Potential Vorticity Analysis and Fine Forecast of Extreme Rainstorm Event in Henan Province in July 2021
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摘要: 2021年7月17～22日，河南省发生一次大范围极端暴雨事件。本文利用ERA5再分析资料，基于湿位涡和倾斜涡度发展理论对此次暴雨过程进行诊断分析，并考察高分辨率全球模式天机系统及欧洲中期天气预报中心业务预报系统对此次降水过程的预报能力。结果表明此次事件是中高—低纬系统相互作用、协同影响的结果。18～19日，来自中高纬度的高湿位涡沿倾斜的湿等熵面自对流层中高层向下入侵，降温和相对湿度增加，导致河南省西北部高涡度带的形成及对流层低层倾斜湿等熵面在河南省北部建立。20日受低纬台风的影响南风增强，沿倾斜湿等熵面的上滑运动增强，导致相对涡度迅猛发展，强降水发生。天机系统对此次强降水过程的预报能力较欧洲中期天气预报中心的业务预报系统好，能为业务部门提前发布预警信号提供科学支撑。湿位涡和倾斜涡度发展理论是暴雨的诊断及预报的有力理论基础，在再分析资料及模式预报数据中与降水强度、降水落区及雨带的移动均有很好的对应。Abstract: During 17–22 July 2021, a large-scale extreme rainstorm occurred in Henan Province. In this study, ERA5 reanalysis data were used to examine the heavy rainfall process based on the theory of moist potential vorticity and slantwise vorticity development. The forecast capabilities of the high-resolution global model Tianji system and the Integrated Forecast System (IFS) of ECMWF (European Centre for Medium-Range Weather Forecasts) were also investigated. The results show that this event results from the interaction and synergistic influence between the middle-high and low-latitude systems. From 18 to 19 July, the high moist potential vorticity in the middle and high latitudes intruded downward from the middle and upper troposphere along the slantwise moist isentropic surfaces. Together with the cooling and increased relative humidity, this led to the formation of the high vorticity zone in northwestern Henan Province and the establishment of the slantwise moist isentropic surfaces in the lower troposphere in northern Henan Province. On 20 July, a low-latitude typhoon strengthened the southerly wind and enhanced the up-sliding movement along the slantwise moist isentropic surface, resulting in the rapid development of relative vorticity and heavy rainfall. The Tianji system can forecast this heavy rain process better than IFS and can provide scientific support for the meteorological departments to issue early warnings. Moist potential vorticity and slantwise vorticity development theory are powerful theoretical bases for heavy rain analysis and forecast, which are well related to precipitation intensity, position, and movement in reanalysis and model forecast data.
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图 1 2021年7月17～22日站点逐日累计降水量（彩色填色，单位：mm）及地形（灰色填色，单位：m）。方框区域表示19～21日降水量最大区域（33°～36°N，112°～115°E），下同
Figure 1. Daily accumulative precipitation (color shadings, units: mm) from meteorological observation station and terrain (gray shadings, units: m) during 17–22 July 2021. The box area represents the maximum precipitation region (33°–36°N, 112°–115°E) during 19–21 July, the same for the below figures
图 2 2021年7月17～22日ERA5资料逐日累计降水量（单位：mm）
Figure 2. Daily accumulative precipitation (units: mm) from ERA5 data during 17–22 July 2021
图 3 2021年7月17～22日ERA5资料日平均500 hPa相对涡度（填色，单位：10−5 s−1）及风矢量（箭头，单位：m s−1）
Figure 3. Daily mean relative vorticity (shadings, units: 10−5 s−1) and wind vector (arrows, units: m s−1) at 500 hPa from ERA5 data during 17–22 July 2021
图 4 2021年7月18～22日ERA5资料600 hPa区域（33°～36°N, 112°～115°E）平均（a）逐小时降水量（黑色线，单位：mm）、相对湿度（绿色线）、24小时变温（红色线，单位：°C），（b）比湿（紫色线）、饱和比湿（蓝色线）、相对湿度（棕色线）的24 h变化百分比，（c）纬向风u、经向风v、垂直速度（
$ \omega $ ）Figure 4. Regional (33°–36°N, 112°–115°E) mean of (a) hourly precipitation (black line, units: mm), relative humidity (green line), and 24-h temperature change (red line, units: °C), (b) percentage change in specific humidity (purple line), saturated specific humidity (blue line), and relative humidity (brown line) over 24 hours, and (c) zonal wind u, meridional wind v, vertical velocity (
$ \omega $ ) at 600 hPa from ERA5 data during 18–22 July 2021
图 5 2021年7月17～22日ERA5资料沿112°～115°E平均的日平均相对湿度（填色）、相当位温（等值线，单位：K）以及经向风v和垂直风
$ \omega $ 的合成风矢量（箭头，v单位：m s−1，$ \omega $ 单位：−0.02 Pa s−1）的高度—纬度剖面。绿色虚线表示目标区域所在的南北纬度，下同Figure 5. Height–latitude cross sections of the daily mean relative humidity (shadings), equivalent potential temperature (isolines, units: K), and composite wind (arrows) of meridional wind v (units: m s−1) and vertical wind
$ \omega $ (units: −0.02 Pa s−1) averaged along 112°–115°E from ERA5 data during 17–22 July 2021. The dashed green lines indicate the latitude of the target area, the same for the below figures
图 6 2021年7月18～21日ERA5资料沿112°～115°E平均的日平均（a1–a4）Pm1（填色，单位：PVU，1 PVU=10−6 K m2 s−1 kg−1）、相当位温（等值线，单位：K）以及经向风v和垂直风
$ \omega $ 的合成风矢量（箭头，v单位：m s−1，$ \omega $ 单位：−0.02 Pa s−1），（b1–b4）Pm2（填色，单位：PVU）及纬向风（等值线，单位：m s−1），（c1–c4）相对涡度（填色，单位：10−5 s−1）的高度—纬度剖面Figure 6. Height–latitude cross sections of the daily mean (a1–a4) Pm1 (vertical term of moist potential vorticity, shadings, units: PVU, 1 PVU=10−6 K m2 s−1 kg−1), equivalent potential temperature (isolines, units: K), composite wind (arrows) of meridional wind v (units: m s−1) and vertical wind
$ \omega $ (units: −0.02 Pa s−1), (b1–b4) Pm2 (horizontal term of moist potential vorticity, shadings, units: PVU) and zonal wind (isolines, units: m s−1), (c1–c4) relative vorticity (shadings, units: 10−5 s−1) averaged along 112°–115°E from ERA5 data during 18–22 July 2021
图 7 2021年7月17～22日ERA5资料日平均的850 hPa Pm1（填色，单位：PVU）及日累计降水量（等值线，单位：mm）
Figure 7. Daily mean 850-hPa Pm1 (shadings, units: PVU) and daily accumulative precipitation (isolines, units: mm) from ERA5 data during 17–22 July 2021
图 8 2021年7月19～22日（a–d）T2资料、（e–h）IFS资料预报逐日累计降水量（单位：mm）。黑框区域表示T2资料、IFS资料预报19～21日最大降水量区域（33.5°～36°N，112°～114.5°E），红框区域表示IFS资料降水过报区域（33.5°～36°N，110°～112.5°E）
Figure 8. Daily accumulative precipitation (units: mm) predicted by (a–d) T2 data and (e–h) IFS data during 19–22 July 2021. The black box area represents the maximum precipitation region (33.5°–36°N, 112°–114.5°E) from T2 data and IFS data during 19–21 July, and the red box area represents the overestimated region (33.5°–36°N, 110°–112.5°E) from IFS data
图 9 2021年7月19～22日区域R1（33.5°～36°N，112°～114.5°E）平均的（a）T2资料、（b）IFS资料的日平均垂直速度（填色），（c）IFS资料区域R2（33.5°～36°N，110°～112.5°E）平均的日平均垂直速度（填色），等值线为区域（33°～36°N，112°～115°E）平均的ERA5资料日平均垂直速度（单位：Pa s−1）。（d–e）同（a–c），但为相对湿度
Figure 9. Daily mean vertical velocity (shadings) obtained from (a) T2 data, (b) IFS data averaged over region R1 (33.5°–36°N, 112°–114.5°E), (c) IFS data averaged over region R2 (33.5°–36°N, 110°–112.5°E) during 19–22 July 2021, the isoline represents daily mean vertical velocity (units: Pa s−1) obtained from ERA5 data averaged over (33°–36°N, 112°–115°E). (d–e) As in (a–c), but for relative humidity
图 10 2021年7月19～21日（a–c）T2资料、（d–f）IFS资料日平均500 hPa相对涡度（填色，单位：10−5 s−1）及水平风矢量（箭头，单位：m s−1）
Figure 10. Daily mean relative vorticity (shadings, units: 10−5 s−1) and horizontal wind vector (arrows, units: m s−1) of (a–c) T2 data and (e–f) IFS data at 500 hPa during 19–21 July 2021
图 11 2021年7月19～21日T2资料沿112°～114.5°E平均的日平均（a1–a3）Pm1（填色，单位：PVU）、相当位温（等值线，单位：K）以及经向风v和垂直风
$ \omega $ 的合成风矢量（箭头，v的单位：m s−1，单位：−0.02 Pa s−1），（b1–b3）Pm2（填色，单位：PVU）及纬向风（等值线，单位：m s−1），（c1–c3）相对涡度（填色，单位：10−5 s−1）的高度—纬度剖面Figure 11. Height–latitude cross sections of the daily mean (a1–a3) Pm1 (shadings, units: PVU), equivalent potential temperature (isolines, units: K), composite wind (arrows) of meridional wind v (units: m s−1) and vertical wind
$ \omega $ (units: −0.02 Pa s−1), (b1–b3) Pm2 (shadings, units: PVU) and zonal wind (isolines, units: m s−1), (c1–c3) relative vorticity (shadings, units: 10−5 s−1) obtained from T2 data averaged along 112°–114.5°E during 19–21 July 2021 -
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