Transcript THE LIFE CYCLE OF A BORE EVENT OVER THE US SOUTHERN GREAT PLAINS DURING IHOP_2002 (Flamant)

THE LIFE CYCLE OF A BORE EVENT
OVER THE US SOUTHERN GREAT
PLAINS DURING IHOP_2002
1
C. Flamant1, S. Koch2
IPSL/SA, CNRS, Paris, France
2
NOAA 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
4 NASA/GSFC, Greenbelt, Maryland
NCAR/ATD, Boulder, Colorado
6 CIMSS, U. of Wisconsin, Madison, Wisconsin
Mc Gill University, Montreal, Canada
7 CIRA, Boulder, Colorado
8 U. degli Studi della Basilicata, Potenza, Italy
3
5
6th ISTP, Leipzig, Germany, 14-20 September 2003
Overview of the presentation
1. Introduction
• Background on bores and solitons
• Expected IHOP-related advances in bore studies
2. The 20 June 2002 mission
• Objectives
• Instruments deployed
3. The 2O June 2002 bore event
• Life cycle (CIDD analyses)
• Vertical structure (LEANDRE 2 and S-POL RHIs)
4. Conclusions and perspectives
Background
Bores may be produced when a cold front or outflow boundary impinge
upon a stable surface layer in the presence of sufficient wind curvature.
A bore is type of gravity wave
disturbance propagating
ahead of a gravity current
(« permanent » displacement
of a layer) …
stable layer
… that may further
evolves into a solitary wave
System (layer is displaced
upward and then returns back
to its original height)
from Locatelli et al. (1998)
These wave events can play a role in convection initiation
and nocturnal convection maintenance
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 !
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.
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
Homestead:
MAPR, ISS,
SRL, GLOW
terrain
Objectives
• Analyse the life cycle of a bore event (how it is triggered,
how it evolves, how it dies…)
• Compare observations with hydraulic theory,
2
• Understand the role of bores in nocturnal convection
3
maintenance,
• Provide validatation for high-resolution numerical
simulations of this event.
terrain
4
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
Gravity current
4
Bore
Soliton
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 scattering associated
with pronounced mixing or Rayleigh scattering due to convergence of insects or dust.
3
1
terrain
Gravity current
4
Bore
Soliton
CIDD analyses
CIDD analyses
1
7
2
3
8
9
5
Homestead
Vertical structure of the bore
The bore was best observed
along a N-S radial coinciding
with P-3 track 1
2
S-POL RHIs: contineous
coverage (0530-0730 UTC)
3
Airborne DIAL LEANDRE 2:
4 overpasses of Homestead
1
terrain
4
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 km
0.3-0.4 g kg-1 near surface
LEANDRE 2 : 2nd pass track 1
Dry layer
0329-0352 UTC
15 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
LEANDRE 2 : 3rd pass track 1
Dry layer
0408-0427 UTC
17 km
0.8 km
0.8 km
h0
h1
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
LEANDRE 2 : 4th pass track 1
Dry layer
0555-0616 UTC
11 km
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
0530 UTC
S-POL RHIs
Azimuth 350°
Horizontal wavelength
consistent with L2
observations of the soliton
0702 UTC
Observations in Homestead
SRL
Bore arrival
Dry
layer
GLOW
June 20, 2002: 50 m, 3 minutes
2
25
1.8
1.6
20
Altitude (km)
1.4
1.2
15
1
0.8
10
0.6
0.4
5
0.2
0
4
4.5
5
5.5
6
6.5
UTC (hrs)
7
7.5
8
0
MAPR
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.
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 above the NBL ?