Characteristics of low-level wind during typical sudden precipitation processes at the northern foot of Qinling Mountains in midsummer

doi:10.11755/j.issn.1006-7639-2025-01-0041

Abstract

Abstract:

The changes of low-level wind field play an important role in the formation of sudden precipitation, which can change the flow field structure in the lower atmosphere, thereby affect the stability and vertical movement of the lower atmosphere and promote the development of convective clouds. Based on wind profile radar data at Chang’an Station, observation data, the fifth generation atmospheric reanalysis data released by the European Center for Medium Range Weather Forecasting, and Doppler radar data, this study analyzed the evolution characteristics of the low-level wind field during three typical sudden precipitation events under the control of the subtropical high at the northern foothills of the Qinling Mountains in midsummer. These events occurred on August 6, 2023, from 11:00 to 12:00 (referred to as “Process I”), July 13, 2023, from 00:00 to 01:00 (referred to as “Process II”), and August 3, 2022, from 18:00 to 19:00 (referred to as “Process III”). The results show that all three events occurred under the circulation background of the subtropical high combined with the intrusion of cold air at low level, exhibiting strong suddenness. For Process I and Process II, the intrusion of cold air at low level was characterized by westerly winds, while for Process III, it was characterized by easterly winds. Before the precipitation, the atmosphere over the Chang’an region was in a significantly unstable state, with weak vertical wind shear in the middle troposphere, which was the main reason for the highly localized nature of these three precipitation events. In midsummer, the multi-year average low-level wind speed at Chang’an Station generally exhibited a single-peak pattern, the wind speed initially increased with height and then decreased. The average wind speed below an altitude of 1 000 meters did not exceed 3.14 m·s?1, and the hourly wind speed shows distinct diurnal variation characteristics. The low-level wind direction displayed a counterclockwise rotation with increasing height, shifting gradually from southwesterly to southeasterly winds.The 4-6 h before the occurrence of three sudden precipitation processes, there was a cold air intrusion process in the low-level over Chang’an, and the wind speed was significantly bigger than the multi-year average. With the continuous invasion of low-level cold air, the 2 m temperature decreased rapidly, the air pressure rose, convection was triggered, and heavy precipitation occurred. The continuous intrusion of low-level cold air could generate strong mesoscale frontogenesis in the lower atmosphere, providing energy and triggering conditions for sudden precipitation. On the other hand, due to the obstruction of the local terrain at the northern foot of the Qinling Mountains and the Guanzhong Basin, the low-level cold air was forced to rise, promoting an increase in precipitation.

Key words: northern foot of the Qinling Mountains, midsummer, sudden precipitation, low-level wind, cold air intrusion

摘要： 低层风场变化对突发性降水的形成有重要作用，其通过改变低层大气的流场结构，进而影响低层大气的稳定性及垂直运动，促使对流云发生发展。基于长安站风廓线雷达资料、地面加密观测资料、欧洲中期天气预报中心发布的第五代大气再分析资料及多普勒雷达资料，分析盛夏（7—8月）副热带高压控制下秦岭北麓3次典型突发性降水过程[2023年8月6日11：00—12：00（简称“过程Ⅰ”）、2023年7月13日00：00—01：00（简称“过程Ⅱ”）和2022年8月3日18：00—19：00（简称“过程Ⅲ”）]中低层风场的演变特征。结果表明：3次过程均发生在副热带高压配合低层冷空气侵入的环流背景下，突发性强。过程Ⅰ和过程Ⅱ低层为偏西风冷空气侵入，过程Ⅲ为偏东风冷空气侵入。降水发生前长安地区大气呈显著不稳定状态，对流层中层垂直风切变较弱，这是3次降水过程局地性强的主要原因。盛夏长安站低层风速多年平均总体呈单峰型变化，风速随高度先增加再减小，1 000 m高度以下平均风速不超过3.14 m·s^-1，小时风速存在明显日变化特征。低层风向随高度增加呈逆时针旋转，由西南风逐渐转为东南风。3次突发性降水过程发生前4~6 h，长安站低层大气存在冷空气侵入过程，且风速较多年平均明显加大。伴随低层冷空气的持续侵入，地面气温迅速下降，气压上升，对流触发，强降水产生。低层冷空气的持续侵入一方面在低层产生强烈的中尺度锋生，为突发性降水提供能量条件和触发条件；另一方面受秦岭北麓及关中盆地局地地形的阻挡作用，迫使低层冷空气强迫抬升，有利于降水增幅。

关键词: 秦岭北麓, 盛夏, 突发性降水, 低层风场, 冷空气侵入

CLC Number:

P468.0+26

LIU Jiahuimin, LI Ming, OUYANG Yu, JI Qing, WANG Qingxia, LI Wenyao, LI Hanyu. Characteristics of low-level wind during typical sudden precipitation processes at the northern foot of Qinling Mountains in midsummer[J]. Journal of Arid Meteorology, 2025, 43(1): 41-53.

刘嘉慧敏, 李明, 欧阳雨, 吉庆, 王青霞, 李文耀, 李涵钰. 秦岭北麓盛夏典型突发性降水过程中低层风特征[J]. 干旱气象, 2025, 43(1): 41-53.

Figures/Tables 13

Fig.1 Schematic diagram of the study area （The red triangle represents the location of Chang’an Station； the color shaded is for terrain，Unit： m）

Tab.1 Wind profile radar parameters at Chang’an Station

类别	参数值
测量范围	水平风速：0~60 m·s^-1；垂直风速：±20 m·s^-1；风向：0~360°
高度分辨率	60、120、240 m
时间分辨率	3波束≤6 min，5波束≤0 min（单位观测时间1 min）
径向速度分辨率	≤0.2 m·s^-1
风速测量精度	≤1.5 m·s^-1
风向测量精度	≤10°

Fig.2 The spatial distribution of 24 h precipitation (color dots, Unit: mm) at different stations at the northern foot of Qinling Mountains from 20:00 on 5 to 20:00 on 6 August 2023 (a), from 20:00 on 12 to 20:00 on 13 July 2023 (c) and from 08:00 on 3 to 08:00 on 4 August 2022 (e), and the evolution of hourly precipitation at Chang’an Station from 08:00 to 17:00 on 6 August 2023 (b), from 20:00 on 12 to 13:00 on 13 July 2023 (d), from 16:00 to 23:00 on 4 August 2022 (f)

Fig.3 The 500 hPa geopotential height field (contours, Unit: gpm), temperature difference between 850 hPa and 500 hPa (the color shaded, Unit: ℃) and 700 hPa wind field (wind vectors, Unit: m·s-1) at 08:00 on 6 August 2023 (a), 20:00 on 12 July 2023 (b), 14:00 on 3 August 2022 (c)

Fig.4 The T-ln P diagram of Chang’an sounding Station at 08：00 on 6 August 2023 （a），20：00 on 12 July 2023 （b），14：00 on 3 August 2022 （c）（The black solid line represents the temperature curve，the green solid line represents the equal saturated mixing ratio curve，and the red line represents stratification curve）

Fig.5 The variation of average wind speed （a） and wind direction （b） with height in the low layer over Chang'an Station from July to August during 2021-2023

Fig.6 Hourly variation of horizontal wind speed （a），wind vectors （b） in lower layer over Chang'an Station from July to August during 2021-2023 （Unit： m·s-1）

Fig.7 Hourly variation of the low layer wind (wind vectors, Unit: m·s-1), precipitation (green column) (a, c, e) and surface 3 h pressure change, air temperature, dew point temperature and temperature change in the past 24 h (b, d, f) at Chang’an Station during Process I (a, b), Process Ⅱ (c, d) and Process Ⅲ (e, f)

Fig.8 The composite reflectivity factor (Unit: dBZ) from Xi’an Doppler radar at different times during Process I (a, b, c), Process Ⅱ (d, e, f) and Process Ⅲ (g, h i)

Fig.9 The spatial distribution of potential pseudo-equivalent temperature (isolines, Unit: K) and frontogenesis function (the color shaded, Unit: 10-9 K·m-1·s-1) at 925 hPa over Chang’an and surrounding areas (a, c, e), the longitude-height sections of the potential pseudo-equivalent temperature (isolines, Unit: K), frontogenesis function (the color shaded, Unit: 10-9 K·m-1·s-1) and wind field (wind vectors, Unit: m·s-1) along the latitude of the precipitation center (b, d, f) at 10:00 on 6 August 2023 (a, b), 19:00 on 12 July 2023 (c, d), 17:00 on 3 August 2022 (e, f) (The red box represents the frontogenetical area at the northern foot of Qinling Mountains, the gray shaded for terrain)

Fig.10 The spatial distribution of wind field （wind vectors，Unit： m·s-1） and horizontal divergence （black dotted lines，only showing the values less than 0，Unit： 10-5s-1） at 925 hPa over Chang’an and surrounding areas at 10：00 on 6 August 2023 （a），19：00 on 12 July 2023 （b），17：00 on 3 August 2022 （c）

Fig.11 Time-height sections of horizontal divergence （the color shaded，Unit： 10-5s-1） and vertical velocity （isolines，Unit： Pa·s-1） along the heavy precipitation center from 06：00 to 14：00 on 6 August 2023 （a），from 12：00 on 12 to 14：00 on 13 July 2023 （b），from 08：00 to 24：00 on 3 August 2022 （c）

Fig.12 The conceptual model of sudden precipitation caused by low layer westerly （a） and easterly （b） cold air intrusion at the northern foot of Qinling Mountain in midsummer （the color shaded for altitude，Unit： m）

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