1

Making upgrades to an operational model : An example

Jongil Han and Hua-Lu PanNCEP/EMC

GRAPES-WRF Joint Workshop

2

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

3

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

4

ISCCP

Old GFS

Low cloud cover (%)

(P>680hPa)

5

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

6

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.

7

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.

8

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

9

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)

10

CTL

New shallow convection scheme

11

12

Cloud depth (mb)

Deep & shallow

Shallow only (<150mb)

13

ISCCP

New shallowOld shallow

14

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

15

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)

16

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

17

No stratocumulus top driven diffusion

With stratocumulus top driven diffusion

Low cloud cover (%)

18

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.

19

24 h accumulated precip ending 12 UTC 15 July 2009

Grid Point StormGrid Point Storm

Observed 72 h GFS Forecast

20

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)

21

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

22

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

23

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

24

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

25

Revised package Old GFS

Total precipitation (grid scale+convective)

26

Revised package Old GFS

Convective precipitation

27

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)

28

29

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

30

31

(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)

32

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.

33

(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

34

NH SH

T574 (~23km)

35

500 MB Anomaly Correlation

Northern Hemisphere Southern Hemisphere

36

(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

37

(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)

38

(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)

39

40

• 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