Characteristics of a Plateau Vortex Precipitation Event on 14 July 2014
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摘要: 利用第三次青藏高原大气科学试验的多种雷达、雨滴谱仪以及MODIS卫星观测资料、常规气象站地面和高空观测资料,针对2014年7月14日发生在青藏高原中部那曲地区的一次降水过程,研究了降水的时空变化特征,触发不同阶段降水的天气尺度和中尺度环流系统以及相关的云降水物理特征。从降水演变特征看,这次降水过程包括3个阶段,即发生在下午的强降水阶段和夜间的两个弱降水阶段。从影响系统看,下午的降水主要由天气尺度的高原低涡发展引起,此时那曲位于低涡中心前部的中尺度辐合线上;发生在晚上的降水主要与高原低涡前部的暖湿东南气流爬越地形有关,东南气流为产生降水提供了有利的水汽、大气不稳定和浅薄的动力抬升条件。从云降水微物理特征看,高原低涡降水初期,低涡前部的上升运动深厚,对流发展明显,而后期的对流性减弱。东南气流爬坡引起的地形降水表现出层状云降水的特征,高原低涡降水的雨滴谱分布较宽(0.3~4.9 mm),而夜间降水过程的雨滴谱分布较窄(0.3~2.1 mm)。Abstract: Using various radar and disdrometer datasets from the Third Tibetan Plateau Atmospheric Science Experiment, MODIS data, surface and sounding datasets, temporal and spatial variations of one rainfall event on 14 July 2014 over Naqu in the central Tibetan Plateau is analyzed, the synoptic and meso-scale atmospheric circulations, and associated cloud-rainfall microphysical characteristics are also investigated. This rainfall process includes three stages, namely, the first stage with heavy rainfall starts in the afternoon (1400 BT) and ends at 1800 BT 14 July, the peak intensity of hourly precipitation occurs during 1500-1600 BT, which reaches 2.1 mm·h-1. The second stage begins at 1900 BT, and the precipitation intensity weakens prominently compared with the first stage. The third stage is from 2200 BT 14 July to 0100 BT 15 July with a weaker precipitation intensity.Rainfall during the first stage is mainly produced by the development of a synoptic-scale plateau vortex and the formation of a meso-scale convergence line in front of the vortex circulation center. The radar echo propagates northeastward, and this stage ends with the weakening of the plateau vortex. Rainfall at night is mainly associated with the warm and moist southeasterly flow passing over the topography near Naqu, which provides favorable conditions of the atmospheric moisture, instability, and shallow dynamic elevation. With the intrusion of the low-level northeasterly flow, the radar echo generally propagates southeastward. Moreover, during the earlier stage of the first rainfall stage, the ascending motion is deep over the east of the vortex, exceeding 3 m·s-1 between 3 km and 11 km above the ground level, which indicates the remarkable development of convections. During the later stage, rainfall is mainly produced by stratiform clouds, with a higher cloud top. Rainfall at night is mainly caused by stratiform clouds. The raindrop size distribution is wider (0.3-4.9 mm) than that size of 0.3-2.1 mm in the topographic rainfall, and the wider raindrop spectrum is closely associated with the larger rainfall rate.
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图 2 2014年7月14日14:00—15日01:00高原上降水量的分布和那曲降水随时间的演变
(a)自动气象站11 h累积降水量,(b)那曲地面自动气象站、雨滴谱仪A和雨滴谱仪B小时降水量,(c)雨滴谱仪B 10 min累积降水量时间变化曲线
Fig. 2 Distribution of accumulate precipitation over the Tibetan Plateau and time series of precipitation at Naqu from 1400 BT 14 Jul to 0100 BT 15 Jul in 2014
(a)accumulated rainfall of rain gauge stations, (b)hourly accumulated rainfall by rain gauge, distrometer A and distrometer B at Naqu, (c)time series of 10 min accumulated rainfall by distrometer B
图 3 2014年7月14日那曲C波段双线偏振雷达2.4°仰角回波强度
(黑色正方形表示雷达位置,黑色三角形代表那曲位置,黑色实线圆表示20 km,40 km和60 km的探测范围)
Fig. 3 C-band mobile polarization radar reflectivity with an elevation angle of 2.4° at Naqu on 14 Jul 2014
(the black square is for the position of the radar, the black triangle is for Naqu, the black circle represents detection ranges of 20 km, 40 km and 60 km)
图 4 2014年7月14日15:00—15日00:00 C波段双线偏振雷达回波强度垂面剖面图
(a)沿31.48°N的时间-纬向剖面(黑色箭头表示回波移动方向,黑色三角形指向那曲所在经度),(b)沿92.06°E的时间-经向剖面(黑色三角形指向那曲所在纬度)
Fig. 4 Cross section of C-band polarization radar reflectivity from 1500 BT 14 Jul to 0000 BT 15 Jul in 2014
(a)cross section along 31.48° N(black lines with arrows indicate the propagation direction of the radar reflectivity and the black triangle is for the longitude of Naqu), (b)cross section along 92.06°E(the black triangle is for the latitude of Naqu)
图 5 基于ERA-Interim再分析资料得到的2014年7月14日02:00—15日02:00 500 hPa位势高度
(黑色实线,单位:dagpm)、涡度(填色)和水平风场(风羽)分布(黑色五角星代表那曲,黑色虚线为水平风辐合线)
Fig. 5 500 hPa geopotential height(the contour, unit:dagpm), vorticity(the shaded), and wind(the barb) of ERA-Interim analysis from 0200 BT 14 Jul to 0200 BT 15 Jul in 2014
(the black pentagram is for Naqu, the black thick dashed line is for a horizontal convergence line)
图 8 2014年7月14日14:00,20:00和15日02:00沿91.94°E的垂直环流(矢量, 纬向风单位:m·s-1;垂直速度单位:10-2 Pa·s-1)和相对湿度(填色)的经向-高度剖面
(黑色五角星为那曲位置,黑色三角形为14日20:00以后那曲附近降水发生位置)
Fig. 8 Cross section of vertical circulation(the vector, unit of zonal wind: m·s-1; unit of vertical velocity: 10-2 Pa·s-1) and relative humidity(the shaded) along 91.94°E at 0800 BT and 1400 BT on 14 Jul and at 0200 BT 15 Jul in 2014
(the black pentagram is for Naqu and the black triangle is the position of rainfall near Naqu after 2000 BT 14 Jul 2014)
图 9 2014年7月14日雨滴谱仪B观测的第1阶段(14:47—17:21,蓝色)、第2阶段(20:03—20:53,绿色)和第3阶段(22:39—23:52,粉色)降水的平均雨滴谱分布
Fig. 9 The raindrop diameter distribution of disdrometer B during the first stage(1447-1721 BT, blue dots), the second stage(2003-2053 BT, green dots) and the third stage(2239-2352 BT, pink dots) on 14 Jul 2014
图 10 2014年7月14日14:00—15日00:00雷达观测时间-高度剖面图
(a)C波段调频连续波雷达回波强度, (b)Ka波段毫米波云雷达回波强度, (c)C波段调频连续波雷达径向速度, (d)Ka波段毫米波云雷达径向速度
Fig. 10 Time-height cross section of radar observation from 1400 BT 14 Jul to 0000 BT 15 Jul in 2014
(a)reflectivity by C-band frequency modulation continuous wave radar, (b)reflectivity by Ka-band cloud radar, (c)radial velocity by C-band frequency modulation continuous wave radar, (d)radial velocity by Ka-band cloud radar
表 1 观测设备名称、观测位置、主要观测的物理量和数据的时间分辨率
Table 1 Equipments, position, main parameters and temporal-resolutions of observations
设备名称 观测仪器的位置 主要观测物理量 输出数据的时间分辨率 C波段双线偏振雷达 31.48°N,91.90°E 反射率因子、径向速度、速度谱宽、差分反射率因子 6 min Ka波段毫米波云雷达 31.48°N,92.01°E 反射率因子、径向速度、速度谱宽、退偏振因子 8.8 s C波段调频连续波雷达 31.48°N,92.07°E 反射率因子、径向速度、速度谱宽、回波功率 3 s 雨滴谱仪A 31.48°N,92.01°E 降水强度、32个直径档和32个速度档 1 min 雨滴谱仪B 31.48°N,92.05°E 降水强度、32个直径档和32个速度档 1 min -
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