hwrf/gfdl pbl model and sensitivity experiments€¦ · hwrf/gfdl pbl model • old operational...
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HWRF/GFDL PBL Model and Sensitivity Experiments
Jongil Han
HFIP Physics Workshop, August 9-11, Clinton, Maryland
Acknowledgment: Sun-Hee Kim
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HWRF/GFDL PBL model
• Old operational NCEP GFS PBL scheme (MRF PBL scheme [Hong & Pan 1996; Troen & Mahrt 1986])
• A non-local scheme where a counter-gradient mixing due to large convective eddies is taken into account.
• A surface driven cubical diffusivity profile up to the PBL height is specified.
• Entrainment flux at the PBL top is given implicitly
• A local scheme (Louis et al. 1982) is used for free atmosphere above the PBL.
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hhz
Kw
2
1h
zzwK sm
mh KK 1Pr
hw
w
s
h0
)(8.7
Heat flux
Non-local counter-
gradient mixing term
by large eddies
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))((
)(2
sv
va
crhg
hURbh
s
vasw
w0
)(8.7
PBL height
h: height of zero heat flux, not height of
minimum heat flux.
m
s
uw *
Rbcr : critical bulk Richardson number
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Vertical diffusivity for free atmosphere
Z
URigflK tmtm )(,
2
,
scalelength asymptotic :
111
0
0
l
lkzl
Current HWRF/GFDL model – 150 m
Current GFS model - 150 m (unstable)
30 m (stable)
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Sensitivity experiments for tunable parameters
Localization
Critical Bulk
Richardson
Number
Mixing length
( )
CTL (current HWRF
PBL scheme)
NON-local diffusion 0.5 150
EXP1 NON-local diffusion 0.25 150
EXP2 NON-local diffusion 0.5 30
EXP3 (current GFS PBL
scheme)
NON-local diffusion 0.25 30
EXP4 Local diffusion - 30
0l
Local diffusion scheme for free atmosphere is applied through all levels
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Momentum diffusivity coefficient (CTL, 48 hour forecast, azimuthally averaged)
2)1(h
zzkwK szm
RMW
( 70 km) near eye
(30 km)
about 2 km
100 km 200 km
In the hurricane environment (near neutral), diffusivity and PBL height are
proportional to wind speed
Momentum diffusivity coef.
Vertical profile of Kzm
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Vertical profile of momentum diffusivity coefficient at RMW (Radius of Maximum wind)
(sensitivity experiment, 48 hour forecast, azimuthally averaged)
Ribc 0.25
Ribc 0.5
l0 30
• Smaller RiBc( e.g. RiBc = 0.25) makes the PBL height lower and the diffusivity weaker (exp1& exp3) .
• Smaller mixing length (e.g. l0=30 m) also makes the diffusivity weaker (exp2 & exp3) but not as much as smaller RiBc .
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• Smaller RiBc( e.g. RiBc = 0.25) makes the PBL height lower (exp1& exp3) .
• Smaller mixing length (e.g. l0=30 m) also makes the PBL height low but only slightly (exp2 & exp3) .
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Central pressure
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Maximum 10m wind
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TC vital
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Experiment summary
• In the hurricane environment (near neutral), diffusivity and PBL height are proportional to wind speed.
• Smaller RiBc (e.g. RiBc=0.25) makes the PBL height lower and the diffusivity weaker, and as a result, hurricane stronger.
• The effect of mixing length in free atmosphere on hurricane intensity is very small.
• The local PBL scheme which has weakest diffusivity gives rise to the strongest hurricane intensity, especially in terms of central pressure.
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Critical reports for the MRF (HWRF/GFDL) PBL scheme from literature
• Too high PBL height
• Too much mixing
• Too weak low level Jet
• Too early development of the PBL in the morning
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Potential improvements for the HWRF/GFDL PBL model
• Reduce RiBc and mixing length in free atmosphere
• Use local scheme for the stable boundary layer
• These modifications have been already implemented into the GFS (2010) and showed improvements in forecast skill
• Need a detailed observation (and LES experiment) for investigating the PBL turbulence structure in hurricane environment so that we can tune the model parameters
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Case (single) • IKE (2008)
• Initial time : 3rd Sep. 2008
930
940
950
960
970
980
990
1000
1010
1020
9/1 9/3 9/5 9/7 9/9 9/11 9/13 9/15
BEST TRACK
Sat (TAFB)
Sat (SAB)
Obj T-Num
AC (sfc)
Surface
Pre
ss
ure
(m
b)
Date (Month/Day)
Hurricane Ike
September 2008
3rd Sep.
3rd Sep.
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Analysis of control experiment ( CTL, 48 hour forecast , azimuthally averaged)
about 2 km
inflow