THE CHARACTERISTICS AND NUMERICAL SIMULATION OF DIURNAL VARIATIONS OF LOW-LEVEL JET AND MEIYU FRONT HEAVY RAINFALL
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摘要: 利用地面加密自动站逐小时观测资料和ERA-Interim再分析资料，分析了2011年6月江淮流域的5次强降水过程和西南低空急流的日变化特征。发现强降水的日变化与西南低空急流的日变化一致：02—08时增强，14时减弱。这主要是由于夜间边界层内的惯性振荡，导致西南低空急流增强从而使得梅雨锋水汽通量辐合增强，降水增强；而白天由于边界层混合摩擦力增大，致使西南低空急流减弱或消失，降水减弱。WRF数值模拟试验不仅重现了观测的日变化特征，而且证实了江淮暴雨和西南低空急流的日变化主要是由非地转风的日变化造成：白天边界层混合强，风为次地转；而夜间边界层混合消失，气压梯度力和科氏力平衡的惯性振荡使得风为超地转Abstract: This study investigates the characteristics of diurnal variations of southwesterly low-level jet (LLJ) and five heavy rainfall events over Yangtze-Huaihe River basin during June 2011 by analyzing the ERA-interim reanalysis data and surface automatic weather stations data. It is found that the southwesterly LLJ and the precipitation have the same evident diurnal variations in the five processes, which is strong in nocturnal-to-early morning hours (0200—0800 BST) and weak in the afternoon (1400 BST) hours. It is mainly attributed to inertial oscillation at nighttime and vertical mixing in the boundary layer during daytime. The enhancement of the southwesterly LLJ leads to the enhancement of the Meiyu front water vapor convergence, which contributes to the enhancement of rainfall at nighttime. Then with the reduction or disappearance of the southwesterly LLJ, the rainfall also weakens. The WRF numerical simulation successfully reproduces the observed evolution of precipitation and the southwesterly LLJ, and it is proved that the diurnal variations of the LLJ and rainfall are mainly attributed to the diurnal variations of ageostrophic winds. With the enhancement of the vertical mixing in the boundary layer during daytime, the winds become subgeostrophic. With the disappearance of the vertical mixing at nighttime, inertial oscillation under the balance of the pressure gradient force and Carioles force turn the winds into supergeotrophic winds
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图 1 2011年6月江淮流域5次降水过程的平均24小时降水分布（彩色填色，单位：mm，灰色填色为地形高度，单位：km，下同）（a~e）以及这5次降水过程达到暴雨和大暴雨的站点数统计（f）
a. 4日02时—5日02时；b. 5日08时—6日20时；c. 9日02时—10日20时；d. 13日20时—15日20时；e. 17日02时—19日08时。
图 2 2011年6月江淮流域的5次降水过程的区域（110~122 °E，26~35 °N）平均小时降水量分布
单位：mm。黑圆圈中的数字表示第几次降水过程，两条虚黑线之间表示一次降水过程。
图 3 2011年6月江淮流域的5次降水过程的850hPa等风速线（红色实线，单位：m/s）和地面6小时累积降水（以图左上角所标时间为中心的前、后3小时累积降水，彩色阴影，单位：mm）
图左上角标注的ddhh表示dd日hh时，下同。
图 4 图 3中黑色长方形框内（水平风速≥12 m/s）的前12小时（前一天20时，灰实线）、前6小时（02时，灰虚线）、当前时刻（08时，黑实线）及后6小时（14时，黑虚线）的区域平均水平风速垂直廓线
图 7 WRF模拟区域的地形高度（阴影，黑色框表示第2重嵌套区域的边界，a）和控制试验（b）、敏感试验（c）模拟的2011年6月14日02时的到达地表向下太阳短波辐射通量单位：w/m2。
图 8 2011年6月13日20时—15日20时的区域（110~122 °E，26~35 °N）平均的观测（黑实线）、控制试验（黑虚线）和敏感试验（灰实线）的逐小时降水单位：mm。
图 9 2011年6月13日20时—15日20时的区域（110~122 °E，26~35 °N）平均的观测（OBS）、控制试验（CTL）和敏感试验（SEN）的风速的垂直时间分布
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