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
terrain
4
3
2
• 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
terrain
1
4
3
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
terrain
1
4
3
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
17
82
3 95
Homestead
terrain
1
4
3
2
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
N
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!!