1. High-resolution Observations and Model Simulations of the Life Cycle of an Intense Mesoscale Snowband over the Northeastern United States Reporter: Prudence Yi-Yun Chien Reference: Novak, D. R., B. A. Colle, and S. E. Yuter, 2008: High-resolution Observations and Model Simulations of the Life Cycle of an Intense Mesoscale Snowband over the Northeastern United States. Mon. Wea. Rev., 136, 1433–1456.

2. 2 Outline Key words Introduction Datasets and methodology Case overview Band life cycle Moisture availability Summary and discussion 2

3. 3 Key words 2D form of frontogenesis equation (Miller, 1948)  Assess frontal forcing for ascent F 2D > 0 3

4. 4 Key words Saturation equivalent potential vorticity (EPV) θ es surface slope > M surface slope EPV<0 Saturation environment 1→21→2 Conditional Symmetric instability CSI condition: 4 Conditional stable Inertial stable, M = fy-u g

5. 5 Introduction Motivation: Advance cool-season QPF => Structural and dynamical evolution of a cool-season mesoscale snowband Using high-resolution obs. & model Case period: 2002/12/25~ 2002/12/26 5

6. 6 Datasets and methodology Dual-Doppler synthesis 1.Upper-air obs 2.Doppler radar 3.Wind profiler 4.ground-based integrated precipitable water vapor (IPW) 5.Conventional surface obs 6.Commercial aircraft Observation 6

7. 7 MM5V3.4 CumulusGrell (1993) MicrophysicsDudhia (1989) PBLMRF ICNCEP - EDAS analysis (0000UTC 25 Dec) BCNCEP – Eta forcast (0000UTC 25 Dec) SSTUS Navy OTIS Model configurations 31 sigma levels 7

8. 8 Case overview Synoptic-scale evolution Mesoscale evolution QPF 8

9. 9 Synoptic-scale evolution 500-hPa Φ (solid) 300-hPa wind (barb) IR brightness temperature (shaded) 9

10. 10 Synoptic-scale evolution isobars (solid) isotherms (gray) wind (barb) 10

11. 11 Mesoscale evolution 700-hPa Φ (thick solid) F 2D (thin solid) reflectivity (shaded) MM5 1800UTC 12/25 2760m 2790m 11

12. 12 Mesoscale evolution 700-hPa θ (thick solid) F 2D (shaded) wind (barb) MM5 700-hPa Φ (thick solid) F 2D (thin solid) reflectivity (shaded) 2100UTC 12/25 2700m 2760m 12

13. 13 Mesoscale evolution 700-hPa Φ (thick solid) F 2D (thin solid) reflectivity (shaded) MM5 0000UTC 12/26 2670m 2728m 13

14. 14 QPF 24-h accumulated precipitation (liquid equivalent) 1200UTC 12/25 ~ 1200UTC 12/26 Max = 76mm Max = 53mm underpredict ~ 30% Max = 59mm underpredict ~ 22% Max = 46mm underpredict ~ 40% 14

15. 15 Band life cycle Band formation Band maturity Band dissipation Model time series 15

16. 16 Band formation Conv. R1 R2 Conv. R1* R2* Conv. R2 Defm. Conv. R2* Defm. 1802UTC WSR-88D 1929UTC WSR-88D 1800UTC MM5 1930UTC MM5 F 2D (shaded) θ (solid) Reflectivity, 1km (shaded) wind, 3km (barb) 16

17. 17 Reflectivity (shaded) Ascent (dot) F 2D (solid) EPV (shaded) RH=100% (thick solid) Negative η (dot) θ es (gray solid) II & CI CI II & CI snowfall F2D max Ascent max 1802UTC WSR-88D 1800UTC MM5 II F2D max Ascent max II CI: conditional instability II: inertial instability 17

18. 18 Band maturity 2101UTC WSR-88D 2100UTC MM5 R2 R2* F 2D (shaded) θ (solid) Reflectivity, 1km (shaded) wind, 3km (barb) 18

19. 19 Reflectivity (shaded) Ascent (dot) F 2D (solid) EPV (shaded) RH=100% (thick solid) Negative η (dot) θ es (gray solid) 2101UTC WSR-88D 2100UTC MM5 CI Weak CSI II F2D max Ascent max F2D max Ascent max CS Weak CSI CS: conditional stability 19

20. 20 2300UTC MM52359UTC WSR-88D 2300UTC MM5 R2 R2* Band dissipation F 2D (shaded) θ (solid) Reflectivity, 1km (shaded) wind, 3km (barb) 20

21. 21 2300UTC MM5 2359UTC WSR-88D Reflectivity (shaded) Ascent (dot) F 2D (solid) EPV (shaded) RH=100% (thick solid) Negative η (dot) θ es (gray solid) II CI II 21

22. 22 Model time series Cross section A-B 22

23. 23 Moisture availability Obs: IPW 23

24. 24 Band formation (1930UTC 12/25 MM5) Band dissipation (2200UTC 12/25 MM5) 1930UTC 2200UTC 1930UTC 2200UTC 24

25. 25 Compare QPF between 2100UTC and 2200UTC Ascent profileΘ e profile(QPF 2200 -QPF 2100 )/QPF 2100 same-6% same-23.5% => Changes in ascent dominated changes in moisture MDMD -0.14

26. 26 Band evolution => forcing, stability, and moisture Forcing (MM5 results) (1) Band formation  deepening of a midlevel trough Increase in deformation, convergence, and frontogenesis (2 )Band dissipation  midlevel trough less defined, and frontogenesis weakened Conditional stability led to weak frontal circulation Summary and discussion

27. 27 Summary and discussion Stability Conventional thinking: CI and SI increase => band formation This study: CI occurred before band formation~1.5h band formation: CI decrease & F 2D increase W max & F 2D max Previous study: W max locate on the warm side of F 2D max ~50-200km This study: W max and F 2D max are nearly coincident

28. 28 Summary and discussion Moisture source: Atlantic Ocean (band formation) change of ascent => change of moisture QPF MM5 underforecast ~30% max preci. (Δx=4km) axis of heaviest preci. ~50km to the SE of the observation location Higher horizontal model resolution => improvement QPF much better improvement on F2D, stability, and moisture

29. Thanks for your listening. & Questions?

30. 30 Conditional instability (CI) (Holton, 2004: An Introduction to Dynamic Meteorology (4 th Ed.), Fig9.10, p.294) (2) Air parcel reach the LFC (1) 30

31. 31 Inertial instability (II) Absolute momentum: M = fy - u g (Holton, 2004: An Introduction to Dynamic Meteorology (4 th Ed.), p.205) In this study, set M = fy – u u=u g +u’ 31

32. 32 Fig.7 Fig.8 Band formation 32

33. 33