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
Presented at IHOP Science Workshop
Toulouse, France
16 June 2004
Gravity Currents in Geophysical Flows
A gravity current is a horizontal
mass flow driven by its greater
density relative to its environment.
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)
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.
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)
Theory
(Rottman and Simpson 1989; Haase and Smith 1989)
Cbore  Cgw 0.5db h0 1  db h0 


Bore speed of propagation
Two parameters determine
whether a bore will be
generated from an intrusive
gravity current:



m > 0.7 is required for bore
Solitary waves require large
Froude Number
Vertical variation of the
Scorer parameter determines
likelihood of wave trapping
 Nh0 0.5db h0 1  db h0 
1
2
U C 
U C 


F

gc
C*
m
Cgw
Cgc

gc
g d0  vw
2Nh0 
Cgc
Nm2
 2U  2 z
m 
 k2
2 
U  Cb  U  Cb 
2
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
IHOP_2002 (International H20 Project)
Surface Observing Sites
Homestead observing systems:

S-Pol Doppler radar with refractivity est.

FM-CW 10-cm radar @ 2-m resolution

MAPR (915 MHz Multiple Antenna Profiler
@30-sec, 60-m resolution)

HARLIE (aerosol backscatter lidar)

GLOW (Doppler lidar)

Scanning Raman Lidar @ 2 min, 60m

AERI (Atmospheric Emitted Radiance
Interferometer @10 min, 50m+)

CLASS (3-hourly soundings)
The Dual Bore Event
of June 4, 2002
Pressure and Temperature Fluctuations
Attending Passage of the Bores
Warming or very slight temperature changes occur with passage of both bores
A
Evolution of gravity currents or bores (white lines) and synoptic cold
front (blue line) as seen in Radar Composite and Mesonet Data
A
Evolution of gravity currents or bores (white lines) and synoptic cold
front (blue line) as seen in Radar Composite and Mesonet Data
B
A
Evolution of gravity currents or bores (white lines) and synoptic cold
front (blue line) as seen in Radar Composite and Mesonet Data
B
A
Evolution of gravity currents or bores (white lines) and synoptic cold
front (blue line) as seen in Radar Composite and Mesonet Data
B
A
Evolution of gravity currents or bores (white lines) and synoptic cold
front (blue line) as seen in Radar Composite and Mesonet Data
B
Evolution of gravity currents or bores (white lines) and synoptic cold
front (blue line) as seen in Radar Composite and Mesonet Data
B
Evolution of gravity currents or bores (white lines) and synoptic cold
front (blue line) as seen in Radar Composite and Mesonet Data
B
Evolution of gravity currents or bores (white lines) and synoptic cold
front (blue line) as seen in Radar Composite and Mesonet Data
Bore A
FM-CW
HARLIE
Bore A
FM-CW
MAPR
Bore B
FM-CW
UWKA Flight-Level Data
Noisy data
Raman Lidar
SE
NW
potential temperature
Bore B seen in
UW King Air Data
at FL 1850 m AGL

vertical air velocity

mixing ratio

Wave propagation
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  (upward
motion leading cooling) and
U (not shown), as in a typical
gravity wave.
Bore A and Gravity Current Properties
Property
Value
Bore depth (db)
1.2 km
Inversion depth & strength (h0)
0.75 km, 10.0 K
Bore strength (db / h0)
1.6
Gravity current depth/speed (d0, Cgc)
1.6 km, 18.0 m/s
Normalized current depth (H = d0 / h0)
2.1
Froude Number (Cgc / Cgw)
1.2
Bore speed pred. vs. obs. (Cb)
20.3 vs. 14.8
Predicted
Observed
Bore B and Gravity Current Properties
Property
Value
Bore depth (db)
1.6 km
Inversion depth & strength (h0)
1.0 km, 3.0 K
Bore strength (db / h0)
1.6
Gravity current depth/speed (d0, Cgc)
0.9 km, 6.9 m/s
Normalized current depth (H = d0 / h0)
0.9
Froude Number (Cgc / Cgw)
0.7
Bore speed pred. vs. obs. (Cb)
7.3 vs. 6.9
Predicted
Observed
AERI Detection of Bores A & B
Potential
Temperature
Relative
Humidity
Mixing
Ratio
A
B
Rapid decrease of refractivity in both S-POL and mesonet data due to
drying caused by passage of bores (particularly bore A): entrainment?
N  3.73  105
e
P

77.6
T
T2
Numerical Simulations
of the Bores
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
q2
 
q2 
u
v
2q3

 2v w
 2 gwv 
 lq
  2u w
t
z 
z 
z
z
l
To the
movie
The Bore-Soliton
Event of June 20, 2002
Gravity Current stage: 0036 UTC
Radar fine line + cooling + pressure increase
Bore stage: 0233 UTC
Radar fine lines + no cooling + pressure increase
Soliton stage: 0600 UTC
Train of wavelike radar fine lines + no cooling + pressure increase
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
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
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
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
Representative Sounding for the Bore
Environment seen in IHOP
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.

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.