超级单体向平流层输送水汽机制的数值模拟

Modeling of the mechanism of water vapor transportation into the stratosphere by a supercell

为了揭示深对流云直接向平流层输送水汽的物理机制,利用WRF中尺度模式的理想个例运行方式对CCOPE(Cooperative Convective Precipitation Experiment)试验期间的一次超级单体进行了数值模拟。选用Thompson云微物理过程方案设置一系列初始云滴数浓度(N_c)进行模拟试验后发现,N_c=175 cm^(-3)情形下模拟云的最大垂直风速与实测结果最为接近,并且模拟出了超级单体。因此,本文利用该模拟结果分析了超级单体向平流层输送水汽的机制。1 min一次的输出结果表明:冻干脱水机制与本次所模拟出的平流层加湿没有直接的关系,超级单体向平流层输送水汽的主要机制可能为湍流输送机制,而升华加湿机制的作用很小。这是由于超级单体云上部的冰晶大部分被消耗而形成雪,因此被输送到平流层的主要是雪这种落速较大粒子,这种粒子不易被向上输送但又容易降落,因此升华所形成的水汽量相比湍流输送的水汽量小很多。湍流造成的水汽输送通量密度的量级约为10^(-9)kg·m^2·s^(-1)。

The water vapor in the lower stratosphere may play an important role in atmospheric radiative processes and chemistry.One source of water vapor in the lower stratosphere is the transport of water vapor from the troposphere via deep convection penetration.The aim of this study was to uncover the physical mechanism for transporting water vapor into the lower stratosphere by deep convection penetration.During the development of a supercell in Montana(US),it was observed that the CCOPE(Cooperative Convective Precipitation Experiment)could be simulated using a WRF-SUPERCELL model performing under idealized conditions,and initialized with soundings obtained from the CCOPE.Simulation tests were conducted,in which a Thompson bulk microphysics scheme with a series of initial cloud droplet number concentration(N_c)was set.The results showed that the test in which N_c=175 cm^(-3)exhibited the closest maximum in-cloud updraft to the observations.On the other hand,the simulation results where N_c=175 cm^(-3)displayed characteristics which were similar in structure to a supercell,such as the bounded weak echo,overhang echo,and mesocyclone.Also,the supercell’s penetration into the lower stratosphere could consequently be used to analyze the physical mechanism of the water vapor transportation into the stratosphere via a supercell penetration.It was determined that the dehydrating effect of the overshooting could be effectively simulated.However,dehydration did not occur on the tropopause and lower stratosphere.This was due to the fact that the supercell consumed so much water vapor in the levels of the lower and middle troposphere that there was not enough remaining water vapor to cause the air to become saturated.This led to no vapor being available to be consumed during the deposition process when the air reached the tropopause and lower stratosphere.Therefore,the freezing dehydration mechanism was found to have no relationship with the water vapor changes in the stratosphere in this modeling process.Furthermore,due to the fact that the ice particle sublimation mass(rate)was much less than the mass(rate)of water vapor change in the lower stratosphere,the sublimation hydration mechanism was determined to be minor.The reason for these finding was that the majority of the ice crystals had been converted into snowflakes in the upper portion of the supercell.Therefore,the majority of the ice particles which had been transported into the stratosphere were in the form of snowflakes.Since snowflakes have greater terminal velocity than ice crystals,the snowflakes ascended to the stratosphere from the troposphere with more difficulty than the ice crystals.Also,the snowflakes tended to descend faster than the ice crystals,which led to fewer ice particles,as well shorter existing times of the ice particles in the lower stratosphere.As a result,the subsequent sublimation processes in the lower stratosphere generated much less water vapor.Nevertheless,there was an obvious stratosphere-troposphere exchange of water vapor observed in the region where the translational kinetic energy was larger than in the surrounding area.On the other hand,the gravity wave break was not observed in the one-minute interval output,and therefore the possibility existed of no transportation of water vapor into the lower stratosphere.It was also considered to be possible that the turbulent transportation mechanism played a primary role in the transportation of the water vapor into the stratosphere via the supercell,and the magnitude of the turbulent transportation water vapor flux density was determined to be approximately 10^(-9)kg·m^(-2)·s^(-1) in this simulation.