# Impact of Surface Potential Vorticity Density Forcing over the Tibetan Plateau on the South China Extreme Precipitation in January 2008. Part ll: Numerical Simulation

• Corresponding author: Guoxiong WU, gxwu@lasg.iap.ac.cn
• Funds:

Supported by the National Key Research and Development Program of China (2018YFC1505706), Key Research Program of Fron-tier Sciences of Chinese Academy of Sciences (QYZDY-SSW-DQC018), and State Key Program of National Natural Science Foundation of China (41730963 and 91637312)

• doi: 10.1007/s13351-019-8606-z
• The surface air convergence on the eastern flank of the Tibetan Plateau (TP) can increase the in situ surface potential vorticity density (PVD). Since the elevated TP intersects with the isentropic surfaces in the lower troposphere, the increased PVD on the eastern flank of TP thus forms a PVD forcing to the intersected isentropic surface in the boundary layer. The influence of surface PVD forcing over the TP on the extreme freezing rain/snow over South China in January 2008 is investigated by using numerical experiments based on the Finite-volume Atmospheric Model of the IAP/LASG (FAMIL). Compared with observations, the simulation results show that, by using a nudging method for assimilating observation data in the initial flow, this model can reasonably reproduce the distribution of precipitation, atmospheric circulation, and PVD propagation over and downstream of the TP during the extreme winter precipitation period. In order to investigate the impact of the increased surface PVD over the TP on the extreme precipitation in South China, a sensitivity experiment with surface PVD reduced over the TP region was performed. Compared with the control experiment, it is found that the precipitation in the TP downstream area, especially in Southeast China, is reduced. The rainband from Guangxi Region to Shandong Province has almost disappeared. In the lower troposphere, the increase of surface PVD over the TP region has generated an anomalous cyclonic circulation over southern China, which plays an important role in increasing southerly wind and the water vapor transport in this area; it also increases the northward negative absolute vorticity advection. In the upper troposphere, the surface PVD generated in eastern TP propagates on isentropic surface along westerly wind and results in positive absolute vorticity advection in the downstream areas. Consequently, due to the development of both ascending motion and water vapor transport in the downstream place of the TP, extremely heavy precipitation occurs over southern China. Thereby, a new mechanism concerning the influence of the increased surface PVD over the eastern TP slopes on the extreme weather event occurring over southern China is revealed.
• Fig. 1.  Distributions of (a) potential vorticity density − $\nabla \cdot \left({{V}W} \right)$ change (10–7 K s–2); (b) divergence and (c) its horizontal component and (d) vertical component (10–5 s–1), averaged over 24–27 January 2008 in the surface layer of the TP. Red and blue lines indicate the 1500- and 3000-m contours of the TP topography, respectively.

Fig. 2.  Location of the areas with surface wind speed altered in the sensitivity experiment (SEN). A represents the area to the south of 40°N and east of 95°E, where the altitude is equal or larger than 1500 m; B denotes the area to the west of 95°E, where the altitude is higher than 3000 m; and C denotes the area to the east of A, to the west of 25°–40°N, 110°E, where the altitude is less than 1500 m. Blue and red lines indicate the 1500- and 3000-m contours of the TP topography, respectively.

Fig. 6.  The 24–27 January mean distributions of (a, d, g, j) near-surface wind (m s–1), (b, e, h, k) divergence (10–6 s–1), and (c, f, i, l) velocity potential (shading; 10–6 m2 s–1) and divergent wind (vector; m s–1), calculated from the (a–c) MERRA2 data, (d–f) control run, (g–i) sensitivity run. Panels (j–l) show the difference between control and sensitivity runs. Blue and red contours indicate the 1500- and 3000-m contours of the TP, respectively.

Fig. 3.  Precipitation (mm day–1) during 24–27 January 2008 from (a) station observation, (b) TRMM retrieval, and (c) FAMIL simulation. The thick blue line indicates the 3000-m contour of terrain height.

Fig. 4.  Distributions of temperature (shading; °C) and wind (vector; m s–1) averaged over 24–27 January 2008 at (a, b) 200, (c, d) 500, and (e, f) 700 hPa. Left columns are from MERRA2 reanalysis data, and right columns are from FAMIL control simulation. The thick blue line indicates the 3000-m contour of TP topography.

Fig. 5.  Evolutions of the distributions of PVD (W) (shading; 10–4 s–1), wind (vector; m s–1), and pressure (contour; interval of 50 hPa) from 24 to 27 January 2008. (a–d) are from MERRA2 data at 295-K isentropic surface; (e–h) are from the FAMIL simulation at 290-K isentropic surface.

Fig. 7.  Time–longitude cross-sections averaged over 30°–35°N for 20–27 January 2008 of the PVD (10–4 s–1) at 295-K isentropic surface from (a) control run, (b) sensitivity run, and (c) their difference (control run minus sensitivity run).

Fig. 8.  The 24–27 January mean distributions of (a, d, g) geopotential height at 500 hPa, (b, e, h) geopotential height at 850 hPa, and (c, f, i) relative humidity (contour; interval of 10%) and divergence of water vapor flux (shaded; 10–7 g s–1 cm–1 hPa–1) at 700 hPa. Vector represents wind (m s–1); geopotential height unit is dagpm. (a–c) Control experiment, (d–f) sensitivity experiment, (g–i) their difference (control minus sensitivity). Blue solid curve indicates the 3000-m contour of terrain height.

Fig. 9.  Spatiotemporal evolutions of absolute vorticity advection (shading; 10–9 s–2) and its components (contour; interval: 0.2×10–9 s–2) during 23–27 January 2008: (a, b) control experiment, (c, d) sensitivity experiment, and (e, f) their difference. (a, c, e) Time–longitude cross-sections of absolute vorticity advection and its zonal component averaged over 20°–40°N on the 310-K isentropic surface. (b, d, f) Time–latitude cross-section of absolute vorticity advection and its meridional component averaged within100°–120°E on the 285-K isentropic surface.

Fig. 10.  Daily evolutions of the distributions of precipitation (shaded; mm day–1) and vertical velocity at 500 hPa (contour; interval: 2 Pa s–1) during 24–27 January 2008 from (a–d) control experiment, (e–h) sensitivity experiment, and (i–l) their difference. Red contour indicates the 3000-m contour of terrain height.

•  [1] Atmospheric Data Service Center of Nanjing University of Information Science & Technology, 2010: Introduction of MERRA satellite analysis data. Trans. Atmos. Sci., 33, 253–256. (in Chinese). [2] Bao, Q., J. Yang, Y. M. Liu, et al., 2010: Roles of anomalous Tibetan Plateau warming on the severe 2008 winter storm in central–southern China. Mon. Wea. Rev., 138, 2375–2384. doi: 10.1175/2009MWR2950.1. [3] Dee, D. P., S. M. Uppala, A. J. Simmons, et al., 2011: The ERA-Interim reanalysis: Configuration and performance of the data assimilation system. Quart. J. Roy. Meteor. Soc., 137, 553–597. doi: 10.1002/qj.828. [4] Ding, Y. H., Z. Y. Wang, Y. F. Song, et al., 2008: Causes of the unprecedented freezing disaster in January 2008 and its possible association with the global warming. Acta Meteor. Sinica, 66, 808–825. (in Chinese) doi: 10.11676/qxxb2008.074. [5] Ertel, H., 1942: Ein neuer hydrodynamischer Wirbelsatz. Meteo-rol. Z., 59, 271–281. [6] Gao, H., L. J. Chen, X. L. Jia, et al., 2008: Analysis of the severe cold surge, ice–snow and frozen disasters in South China during January 2008. Ⅱ. Possible climatic causes. Meteor. Mon., 34, 101–106. (in Chinese). [7] Gao, Y., T. W. Wu, and B. D. Chen, 2011: Anomalous thermodynamic conditions for freezing rain in southern China in January 2008 and their causes. Plateau Meteor., 30, 1526–1533. (in Chinese). [8] Gu, L., K. Wei, and R. H. Huang, 2008: Severe disaster of blizzard, freezing rain and low temperature in January 2008 in China and its association with the anomalies of East Asian monsoon system. Climatic Environ. Res., 13, 405–418. (in Chinese). [9] Gu, X. Z., 2011: Diagnostic analysis on severe cold surge, rain and ice–snow in South China in January 2008. Plateau Meteor., 30, 150–157. (in Chinese). [10] Haynes, P. H., and M. E. McIntyre, 1987: On the evolution of vorticity and potential vorticity in the presence of diabatic heating and frictional or other forces. J. Atmos. Sci., 44, 828–841. doi: 10.1175/1520-0469(1987)044<0828:OTEOVA>2.0.CO;2. [11] Haynes, P. H., and M. E. McIntyre, 1990: On the conservation and impermeability theorems for potential vorticity. J. Atmos. Sci., 47, 2021–2031. doi: 10.1175/1520-0469(1990)047<2021:OTCAIT>2.0.CO;2. [12] Held, I. M., and T. Schneider, 1999: The surface branch of the zonally averaged mass transport in the troposphere. J. Atmos. Sci., 56, 1688–1697. doi: 10.1175/1520-0469(1999)056<1688:TSBOTZ>2.0.CO;2. [13] Holton, J., 1992: Chapter 3: Elementary applications of the basic equations. An Introduction to Dynamic Meteorology, 3rd Ed., Academic Press, New York, 75–77. [14] Hoskins, B. J., 1991: Towards a PV-θ view of the general circulation. Tellus A, 43, 27–35. doi: 10.3402/tellusb.v43i4.15396. [15] Hoskins, B. J., 1997: A potential vorticity view of synoptic development. Meteor. Appl., 4, 325–334. doi: 10.1017/S1350482797000716. [16] Hoskins, B. J., 2015: Potential vorticity and the PV perspective. Adv. Atmos. Sci., 32, 2–9. doi: 10.1007/s00376-014-0007-8. [17] Hoskins, B. J., I. Draghici, and H. C. Davies, 1978: A new look at the ω-equation. Quart. J. Roy. Meteor. Soc., 104, 31–38. doi: 10.1002/qj.49710443903. [18] Hoskins, B. J., M. E. McIntyre, and A. W. Robertson, 1985: On the use and significance of isentropic potential vorticity maps. Quart. J. Roy. Meteor. Soc., 111, 877–946. doi: 10.1002/qj.49711147002. [19] Huffman, G. J., D. T. Bolvin, and E. J. Nelkin, et al., 2007: The TRMM multi-satellite precipitation analysis: Quasi-global, multi-year, combined-sensor precipitation estimates at fine scales. J. Hydrometeor., 8, 38–55. doi: 10.1175/JHM560.1. [20] Li, C. Y., and W. Gu, 2010: Analysis of the anomalous activity of blocking high over the Ural Mountains in January 2008. Chinese J. Atmos. Sci., 34, 865–874. (in Chinese) doi: 10.3878/j.issn.1006-9895.2010.05.02. [21] Li, J. X., Q. Bao, Y. M. Liu, et al., 2017: Evaluation of the computational performance of the finite-volume atmospheric model of the IAP/LASG (FAMIL) on a high-performance computer. Atmos. Ocean. Sci. Lett., 10, 329–336. doi: 10.1080/16742834.2017.1331111. [22] Li, L. F., Y. M. Liu, and C. Y. Bo, 2011: Impacts of diabatic heating anomalies on an extreme snow event over South China in January 2008. Climatic Environ. Res., 16, 126–136. (in Chinese). [23] Liu, X., G. X. Wu, and W. P. Li, 2006: The diurnal variation of the atmospheric circulation and diabatic heating over the Tibetan Plateau. Adv. Earth Sci., 21, 69–78. (in Chinese). [24] Liu, Y., Y. H. Zhao, and Z. Y. Guan, 2008: Influences of stratospheric circulation anomalies on tropospheric weather of the heavy snowfall in January 2008. Climatic Environ. Res., 13, 548–555. (in Chinese). [25] Lucchesi, R., 2012: File Specification for MERRA Products. GMAO Office Note No. 1 (Version 2.3), 87 pp, available at https://gmao.gsfc.nasa.gov/pubs/docs/Lucchesi528.pdf. Accessed on 18 April 2019. [26] Ma, T. T., G. X. Wu, Y. M. Liu, et al., 2019: Impact of surface potential vorticity density forcing over the Tibetan Plateau on the South China extreme precipitation in January 2008. Part I: Data analysis. J. Meteor. Res., 33, 400–415. doi: 10.1007/s13351-019-8604-1. [27] Nan, S., and P. Zhao, 2012: Snowfall over central–eastern China and Asian atmospheric cold source in January. J. Climate, 32, 888–899. doi: 10.1002/joc.2318. [28] Rienecker, M. M., M. J. Suarez, R. Gelaro, et al., 2011: MERRA: NASA’s modern-Era retrospective analysis for research and applications. J. Climate, 24, 3624–3648. doi: 10.1175/JCLI-D-11-00015.1. [29] Schneider, T., 2005: Zonal momentum balance, potential vorticity dynamics, and mass fluxes on near-surface isentropes. J. Atmos. Sci., 62, 1884–1900. doi: 10.1175/JAS3341.1. [30] Schneider, T., I. M. Held, and S. T. Garner, 2003: Boundary effects in potential vorticity dynamics. J. Atmos. Sci., 60, 1024–1040. doi: 10.1175/1520-0469(2003)60<1024:BEIPVD>2.0.CO;2. [31] Shaw, S. N., 1930: Manual of Meteorology, Volume III. The Physical Processes of Weather. Cambridge University Press, 473 pp. [32] Tan, G. R., H. S. Chen, Z. B. Sun, et al., 2010: Linkage of the cold event in January 2008 over China to the North Atlantic Oscillation and stratospheric circulation anomalies. Chinese J. Atmos. Sci., 34, 175–183. (in Chinese) doi: 10.3878/j.issn.1006-9895.2010.01.16. [33] Tao, S. Y., and J. Wei, 2008: The severe snow and freezing-rain in January 2008 in southern China. Climatic Environ. Res., 13, 337–350. (in Chinese). [34] Tao, Z. Y., Y. G. Zheng, and X. L. Zhan, 2008: The southern China quasi-stationary front during the ice–snow disaster of January 2008. Acta Meteor. Sinica, 66, 850–854. (in Chinese) doi: 10.11676/qxxb2008.077. [35] Wang, D. H., C. J. Liu, Y. Liu, et al., 2008: A preliminary analy-sis of features and causes of the snow storm event over the southern China in January 2008. Acta Meteor. Sinica, 66, 405–422. (in Chinese) doi: 10.11676/qxxb2008.038. [36] Wang, L., G. Gao, and Q. Zhang, 2008: Analysis of the severe cold surge, ice–snow and frozen disasters in South China during January 2008. I: Climatic features and its impact. Meteor. Mon., 34, 95–100. (in Chinese). [37] Wang, Y. F., Y. Li, and P. Y. Li, 2008: The large scale circulation of the snow disaster in South China in the beginning of 2008. Acta Meteor. Sinica, 66, 826–835. (in Chinese) doi: 10.11676/qxxb2008.075. [38] Wang, Z. Y., Q. Zhang, Y. Chen, et al., 2008: Characters of meteorological disasters caused by the extreme synoptic process in early 2008 over China. Adv. Climate Change Res., 4, 63–67. (in Chinese). [39] Wen, M., S. Yang, A. Kumar, et al., 2009: An analysis of the large-scale climate anomalies associated with the snowstorms affecting China in January 2008. Mon. Wea. Rev., 137, 1111–1131. doi: 10.1175/2008MWR2638.1. [40] Yang, G. M., Q. Kong, D. Y. Mao, et al., 2008: Analysis of the long-lasting cryogenic freezing rain and snow weather in the beginning of 2008. Acta Meteor. Sinica, 66, 836–849. (in Chinese) doi: 10.11676/qxxb2008.076. [41] Zeng, M. J., W. S. Lu, X. Z. Liang, et al., 2008: Analysis of temperature structure for persistent disastrous freezing rain and snow over southern China in early 2008. Acta Meteor. Sinica, 66, 1043–1052. (in Chinese) doi: 10.11676/qxxb2008.093. [42] Zhu, Q. G., J. R. Lin, and S. W. Shou, 2007: Chapter 3. Synoptic Principles and Methods. China Meteorological Press, Beijing, 120–122. (in Chinese).
•  [1] Tingting MA, Guoxiong WU, Yimin LIU, Zhihong JIANG, Jiahui YU. Impact of Surface Potential Vorticity Density Forcing over the Tibetan Plateau on the South China Extreme Precipitation in January 2008. Part I: Data Analysis. Journal of Meteorological Research, 2019, 33(3): 400-415.  doi: 10.1007/s13351-019-8604-1. [2] Jie TANG, Xueliang GUO, Yi CHANG. A Numerical Investigation on Microphysical Properties of Clouds and Precipitation over the Tibetan Plateau in Summer 2014. Journal of Meteorological Research, 2019, 33(3): 463-477.  doi: 10.1007/s13351-019-8614-z. [3] Ping ZHAO, Yueqing LI, Xueliang GUO, Xiangde XU, Yimin LIU, Shihao TANG, Wenming XIAO, Chunxiang SHI, Yaoming MA, Xing YU, Huizhi LIU, La JIA, Yun CHEN, Yanju LIU, Jian LI, Dabiao LUO, Yunchang CAO, Xiangdong ZHENG, Junming CHEN, An XIAO, Fang YUAN, Donghui CHEN, Yang PANG, Zhiqun HU, Shengjun ZHANG, Lixin DONG, Juyang HU, Shuai HAN, Xiuji ZHOU. The Tibetan Plateau Surface–Atmosphere Coupling System and Its Weather and Climate Effects: The Third Tibetan Plateau Atmospheric Science Experiment. Journal of Meteorological Research, 2019, 33(3): 375-399.  doi: 10.1007/s13351-019-8602-3. [4] Sun Guowu, Yu Yaxun, Wang Baoling, Feng Jianying. NUMERICAL EXPERIMENTS ON THE INFLUENCES OF GENERAL CIRCULATION ANOMALY OVER THE TIBETAN PLATEAU AND SURFACE ALBEDO CHANGE IN NORTHWEST CHINA ON SUMMER PRECIPITATION. Journal of Meteorological Research, 1998, 12(3): 311-320. [5] Niu Guoyue, C. Carssardo, Hong Zhongxiang, A. Longhetto. NUMERICAL SIMULATION ON THE RESPONSE OF LAND SURFACE TO SEVERE WEATHER*. Journal of Meteorological Research, 1998, 12(4): 410-424. [6] Wei HUA, Guangzhou FAN, Yiwei ZHANG, Lihua ZHU, Xiaohang WEN, Yongli ZHANG, Xin LAI, Binyun WANG, Mingjun ZHANG, Yao HU, Qiuyue WU. Trends and Uncertainties in Surface Air Temperature over the Tibetan Plateau, 1951–2013. Journal of Meteorological Research, 2017, 31(2): 420-430.  doi: 10.1007/s13351-017-6013-x. [7] Zhong Qiang. CHARACTERISTICS OF VARIATION IN SURFACE ALBEDO AND SNOW FORCING OVER THE TIBETAN PLATEAU*. Journal of Meteorological Research, 1998, 12(2): 177-189. [8] WU Qiuxia, NI Yunqi. DIAGNOSTIC ANALYSIS OF THE INFLUENCE OF ANOMALOUS SURFACE SENSIBLE HEAT FLUX IN THE TIBETAN PLATEAU AND ITS VICINITY ON THE EAST ASIAN WINTER MONSOON*. Journal of Meteorological Research, 2004, 18(2): 147-166. [9] REN Rongcai, WU Guoxiong, CAI Ming, SUN Shuyue, LIU Xin, LI Weiping. Progress in Research of Stratosphere-Troposphere Interactions： Application of Isentropic Potential Vorticity Dynamics and the Effects of the Tibetan Plateau. Journal of Meteorological Research, 2014, 28(5): 714-731.  doi: 10.1007/s13351-014-4026-2. [10] WENG Yonghui, XU Xiangde, BAI Jingyu, DONG Chaohua. APPLICATION OF THE VARIATIONAL METHOD OF TOVS DATA OVER THE TIBETAN PLATEAU IN IMPROVEMENT OF THE INITIAL FIELD OF NUMERICAL MODELS*. Journal of Meteorological Research, 2001, 15(3): 362-376. [11] Dong Haiping, Wei Shaoyuan, Pan Xiaobin, He Hongrang. NUMERICAL STUDY OF A MESO-β SCALE RAINSTORM EVENT*. Journal of Meteorological Research, 1999, 13(1): 64-72. [12] Zheng Qinglin, Gu Yu, Song Qingli, Jiang Ping. NUMERICAL STUDY ON EFFECTS OF THE QINGHAIXIZANG PLATEAU ON FORMATION OF THE URAL BLOCKING HIGH. Journal of Meteorological Research, 1993, 7(1): 61-69. [13] Yaohui LI, Cailing ZHAO, Tiejun ZHANG, Wei WANG, Haixia DUAN, Yuanpu LIU, Yulong REN, Zhaoxia PU. Impacts of Land-Use Data on the Simulation of Surface Air Temperature in Northwest China. Journal of Meteorological Research, 2018, 32(6): 896-908.  doi: 10.1007/s13351-018-7151-5. [14] ZOU Han, ZHU Jinhuan, ZHOU Libo, LI Peng, MA Shupo. Validation and Application of Reanalysis Temperature Data over the Tibetan Plateau. Journal of Meteorological Research, 2014, 28(1): 139-149.  doi: 10.1007/s13351-014-3027-5. [15] CHEN Xing, YU Ge, LIU Jian. AN AGCM +SSiB MODEL SIMULATION ON CHANGES IN PALAEOMONSOON CLIMATE AT 21 KA BP IN CHINA*. Journal of Meteorological Research, 2001, 15(3): 333-345. [16] LI Chiqin, ZUO Qunjie, XU Xiangde, GAO Shouting. Water Vapor Transport around the Tibetan Plateau and Its Effect on Summer Rainfall over the Yangtze River Valley. Journal of Meteorological Research, 2016, 30(4): 472-482.  doi: 10.1007/s13351-016-5123-1. [17] ZHANG Xia, YAO Xiuping, MA Jiali, MIMA-Zhuoga. Climatology of Transverse Shear Lines Related to Heavy Rainfall over the Tibetan Plateau during Boreal Summer. Journal of Meteorological Research, 2016, 30(6): 915-926.  doi: 10.1007/s13351-016-6952-7. [18] BAO Yan, GAO Yanhong, Lü Shihua, WANG Qingxia, ZHANG Shaobo, XU Jianwei, LI Ruiqing, LI Suosuo, MA Di, MENG Xianhong, CHEN Hao, CHANG Yan. Evaluation of CMIP5 Earth System Models in Reproducing Leaf Area Index and Vegetation Cover over the Tibetan Plateau. Journal of Meteorological Research, 2014, 28(6): 1041-1060.  doi: 10.1007/s13351-014-4032-4. [19] Sijia ZHANG, Donghai WANG, Zhengkun QIN, Yaoyao ZHENG, Jianping GUO. Assessment of the GPM and TRMM Precipitation Products Using the Rain Gauge Network over the Tibetan Plateau. Journal of Meteorological Research, 2018, 32(2): 324-336.  doi: 10.1007/s13351-018-7067-0. [20] Siqiong LUO, Xuewei FANG, Shihua LYU, Yu ZHANG, Boli CHEN. Improving CLM4.5 Simulations of Land–Atmosphere Exchange during Freeze–Thaw Processes on the Tibetan Plateau. Journal of Meteorological Research, 2017, 31(5): 916-930.  doi: 10.1007/s13351-017-6063-0.
###### 通讯作者: 陈斌, bchen63@163.com
• 1.

沈阳化工大学材料科学与工程学院 沈阳 110142

## Impact of Surface Potential Vorticity Density Forcing over the Tibetan Plateau on the South China Extreme Precipitation in January 2008. Part ll: Numerical Simulation

###### Corresponding author: Guoxiong WU, gxwu@lasg.iap.ac.cn
• 1. State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences, Beijing 100029
• 2. Tianjin Meteorological Service Center, Tianjin 300074
• 3. College of Earth Science, University of Chinese Academy of Sciences, Beijing 100049
Funds: Supported by the National Key Research and Development Program of China (2018YFC1505706), Key Research Program of Fron-tier Sciences of Chinese Academy of Sciences (QYZDY-SSW-DQC018), and State Key Program of National Natural Science Foundation of China (41730963 and 91637312)

Abstract: The surface air convergence on the eastern flank of the Tibetan Plateau (TP) can increase the in situ surface potential vorticity density (PVD). Since the elevated TP intersects with the isentropic surfaces in the lower troposphere, the increased PVD on the eastern flank of TP thus forms a PVD forcing to the intersected isentropic surface in the boundary layer. The influence of surface PVD forcing over the TP on the extreme freezing rain/snow over South China in January 2008 is investigated by using numerical experiments based on the Finite-volume Atmospheric Model of the IAP/LASG (FAMIL). Compared with observations, the simulation results show that, by using a nudging method for assimilating observation data in the initial flow, this model can reasonably reproduce the distribution of precipitation, atmospheric circulation, and PVD propagation over and downstream of the TP during the extreme winter precipitation period. In order to investigate the impact of the increased surface PVD over the TP on the extreme precipitation in South China, a sensitivity experiment with surface PVD reduced over the TP region was performed. Compared with the control experiment, it is found that the precipitation in the TP downstream area, especially in Southeast China, is reduced. The rainband from Guangxi Region to Shandong Province has almost disappeared. In the lower troposphere, the increase of surface PVD over the TP region has generated an anomalous cyclonic circulation over southern China, which plays an important role in increasing southerly wind and the water vapor transport in this area; it also increases the northward negative absolute vorticity advection. In the upper troposphere, the surface PVD generated in eastern TP propagates on isentropic surface along westerly wind and results in positive absolute vorticity advection in the downstream areas. Consequently, due to the development of both ascending motion and water vapor transport in the downstream place of the TP, extremely heavy precipitation occurs over southern China. Thereby, a new mechanism concerning the influence of the increased surface PVD over the eastern TP slopes on the extreme weather event occurring over southern China is revealed.

Reference (42)

/