Background on Mellor-Yamada- Nakanishi-Niino (MYNN) PBL Scheme
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Transcript of Background on Mellor-Yamada- Nakanishi-Niino (MYNN) PBL Scheme
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Modifications to the MYNN PBL and Surface Layer Scheme
for WRF-ARW
Joseph Olson1,2
John M. Brown1
1NOAA-ESRL/GSD/AMB2Cooperative Institute for Research
in Environmental Sciences
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Background on Mellor-Yamada-Nakanishi-Niino (MYNN) PBL Scheme
• Implemented into WRF-ARW in 2008 (v3.0). • Main features of the MYNN include:
• Turbulent kinetic energy (TKE)-based local mixing scheme (like MYJ)
• Option to run at level 2.5 or 3.0 closure.
• Liquid water potential temperature, θl (= θ - (θ/T)(Lv/cp)ql), and total water content, qw (= qv + ql), are used as thermodynamic variables.
• Tuned to a database of LES simulations in order to overcome the typical biases associated with other MY-type schemes (insufficient growth of convective boundary layer and underestimated TKE).
• More elaborate mixing length formulations to flexibly change behavior across the stability spectrum.
• Initially adopted a very simple surface layer scheme, taken from an old version of the YSU surface layer scheme, with very little customization to the PBL scheme.
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Known Problems with MYNN• warm-bias over desert/bare soil regions • production of negative TKE • high 10- and 60-m wind speed bias • excessive low-level clouds over the ocean/arctic
Wind Vector RMSE
MYNN
MYNN
MYJ
MYJ
6-hr forecasts from MYNN and MYJ between 20120410-20120510 verified against rawinsonde data over CONUS region
Temperature Bias
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• Adjustment of closure constants to remove negative TKE problem (Canuto et al. 2008 and Kitamura 2010) but also removes critical Richardson number.
• Subsequent modifications to closure constants C2 and C3 to help reduce over-diffusive behavior
• Further modifications to mixing length formulae (surface layer and buoyancy length scales) to compensate increased diffusivity (shown later).
after
before
Recent changes to MYNN PBL scheme
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and the turbulent length scale lt is:
and the buoyancy length scale lb is:
The mixing length is designed such that the shortest length scale among, ls, lt, and lb will dominate:
where the surface layer length scale ls is a function of the stability parameter(ζ=z/L; L in the M-O length):
if 0 ≤ ζ ≤ 1
if ζ < 0
MYNN Mixing Length Formulation
ls ={
Stable Conditions
Unstable Conditions
where qc is a turbulent velocity scale ~O(w*)
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Changes to Surface Layer Length Scale (ls)
At z/L = -1, ls ~ 1.0z
At z/L = 1, ls = kz/3.7
At z/L = 1, ls = kz/3.1
At z/L = -1, ls ~ 0.75z
α4 = 100, cns = 2.7 α4 = 20, cns = 2.1
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• Iterative - accurate solution of z/L, u* and θ*.• Updated thermal/moisture roughness lengths over land (Zilitinkevich
(1995), Air Pollution III – Vol. I.)
• Updates thermal/moisture roughness lengths over water, taken from the COARE 3.0 bulk algorithm (Fairall et al. 2003, J. of Climate).
• Use of consistent flux-profile relationships with those used to formulate the surface layer length scale in the PBL scheme.
• Relaxed some arbitrary limits, such as lower limit for u* (0.1 0.01).
Changes to MYNN surface layer scheme
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2 m Temperature Verification in RAP(all metars across CONUS region)
BIAS RMSEMYJ
MYNN
• Afternoon warm bias reduced by up to 1oC in MYNN compared to MYJ
• Both Biases and RMSEs are comparable at night.
coldwarm
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10 m Wind Speed Verification in RAP 9 hr forecasts – 01-12 June 2012
verified against all metars across the CONUS region
MYJMYNN
OBS-MODEL(negative means high wind speed bias)
α4=100 α4=20
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Evolution of the Low-Level Jet (3-km nest)Loop was made from 1-hr output intervals between 00-18 UTC 06 Sept 2011.
. .... ....
Wind Speed (m s-1)
Wind speed and θ (Leeds, ND)
TKE and θ (Leeds, ND)
m2 s-2
m s-1
80-m wind speed (color)WFIP-North Profilers
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Profile Evolution (Leeds, ND) – Lb Tests
MYJ
MYNNα2=0.53
MYNNα2=0.75
MYNNα2=0.64
• All model profiles are taken from the 3 km simulations.
• MYNN has stronger LLJ but too strong under 100 m AGL.
• MYNN is improved with increased α2.
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Mean profiles (LDS) 03-09 UTC (MYNN-lb tests)
• α2 = 0.75 produces a weaker LLJ (~1.4 m s-1 weaker than α2=0.53).
• α2 = 0.64 produces a weaker LLJ (~0.7 m s-1 weaker than α2=0.53).
• Largest differences in TKE & lm are above the jet max.
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Profile Evolution (Leeds, ND) – Ls Tests
MYJ
MYNNcns=2.7
MYNNcns=1.5
MYNNcns=2.1
• All model profiles are taken from the 3 km simulations.
• Parameter cns has a powerful impact on the LLJ strength.
• MYNN is improved with decreased cns..
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Mean profiles (LDS) 03-09 UTC (MYNN-ls tests)
• cns = 1.5 produces a weaker LLJ (~1.8 m s-1 weaker than cns = 2.7).
• cns = 2.1 produces a weaker LLJ (~0.9 m s-1 weaker than cns = 2.7).
• Large differences in TKE & lm are below the jet max.
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Wind Speed (m s-1)
Wind speed and θ (Ainsworth, NE)
TKE and θ (Ainsworth, NE)
m2 s-2
m s-1
80-m wind speed (color) WFIP-North Profilers
Evolution of the Low-Level Jet (3-km nest)Loop was made from 1-hr output intervals between 00-23 UTC 10 June 2012.
... ..
..
..MYNN PBL scheme configured with: α2 = 0.64, cns = 2.1, and α4 = 20.
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Profile Evolution (Ainsworth, NE)
MYNN
MYJ
Profiler Winds
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6-hr forecasts from RAP with MYNN and MYJ (including all modifications) between 20120607-20120612
verified against rawinsonde data over CONUS region
Wind Speed Bias Relative Humidity BiasTemperature Bias
Wind Vector RMSE Relative Humidity RMSETemperature RMSE
MYJMYNN
MYJMYNN
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Summary
• Important modifications were made to improve the MYNN PBL scheme: (1) modified lb (α2), (2) modified ls (cns) , and (2) modified ls (α4).
• A small change to all 3 parameters can collectively reduce the high wind speed bias in the lowest 100 m, while maintaining better forecast skill of wind speed in the rest of the PBL.
• Many improvements to the surface layer scheme reduced the daytime warm bias, while remaining competitive with the MYJ at night.
• Too much tuning to improve the low-level wind speeds at 13 km grid spacing may degrade wind forecasts at higher resolution (or other altitudes). Cliff Mass will discuss this in the next talk.
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Not Shown
• Hybrid PBLH: uses θ-based PBLH in neutral/unstable conditions and TKE-based PBLH in stable conditions.
• TKE budgets are available output fields by configuring Registry.EM.
•Make 3D TKE-budgets output fields dependent on namelist option, to reduce memory useage when not needed.
•Implement changes into v3.4.1.
Future Work
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Extra Slides
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Background on Rapid Refresh (RAP)• Hourly data assimilation system which uses the WRF-ARW as the
model forecast component (Weygandt et al., 2.1). • Parent model of the High-Resolution RAP (HRRR). • Testing different parameterizations within the WRF-ARW against
our current configuration. • For wind energy application, comparison of current PBL scheme,
Mellor-Yamada-Janjic (MYJ) to Mellor-Yamada-Nakanishi-Niino (MYNN). Main features of the MYNN include:
• Turbulent kinetic energy (TKE)-based local mixing scheme (like MYJ)
• Option to run at level 2.5 or 3.0 closure.
• Tuned to a database of LES simulations in order to overcome the typical biases associated with other MY-type schemes (insufficient growth of convective boundary layer and underestimated TKE).
• More elaborate mixing length formulations to flexibly change behavior across the stability spectrum.
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Hybrid PBLH in MYNN Coastal Jet CaseWind speed parallel to the barrier (color), potential temperature (red contours) and TKE (black contours).
ziTKE
ziθv
zihybrid
zihybrid + 0.3zihybrid
22 UTC 00 UTC23 UTC
• If we take the maximum wind speed of the jet as the PBL top, then ziθv significantly underestimates the PBL height.
• ziTKE overestimates the PBL height, especially during the period of elevated mixing associated with the strong vertical wind shear on the outer edge of the jet.
• The hybrid PBLH, zihybrid, best follows the level of maximum wind speeds, but is also shallow-biased prior to the period enhanced mixing.
• The height, zihybrid + 0.3zihybrid, is used as the level for which the turbulent length scale is integrated to (between the surface and zihybrid + 0.3zihybrid). This allows the TKE within the “entrainment layer” to be accounted for when determining the turbulent length scale.
Mt.
Fairw
eath
er
Mt.
Fairw
eath
er
Mt.
Fairw
eath
er
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2 m Temperature Verification(8-day means of 9 hr forecasts – valid at 21 UTC)
MYJ MYNN
East Bias: 1.78East MAE: 2.69West Bias: 0.22West MAE: 2.33
East Bias: 2.41East MAE: 3.03West Bias: 1.33West MAE: 2.38