Post on 19-Jan-2016
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Making upgrades to an operational model : An example
Jongil Han and Hua-Lu PanNCEP/EMC
GRAPES-WRF Joint Workshop
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Recent upgrade in the operational GFS(Effective at July 27th 2010 12 UTC)
• Resolution increase: T382L64 (~35km) => T574L64 (~23km)
• Major physics change:
- Shallow and deep convection, PBL schemes
- Radiation:
- SW: NCEP0 => RRTM (Rapid Radiative Transfer Model)
- LW computation frequency: 3 hrs => 1 hr
- SW cloud overlap: random => maximum-random overlap
- Positive-definite vertical tracer transport scheme (remove negative water vapor)
- Minor changes in mountain blocking parameterization
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Revision of shallow cumulus convection scheme
• One of long standing problems in the GFS was the systematic underestimation of stratocumulus clouds over off-coast regions in the eastern Pacific and Atlantic Oceans.
• This problem has been attributed to the shallow convection scheme which uses a turbulent diffusion approach
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ISCCP
Old GFS
Low cloud cover (%)
(P>680hPa)
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Seek conditionally unstable layer
K
Kmax=5m2s-1
Old operational shallow convection scheme in the GFS (Tiedke, 1983)
LCL
z
TK
zt
T 1
z
qK
zt
q 1
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Old shallow convection scheme (Diffusion scheme)
New shallow convection scheme (Mass flux scheme)
Mass flux analogy(de Roode et al., 2000) :
Au (updraft area)=0.5
Ad (downdraft area)=0.5
Au~0.0; Ad~1.0
Environment is dominated by subsidence resulting in environmental warming and drying.
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Revised shallow convection scheme
• Use a bulk mass-flux parameterization same as deep convection scheme.
• Separation of deep and shallow convection is determined by cloud depth (currently 150 mb).
• Entrainment rate is given to be inversely proportional to height (which is based on the LES studies) and much larger than that in the deep convection scheme.
• Mass flux at cloud base is given as a function of the surface buoyancy flux (Grant, 2001). This differs from the deep convection scheme, which uses a quasi-equilibrium closure of Arakawa and Shubert (1974) where the destabilization of an air column by the large-scale atmosphere is nearly balanced by the stabilization due to the cumulus.
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Revised shallow convection scheme
• Entrainment rate:
Siebesma et al.2003:
• Detrainment rate = Entrainment rate at cloud base
zce
1 ce =0.3 in this study
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Revised shallow convection scheme
Mass flux at cloud base:
Mb=0.03 w* (Grant, 2001)
3/1
00* ))(/( hwTgw v
(Convective boundary layer velocity scale)
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CTL
New shallow convection scheme
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Cloud depth (mb)
Deep & shallow
Shallow only (<150mb)
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ISCCP
New shallowOld shallow
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Revised PBL scheme
• Include stratocumulus-top driven turbulence mixing based on Lock et al.’s (2000) study
• Enhance stratocumulus top driven diffusion when the condition for cloud top entrainment instability is met
• Use local diffusion for the nighttime stable PBL rather than a surface layer stability based diffusion profile
• Background diffusivity for momentum has been substantially increased to 3.0 m2s-1 everywhere, which helped reduce the wind forecast errors significantly
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hsurfh z
Kw
2
1
h
zzwK s
surfm
surfm
surfh KK 1Pr
hw
w
sh
0)(5.6
(Buoyancy reversal term is neglected)
MRF PBL Revised model
Heat flux
hsurfh
Sch
surfh K
zKKw
)/()(0
3pbbSc cRzh
gV
2/12
185.0
bb
b
bb
bSc
Sch zh
zz
zh
zzVK
phv c
Rcw
b
)(
,7.0 tep qLcif C=1.0
where c=0.2
(CTEI condition)
(Simplified after Lock et al., 2000)
(Moeng et al., 1999)
(MacVean and Mason, 1990)
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Liquid water (q l ) at initial time
0
200
400
600
800
1000
1200
0 0.1 0.2 0.3 0.4 0.5 0.6[g/kg]
z (m
)Liquid water potential temp.
at initial time
0
200
400
600
800
1000
1200
285 290 295 300 305[K]
z (m
)
Total liquid water (q+q l) at initial time
0
200
400
600
800
1000
1200
0 1 2 3 4 5 6 7 8 9 10[g/kg]
z (m
)
Diffusivity at initial time
0
200
400
600
800
1000
1200
0 20 40 60 80 100 120
[m2/s]
z (m
)
Kscu+Ksfc
Kscu
Ksfc
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No stratocumulus top driven diffusion
With stratocumulus top driven diffusion
Low cloud cover (%)
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GFS Grid Point Storms (bull’s eye precip)
• The GFS suffered from grid point storms during the convective season, which was another long standing problem in the GFS forecasts.
• The old deep convection scheme did not appear to fully eliminate the instability and consequently, an explicit convective ascent occurred on the grid scale, producing unrealistically large precipitation.
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24 h accumulated precip ending 12 UTC 15 July 2009
Grid Point StormGrid Point Storm
Observed 72 h GFS Forecast
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Revised deep convection scheme
• Increase cloud top
- Random cloud top selection => single deepest cloud
- A convective overshooting is parameterized in terms of cloud work function
• Increase maximum allowable cloud base mass flux
• Include the effect of convection-induced pressure gradient force to reduce convective momentum transport (reduced about half)
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Old GFS deep convection scheme (SAS)
SL
DL
LFC
CTOP
h hs
Updraft mass flux
0.5
1.0
Downdraft mass flux
1.0
0.05
Entrainment
EntrainmentDetrainment
Environmental moist static energy
150mb
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A
hs
hc
0.1A
The overshoot of the cloud top is stopped at the height where a parcel lifted from the neutral buoyancy level with energy equal to 10% of the cloud work function (A) would first have zero energy.
T
B
T
B
z
zp
uz
z
u
dzTc
ssgdz
T
TTgA
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Revised deep convection scheme
Maximum cloud base mass flux [currently 0.1 kg/(m2s)] is defined for the local Courant-Friedrichs-Lewy (CFL) criterion to be satisfied (Jacob and Siebesman, 2003);
tg
pM b
max
Then, maximum mass flux is as large as 0.5 kg/(m2s) for T382 (35km) resolution
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Revised deep convection scheme
Organized entrainment (Betchtold et al., 2008)
1100 )1()( FRHcFz
zz
1.0)(0
)()( 00 bzzz 4
1 100.1 c
)(0 bzz 3
1
2
0 ,
sb
s
sb
s
q
qF
q
qF
turb. org.
in sub-cloud layers
above cloud base
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Revised package Old GFS
Total precipitation (grid scale+convective)
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Revised package Old GFS
Convective precipitation
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Revised package24 h accumulated precipitation ending at 12 UTC, July 24, 2008 from (a) observation and 12-36 h forecasts with (b) control GFS and (c) revised model
Total precipitation (grid scale+convective)
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Reduction of convective momentum transport due to convection-induced pressure gradient force
(Han and Pan, 2006)
VVz
VMc
t
Vuu
1
)1(
c: effect of convection-induced pressure gradient force
c=0.0 in the current operational GFS convection scheme
c=0.55 in the revision
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(a) Atlantic
0
5
10
15
20
25
30
35
40
0 12 24 36 48 72 96 120Forecast hour
Me
an
in
ten
sit
y e
rro
r (k
ts)
CTL
New Shal
(b) East Pacific
0
5
10
15
20
25
30
35
40
0 12 24 36 48 72 96 120Forecast hour
Me
an
in
ten
sit
y e
rro
r (k
ts)
CTL
New Shal
#CASES (350) (323) (300) (275) (245) (199) (158) (129)
#CASES (257) (230) (194) (163) (135) (91) (55) (27)
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Medium-range forecast experiments with data assimilation
• Resolution: T382L64 (about 35km at equator)
• Test period: June 2 – Nov. 10, 2008 (7-day forecasts at each 00Z cycle), which includes the whole 2008 hurricane season.
• A spin-up series of forecasts for the previous 19 days has been discarded from the analysis.
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(a) NH
0
10
20
30
40
50
60
70
0 24 48 72 96 120 144 168Forecast hour
RM
SE
(m
)
CTLNew Shal
(b) SH
0
10
20
30
40
50
60
70
80
90
100
0 24 48 72 96 120 144 168Forecast hour
RM
SE
(m
)
CTL
New Shal
500 mb height forecast skill
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NH SH
T574 (~23km)
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500 MB Anomaly Correlation
Northern Hemisphere Southern Hemisphere
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(a) 12-36 hrs
0
0.1
0.2
0.3
0.4
0.5
0.2 2 5 10 15 25 35 50 75
Threshold (mm/24 hrs)
Eq
uit
ab
le t
hre
at
sc
ore
CTL
New Shal
(b) 36-60 hrs
0
0.1
0.2
0.3
0.4
0.5
0.2 2 5 10 15 25 35 50 75
Threshold (mm/24 hrs)
Eq
uit
ab
le t
hre
at
sc
ore CTL
New Shal
(c) 60-84 hrs
0
0.1
0.2
0.3
0.4
0.5
0.2 2 5 10 15 25 35 50 75
Threshold (mm/24 hrs)
Eq
uit
ab
le t
hre
at
sc
ore CTL
New Shal
Precip. score over continental US
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(a) 850 hPa (Tropics: 20S-20N)
0
1
2
3
4
5
6
0 24 48 72 96 120 144 168
Forecast hour
RM
SE
(m
/s)
CTLNew Shal
(b) 200 hPa (Tropics: 20S-20N)
0
2
4
6
8
10
12
0 24 48 72 96 120 144 168
Forecast hour
RM
SE
(m
/s)
CTLRevised
Vector wind error (Tropics)
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(a) Atlantic
0
50
100
150
200
250
0 12 24 36 48 72 96 120Forecast hour
Me
an
tra
ck
err
or
(nm
) CTL
New Shal
(b) East Pacific
0
50
100
150
200
250
0 12 24 36 48 72 96 120Forecast hour
Me
an
tra
ck
err
or
(nm
) CTL
New Shal
#CASES (359) (331) (307) (281) (254) (205) (161) (131)
#CASES (261) (233) (197) (166) (138) (92) (56) (28)
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• Turbulence parameterization : moist-process conserved variable mixing (Tl and Qt)
• Macro and micro physics : Ferrier scheme with partial clouds
• Consistent cloud fraction formulation for macro-physics, turbulence, and radiation
Future plan