Bores During IHOP_2002 and Speculation on Nocturnal Convection

1. Bores During IHOP_2002 and Speculation on Nocturnal Convection Or Things that go Bump in the Night 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. IHOP_2002 Design and Nocturnal Convection • Nocturnal precipitation recognized in planning through evening low-level jet flights designed to better understand nocturnal systems (Parsons) • Bores flight mission added (Koch and Geerts) • Much of bore data collected via ground-based measurements and fortuitous circumstances (e.g., Demoz and Raman lidar calibration, evening LLJ flights)

5. Primary Motivation for this Study Some long known facts……. • The Southern Great Plains region has a nocturnal maximum in warm season precipitation. • Long-wave radiation cooling and an absence of solar heating means cooler temperatures at night and likely less potential for convection. • There is no shortage of theories proposed to explain this nocturnal maximum. • Some form of propagation (latest proponent—Carbone and colleagues) • Some form of low-level dynamic forcing (tides, LLJ and regional circulations, etc – dates back to Pleagle et al, recently Dai) • Dynamic forcing north of the stationary front and above the stationary front

6. Hypothesis: If nocturnal convection was forced by persistent regional forcing (LLJ-heating on sloped terrain, tides, etc)………..Then some pronounced signal should appear in the diurnal signal of CAPE and CIN

7. CAPE/CIN: mean Speculation:If nocturnal low-level forcing occurs it is weak and perhaps insignificant in forcing the nocturnal convection. • Larger CAPE for LLJ throughout the diurnal cycle • Maximum CAPE but minimum CIN in the afternoon for LLJ • The 2nd small maximum at ~0.5 km around early morning

8. 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

9. 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)

10. GOAL: Attempt to answer the conundrum of why there is a nocturnal precipitation maximum over the Southern Great Plains when the convective stability is less favorable. METHOD: Focus on nocturnal precipitation systems. Link together IHOP_2002 remote sensing data from (at this point) radar, lidar, and profiler with stability measurements from radisondes.

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

12. Nocturnal MCS 20 June

13. 20 June An example of a nocturnal undular bore

14. 20 June – Surface Data No corresponding temperature change Arrival of wave train in pressure field

15. 4 June

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

17. 18 June 2002

18. 2 June Bore/Wave Event

19. 12 June Bore/Wave Event

20. 13 June Bore/Wave Event

21. 21 June Bore/Wave Event

22. 25 June Bore/Wave Event

23. Question #1 How do nocturnal convective systems “behave”? They behave badly by the standards of daytime convection !! They often produce bores, while daytime convection has long been known to favor gust fronts. Answer #1

24. Question #2 How “common” are these so-called bores and what are their characteristics?

25. BORES!!!!!

27. S-Pol and MAPRbore/wave events ~18 bore and 8 wave events were observed in the S-Pol and MAPR data sets. Bore events are observed in the later stages of LLJ periods when precipitation occurs.

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

29. Answer #2: Bore Charateristics • 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) • A single radars can undercount bore/wave events, since the lifting can be limited to heights above the PBL. Thus, ~26 events may be an undercount. • These disturbances are (almost) always initiated by convection (evidence for both a secondary evening and larger nocturnal initiation). Bores occur later in the program and we did not see bore triggering by dry fronts.

30. Bore Characteristics • 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. • The winds just ahead of the bores are typically a strong low-level jet.

31. Question #3: So what or why are bores important (e.g., Jim Wilson only found three initiation events)?

32. 20 June (MAPR)

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

34. Water Vapor: 20 June

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

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

37. “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

38. IHOP_2002 Sounding Western OK1730 pm LST CAPE CIN

39. 20 June: 3 am Sounding Dramatic moisture increase

40. 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.

41. 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

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

43. 2 June Bore/Wave Event

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

45. 12 June Bore/Wave Event

46. 13 June Bore/Wave Event

47. 21 June Bore/Wave Event

48. 25 June Bore/Wave Event

49. 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.

50. Time of Occurrence