THE LIFE CYCLE OF A BORE EVENT OVER THE US SOUTHERN GREAT PLAINS DURING IHOP_2002 IHOP Science workshop, Toulouse, June 2004 C. Flamant 1, S. Koch 2, M. Pagowski 3 1 IPSL/SA, CNRS, Paris, France 2 NOAA FSL, Boulder, Colorado 3 CIRA, Boulder, Colorado T. Weckwerth 4, J. Wilson 4, D. Parsons 4, B. Demoz 5, B. Gentry 5, D. Whiteman 5, G. Schwemmer 5, F. Fabry 6, W. Feltz 7, P. Di Girolamo 8 4 NCAR/ATD, Boulder, Colorado 5 NASA/GSFC, Greenbelt, Maryland 6 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 mission On 20 June 2002, the life cycle of a bore (i.e. triggering, evolution and break-down) was sampled in the course of night time ELLJ mission during which 2 aircraft and a number of ground- based facilities were deployed. RUC 20 km (0300 UTC) terrain S-POL Homestead: MAPR, ISS, SRL, GLOW NRL P-3 (LEANDRE 2 and ELDORA) MCS LearJet dropsondes The bore was triggered by a thunderstorm outflow

Objectives terrain 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 numerical simulations of this event. Observations and simulation

The 20 June 2002 bore event terrain Gravity currentBoreSoliton 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 Gravity currentBoreSoliton 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 associated with pronounced mixing or Rayleigh scattering due to convergence of insects or dust.

CIDD analyses

Homestead

terrain Vertical structure of the bore The bore was best observed along a N-S radial coinciding with P-3 track 1 S-POL RHIs: contineous coverage ( UTC) Airborne DIAL LEANDRE 2: 4 overpasses of Homestead 3 legs of LearJet dropsondes Homestead Profiling Site: SRL, GLOW, MAPR

LEANDRE 2 : 1 st pass track UTC Moistening L2 WVMR retrievals: 100 shots (10 sec.) 800 m horizontal resolution 300 m vertical resolution Precision: g kg -1 at 3.5 km g kg -1 near surface

UTC LEANDRE 2 : 2 nd pass track 1 15 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

UTC LEANDRE 2 : 3 rd pass track 1 17 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 h0 h1 h1/h0~2.1 Dry layer

UTC LEANDRE 2 : 4 th pass track 1 11 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 RHIs Azimuth 350° Horizontal wavelength consistent with L2 observations of the soliton 0530 UTC Strong Low-level Jet : 27 m/s jet core at ~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 km MSL layer is strongest.

S-POL RHIs Azimuth 350° LLJ still present The 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 SRL Bore arrival Dry layer

Observations in Homestead N LLJ max v 0220, 400 m Bore arrival GLOW

Summary - observations  The life cycle of a « bore » event was observed as fine lines in S-POL reflectivity and 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 a speed 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 the theoretical transition region appears at h1/h0=2…)  Solitary waves developed to the rear of the leading fine line atop a 700 – 900 m deep surface stable layer. Depth of stable layer increased by 600 m with passage of leading wave. The inversion was then lifted by each passing wave. Similar trends are 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 to 0430 UTC, and 5 m/s afterward. Waves exhibited amplitude-ordering (leading wave 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 hydraulic theory (propagation speed of GC and bore; cooling associated with the GC; pressure increase associated with the GC and bore; lifting; horizontal wavelength). Assess the trapping mechanisms allowing the bore to maintain all the way to Homestead We are (or will be) investigating this using Scorer parameter (RDS) and cross-spectral analyses (in situ and L2). Possible generation of KH waves by wind shear will also be investigated. Understand the mechanisms leading to the bore breakdown south of Homestead Is this caused by orography, the presence of the strong, very narrow jet or the fact that we no longer have stably stratified conditions. In the latter case, is this related to the CAPE and CIN redistribution with altitude (induced by the bore itself), leading to the injection 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 temperature through the bore 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 as in 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 consistent with observation Elevated moist layer not reproduced Elevated inversion close to the surface is tilted Summary - simulations We still have a long way to go!!