THE LIFE CYCLE OF A BORE EVENT OVER THE US SOUTHERN GREAT

PLAINS DURING IHOP_2002

IHOP Science workshop, Toulouse, 14-17 June 2004

C. Flamant1, S. Koch2, M. Pagowski3

1 IPSL/SA, CNRS, Paris, France 2 NOAA FSL, Boulder, Colorado 3 CIRA, Boulder, Colorado

T. Weckwerth4, J. Wilson4, D. Parsons4, B. Demoz5, B. Gentry5, D. Whiteman5,G. Schwemmer5, F. Fabry6, W. Feltz7, P. Di Girolamo8

4 NCAR/ATD, Boulder, Colorado 5 NASA/GSFC, Greenbelt, Maryland6 Mc Gill University, Montreal, Canada 7 CIMSS, U. of Wisconsin, Madison, Wisconsin

8 U. degli Studi della Basilicata, Potenza, Italy

The 20 June 2002 ELLJ missionOn 20 June 2002, the life cycle of a bore (i.e. triggering, evolution andbreak-down) was sampled in the course of night time ELLJ mission duringwhich 2 aircraft and a number of ground- based facilities were deployed.

RUC 20 km (0300 UTC)

terrainS-POL

Homestead:MAPR, ISS,SRL, GLOW

NRL P-3(LEANDRE 2and ELDORA)

MCS

LearJetdropsondes

The bore was triggered by a thunderstorm outflow

Objectives

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• Analyse the life cycle of a bore event (how it is triggered,how it evolves, how it dies…)

• Compare observations with hydraulic theory,

• Provide validatation for high-resolution numericalsimulations of this event.

Observations and simulation

The 20 June 2002 bore event

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Gravity current Bore Soliton

CIDD analyses (S-POL and DDC radar reflectivity + surface mesonets)

Data used to analyse the « bore » event life cycle:• Triggering (gravity current): DDC and S-POL radars, surface mesonets• Temporal evolution: airborne DIAL LEANDRE 2, DDC and S-POL radars,

surface mesonets, dropsondes, in situ P-3• Break-down: Profiling in Homestead (SRL, GLOW, MAPR), ISS soundings,

S-POL radar, surface mesonets

CIDD analyses

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Gravity current Bore Soliton

CIDD analyses (S-POL and DDC radar reflectivity + surface mesonets)

The different stages of the event:• Gravity current: radar fine line + cooling + pressure increase• Bore: 1 or 2 radar fine lines + no cooling + pressure increase• Soliton: train of wavelike radar fine lines + no cooling + pressure increase

A fine line in the radar reflectivity fields is indicative of either Bragg scattering associatedwith pronounced mixing or Rayleigh scattering due to convergence of insects or dust.

CIDD analyses

CIDD analyses

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Homestead

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Vertical structure of the bore

The bore was best observedalong a N-S radial coincidingwith P-3 track 1

S-POL RHIs: contineouscoverage (0530-0730 UTC)

Airborne DIAL LEANDRE 2:4 overpasses of Homestead

3 legs of LearJet dropsondes

Homestead Profiling Site:SRL, GLOW, MAPR

LEANDRE 2 : 1st pass track 1

0141-0209 UTC

Moistening

L2 WVMR retrievals:

100 shots (10 sec.) 800 m horizontal resolution 300 m vertical resolution Precision:0.05-0.1 g kg-1 at 3.5 km0.3-0.4 g kg-1 near surface

0329-0352 UTC

LEANDRE 2 : 2nd pass track 115 km

0.8 km

0.8 km

• Amplitude ordered waves• Inversion surfaces lifted successfully higher by each passing wave • Trapping mechanism suggested by lack of tilt between the 2 inversion layers

Dry layer

0408-0427 UTC

LEANDRE 2 : 3rd pass track 117 km

0.8 km

0.8 km

• Amplitude ordered waves• Inversion surfaces lifted successfully higher by each passing wave • Trapping mechanism suggested by lack of tilt between the 2 inversion layers

h0h1

h1/h0~2.1

Dry layer

0555-0616 UTC

LEANDRE 2 : 4th pass track 111 km

• Waves are no longer amplitude ordered• Inversion surfaces lifted successfully higher by each passing wave (not expected)• Lifting weaker than previously• Trapping mechanism suggested by lack of tilt between the 2 inversion layers

0.6 km

Dry layer

S-POL RHIsAzimuth 350°

Horizontal wavelengthconsistent with L2observations of the soliton

0530 UTC

Strong Low-level Jet : 27 m/s jet coreat ~0.5 km AGL (1.4 km MSL). Agrees with best with Homestead 0600 UTC sounding.

The strong jet is created in response to nocturnal cooling. The jet is strongest at the time when the static stability in the 1.2-1.8 km MSL layer is strongest.

S-POL RHIsAzimuth 350°

LLJ still presentThe soliton is no longer seen

0702 UTC

Note existence of a Low-Level Jet (25-30 m/s magnitude), but the absence of the waves seen in S-POL & Leandre.

MAPR

Observations in Homestead

SRLBore arrival

Drylayer

Observations in Homestead

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LLJ max v0220, 400 m

Bore arrival

GLOW

Summary - observationsThe life cycle of a « bore » event was observed as fine lines in S-POL reflectivityand Mesonet data (CIDD analyses) as well as by LEANDRE 2, S-POL RHIs, ISS,and MAPR: it occured along an outflow boundary that propagated southward at aspeed of ~11 m/s from SW KS into the Oklahoma panhandle

The GC that initiated the bore disapeared shortly after 0130 UTC over SW KS.The bore then propagated southward, and evolved in a soliton)

With h1/h0~2.1, the bore can be classified as a strong bore (however thetheoretical transition region appears at h1/h0=2…)

Solitary waves developed to the rear of the leading fine line atop a 700 – 900 mdeep surface stable layer. Depth of stable layer increased by 600 m with passageof leading wave. The inversion was then lifted by each passing wave. Similar trendsare observed in the elevated moist layer above

Solitary waves characteristics: horizontal wavelength = 16 km at an early stage,decreasing to 11 km upon reaching Homestead; phase speed = 8.8 m/s prior to0430 UTC, and 5 m/s afterward. Waves exhibited amplitude-ordering (leadingwave always the largest one) except at a latter stage. Evidence of wave trapping.

Where do we go from here?• Verify to what extend observations are compatible with theory(Simpson, 1987; Rottman and Simpson, 1989; Koch et al., 1991;Egger 1984 – Kortewegeg-deVries-Burgers equation)

We have assessed a number of CG and bore related quantities need to confront hydraulictheory (propagation speed of GC and bore; cooling associated with the GC; pressureincrease associated with the GC and bore; lifting; horizontal wavelength).

• Assess the trapping mechanisms allowing the bore to maintain all theway to Homestead

We are (or will be) investigating this using Scorer parameter (RDS) and cross-spectralanalyses (in situ and L2). Possible generation of KH waves by wind shear will also beinvestigated.

• Understand the mechanisms leading to the bore breakdown south ofHomestead

Is this caused by orography, the presence of the strong, very narrow jet or the fact thatwe no longer have stably stratified conditions. In the latter case, is this related to theCAPE and CIN redistribution with altitude (induced by the bore itself), leading to theinjection of water vapor above the NBL ?

First attempt to simulate the event using MM5

• Hourly LAPS analyses (initialization + forcing)• 2-km resolution domain nested (1-way) in a 6-km domain• 44 levels:

• 20 levels below 1500 m• 10 m vertical resolution at surface• 250 m vertical resolution at the top of the BL

2D horizontal fields of :• temperature• precipitation• divergence

2D vertical cross sections of RH and potential temperaturethrough the bore

0100-0730 UTC

• Triggering mechanisms seems to be OK• The bore is produced to far to the north• The bore is triggered and dissipates at the same time asin the observations (pure luck??) •Wavelength (~15 km) in agreement with observations• Number of waves too small (5 at most)• Trapped waves are observed up to 3 km MSL which is consistentwith observation • Elevated moist layer not reproduced• Elevated inversion close to the surface is tilted

Summary - simulations

We still have a long way to go!!