2017 International Conference on Mathematics, Modelling and Simulation Technologies and Applications (MMSTA 2017) ISBN: 978-1-60595-530-8

Fine Resolution Simulation of a Cold Vortex in Northeast by Using a Meso-Scale Meteorological Model Xiao-yun SUN1, Ju HU2, Tian-liang ZHAO1,*, Shu WANG2 and Zhou-xiang ZHANG2 1Key Laboratory for Aerosol-Cloud-Precipitation of China Meteorological Administration, University of Information Science and Technology, Nanjing 210044, China 2Renewable Energy Research Center, China Electric Power Research Institute (CEPRI, State Key Laboratory of Operation and Control of Renewable Energy & Storage Systems, 100192, China *Corresponding author

Keywords: Modeling, Cold vortex, Precipitation, Northeast China.

Abstract. In this paper, a cold vortex of June 16-19, 2016 was simulated with fine resolution model (Climate-FDDA) to investigate the mechanism of precipitation and strong wind induced by a cold vortex in the Northeast Plain. The cold vortex process was reasonably captured by the fine resolution simulation. Due to the severe cyclonic circulation caused by the cold vortex process, water vapor was transported to the Northeast China Plain, providing a wealth of water vapor conditions. As the cold vortex is a deep convective synoptic system, unstable stratification and vigorous mixing action in the vertical direction could promote the occurrence of precipitation events over the region of northeast China. The cold vortex also lead the strong winds in the plain. During this case, the maximum surface wind speed reached 18m/s, which could be beneficial to the utilization of wind energy.

Introduction Meteorologically, a cold vortex is defined as a cyclone with a cold core and a closed isoline at a geopotential height of 500 hPa on the isobaric map, the cold vortex system is located between 35°N– 60°N and 115°E–145°E and lasts for at least two days [1]. The Northeast China cold vortex is a major high-impact weather system affecting the industry and agriculture in Northeast China [2-4]. According to existing statistical data [5], during the summer, the formation of the cold vortex is generally cut off from the bottom of the deepened westerly trough with a cut-off low [6]. The Northeast China cold vortex creates a circulation that can eventually lead to heavy rains or thunderstorms in the northeastern and northern parts of China [7]. Various aspects of the Northeast China cold vortex have been studied, including definition, energy cycle, mesoscale features relevant to heavy rains, especially the dynamic mechanisms for its formation [5, 8-10]. Due to the low temporal and spatial resolution of conventional observed data, and the diagnostic results have limitations for understanding the structural characteristics of Northeast China cold vortex and the meso scale convection caused by cold vortex [11]. In this study with the fine resolution dynamical climate analysis downscaling system, a case of cold vortex in northeast China from June 16 to 19, 2016 was investigated to analyze the cut-off low systems and the rainstorm during the cold vortex formation.

Model Description This study employed the NCAR Weather Research and Forecasting (WRF) -based Climate-FDDA weather downscaling tool to generate 20 years of dynamical downscale regional reanalysis for China at 9-km grid intervals.

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WRF is a next-generation mesoscale numerical weather prediction system designed to serve both operational forecasting and atmospheric research needs. NCAR’s Research Applications Laboratory developed a WRF-based real-time 4-dimensional data assimilation (WRF-RTFDDA) and forecasting system, specifically for supporting mesoscale weather-sensitive applications. NCAR Climate-FDDA uses global reanalysis as a background large-scale driver, assimilates regional observation, and simulates high-resolution processes with detailed terrain and land-surface forcing. The modeling system is specially formulated for effectively downscaling to produce high-resolution, high-fidelity regional climate reanalysis with assimilation of all available observations. The CEPRI-NCAR China-9km Climate-FDDA system effectively combines the NASA MERRA version 2 global weather reanalysis datasets (at 60-km grid resolution), high-resolution topography, landscape (land use) and soil property data, and regional weather observations to produce dynamically balanced and physically consistent high-resolution downscaling climate reanalysis over China. In this paper, using the Climate-FDDA model, we conducted a simulation with high temporal and spatial resolution. With the simulation results and NCEP (National Centers for Environmental Prediction) FNL (Final) Operational Global Analysis data, we analyzed the precipitation induced by the cold vortex that occurred from June 16 to 19, 2016.

Modeling Results Meteorological Analysis Figure 1 shows the evolution of the 500 hPa temperature and geopotential height during the lifecycle of the cold vortex. At 20:00 CST on June 16, a cut-off low occurred over eastern Lake Baikal (Fig. 1a). There was a tilted westerly trough to the north of China at 20:00 CST on June 17, and the cut-off low controlled Northeast Plain completely at 20:00 CST on June 18 (Figs. 1b and 1c). The southeastward extension of the cold trough led to pronounced cooling in Northeast China, with a cold core over Northeast China. In the south and west of the vortex. Cold advections were present in the southwest of the vortex, implying the prominent baroclinicity of the local atmosphere. The cold vortex entered its decay stage and moved out of Northeast China at 20:00 CST on June 19 (Fig. 1d). As the cold vortex is a deep convection system, cyclonic vorticity deeply extended upward into the middle troposphere from surface. Figure 2 shows the surface streamline and the surface temperature. When cold vortex reached the northeast plain, there is an obvious cyclonic circulation with cold core, leading to pronounced cooling in Northeast China. There were strong winds around the cold vortex (Figs. 2b and 2c). Most of the wind direction in the south and west of the vortex are southerly wind, blowing from the ocean to the land. During the cold vortex controlling the Northeast China Plain, the maximum of observed wind speed is 24m/s and the simulated value of surface wind speed reached 18 m/s. The wind speed is reasonably captured by the simulation. The days of Northeast China Plain affected by the cold vortex accounted for 42% in summer [12], resulting in the rich in wind energy resources.

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Figure 1. 500 hPa geopotential height (solid contour lines, unit: gpm) and the 500-hPa temperature (dash-dot contour lines, unit: K) at (a) 20:00 CST 16 June, (b) 20:00 CST 17 June, (c) 20:00 CST 18 June, and (d) 20:00 CST 19 June.

Figure 2. Surface wind field from the simulation data at (a)12:00 CST 16 June, (b)12:00 CST 17 June, (c)12:00 CST 18 June, and (d)12:00 CST 19 June. The plots show the surface streamline (vectors, unit: m s-1) and the surface temperature (color contours, unit: K). Precipitation Distribution The precipitation conditions were analyzed at 16-19 June during the period of cold vortex through the northeast plain. During this precipitation period, the daily precipitation reached 122mm. Figure 3

106 shows the simulated daily precipitation distribution. On the 17th, a wide range of precipitation occurred in the Northeast Plain, and a high value precipitation center occurred in Province (Fig. 3a). The large precipitation centers moved southward to the Bohai Sea on the 19th (Figs. 3b). The precipitation in the Northeast Plains mainly concentrated in Liaoning province, the southern part of Jilin province and Heilongjiang province with another precipitation center appeared in the northwest of junction of Inner Mongolia, Heilongjiang province and Jilin province. We compared the precipitation value with the observation data of five sites (Haerbin, Tonghe, Tongliao, , Linjiang). The color circles in Figure 3 were the daily cumulative precipitation of the site, the color represent the value. The precipitation was mainly concentrated in the southeast of the cold vortex. The high value center moved southward along with the movement of the cold vortex (Figs. 3b). Figure 3 shows that the high-resolution model had a reasonable prediction of the movement of the precipitation center.

Figure 3. Daily precipitation from the simulation data at (a) 17 June and (b) 19 June (unit: mm). The color circles represent the observed precipitation (mm) of sites. Vapor Conditions Figure 4 shows the change of specific humidity over northeast China at 200m. With the development and movement of cold vortex, the moist air constantly transported to the northeast China. At 12:00 CST on June 17 and 19 (Figs. 4a and 4b), the distribution of water vapor in the Northeast China Plain shows an increasing trend from northwest to southeast, implying that the water vapor transported by the southerly wind on the southeast periphery of the cold vortex. Precipitation and water vapor are concentrated in Liaoning province, the southeast of the cold vortex.

Figure 4. QV at 200 m from the simulation data at (a) 12:00 CST 17 June and (b) 12:00 CST 19 June (unit: kg kg-1).

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U* is the friction velocity in similarity theory, and a high U* value means that the atmosphere stratification is unstable. The larger the U*, the more unstable the atmospheric stratification and the more unstable energy in the atmosphere. The regions corresponding to the high value regions of precipitation in the U* variations (Fig. 5) were the high U* centers. The unstable energy in the atmosphere can be released under a rising condition. The energy conversion in the atmosphere is mainly realized through the vertical movement, and the vertical transmission of the physical variables such as water vapor, heat, momentum and vortices resulting from the vertical movement has a great impacts on development of the weather system [13].The presence of cold vortex also caused an increase in U*, which increased the available wind energy of this area, which is beneficial for developing wind energy.

Figure 5. U* from the simulation data at (a) 12:00 CST 17 June and (b) 12:00 CST 19 June (unit: m s-1).

Conclusions With fine resolution model (Climate-FDDA) a cold vortex of June 16-19, 2016 was simulated to analyze the precipitation and strong wind induced by a cold vortex in Northeast China Plain. The cold vortex process was reasonably captured. Due to the severe cyclonic circulation caused by the cold vortex process, water vapor was transported to the Northeast China Plain, providing a wealth of water vapor conditions. As the cold vortex is a deep convective synoptic system, unstable stratification and vigorous mixing action in the vertical direction could promote the occurrence of precipitation events over the northeast China.

Acknowledgements This work was supported by Science and Technology Project of State Grid Corporation of China

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