THE LIFE CYCLE OF A BORE EVENT OVER THE US SOUTHERN GREAT PLAINS DURING IHOP_2002

1. THE LIFE CYCLE OF A BORE EVENT OVER THE US SOUTHERN GREAT PLAINSDURING IHOP_2002 C. Flamant1, S. Koch2 1IPSL/SA, CNRS, Paris, France 2NOAA FSL, Boulder, Colorado T. Weckwerth3, J. Wilson3, D. Parsons3, B. Demoz4, B. Gentry4, D. Whiteman4, G. Schwemmer4, F. Fabry5, W. Feltz6, M. Pagowski7, P. Di Girolamo8 3 NCAR/ATD, Boulder, Colorado4NASA/GSFC, Greenbelt, Maryland 5Mc Gill University, Montreal, Canada6 CIMSS, U. of Wisconsin, Madison, Wisconsin 7 CIRA, Boulder, Colorado 8 U. degli Studi della Basilicata, Potenza, Italy 6thISTP, Leipzig, Germany, 14-20 September 2003

2. Overview of the presentation • Introduction • Background on bores and solitons • Expected IHOP-related advances in bore studies • The 20 June 2002 mission • Objectives • Instruments deployed • The 2O June 2002 bore event • Life cycle (CIDD analyses) • Vertical structure (LEANDRE 2 and S-POL RHIs) • Conclusions and perspectives

3. stable layer from Locatelli et al. (1998) Background Bores may be produced when a cold front or outflow boundaryimpinge upon a stable surface layer in thepresence of sufficient windcurvature. A bore is type of gravitywave disturbance propagating ahead of a gravity current (« permanent » displacement of a layer) … … that may further evolves into a solitary wave System (layer is displaced upward and then returns back to its original height) These wave events can play a role in convection initiation and nocturnal convection maintenance

4. Expected IHOP-related advances Until now, observational investigations of solitary waves/bore events over the SGP have been primarily limited to individual case studies often using detailed measurements taken at a single location. • What makes IHOP_2002 so special: • Wide spread networks of instruments: • WSR-88D radars • surface mesonets (OK, SWKS, ASOS, AWOS, etc…) • Daily forecast of bore events • Systematic measurements from Homestead Profiling site • Aircraft pool deployment (in situ and remote sensing) • « Bore » life cycle IHOP_2002: bore events are common features in the SGP !

5. 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 timeELLJ mission during which 2 aircraft and a number of ground- based facilities were deployed. The bore was triggered by a thunderstorm outflow RUC 20 km (0300 UTC) LearJet dropsondes MCS NRL P-3 (LEANDRE 2 and ELDORA) S-POL terrain Homestead: MAPR, ISS, SRL, GLOW

6. Objectives • Analyse the life cycle of a bore event (how it is triggered, • how it evolves, how it dies…) • Compare observations with hydraulic theory, • Understand the role of bores in nocturnal convection • maintenance, • Provide validatation for high-resolution numerical • simulations of this event. 2 3 terrain 4

7. The 20 June 2002 bore event • 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 (S-POL and DDC radar reflectivity + surface mesonets) 3 1 terrain 4 Gravity current Bore Soliton

8. CIDD analyses 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 scatteringassociated withpronounced mixing or Rayleigh scattering due to convergence of insects or dust. 3 1 terrain 4 Gravity current Bore Soliton

9. CIDD analyses

10. CIDD analyses 1 7 8 2 5 9 3 Homestead

11. 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 (0530-0730 UTC) Airborne DIAL LEANDRE 2: 4 overpasses of Homestead 3 legs of LearJet dropsondes Homestead Profiling Site: SRL, GLOW, MAPR 2 3 1 terrain 4

12. 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 km 0.3-0.4 g kg-1 near surface

13. LEANDRE 2 : 2ndpass track 1 15 km Dry layer 0329-0352 UTC 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

14. LEANDRE 2 : 3rdpass track 1 Dry layer 17 km 0408-0427 UTC 0.8 km 0.8 km h1 h0 h1/h0~2.1 • Amplitude ordered waves • Inversion surfaces lifted successfully higher by each passing wave • Trapping mechanism suggested by lack of tilt between the 2 inversion layers

15. LEANDRE 2 : 4thpass track 1 11 km Dry layer 0555-0616 UTC 0.6 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

16. 0530 UTC S-POL RHIs Azimuth 350° Horizontal wavelength consistent with L2 observations of the soliton 0702 UTC

17. Observations in Homestead SRL Bore arrival Dry layer GLOW MAPR

18. Summary • 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.

19. 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 ) • 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 abovethe NBL ?