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Group Report

Group Report. 時間 ： 2013/10/08 地點： S1-802 講 員：簡文鴻 指導 教授：楊明仁 老師. Impacts of typhoon track and island topography on the heavy rainfalls in Taiwan associated with Morakot (2009). BAOGUO XIE

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Group Report

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  1. Group Report 時間：2013/10/08地點：S1-802講員：簡文鴻 指導教授：楊明仁 老師

  2. Impacts of typhoon track and island topography on the heavy rainfalls in Taiwan associated with Morakot (2009) BAOGUO XIE Laboratory for Climate and Ocean-Atmosphere Studies, Department of Atmospheric and Oceanic Sciences, Peking University, Beijing, China, and Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania FUQING ZHANG Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania

  3. Outline • Keywords. • Introduction.(Section 1) • Model configuration and experimental design. (Section 2) • Overview of Morakot and performance of the WRF control and reference simulations.(Section 3) • Sensitivity of the rainfall to Morakot’s track. (Section 4) • Effects of island topography on rainfall predictability. (Section 5) • Monsoon–typhoon interaction. (Section 6) • Summary and conclusions(Section 7) • Reference

  4. Keywords • Rainfall • Topography • Southwest Monsoon

  5. Introduction • Taiwan suffers from heavy rains and flood disasters caused by TCs each year. Because of the complex topographic features and their interactions with TCs. • The predictability of rainfall associated with TCs is strongly dependent on an accurate forecast of the TC track, which may be significantly deflected by the complex high topography. • The southwesterly monsoon flow is believed to be responsible for supplying large amounts of moisture to the TC environments during the monsoon season. However, monsoon plays in the heavy rainfall is difficult to elucidate since the monsoon is a large-scale system, acting as a background environment.

  6. Z10[Zhang et al. 2010]: Examined the predictability of Morakot with a cloud-resolving ensemble initialized with analysis andflow-dependent perturbations obtained from a real time global ensemble data assimilation (EDA) system. And they were able to predict this record-breaking rainfall event. • F11[Fang et al. 2011]: Found that the orography is crucial in determining the structure, intensity, and variability of Morakot’s rainfall. • Both of them concluded that the forecasting of extreme rainfall events such as Morakot would benefit from probabilistic prediction provided by a high-resolution mesoscale ensemble forecast system.

  7. The primary focuses of the current study are (1) Relationship between predictability of the typhoon track and precipitation. (2) Enhancement of orographically induced rainfall. (3) Enteractions between the southwest monsoon and Morakot.

  8. Model configuration and experimental design • WRF(Skamarock et al. 2007) • - 2 two-way-nested model domains. • - Initiation: • 1) 0000 UTC 5 Aug. (CNTL00) • 2) from Global EDA system[Z10] • Global EDA system: • - 60 members (flow-dependence uncertainty) • - Period: 0000 UTC 5 Aug. ~ 1200 UTC 10 Aug. Total:132h • - No cumulus parameterization. • - Microphysics scheme: the WRF Single-Moment 6-class(WSM6) graupel. • Observed track and intensity data : • - JTWC • The Final Operation Global (FNL) analysis data: • -NCEP(for synoptic analysis and comparison.)

  9. d01 D01 D02

  10. Overview of Morakot and erformanceof the WRF control and reference simulations • 2 August 2009 (tropical depression) 5 August (T.C) 7 August (945hPa)(before landfall) • Effects on Taiwan lasted from 6 to 10 August • Made landfall at 1800 UTC 7 August and August 11 apartedto China • The strongest rainfall occurred between 0000 UTC 8 August and 0000 UTC 9 August (Z10; Hong et al. 2010; F11).

  11. H H 850-hPa wind (full wind barb is 10 m /s) 850-hPa Geopotential height (solid contour lines; from 1150 to1530 m by every 40 m) 500-hPa height (dashed contour lines; from 5440 to 5880 m by every 40 m) Precipitable water (PW,shaded; kg /m^2)

  12. Worst performance. Good performance. Forecast from ensemble mean. Best performance. The 72-h accumulated rainfall (shaded; mm) of Typhoon Morakot over Taiwan(from 0000UTC6 Aug to 0000UTC9 Aug)

  13. Sensitivity of the rainfall to morakot’strack • NAPE (Normalized absolute precipitation error ) N- the total number of grid points with precipitation in either the observation and/or the simulation. Ri- the precipitation in an ensemble member. Roi- the precipitation in an observation. A smaller value of NAPE indicates a better forecast of precipitation.

  14. a) Ensemble-based analysis The members with good track forecasts also produced good rainfall forecasts. [Z10] Track error: distance between the positions of the ensemble member and best-track observations. (averaged from the 3-h a prior and posterior to landfall time.) NAPE: 72-h accumulated precipitation error. (from 0000UTC6 Aug to 0000UTC9 Aug) The accuracy of the quantitative precipitation forecast (QPF) for Morakotis closely related to the predictability of its track.

  15. Sensitivity experiment of moved terrain • MOVE00 • MOVE54 200km 160km

  16. Orographic precipitation enhancement a. No-terrain experiments - OCEAN CNTL00*45% Morakot brings extremely heavy and asymmetrically distributed rainfall to Taiwan Island even if there is no topography or land. (Same as F11.) The 72-h accumulated rainfall (shaded; mm) of Typhoon Morakot over Taiwan(from 0000UTC6 Aug to 0000UTC9 Aug)

  17. a. No-terrain experiments - LAND

  18. the vertical wind shear can influence the storm motion and the distribution of associated convection (Corbosiero and Molinari 2002, 2003;Cecil 2007; Gao et al. 2009). • The rainfallmay also appear more substantial on the downshear left if the falling hydrometers in this sector are advected by the TC’s cyclonic circulation (e.g., Frank and Ritchie 1999).

  19. vector; m/(s*hPa) The vertical wind shear between 200 and 700 hPa, shaded; mm 3-h accumulated rainfall

  20. in the inner domain over Taiwan maximum rainfall -30% -10% 3 2247mm,-36% -30% -3% 1410mm,-49% 1067mm,-42% Over Taiwan Maximum rainfall In the inner domain (4.5km) The 72-h accumulated rainfall (shaded; mm) of Typhoon Morakot over Taiwan(from 0000UTC6 Aug to 0000UTC9 Aug)

  21. Meridionallyaveraged 72-h accumulated rainfall distribution in a west–east direction 00 15 23

  22. Scatterplots of 72-h accumulated gridpoint rainfall over Taiwan CNTL-HALF vs. CNTL LAND-OCEAN vs. LAND HALF-LAND vs. HALF leeward windward

  23. 1D model for terrain effects 1 2 3 4 5

  24. Meridionalmeans over the heavy rainfall region at 0000 UTC 8 August horizontal wind water vapor 1D model–simulated rainfall

  25. The monsoon–typhoon interaction • The southwest monsoon is a large-scale system that is believed to have significant impacts on the development of tropical cyclones in the northwest PacificOcean (Carr and Elsberry 1995; Briegeland Frank 1997; Chen et al. 2004; Wu et al. 2011). • Lee et al. (2011) and Chien and Kuo (2011) highlighted the convergence of the typhoon circulation with the monsoon flow as an important factor for creating a quasi-stationary zonal convective band, which induced the heavy rainfall over Taiwan. • Nguyen and Chen (2011) documented that the combined circulations associated with the tropical storms and monsoon gyre brought in moisture-laden flows toward the western slopes of southern Taiwan that result in heavy rainfall. • Wuetal. (2011) showed thatMorakotcoincided with a quasi-biweekly oscillation and the Madden–Julianoscillation, which enhanced the synoptic-scale southwesterly winds of Morakot and thus decreased its westward movement, leading to a long residence time in thevicinityof Taiwan.

  26. The 850-hPa wind and height at 0000 UTC 8 Aug Mean 850-hPa wind speed of 96-h forecast in the rectangular region Tracks for CNTL00,CNTL54, and INNER00 1.5m/s

  27. Summary and conclusions • Good rainfall forecast foremost requires good track forecast. • The rainfall forecast will be greatly degraded if the island is moved away from the typhoon’s path (MOVE00 versus CNTL00) while the forecast will improve immensely if the island is relocated to the direct path of the storm (MOVE54 versus CNTL54).

  28. The impacts of the island topography on rainfall intensity and distribution were then examined through a series of sensitivity experiments that differ from the three reference simulations (CNTL) by HALF,LAND and OCEAN • These sensitivity experiments show that (i) The presence of Taiwan leads to the redistribution of rainfall from the ocean to the island but does not necessarily extract more total water from the atmosphere. (ii) It is primarily the island topography, not the land interaction, that causes the redistribution of the rainfall. (iii) The impact of the topography is by no means a linear function of terrain height or terrain slope.

  29. The strength of the southwesterly monsoon gyre could be strongly enhanced by the approaching typhoon circulation, which leads to stronger convergence and moisture transport near Taiwan. These sensitivity experiments show that the interaction between the typhoon and topography, rather than the location of the monsoon flow, contribute the most to the heavy rainfall caused by Morakot.

  30. Reference • Xie, B., and F. Zhang, 2012: Impacts of typhoon track and island topography on the heavy rainfalls in Taiwan associated with Morakot(2009). Mon. Wea. Rev., 140, 3379–3394. • Zhang, F., Y. Weng, Y.-H. Kuo, J. S. Whitaker, and B. Xie, 2010:Predicting Typhoon Morakot’s catastrophic rainfall with a convection-permitting mesoscale ensemble system. Wea.Forecasting, 25, 1816–1825. • Fang, X., Y.-H. Kuo, and A. Wang, 2011: The impact of Taiwan topography on the predictability of Typhoon Morakot’srecord-breaking rainfall: A high-resolution ensemble simulation. Wea. Forecasting, 26, 613–633. • Braun, S. A., and L. Wu, 2007: A numerical study of Hurricane Erin (2001). Part II: Shear and the organization of eyewall vertical motion. Mon. Wea. Rev., 135, 1179–1194.Meteorological WindowWebsite • Importance of the upper-level warm core in the rapid intensification of a tropical cyclone Da-Lin Zhang1 and HuaChen1Received 6 December 2011; revised 25 December 2011; accepted 27 December 2011; published 28 January 2012 • Roles of upper-level processes in tropical cyclogenesis Da-Lin Zhang1 and Lin Zhu1 Received 16 July 2012; revised 23 July 2012; accepted 1 August 2012; published 12 September 2012. • 台灣南部地區地形降雨特性初步研究,李宗融、于宜強,國家災害防救科技中心 • 侵台颱風之定量降水預報研究：模式範圍與解析度之影響,王重傑,國立台灣師範大學地球科學系

  31. END

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