2002年7月20-25日揭示的热带水汽羽和暴雨的关系

THE RELATIONSHIP BETWEEN TROPICAL WATER VAPOR PLUME AND HEAVY RAINFALL DURING 20-25 JULY 2002

利用GMS-5水汽图像和NCEP/NCAR 1°×1°再分析资料,分析了2002年7月20～25日梅雨暴雨过程中热带水汽羽的变化及其与物理量场的配置.结果表明热带水汽羽和暴雨之间存在密切联系,(1)有一条热带水汽羽始终和暴雨相伴,其走向从孟加拉湾向东北方向延伸到朝鲜半岛,热带水汽羽不单体现了水汽在对流层中、高层的平流,实际上还反映了对流层整层深厚的水汽沿着水汽通道向北输送.其中,低空急流对水汽涌的输送起到了积极的作用,每一次水汽向东北方向涌动时,其东南侧都伴有低空急流,并且急流核跟随水汽涌一起移动.中尺度对流云团在急流的左前方生成和发展,它们也跟随水汽涌一起移动.(2)热带水汽羽的北部边界大致与高空急流轴平行,暴雨云团一般出现在西南风高空急流入口区的右后方,距离急流轴约3个纬距的地方.高空急流的存在为MCS提供了很好的质量外流途径,即辐散机制,有利于MCS的发展.(3)在暴雨过程期间,热带水汽羽内维持有一条θse≥350 K的脊轴,走向和热带水汽羽平行.低空θse脊轴不单指示了低空高能量的位置,其上或附近也最有可能存在明显不稳定的区域,因此也是暴雨容易出现的地方.另外,在热带水汽羽中也维持着一条窄而强的正涡度带,位置和走向均和低空θse脊轴相吻合,体现了低层的动力抬升机制,正涡度中心基本和MCS相对应.

Basically, the low level moist condition is regarded as one of the most important prerequisites to heavy-rain forecast, so it is very often to ignore the upper level moist condition, which is depicted clearly on satellite water vapor imagery. Although many studies have declared some relationship between the heavy rainfall and the water vapor plume （WVP）, however how and why the upper level moist condition contributes to heavy rainfall are still unclear. The relationship between the tropical water vapor plume and heavy rainfall during 20 - 25 July, 2002 was analyzed using GMS-5 infrared black body temperature （TBB） data as well as NCEP/NCAR 1°× 1° reanalysis data. Results demonstrate a close relationship between WVP and heavy rainfall as follows. In the storm period, （1）a remarkable tropical WVP stretches northeastward from the Bay of Bengal to the Korea Peninsula, and moisture clusters were found surging one after another from lower to higher latitudes in the WVP. The moist-surges were not only depicted as the horizontal advection in the middle and upper levels but also meant the whole levels＇ moisture transportation in the moist channel, since each time the low level jet （LLJ） was found accompanying to the southeast of the moist-surge when it burst northeastward, and the wind core also moves with the moist surge. （2）the north edge of the WVP was ahnost parallel to the upper level jet （ULJ） axis, and the meso-scale convective clusters （MCS） developed in the right back side of the southwesterly ULJ, within 3 latitudes away from the ULJ axis. The existing ULJ provided well outflows of air in the upper levels （divergence mechanism） which obviously benefit to MCS evolution. （3）A high θse tongue （θse≥350 K） maintains within and in parallel to the direction of the WVP during the heavy rain period. The low level θse tongue indicated the area of high energy accumulation, above and around which was the most possible unstable area, therefore the heavy rainfall easily occurs. （4）Similar to the θse distribution in the WVP, a narrow and strong 850 hPa positive vorticity belt stayed in the WVP, which indicated the lifting mechanism in the low levels, and the cores of the positive vorticity correspond to MCS development.