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What we have learned about Orographic Precipitation Mechanisms from MAP and IMPROVE-2: MODELING Socorro Medina, Robert Houze, Brad Smull University of Washington Matthias Steiner Princeton University Nicole Asencio Meteo-France. RADIAL VELOCITY. Height ( km ). Distance (km). - PowerPoint PPT Presentation
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What we have learned about Orographic Precipitation
Mechanisms from MAP and IMPROVE-2:
MODELING
Socorro Medina, Robert Houze, Brad Smull University of Washington
Matthias SteinerPrinceton University
Nicole AsencioMeteo-France
Windward Shear Layer –Repeatable pattern in different
storms/mountain ranges
Medina, Smull, Houze, and Steiner (2005); JAS - IMPROVE Special Issue
RADIAL VELOCITY
Hei
gh
t (k
m)
Distance (km)
Objective #1
Investigate how the shear layer
develops. Explore the role of:
–Pre-existing baroclinic shear
–Surface friction
– Stable flow retarded by steep terrain
Approach – 2D Idealized simulations• Weather Research and Forecasting (WRF) model version
1.3 in Eulerian mass coordinates• Domain: 800 km x 30 km (120 vertical layers)• 2 km horizontal resolution; ~250 m vertical resolution• Lin et al. (1983) microphysical scheme• Land surface:
– Option 1: Frictionless “free-slip” surface– Option 2: Non-dimensional surface drag coefficient Cd =
0.01• 2D bell-shaped mountain (characterized by height h and
half-width a) placed in the center of horizontal domain• Alpine-like simulations: h=3.1 km; a=44 km• Cascade-like simulations: h=1.9 km; a=32 km• Results shown after 30 hours of initialization
Initialized with vertically uniform wind speed (10 m/s) and stability; saturated atmosphere with Ts = 283 K
Stability
Friction
Color- Horizontal windContours-Wind shear
Nm2= 0.03x10-4 s-2
Nm2= 0.3x10-4 s-2
Nm2= 1.0x10-4 s-2
Free-slip Cd=0.01
Medina, Smull, Houze, and Steiner
(2005); JAS - IMPROVE Special
Issue
ALPS-like mountain
Distance (km)
Hei
gh
t (k
m)
Friction
Color- Horizontal windContours-Wind shear
Nm2= 0.03x10-4 s-2
Nm2= 0.3x10-4 s-2
Nm2= 1.0x10-4 s-2
Stability
Medina, Smull, Houze, and Steiner
(2005); JAS - IMPROVE Special
Issue
Initialized with vertically uniform wind speed (10 m/s) and stability; saturated atmosphere with Ts = 283 K
CASCADE-like mountain
Free-slip Cd=0.01Distance (km)
Hei
gh
t (k
m)
Idealized Simulation of Case 13-14 Dec 2001
Initial conditions: Solid lines
HORIZONTAL WIND
WIND SHEAR
Medina, Smull, Houze, and Steiner (2005); JAS -
IMPROVE Special Issue
RH (%) T (°C) Zonal Wind (m/s)H
eig
ht
(km
)
Distance (km)
Hei
gh
t (k
m)
Shear = 12.5 m s-1 km-1
Conclusions # 1
Idealized simulations show that orographic effects alone are sufficient to produce a shear layer on the windward side of the terrain when the stability is high enough (e.g. Alpine cases)
Simulations based on IMPROVE-2 environmental and terrain condition indicate that surface friction and/or pre-existing shear were necessary to produce an enhanced layer of shear
Objective #2
Investigate if mechanisms of orographic precipitation
enhancement deduced from observations are also present in mesoscale
models
FLOW-OVER Precipitation enhancement by coalescence & riming
over first peak
TERRAIN
snow
rain
0ºC
cloud dropletsgraupel growingby riming
rain growingby coalescence
Slightly unstable
air
Medina and Houze (2003)
Approach• Focus on MAP – IOP2b• Meso-NH: mesoscale non-hydrostatic model used by
French research community (Lafore et al. 1998)• 2.5-km horizontal resolution nested in a 10-km horizontal
resolution domain• Initial and lateral conditions:
– Given by linearly interpolating in time French Operational Analysis (ARPEGE) for 10-km resolution domain
– Given by 10-km resolution domain for 2.5 km resolution domain
• 2.5-km horizontal resolution domain: Microphysical bulk parameterization including cloud, rain, ice, snow, and graupel (Pinty and Jabouille 1998)
• Validation of simulation conducted by Asencio et al. 2003 (QJMRS)
Comparison of IOP2b radar observations and simulation
20 SEP OBSERVED RAIN ACCUMULATION (mm)
20 SEP SIMULATED RAIN ACCUMULATION (mm)
20 SEP OBSERVED RADIAL VELOCITY (m/s)
20 SEP SIMULATED RADIAL VELOCITY (m/s)
(Provided by J. Vivekanandan)
Observed and Simulated Mean Hydrometeors (over 7h)
FREQUENCY OF OCCURRENCE OF OBSERVED LIGHT RAIN (%)
FREQUENCY OF OCCURRENCE OF OBSERVED MODERATE RAIN (%)
FREQUENCY OF OCCURRENCE OF OBSERVED HEAVY RAIN (%)
MIXING RATIO OF SIMULATED RAIN (kg/kg)
Observed and Simulated Mean Hydrometeors (over 7h)
FREQUENCY OF OCCURRENCE OF OBSERVED GRAUPEL (%)
MIXING RATIO OF SIMULATED GRAUPEL (kg/kg)
FREQUENCY OF OCCURRENCE OF OBSERVED DRY SNOW (%)
MIXING RATIO OF SIMULATED SNOW (kg/kg)
MIXING RATIO OF CLOUD (kg/kg) RATE OF CLOUD GROWTH BYCONDENSATION (S-1)
Mean Microphysical Processes –CLOUD (over 7h)
RATE OF GRAUPEL GROWTH BY COLLECTION OF CLOUD AND SNOW (S-1)
MIXING RATIO OF GRAUPEL(kg/kg)
Mean Microphysical Processes –GRAUPEL (over 7h)
RATE OF GRAUPEL GROWTH BY SNOW RIMING CLOUD (S-1)
RATE OF RAIN GROWTH BYACCRETION OF CLOUD (S-1)
MIXING RATIO OF RAIN (kg/kg)
Mean Microphysical Processes –RAIN (over 7h)
RATE OF RAIN GROWTH BY GRAUPEL AND SNOW MELT (S-1)
Conclusions # 2
A Meso-NH simulated cross-barrier flow of IOP2b had the correct structure but the speed was overestimated.
The Meso-NH simulation produced precipitation patterns comparable with the radar observations.
The location and occurrence of simulated microphysical processes of orographic precipitation enhancement are consistent with the S-Pol polarimetric radar data.
Graupel is created by riming of cloud and it grows by collection of snow and cloud.
Rain is produced via melting of graupel (& snow) followed by cloud accretion.
The model suggests that hydrometeor growth rates can be ~4-7 times larger over the mountains than over the low elevations.
FIN
Run Stability (Nm2; x10-4 s-2) Wind speed
perpendicular to terrain (u; m s-1)
h; x103m a; x103 m Lr
(x103 m)
Ro Fr
IOP2b 0.03 12.5d 3.1 44 54 2.84 2.33
IOP8 1.00 10 3.1 44 310 2.27 0.32
ALPS a-b
0.03 10 3.1 44 54 2.27 1.86
ALPS c-d
0.30 10 3.1 44 170 2.27 0.59
ALPS e-f
1.00 10 3.1 44 310 2.27 0.32
CASC. a-b
0.03 10 1.9 32 33 3.12 3.04
CASC. c-d
0.30 10 1.9 32 104 3.12 0.96
CASC. e-f
1.00 10 1.9 32 190 3.12 0.53
U=20m/s
0.30 20 1.9 32 104 6.25 1.92
U=30m/s
0.30 30 1.9 32 104 9.38 2.88
13 Dec 0.37d 20d 1.9 32 116 6.25 1.73
a Lr = (N h) f-1; f=Coriolis parameterb Ro = u (f a)-1
c Fr = u (N h)-1 d Vertically averaged over the lowest 3 km.
Garvert et al. 2005
IOP2b Wind profiler data
OBSERVATION
SIMULATION
MIXING RATIO OF SNOW (kg/kg)MIXING RATIO OF GRAUPEL(kg/kg)
Mean Microphysical Processes –GRAUPEL (over 7h)
RATE OF GRAUPEL GROWTH BY COLLECTION OF CLOUD AND SNOW (S-1)
RATE OF GRAUPEL GROWTH BY SNOW RIMING CLOUD (S-1)
RATE OF RAIN FALLOUT (S-1) RATE OF RAIN GROWTH BYACCRETION OF CLOUD (S-1)
MIXING RATIO OF RAIN (kg/kg)
Mean Microphysical Processes –RAIN (over 7h)
RATE OF RAIN GROWTH BY GRAUPEL MELTING (S-1)
Observations Simulation
2D Simulation with 100 m resolution of stable flow over a 2 km ridge conducted by
with Bryan and Fritsch (2002) model
Simulation conducted by D. Kirshbaum
Precipitation
Precipitation
Precipitation
N_m^2=(g/T)(dT/dz + Gamma_m)(1+Lq_s/RT)
Gamma_m=Gamma_d(1+q_w)(11+Lq_s/RT)*f(T,q_s,q_L)
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