Mountain wave structures occurring

1
Mountain wave structures occurring within a
major orographic precipitation event Part 1 -
Observed and Modeled Kinematic Fields Brad
Smull, Matt Garvert and Cliff Mass IMPROVE-Meetin
g 13 July 2005
2
Part-1 Discussion Topics

• Analysis capitalizing on fully 3-D kinematic
  fields over complex terrain (unprecedented in
  their detail) is made possible by airborne
  Doppler radar observations spanning Cascade
  crest, and reveals key features including
• Sloped shear layer blanketing windward slopes
• Underlying stable, low-?e flow parallel to
  barrier
• Profound mountain-wave over mean crest
• Allows evaluation/verification of MM5 output
  fields against the most spatially extensive
  observational dataset available.
• Model-based trajectory analysis illuminates
  origins of windward shear-layer and contrasting
  thermodynamic properties.
• Sensitivity of shear layer and mountain wave
  amplitude to PBL physics is evaluated via
  multiple parameterizations available within MM5.

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13 -14 December 2001 Case

• Strong frontal system impacted IMPROVE-2 study
  area.
• Event has been extensively studied (JAS Special
  Issue).
• PSU-NCAR MM5 v3.6 has been used to simulate the
  event (Garvert et al. 2005a,b).
• Here we focus on a two-hour period of relatively
  steady airflow and precipitation structure from
  2300-0100 UTC on 13-14 December 2001.
• Highlight results from MM5 Control Run utilizing
  MRF-type PBL parameterization.
• All runs employ Reisner2? Thompson BMP scheme

180 W
160 W
140 W
120 W
100 W
60 N
50 N
40 N
IMPROVE-2 Study Area
30 N
500-hPa Heights and Temperature 0000 UTC 14
December 2001
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IMPROVE-2 study area including terrain and P-3
track
Willamette Valley
Cascade Crest
Latitude (Deg)
SPL
IB
MB SBAND
MB
IB Profiler UW Sounding SPL Spol
Radar
UW
Distance from Crest (km)
Distance from Crest (km)
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U- and V-wind profiles 120 km upstream of
Cascade crest (Irish Bend Profiler)
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Radar reflectivity andwind vectors from
composite grid
Height 3 km MSL
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Doppler-derived wind speed on composite grid at 3
km MSL
SPL
IB
Latitude (Deg)
MB
UW
Height 3 km MSL
Distance from Crest (km)
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Observed Mean E-W Cross Section of U- and
V-component Flow and Mean Terrain
U-Wind
V-Wind
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Analogous Mean Section of MM5-Simulated U and V
(2h temporal average from Control Run)
U-Wind
V-Wind
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Corresponding MM5-Simulated Thermodynamics (?e)
U-Wind
V-Wind
Theta-e (K)
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MM5 U-component flow pattern andsuperposed
trajectory endpoint locations
x
x
x
x
08-12
12-16
16-20
20-24
24-28
28-32
32-36
36-40
gt40
lt08
(m s-1)
U-Wind
Theta-e (K)
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Contrasting sources of low- vs. mid-tropospheric
airstreams illustrated via MM5-based trajectory
analysis
2300
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PBL Parameterizations

• Two basic approaches exist for evaluation of
• wa -?a da/dz
• First order closure ?a is specified from the
  vertical shear and static stability
• 1.5 order closure or TKE closure TKE is
  predicted with a prognostic energy equation ?a
  is specified using TKE and some appropriate
  length scale.
• Experience indicates that the PBL
  parameterization can significantly impact the
  structure and strength of mountain waves (Peng
  and Thompson 2003)

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Airborne Dual-Doppler Radar Observed Mean Cross-
and Along-Barrier Flow (for reference)
U-Wind
V-Wind
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Horizontal Flow ?e from MM5 Control Run
Utilizing MRF-type PBL Closure
V-Wind
U-Wind
Theta-e (K)
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MM5 Experimental Run Utilizing ETA-type
PBL(Mellor-Yamada 1.5 Closure)
U-Wind
V-Wind
Theta-e (K)
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Corresponding Mean Vertical Motions fromMM5
Control Run Utilizing MRF-PBL
Theta-e (K)
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ETA-PBL Experiment yields notable differences in
mountain wave behavior
Theta-e (K)
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Impacts of PBL Parameterization Extend
toSimulated Precipitation Microphysics
Snow-diff
CLW-diff
Graupel-diff
Microphysics Differences ETA - MRF
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Key Accomplishments and Conclusions

• 13-14 December IMPROVE-2 observational analysis
  is likely the best in existence for
  representation of airflow over complex terrain.
• Provides unique context for BMP evaluation, and
  unprecedented depiction of mountain wave
  structure in a moist (saturated) environment.
• Low ?e air banked up against Cascade foothills
  acted as an effective barrier to higher ?e flow
  coming more directly off Pacific, which
  accelerated and ascended this effective barrier,
  contributing to production of significant shear.

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Conclusions (cont.)

• Strength of U-component (cross-barrier) shear is
  underdone in MM5 control simulation (i.e., model
  produces comparatively shallow and weak shear
  layer over windward slopes).
• Switch to ETL-type PBL yields notable
  kinematic/microphysical changes, though not
  necessarily improvements.
• Intensity of mountain wave above mean crest is
  very sensitive to the upstream shear and hence to
  choice of model PBL parameterization.

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Future Work

• Re-simulate this case with WRF to capitalize on
  improved numerics/microphysics.
• Examine additional events exhibiting similar
  structures, e.g. IMPROVE-2 29-30 November event
  and/or MAP IOP-8.
• Further quantify sensitivity of simulated
  flow/precip fields and underlying microphysical
  processes to differing PBL parameterizations.

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End
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Observed Mean E-W Cross Section of U- and
V-component Flow
U-Wind
V-Wind
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Corresponding Perturbation Flow Patterns
U
V
-13 -11
-15 -13
-11 - 9
- 9 - 7
- 7 - 5
- 5 - 3
- 3 - 1
- 1 1
1 3
3 5
5 7
7 9
gt 9
lt-15
(m s-1)