George Tai Jen Chen Department of Atmospheric Sciences National Taiwan University ( August 2006 )

1. The role of latent heating on the development and evolution of Mei-yu frontal systems George Tai Jen Chen Department of Atmospheric Sciences National Taiwan University ( August 2006 )

2. Mei-yu Season • Transition period from prevailing winter northeasterlies to summer southwesterlies over east Asia. • Quasi-stationary frontal system. (Mei-yu front, Baiu by Japanese) • Mid-May to mid-June for Taiwan and southern China.

3. Mei-yu Front (MYF) • Some are similar to midlatitude polar fronts. (Trier et al. 1990, Mon. Wea. Rev.) • Most other cases: weaker horizontal temperature gradient, strong cyclonic shear. (Akiyama 1973, Pap. Meteor. Geophys.; Kato 1985, J. Meteor. Soc. Japan) • Chen and Chang (1980, Mon. Wea. Rev.): structural difference between the eastern and western sections of MYF.

4. The western section of MYF • Chen and Chang (1980, Mon. Wea. Rev.): Similar to tropical disturbance: quasi-barotropic warm core structure, weak ▽T, strong wind shear. • Kuo and Anthes (1982, Pap. Meteor. Res.): Relative vorticity can be utilized as an indicator for frontal intensity. • Cho and Chen (1995, J. Atmos. Sci.): strong potential vorticity anomaly with mature MYF, Mei-Yu frontogenesis by CISK.

5. Low-level Jet (LLJ) • Strongly correlated with heavy rainfall. (Akiyama 1973, Pap. Meteor. Geophys.; Chen and Yu 1988, Mon. Wea. Rev.) • Early studies: downward transport of momentum by cumulus convection. (Matsumoto 1973, J. Meteor. Soc. Japan; Ninomiya and Akiyama 1974, J. Meteor. Soc. Japan) • Cumulus latent heating on LLJ formation. (Chou et al. 1990, Mon. Wea. Rev.)

6. (a) Case 1: 12-13 June 1990(Chen et al. 2003, Mon. Wea. Rev.) Mei-yu frontogenesis 850 hPa weather map and PV at 12Z 12 June Wind shear and PV (10-2 PVU) accompanying the front.

7. (b) 850 hPa weather map and PV at 00Z 13 June PV along the front significantly increase (frontogenesis) with a LLJ formation to the south of the front during the 12 h.

8. PV inversion techniques(Davis and Emanuel 1991, Mon. Wea. Rev.) • PV: conserved property and invertibility. • Nonlinear balance equation (Charney 1962, Proc. Symp. Numerical Weather Prediction, Tokyo) • Given a known distribution of PV and specified boundary conditions, the system can be solved to give height and wind fields under nonlinear balanced relationship.

9. Piecewise inversion The PV anomalies can be divided into any number of parts and the height and the wind field associated with each part can be obtained. • Prognostic system q/t, /t, /t, , and  under nonlinear balanced condition can be obtained.

10. Scheme for q’ partitioning and contributions to frontal intensity from all processes at 850 hPa PV anomaly (109.125-117E; 29.25-30.375N ) associated with latent heat release (ms) were responsible for the frontogenesis.

11. B A w GMS IR imagery and vertical motion as computed by PV prognostic system along AB at 00Z 13 June Upward motion (cm s-1) computed by prognostic system was closely matching the position of deep convection on cloud imagery.

12. - + - + q/t /t PV tendency and height tendency as computed by PV prognostic system along AB at 00Z 13 June Positive PV tendency and negative height tendency (frontogenesis) at low level were related to the MCSs.

13. Mei-yu fronotogenesis by CISK • q/t is directly proportional to both the vertical gradient of heating/cooling rate and the absolute vorticity. • In a quasi-barotropic system, the vertical component of η is rather close to q. • q/t is proportional to q→ nonlinear interaction.

14. w q/t If ms is reduced by ½ at 00Z 13 June - + Similar vertical motion pattern with much less PV generation at the low level.

15. Conclusion • PV perturbations related to latent heat release from MCSs were responsible for the frontogenesis. • CISK mechanism proposed by Cho and Chen (1995) was observed to be responsible for the Mei-yu frontogenesis.

16. Case 2 : 7-8 June 1998(Chen et al. 2006, Mon. Wea. Rev.) Northward retreating Mei-yu front • Although this phenomenon is not rare, the mechanism has never been investigated.

17. GMS IR imagies Frontal cloud band with an organized MCS over the frontal disturbance moved northeastward.

18. Synoptic maps at 850 hPa between 12Z 7 and 06Z 8 June Trough deepened in association with the organized MCS, and the southwesterly winds intensified (LLJ formation) to the south of the MCS.

19. Composite vorticoty and divergence at 925 and 850 hPa normal to and across the MYF (at 0) during 12Z 7 - 06Z 8 June Vorticity in phase with convergence. Comparable values of vorticoty at both levels. Nearly no vertical tilt.

20. Retreat of the front Time variations of vorticity budget across the front at 850 hPa Effect of horizontal vorticity advection (10-5 s-1(6h)-1) mainly caused the northward retreat of the front. (The vital role of the LLJ to the southwest of the front.)

21. Scheme for q’ partitioning and contributions to frontal intensity at 850 hPa from all processes PV anomaly associated with latent heat release (LLh) were mainly responsible for the frontogenesis.

22. 12Z 7 June 18Z 7 June 06Z 8 June 00Z 8 June LLJ formed and intensified largely through the Coriolis acceleration of ageostrophic winds( z). ( //z shaded) T The formation of LLJ: ageostrophic wind analysis

23. 12Z 7 June 00Z 8 June 06Z 8 June The formation of LLJ: PV perspective PV anomaly due to latent heating (LLh) and the associated (inverted) balanced winds at 850 hPa • Front intensified through latent heat release. • LLh caused the increase of southwesterly wind components to the southeast of the MCS.

24. Wind vectors averaged over a hexagonal domain centered along the axis of the LLJ from different PV anomaly components at 00Z 8 June When southwesterlies associated with LLh are superimposed upon the background SW monsoonal flows → LLJ formation.

25. Conclusion • Strong southwesterly flow (LLJ) led to rapid retreat of the front while the movement was dominated by horizontal advection. • Enhanced gradient of height tendency induced ageostrophic winds, and the LLJ formed through Coriolis acceleration of these winds.

26. Case 3: 6-7 June 2003(Chen et al. 2006, submitted to Mon. Wea. Rev.) (c) 18Z 6 June (a) 00Z 6 June (d) 00Z 7 June (b) 12Z 6 June Cyclogenesis A Mei-yu front over southern China intensified with a development of frontal disturbance and an LLJ formation at 850 hPa within a 24-h period Synoptic maps at 850 hPa between 00Z 6 and 00Z 7 June

27. Synoptic maps at 500 and 300 hPa at 00Z 6 June 500 hPa 300 hPa No favorable synoptic-scale system at upper levels

28. Question Was the CISK mechanism responsible for Development of frontal disturbance? Mei-yu frontogenesis? 3 Formation of the LLJ? Data and methodology Diagnosis using ECMWF 1.125 data andmethods including the piecewise PVIT and vorticity budget analysis.

29. Wave-like structure of the frontal disturbances(studied by Kuo and Horng 1994, Terr. Atmos. Ocean; Du and Cho 1996, J. Meteor. Soc. Japan → Barotropic instability) 12Z 6 June 00Z 7 June 18Z 6 June B C A C B A C B A D D Relative vorticity (10-5s-1) at 850 hPa and Satellite (GOES-9) imageries Individual vorticity maxima along the front about 400 km apart (wave-like), in good agreement with the MCSs

30. Some fields at 18Z 6 June 6-h sfc rainfall (mm), 12-18Z 6 June ω (Pa s-1) at 700 hPa  Relative vorticity (10-5s-1) at 850 hPa • Strongest 700-hPa vertical velocity and surface rainfall also along Meiyu front, consistent with vorticity centers • Convection did occur along the front, as well as south of the front

31. Vorticity bueget analyse (18Z 6 June) local horizontal vertical convergence/ tilting residual tendency advection advection stretching Eastward movements of vorticity centers Eastward movements of frontal disturbances

32. local horizontal vertical convergence/ tilting residual tendency advection advection stretching Major contributor toward the generation of frontal vorticity Southward movements of the front at later stage

33. 125 hPa ’ 150 hPa q’ 200 hPa q’ 250 hPa q’ 300 hPa q’ 400 hPa q’ 500 hPa q’ 700 hPa q’ 850 hPa q’ 925 hPa q’ 962 hPa ’ 12Z 6 June 06Z 6 June 18Z 6 June 06Z 6 Jun 00Z 7 June ul ms mu RH  70% RH < 70% and q’ 0 or q’ < 0 lb 18Z 6 June 00Z 7 June Piecewise PV inversion Domain of PV inversion Area of interest Front LLJ Mean field: One-month mean from 15 May to 15 June, 2003

34. Frontogenesis and cyclogenesis Height (gpm), wind (ms-1), and vorticity (10-5s-1, shaded) associated with ms at 00Z 7 June PV anomaly associated with latent heat release (ms) were responsible for the frontogenesis and cyclogenesis.

35. Ageostrophic flow C B A D 18Z 6 June 00Z 7 June Northward-directed ageostrophic flow at 850 hPa to the south of developing MCSs, producing local wave like LLJ maxima through Coriolis torque

36. Apparent heat source (Q1) computed for the frontal zone Pressure (hPa) 12Z 6 June 18Z 6 June 00Z 7 June Heating rate (C per 6 h) Intense heating throughout troposphere and strongest at 400 hPa, reaching 30C per day at 18Z June 6

37. Heating efficiency • Heating efficiency related to horizontal and time scale of convection: • Rossby radius of deformation (LR): N ~ 1.3 102 s1,h ~ 7 km, ~ 1.8  104 s1 LR= Nh/~ 500 km • Horizontal scale of MCSs L LR • Latent energy released inside the frontal MCSs could heat the atmosphere effectively → Wind field adjusted toward the mass field → Cyclogenesis

38. Conclusion • The CISK mechanism (cyclone-cumulus feedback) was responsible for development of the wave–like disturbances (cyclogenesis) along the Mei-yu front. • Frontal strength was maintained by stretching (convergence) effect. Eastward development was due to horizontal advection, and slowly southward migration at later stages was due to tilting effect.

39. Northward-directed ageostrophic flow at 850 hPa to the south of developing MCSs produced local LLJ maxima through Coriolis torque. • Both frontal strengthening and LLJ development were largely attributed to PV perturbations associated with latent heat release (“ms”), and minimum effects were from adiabatic processes.