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 Or Things that go Bump in the Night

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

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) Diurnal Cycle of Rainfall

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

US Warm Season Precipitation Eastward propagation of mountain-generated systems from the previous afternoon (Riley et al. 1987, Carbone et al. 2002) 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?

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

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:Caveat: Additional changes present, low-level moisture content increases with SE flow

Nocturnal MCS 20 June

An example of a nocturnal undular bore 20 June

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

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

20 June (MAPR)

Water Vapor: 20 June

20 June Event (cont.)

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:Caveat: Additional changes present, low-level moisture content increases with SE flow

4 June

S-Pol Bore/Wave Events 27 MAY 11 June

18 June 2002

21 June Bore/Wave Event

2 June Bore/Wave Event

12 June Bore/Wave Event

13 June Bore/Wave Event

25 June Bore/Wave Event

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

BORE STATS

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

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

IHOP_2002 Sounding Western OK 1730 pm LST CAPE CIN

20 June: 3 am Sounding Dramatic moisture increase

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

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

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

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.

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.