Velocity Fields in Cumulus Derived from Airborne DualDoppler Measurements

1
Velocity Fields in Cumulus Derived from
AirborneDual-Doppler Measurements
RICK DAMIANI SAMUEL HAIMOV GABOR VALI Dept. of
Atmos. Sci. University of Wyoming Laramie, WY,
82071, USA

• High resolution airborne dual-Doppler analysis of
  Cu kinematics ('High-Plains Cumulus Experiment'
  (HiCu), summer 2003).
• Wyoming Cloud Radar (WCR) on-board the UWyo
  KingAir (UWKA) research aircraft and in situ
  probes.
• Cu evolution model and entrainment implications

contactrickdami_at_uwyo.edu
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WCR cloud scanning modes
Up/Down profiling mode
Vertical Plane Dual-Doppler
Horizontal Beam Dual-Doppler
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Airborne Dual-Doppler Basic Concept

• Time lag ?t1 - 15 s
• Lifetime of physical features assumed greater
  than ?t
• Plane of the beams determines the resolvable
  components of the velocity

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Conceptual Model of Cu Growth Dynamics
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VPDD
20030826, 1823UTC

• Two counter-rotating vortices are visible in the
  ascending cloud-top.

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Conceptual Model of Cu Growth Dynamics
asymmetric vorticity structures in stronger winds
(gt10 m/s) vortex loops or tilted vortex rings
may be the result of rising thermals in
cross-winds
7
VPDD
20030717, 2142UTC

• A structure resembling a tilted vortex ring is
  visible at the top.
• Intense entrainment is driven by the clockwise
  circulation.

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Conceptual Model of Cu Growth Dynamics
Potential Entrainment Sites primary and
secondary circulations play an important role in
the entrainment, driving dry air into the cloud
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Up/Down Profile Mode
density temperature (Tr) 5 liquid water
content (lwc100)
gust-probe vertical velocity 1-s gust vectors

• Convergence and lwc drop indicate ambient air
  entrainment and mixing driven by the circulation.

WCR retrieved vertical velocity reflectivity
20030713, 2058UTC
Flight level
0 2000
4000
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Conceptual Model of Cu Growth Dynamics
horizontal cross-section
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HBDD
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Vertical Vorticity

• Vortex ring tilting a possible mechanism to
  generate vertical vorticity (wz) in
  counter-rotating vortex pairs (CVP).
• Other mechanisms include CVP forming at the sides
  of a plume in cross-flow, and slanted rising
  motion interacting with precipitating parcels.

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Conclusions

• High resolution (30-50 m) airborne dual-Doppler
  analysis has been developed()
• The analysis reveals the profound
  three-dimensionality of the Cu fluiddynamics
• Vorticity structures (500 m scale) are key
  players in the Cu development and entrainment
  processes
• Horizontal kinematics shows length scales and
  velocity gradients comparable to the vertical
  counterpart. Tilting of horizontal vorticity and
  vortex rings may be responsible for the large and
  persistent vertical vorticity.

() for more info contact rickdami_at_uwyo.edu
14

• Acknowledgments
• This study was supported by NSF Grant
  ATM-0094956. We appreciate the contributions from
  the colleagues and the UWKA facility team
  involved in the HiCu experiment.
• References
• 1 Vali, G. et al., 1998 Fine scale structure
  and microphysics of coastal conver-stratus. J.
  Atmos. Sci., 55, 35403564.
• 2 Johari, H., 1989 An experimental
  investigation of mixing in buoyant flows. Ph.D.
  thesis, University of Washington, available from
  University Microfilms, 300 N. Zeeb Rd., Ann
  Arbor, MI 48106, 110 pages.
• 3 Carpenter, R. L. Jr., K. K. Droegemeier, and
  A. M. Blyth, 1998 Entrainment and detrainment in
  numerically simulated cumulus congestus clouds.
  Part III Parcel analysis. J. Atmos. Sci., 55,
  34403455.
• 4 Zhao, M. and P. H. Austin, 2004 Life cycle
  of numerically simulated shallow cumulus clouds.
  Part II Mixing dynamics. J. Atmos. Sci.,
  submitted.
• 5 Emanuel, K. A., 1994 Atmospheric Convection.
  Oxford, 580 pp.

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Conclusions 1/2

• High resolution Airborne dual-Doppler analysis
  has been developed()
• The analysis reveals the profound
  three-dimensionality of the Cu fluiddynamics
• Cloud scale vorticity structures are often
  identifiable in ascending thermals
• a primary vorticity structure, resembling the
  classic vortex-ring, is visible in low-wind
  conditions in the ascending cloud top
• secondary vortices are shed along the updraft
  flanks (shear layer instability)

() for more info contact rickdami_at_uwyo.edu
16
Conclusions 2/2

• The dynamics affects the hydrometeor distribution
  (reflectivity fields), the entrainment
  mechanisms, and potentially the production rate
  of precipitation (re-ingestion of particles into
  the thermal core)
• Horizontal kinematics shows length scales and
  velocity gradients comparable to the vertical
  counterpart
• Tilting of horizontal vorticity and vortex rings
  may be responsible for the large and persistent
  vertical vorticity
• Slanted shear in the vertical and interactions
  between updraft and descending flow may cause
  vertical vorticity

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Dual-Doppler Analysis Procedure

• 0. Determine beam directional angles with respect
  to the airframe
• Select the flight segment suitable for a
  dual-Doppler retrieval
• Read synchronize data from WCR and AC data
  system
• Apply calibration and thresholding to the
  reflectivity data
• Correct retrieved Doppler velocities for Aircraft
  motion, via INS (Inertial Navigation System) GPS
  data
• Get rid of possible foldings in the Doppler
  retrieval, based on the estimate of a mean wind
  velocity
• Transform each data point from both beams to a
  common coordinate system
• Construct a grid (mesh) onto which the data
  coming from the two beams will be interpolated.
  The grid advects with the estimated mean wind
  velocity field
• Weighting data points from each beam in the
  vicinity of every grid point according to a
  selected criterium
• Solve the dual-Doppler decomposition inverse
  problem determine the velocity for each grid
  point from the measured components
• Store grid and processing statistics and
  information parameters

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Dual-Doppler Retrieval

• Aircraft motion removal and velocity unfolding
• Data Regridding (advecting grid) and weighting
• Velocity decomposition inverse problem return
  signals associated with two radar antennas are
  combined together to provide independent
  components of the scatterers' mean velocity in a
  given illuminated volume.

System (1) is solved by a weighted least-squares
method using an estimate of the cross-plane'
component, usually derived from in-situ-measured
horizontal winds. The overall uncertainty
associated with the evaluation of the
cross-plane component contribution, beam
pointing angles and removal of the UWKA motion is
in the order of 1 m/s at WCR signal-to-noise
ratio gt 0 dB.
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Dual-Doppler Velocity retrieval Weighted
Least-Square Method
SVDC
weighting/thresholding
Minimum Norm Least-Square Solution
Cross-plane component
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VPDD

• WCR in situ data air temperature (trf), liquid
  water content (lwc), gust probe vertical velocity
  (hw) horizontal gust vectors, FSSP total
  number concentration (FSSP ).

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WCR specs.

• Coherent polarized transmitter
• Airborne Platform (UWKingAir?4antennas
  NCAR C-130?2antennas, CONVAIR?2antennas)
• Frequency 94.92GHz (lambda3.16mm)
• Peak Power 1.6 kW (duty cycle1)
• Pulse Repetition Frequency 1-20kHz
• Pulse Length 100, 200, 500 ns
• Beamwidth lt1º

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WCR specs.
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WCR Airborne Configurations 1/2
Vertical Plane Dual-Doppler
Horizontal Beam Dual-Doppler

• The effective plane of scan also depends on the
  AC attitude

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WCR Airborne Configurations 2/2

• Profiling modes
• Side/down mode is achieved using beam 1 from both
  HBDD and VPDD configurations
• Profiling Mode (Up/Down) is achieved by
  redirecting the HBDD beam 1 upward via mirror
  plate

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WCR Airborne Configurations
Vertical Plane Dual-Doppler
Horizontal Beam Dual-Doppler

• 4 antennas are mounted on the aircraft (AC) two
  point downward and allow the retrieval of
  velocities in a vertical plane aligned with the
  aircraft track (Vertical Plane Dual-Doppler,
  VPDD, (a)) the other two scan a horizontal plane
  (Horizontal Beam Dual-Doppler, HBDD, (b)). One of
  the horizontal beams can be redirected upward and
  used in combination with the nadir-pointing one
  to provide single-Doppler scans above and below
  flight level (Up/Down profile).