凝结潜热释放和地表热通量对一次飑线过程的影响

Effects of latent heating and surface heat fluxes on a squall line process

针对2013年7月4—5日一次产生大风、短时强降水强对流天气的飑线过程进行数值试验,研究了凝结潜热释放和地表热通量在中尺度对流系统的发生发展过程中的影响。结果表明:(1)凝结潜热释放对飑线系统有重要的作用,对飑线发展维持、移动及成熟阶段的垂直结构都有着一定影响。(2)当飑线系统进入成熟阶段后,小尺度的积云对流中的凝结加热作用于中高层大气,加强了高层辐散低层辐合的配置,而对流系统内垂直上升运动的加强又导致凝结加热作用更强。(3)凝结加热作用极大地促进了飑线的低层入流、高层出流的垂直结构,以及系统前方中层入流和高层出流之间形成的间接垂直反环流,促进高空动能下传的同时,使系统前方对流不稳定性增加,新的对流单体易于触发。(4)潜热释放间接增强了对流内部的冷性下沉气流,进而加强了低层的阵风锋,使得新的对流单体能在飑锋处触发,飑线以新老对流单体交替的方式向前移动。(5)在高低空急流的垂直耦合促进飑线发展的同时,成熟阶段飑线中较强的凝结加热对高低层急流有反馈作用,一定程度上增强了这种垂直耦合。(6)地表感热、潜热通量等边界层非绝热过程在对流系统的触发和发展中起到了较为重要的作用。地表热通量在白天加强了对流边界层的湍流混合作用,由此产生的特征维持到了夜间,形成了适合飑线触发的层结条件。另外,在对流形成之前,与地表潜热通量相关的边界层加湿作用为对流的爆发贮存了丰富的水汽和不稳定能量。

A squall line system causing gale and short-term heavy rainfall during 4-5 July, 2013 was numerically simulated to analyze the effects of latent heating and surface heat fluxes on mesoscale convective system. Results reveal that： （1） Latent heat releasing plays an important role in squall line＇s development, movement and vertical structure at mature stage. （2） At mature stage, the heating effect on the middle and high level atmosphere by condensation latent heating within cumulus convection intensifies the low-level convergence and high-level divergence, and then the powerful updraft flow in convective system results in stronger latent heating. （3） Condensation latent heating significantly contributes to the low-level inflow and high-level outflow of squall line system. Indirect vertical circulation leads to the en- hancement of spreading downward of high-level momentum. （4） The cold down＆aft is strengthened by condensation latent heating, which enhances the gust front, and makes new convective cells trigger in front of the squall line system. This system keeps moving forward with dissipation of old cells and continu- ous generation of new cells. （5） While the vertical coupling structure of high-level and low-level jets promotes the development of squall line, the stronger condensation heating effect of system at mature stage provides a feedback effect on enhancing this vertical coupling to a certain extent. （6） By affecting the antecedent environmental circulation, the surface heating fluxes also play important roles in triggering and maintaining squall line. Firstly, the surface heat flux in the daytime enhances the convective bounda- ry layer turbulent mixing, which gradually develops an unstable stratification that maintains until mid- night. Secondly, the boundary layer humidification effect related with surface latent heating flux stores a- bundant vapor and unstable energy for system＇s outburst.