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, M. Pagowski3
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
5 NASA/GSFC, Greenbelt, Maryland
NCAR/ATD, Boulder, Colorado
7 CIMSS, U. of Wisconsin, Madison, Wisconsin
Mc Gill University, Montreal, Canada
8 U. degli Studi della Basilicata, Potenza, Italy
4
6
IHOP Science workshop, Toulouse, 14-17 June 2004
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
• Provide validatation for high-resolution numerical
simulations of this event. 3
terrain
Observations and simulation
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
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
1.2-1.8 km MSL layer is strongest.
0702 UTC
S-POL RHIs
Azimuth 350°
LLJ still present
The soliton is no longer seen
MAPR
Note existence of a Low-Level Jet
(25-30 m/s magnitude), but the
absence of the waves seen in S-POL
& Leandre.
Observations in Homestead
SRL
Bore arrival
Dry
layer
Observations in Homestead
GLOW
LLJ max v
0220, 400 m
Bore arrival
N
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
0100-0730 UTC
Summary - simulations
• 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
We still have a long way to go!!