强垂直风切变环境下对流单体对飓风强度的影响

THE EFFECTS OF CONVECTIVE CELLS ON HURRICANE'S INTENSITY IN HIGHLY-SHEARED WIND ENVIRONMENTS

利用高分辨率模式输出资料,诊断分析强垂直风切变环境下飓风Bonnie(1998)中风暴相对螺旋度的分布特征,再现了Molinari等(2008)利用下投式探空仪获得的该飓风内部风暴相对螺旋度的离散观测结果。通过对比不同垂直风切变环境下,不同区域风暴的相对螺旋度、对流有效位能及风速的水平分布,揭示出与高值风暴相对螺旋度相联系的强对流单体的分布与环境垂直风切变的密切联系。基于风暴相对螺旋度和对流有效位能的配置分析,研究强环境垂直风切变时段,眼壁附近的深厚涡旋对流以及螺旋雨带中的小型对流单体的三维结构和演变特征。分析表明,环境垂直风切变较强时,在眼壁附近的顺切变区存在典型的深厚涡旋对流系统,这类深厚涡旋系统能够激发二级垂直环流,有利于旋转上升运动的维持,并在近眼心区域引发补偿性的干暖下沉气流,有助于飓风暖心的维持和加强;同时,螺旋雨带中也存在以涡度为特征的小型对流单体,这些对流单体随着平流不断移入飓风中心,使得飓风中心垂直涡度增加,最终导致飓风强度的增强。

The high-resolution simulation data of Hurricane Bonnie （1998） is used to study the distribution of storm relative environmental helicity （SREH） in highly-sheared wind environments, which matches well with the discrete observations by Molinari and Vollaro （2008） with dropsondes. By analyzing the distribution of SREH, convective available potential energy （CAPE） and horizontal winds under different environmental vertical wind shear （VWS） in different areas, we found that the distribution of convective cells associated with high SREH is in close contact with environmental VWS. The three-dimensional structure and evolution of the deep vortical convections in the eyewall and small convective cells in spiral rain bands are also analyzed in the highly-sheared wind environmental based on the configuration of SREH and CAPE. It is indicated that typical vortical convective cells mainly exist in the downshear quadrant near the eyewall when the environmental VWS is strong. These convective cells may generate secondary vertical circulation, which is in favor of maintaining the vortical strong updrafts, and produce the compensated downdrafts near the hurricane eye, which strengthen the warm core of Bonnie. Meanwhile, the small convective cells indicated by vorticity couples in spiral rain bands move into the eye with the advection and increase the vorticity of the vortex and enhance the intensity of the hurricane.