-type Sea-breeze Front And Associated Convection Initiation Mechanism In Bohai Bay Region

Sea-breeze front(SBF)is one of the boundary layer systems that is closely related to the severe convective weather in coastal region.The merger-type SBF can more easily initiate new convective systems,or intensify the original convective systems merged with the SBFs.Bohai Bay is one of the regions where SBF frequently occurs in China.Examination of merger-type SBF in the Bohai Bay region can help better understand the genesis mechanisms of severe convective weather and improve the ability of disaster prevention and mitigation.This work studies the statistical characteristics of merger-type SBF in the Bohai Bay region.The associated atypical occlusion process and convection initiation(CI)mechanism are also investigated.Using the Tianjin operational Doppler radar and automated ground-based weather station in the Bohai Bay region from May to September during 2009-2015,131 cases of merger-type SBFs were discovered,among which 85(46)cases are merged with GFs(convective systems).The mergers mainly occur on the northern and western sides of Tianjin radar,in particular within 20-30 km of two cities(Tianjin and Tangshan).The merged SBFs and GFs have horizontal scales of about 50-300 km,while more than half of the convective systems are less than 50 km.The occurrence of merger-type SBFs shows a significant intra-annual variability,with 26 cases in 2009 and 13 cases in 2011,while there i s 18.7 cases per year on average.More than 90%of merger-type SBFs occur between June and August,especially in July.Moreover,about 95%of the merger-type SBFs take place during 1200-1900 local standard time.Convection initiation/intensification is found in about 85%of the merger-type SBFs after the merging process.CI prior to the merger occurs in 39%of the cases merged with GFs,which is increased to 75%when the horizontal scales of the SBF and GF are both larger than 150 km.It also shows that 88.2%of the cases are in the form of two fronts moving towards each other.A numerical simulation of a merger process of SBF and GF occurred on 14 July 2011(hereafter,110714 case)is conducted using the Weather Research and Forecasting(WRF)model with a high horizontal resolution(1.333 km).The results show that an atypical occlusion process occurs in this merger case,which differs significantly from the traditional,synoptic-scale occlusion process within extratropical cyclones.This atypical occlusion process is caused by the merger of two cold-type mesoscale fronts.One of the two fronts is the gust front of convective storms,while the other is a sea-breeze front.As the two fronts move towards each other,the warm air between them is squeezed and separated from the surface.An atypical occluded front is formed when the two fronts merge,with the warm air forced aloft.This kind of occlusion is termed as "merger" process,different from the well-known "catch-up"and "wrap-up" processes.Moreover,local convection is enhanced during the merger process,with severe convective weather produced in the merger area.CI prior to the merger of the SFB and GF in the 110714 case is studied using WRF simulation with a higher horizontal resolution of 444.4 m.The results show that,CI is generated between the two mesoscale fronts even though they are still about 25-30 km far away from each other.During the development of convective cells,the low-level convergence and conditional instability averaged within the intermediate area between the two fronts are enhanced significantly,both of which favor the initiation of convection.Vertical momentum budgets are conducted in the intermediate area as well as along the backward trajectories of representative parcels within a selected convective cell.The vertical acceleration is decomposed into dynamic and buoyant components,respectively.The diagnostic results show that the dynamic acceleration dominates in the lower boundary layer,which is one order larger than its buoyant counterpart.Therefore the dynamic forcing appears to be more relevant to CI.Based on anelastic approximation,the dynamic acceleration is further decomposed into four terms including extension,horizontal curving,vertical twisting and a combination of vertical velocity and vertical variation of density.The positive dynamic acceleration is mainly caused by the extension term.Consequently,CI prior to the merger of the SBF and GF is mainly attributed to the dynamical lifting effect in association with the low-level horizontal convergence of the converging two meso-scale fronts.