Bores During IHOP_2002 and Speculation on Nocturnal Convection

1. Or Things that go Bump in the Night Bores During IHOP_2002 and Speculation on Nocturnal Convection David B. Parsons, Crystal Pettet and June Wang NCAR/ATD Acknowledgements to Tammy Weckwerth, Ed Browell et al., Cyrille Flamant et al., and Steve Koch and the bore working group

2. Primary Motivation for this Study Some long known facts……. • The Southern Great Plains region has a nocturnal maximum in warm season precipitation.

3. Diurnal Cycle ofRainfall Diurnal variation of hourly thunderstorm frequency over the United States. Normalized amplitude of the diurnal cycle is given by the length of the arrows in relation to the scale at bottom left. (Amplitudes are normalized by dividing by the mean hourly thunderstorm frequency averaged over the 24 hr of the day at each station.) Phase (time of maximum thunderstorm frequency) is indicated by the orientation of the arrows. Arrows directed from north to south denote a midnight maximum, arrows directed from east to west denote a 6 a.m. maximum, those from south to north denote a midday maximum, etc. [Based on data in Mon. Wea. Rev., 103, 409 (1975).] (From J.M. Wallace & P.V. Hobbs, “Atmospheric Science An Introductory Survey”, Academic Press, New York, NY, 1977, pp.43)

4. Sounding-based Schematic of Nocturnal Convection Initiation Cases of this type were few during IHOP_2002 and not yet analyzed. Future talk. From Trier and Parsons 1993

5. US Warm Season Precipitation Speculation: Since there are no strong signals in the mean CAPEs and CINS, perhaps convection itself may hold the key to propagation.How do nocturnal convective systems behave? • Eastward propagation of mountain-generated systems from the previous afternoon (Riley et al. 1987, Carbone et al. 2002)

6. Question #1 How do nocturnal convective systems “behave”?

7. 20 June Case • Undular-bore like structure present in radar and profiler data (actually 3 events were present) • Net effect of the bore is a (~200 hPa) deepening of moisture and a reduction in convective inhibition • Now examining additional cases • Caveat: Additional changes present, low-level moisture content increases with SE flow

8. Nocturnal MCS 20 June

9. 20 June An example of a nocturnal undular bore

10. 20 June – Surface Data No corresponding temperature change Arrival of wave train in pressure field

11. 20 June Doppler Velocity Doppler Velocity Example of a Nocturnal Undular Bore

12. 20 June (MAPR)

13. Water Vapor: 20 June

14. 20 June Event (cont.)

15. 20 June Case • Undular-bore like structure present in radar and profiler data (actually 3 events were present) • Net effect of the bore is a (~200 hPa) deepening of moisture and a reduction in convective inhibition • Now examining additional cases • Caveat: Additional changes present, low-level moisture content increases with SE flow

16. 4 June

17. S-Pol Bore/Wave Events 27 MAY 11 June

18. 18 June 2002

19. 21 June Bore/Wave Event

20. 2 June Bore/Wave Event

21. 12 June Bore/Wave Event

22. 13 June Bore/Wave Event

23. 25 June Bore/Wave Event

24. BORE Example From MAPR 4 June Post height Pre-bore height

25. BORESTATS

27. Pre-bore Winds: Composite 1000 m 800 m 1300 m 2700 km

28. Bore Height Displacements Scattering Layer Height (km) Reference slope of .5 m/s Reference slope of .5 m/s Time (mins)

29. IHOP_2002 Sounding Western OK1730 pm LST CAPE CIN

30. 20 June: 3 am Sounding Dramatic moisture increase

31. Post-bore: Elevated convection is preferred (high CAPE, low CIN) Day-time: Surface-based convection is preferred but high CIN

32. “Surface”-based Parcel 20TH June Unstable, capped env. 1730 pm Dramatic stabilization, expected due to radiational cooling ! 0301 am Very stable

33. “Surface” and Inversion Parcels 0301 am 1730 pm 1730 pm 0301 am Opposite trends In fact the parcels are easier to convect than during the day!!!! Instability increases during the night

34. Question #3: Why are bores important? • Bores provide extremely strong lifting that leaves an environment in their wake that can be unstable to convective lifting aloft. • Since this wake air feeds nocturnal convection, bores are a possible mechanism for maintaining deep convection in the presence of unstable surface conditions. • Large stability and moisture variations are found during the subsequent day. SPC forecaster feel bores likely explain these variations.

35. Findings • Bore/wave disturbances are ubiquitous over this region at night when convection is present. ~26 event. Most events occur at the end of LLJ moisture return periods (when convection is present) • These disturbances can promote intense lifting with net displacements of up to ~1-2 km. They creating a deeper moist inflow and favorably impact stability. Peak vertical motions are >1-2 m/s. • Surface radars undercount bore/wave events (at a fixed location), since the lifting can be limited to heights above the PBL. Thus, ~26 events is likely a severe undercount! • These disturbances are (almost) always initiated by convection (slight evidence for both a secondary evening and larger nocturnal initiation). Later in the program and initiation is not by dry fronts. • Typical spacings of waves ~10-14 km, surface evidence (pressure disturbances (.25 – 1.5 hpa) with some closed circulations, typical duration is ~3-6 hrs with mesoscale to synoptic coverage areas.