The Structure and Dynamics of Solitons and Bores in IHOP

1
The Structure and Dynamics of Solitons and Bores
in IHOP

• Steven E. Koch
• NOAA Research - Forecast Systems Laboratory
• Collaborators
• Mariusz Pagowski1, Cyrille Flamant2, James W.
  Wilson3,
• Frederic Fabry4, Wayne Feltz5, Geary Schwemmer6,
• Bart Geerts7, Belay Demoz6, Bruce Gentry6, Dave
  Whiteman6
• 1 CIRA / Colorado State University
• 2 Centre National de la Recherche Scientifique
  (CNRS), Paris, France
• 3 National Center for Atmospheric Research
• 4 Radar Observatory, McGill University
• 5 CIMSS / University of Wisconsin
• 6 NASA / Goddard Space Flight Center
• 7 Department of Atmospheric Sciences, University
  of Wyoming

2
Gravity Currents in Geophysical Flows
A gravity current is a horizontal mass flow
driven by its greater density relative to its
environment.
3
Evolution of a Gravity Current into a Bore
An internal bore in the atmosphere is a kind of
gravity wave generated by the intrusion of a
gravity current into a low-level stable layer.
Passage of the bore results in a sustained
elevation of the stable layer. Unlike gravity
currents, bores do not transport mass.
Simpson (1987)
4
Evolution of Soliton from Bore
A train of amplitude-ordered solitary waves (or
soliton) can evolve from bores in some instances.
Wave amplitudes vary inversely with their width
and are highly dispersive.
The number of waves increases with time, but is
limited by turbulent dissipation. The energy of
the wave system tends to be concentrated in the
first few solitary waves.
5
Types of Bores
Transition of an undular bore into a turbulent
bore depends upon its strength (db / h0). Bore
strength is determined by the Froude Number and
the ratio of the gravity current depth to the
inversion depth (d0 / h0) Houghton and Kasahara
(1968)
6
Theory(Rottman and Simpson 1989 Haase and Smith
1989)

• Bore speed of propagation
• Two parameters determine whether a bore will be
  generated from an intrusive gravity current
• m gt 0.7 is required for bore
• Solitary waves require large Froude Number
• Vertical variation of the Scorer parameter
  determines likelihood of wave trapping

7
Complications

• Gravity currents may generate other kinds of
  phenomena in addition to bores and solitons
• Kelvin-Helmholtz waves (strongly trapped waves
  that propagate rearward relative to the current
  head)
• Trapped lee waves (display no tilt nor relative
  motion)
• Intermediate structures during early stage of
  bore formation composed of some combination of
  current and inversion air
• Bore properties may not compare well with theory
  when
• Vertical wind shear is present
• The inversion is elevated or stratification is
  complex
• Gravity current is unsteady or multiply-structured
  

8
IHOP_2002 (International H20 Project)Surface
Observing Sites

• Homestead observing systems
• S-Pol Doppler radar with refractivity est.
• FM-CW 10-cm radar _at_ 2-m resolution
• MAPR (915 MHz Multiple Antenna Profiler _at_30-sec,
  60-m resolution)
• HARLIE (aerosol backscatter lidar)
• GLOW (Doppler lidar)
• Scanning Raman Lidar _at_ 2 min, 60m
• AERI (Atmospheric Emitted Radiance Interferometer
  _at_10 min, 50m)
• CLASS (3-hourly soundings)

9
The Dual Bore Event of June 4, 2002
10
Pressure and Temperature Fluctuations Attending
Passage of the Bores

• Warming or very slight temperature changes occur
  with passage of both bores

11
Evolution of gravity currents or bores (white
lines) and synoptic cold front (blue line) as
seen in Radar Composite and Mesonet Data
A
12
Evolution of gravity currents or bores (white
lines) and synoptic cold front (blue line) as
seen in Radar Composite and Mesonet Data
A
13
Evolution of gravity currents or bores (white
lines) and synoptic cold front (blue line) as
seen in Radar Composite and Mesonet Data
B
A
14
Evolution of gravity currents or bores (white
lines) and synoptic cold front (blue line) as
seen in Radar Composite and Mesonet Data
B
A
15
Evolution of gravity currents or bores (white
lines) and synoptic cold front (blue line) as
seen in Radar Composite and Mesonet Data
B
A
16
Evolution of gravity currents or bores (white
lines) and synoptic cold front (blue line) as
seen in Radar Composite and Mesonet Data
B
17
Evolution of gravity currents or bores (white
lines) and synoptic cold front (blue line) as
seen in Radar Composite and Mesonet Data
B
18
Evolution of gravity currents or bores (white
lines) and synoptic cold front (blue line) as
seen in Radar Composite and Mesonet Data
B
19
Bore A
FM-CW
HARLIE
20
Bore A
FM-CW
MAPR
21
Bore B
FM-CW
UWKA Flight-Level Data
Noisy data
Raman Lidar
22
SE
NW
potential temperature

• Bore B seen in
• UW King Air Data
• at FL 1850 m AGL
• 3C cooling and 4 g/kg more moisture are found at
  this level behind the bore (NW).
• Amplitude-ordered solitary waves were penetrated
  by the UWKA at the top of the bore.
• Vertical motions are in phase quadrature with q
  (upward motion leading cooling) and U (not
  shown), as in a typical gravity wave.

vertical air velocity
mixing ratio
Wave propagation
23
Bore A and Gravity Current Properties
24
Bore B and Gravity Current Properties
25
AERI Detection of Bores A B
Potential Temperature
Relative Humidity
Mixing Ratio
26
Rapid decrease of refractivity in both S-POL and
mesonet data due to drying caused by passage of
bores (particularly bore A) entrainment?
27
Numerical Simulations of the Bores
28
Numerical Simulations of Turbulent Kinetic Energy
(TKE) Generation and Mixing by Bore

• Nested MM5 model (18, 6, 2, 0.666 km) initialized
  at 00Z 4 June 02
• Initial / boundary conditions from RUC-20 model
• 44 vertical levels (22 below 1500 m)
• Three PBL experiments (all use Mellor-Yamada 2.5
  closures)
• BT (Burk Thompson 1989) uses diagnostic mixing
  length (shown)
• ETA (Janjic 1994) similar to B-T scheme but with
  limit upon mixing length in statically stable
  layers resulting in less TKE generation
• QL (Kantha Clayson 1994) offers two advantages
• Richardson number-dependent shear instability
  mixing term in strongly stratified layers
  enhances TKE in stable layer above a well-mixed
  PBL
• Prognostic equations for TKE and for mixing length

To the movie
29
The Bore-Soliton Event of June 20, 2002
30
Gravity Current stage 0036 UTCRadar fine line
cooling pressure increase
31
Bore stage 0233 UTCRadar fine lines no
cooling pressure increase
32
Soliton stage 0600 UTCTrain of wavelike radar
fine lines no cooling pressure increase
33
Vertical structure of the bore as measured by
Leandre2 DIAL water vapor system
The evolution of the bore was observed by the
LEANDRE 2 DIAL system on the P-3 aircraft along
N-S sections normal to the bore. S-POL provided
PPI RHI coverage Four P-3 overpasses occurred
over the Homestead Profiling Site, offering
comparisons with SRL, GLOW, and MAPR
L2 WVMR retrievals 800 m horizontal
resolution 300 m vertical resolution
34
LEANDRE 2 3rd pass (0408-0427 UTC)
Dry layer
17 km
hB
h0

• Amplitude ordered waves
• Inversion surfaces lifted successfully higher by
  each passing wave
• Trapping mechanism suggested by lack of tilt
  between the 2 inversion layers
• Bore intensity (hB/h0) 2.1

35
LEANDRE 2 4th pass (0555-0616 UTC)
Dry layer
11 km
0.6 km

• Waves are no longer amplitude ordered
• Inversion surfaces lifted successfully higher by
  each passing wave
• Leading wave is weaker, but clouds have formed
  aloft above each wave
• Trapping mechanism suggested by lack of tilt
  between the 2 inversion layers

36
S-POL RHIs at 0530 UTC along azimuth 350
11-km horizontal wavelength at 2.5-km level and
27 m s-1 LLJ seen in S-Pol data are consistent
with Leandre-II and ISS observations, respectively
37
Representative Sounding for the Bore Environment
seen in IHOP
38
Summary of Findings

• Bores and solitons appeared as fine lines in
  S-POL reflectivity displays and their vertical
  structures were readily detected by remote
  sensing systems (DIAL, Raman lidar, HARLIE, GLOW,
  LEANDRE2, MAPR, etc). An unprecedented set of
  observations has been collected on the
  time-varying structure of bores and solitons in
  IHOP 18 bore events were logged during the
  six-week IHOP experiment, allowing for common
  aspects of their environment to be determined!
• Solitary waves developed to the rear of the
  leading fine line atop a 0.7-1.0 km deep surface
  stable layer. This layer increased in depth but
  weakened with passage of the leading wave. The
  inversion was then further lifted by each passing
  wave.
• Nature of wave propagation does not suggest wave
  origin is intrinsic to bore dynamics as expected
  from bore theory, but rather, that lee-wave
  activity was the actual mechanism.
• Solitary wave characteristics
• Horizontal wavelength 10-20 km (4 June) 16 km
  decreasing to 11 km (20 June) why?
• Phase speed 7.3 20.3 m/s (4 June) 8 m/s
  decreasing to 5 m/s (20 June) why?
• Waves exhibited amplitude-ordering (except in
  later stages of 20 June soliton)
• Suggestion of wave trapping seen in Leandre,
  Raman Lidar data really?
• Pronounced reduction in refractivity occurred due
  to drying in the surface layer (June 4 only), but
  cooling moistening seen aloft in both cases
  (AERI, UWKA data for Bore B on 4 June, Leandre on
  20 June) was likely a result of adiabatic lifting.

39
Where do we go from here?

• Attempt to synthesize MAPR and GLOW data to
  obtain 2D circulations in 20 June event, produce
  detailed Scorer parameter analyses for both
  events, and compare to moisture structures
• Reexamine to what extent the observations are in
  agreement with the hydraulic and weakly nonlinear
  theories for gravity currents, bores, and
  solitons
• Assess the trapping mechanisms allowing the bore
  to be maintained for such long distances (the
  Scorer parameter) and whether suggestions of wave
  trapping seen in Raman Lidar and Leandre-II data
  are correct interpretations
• Understand the reason why the number of waves
  within the solitons varied with time and the
  mechanism for soliton breakdown
• Determine whether waves seen in the remotely
  sensed data were truly dispersive solitary waves,
  or were instead lee waves, K-H waves, or some
  other wave phenomenon
• Complete the numerical simulations to better
  understand these issues and also why drying
  (reduction of refractivity) only occurs
  sometimes. Bore forcing mechanisms are very
  sensitive to model physics, and details
  concerning entrainment and turbulence depend upon
  PBL parameterization suggesting need for LES
  studies.