Motivation: What is the role of moist physics in the Hadley ...

Motivation: What is the role of moist physics in the Hadley ...

Downscaling Ability of the NCEP Regional Spectral Model v.97: The
Big Brother Experiment
Motivation: The Big Brother Experiment methodology allows us to assess the

Tropical Domain Results

errors in regional climate simulations which are solely due to the nesting
strategy.

BIG BROTHER
BROTHER

LITTLE

PRECIPITATION

Setup of the Big Brother
Experiment:

SEA+LAND
Stat. Tran
Corr
s.
.
Corr.

This experiment was first performed by Denis et al.
(2001), using a grid point model for a midlatitude
winter simulation. Our study extends theirs by
using both a tropical and a midlatitude domain case,
and using a different (in this case, spectral) model.

STEPS:

1.We first perform a large-domain high
resolution regional model simulation, the
Big Brother.
2.This reference simulation is then degraded
by filtering out short-scales which would be
unresolved in the GCM
3.This filtered reference is then used to drive
the same regional climate model, integrated
at the same resolution as the Big Brother,
but over a smaller domain embedded in the
large domain, called the Little Brother

The Big Brother Experiment
(BBE)
Global LowResolution Data
Set

0.81
47

0.425
7

LW

0.85
34

0.538
2

SW

0.22
47

0.12
93

Stat.
Var.
Ratio

SEA
Trans.
Var.
Ratio

1.317

1.4374

1.2967
1.7362

LEGEND:
IC = Initial
Condition

Trans.
Corr.

0.405

0.884
7

0.4834
0.577
0.125
8

1.3442

0.8227

0.5061

0.927
2

1.8379

0.207
5

0.092
1

0.26
18

Filter Small
Scales

Stat.
Corr
.

Tran
s.
Corr.

Tot
al

0.99
35

0.933
6

LW

0.99
36

0.939
1

SW

0.98
41

0.750
4

Correspond
ing Data
Set
Regiona
l
Climate
Validati
on

Trans.
Var.
Ratio

1.0061

1.0188

1.0064
0.9834

Stat.
Corr.
0.9938

1.0182

0.994

1.072

0.984
8

0.9345

0.97
75

0.835

0.9363

0.97
92

0.8448

0.845

0.98
1

0.830
5

ZONAL WIND (850 MBAR)
SEA+LAND
Stat.
Corr
.

Small regional
High Resolution
Simulation

Tran
s.
Corr.

Tot
al

0.97
62

0.918

LW

0.97
68

0.922
7

SW

0.96
00

0.78
6

Stat.
Var.
Ratio

SEA
Trans.
Var.
Ratio

0.8878

0.9766

0.8879
0.8726

LAND

Stat.
Corr.

Trans. Stat.
Corr. Corr.

Trans.
Corr.

0.977

0.9168

0.983
6

0.8754
0.8821
0.802
8

0.9748

0.9773

0.9194

0.984
7

1.0451

0.966
7

0.844
0

0.94
89

Midlatitude Domain Results
BIG BROTHER

LITTLE BROTHER

MIDLATITUDE
DOMAIN:
One month simulation
completed for February,
2001
Both domains centered at
70W and 47N
Horizontal resolution of
50 km
Big Brother domain at
151 x 142 grid points
Little Brother domain at
81 x 77 grid points
18 vertical layers

LITTLE
BROTHER
BIG
BROTHER

PRECIPITATION
SEA+LAND

SEA

Stat.
Corr
.

Tra Stat.
ns.
Var.
Corr Rati
.
o

Tran
s.
Var.
Rati
o

Stat.
Corr.

Tot
al

0.94
52

0.73
82

1.06
39

1.05
02

0.966
3

LW

0.95
89

0.79
41

1.04
92

1.03
88

SW

0.82
11

0.41
40

1.18
63

1.18
89

Stat.
Corr.
T=06
HR

T=12
HR

T=18
HR

T=24
HR

Stat.
Var.
Ratio

Trans.
Corr.

Trans.
Var.
Ratio

Tot
al

0.9025

0.9382

0.4744

1.0981

LW

0.9400

0.9396

0.5905

1.1085

SW

0.3561

0.9120

0.1484

1.0779

Tot
al

0.8742

0.8398

0.4346

1.0747

LW

0.9141

0.8410

0.5406

1.0809

SW

0.3158

0.8013

0.1231

1.0836

Tot
al

0.7747

0.9637

0.3590

1.2473

LW

0.8176

0.9540

0.4544

1.2457

SW

0.2266

0.9423

0.0783

1.3260

Tot
al

0.8845

1.0883

0.4398

1.0224

LW

0.9274

1.0969

0.5601

1.0428

SW

0.2343

1.0506

0.0765

1.0228

PRECIPITATION FIELD
ENSEMBLES

The table on the left
shows the ensemble
results comparing the
identically forced Little
Brother simulations, but
initialized at different
times differing by 6-24
hours; the correlations of
the small-scale
precipitation between
these different
simulations was almost as
low as that between the
Big and Little Brother
ensemble simulations
th
(not shown
here).

NOAA 29

Trans.
Corr.

Stat.
Corr
.

Trans.
Corr.

0.7258

0.88
44

0.6980

0.975
0

0.7944

0.91
53

0.7524

0.823
2

0.414
9

0.81
96

0.475
0

SEA

Tran Stat.
Stat.
s.
Var.
Corr Corr Rati
.
.
o
Tot
al

0.99
72

0.96
41

1.06
82

LW

0.99
73

0.96
45

SW

0.99
72

0.95
88

Ensemble Simulations Results

The Model: NCEP Regional Spectral Model, v. 97 Juang et al.,
1997.

LAND

SPECIFIC HUMIDITY (850 MBAR)
Tran
s.
Var.
Rati
o

LAND

Stat.
Corr.

Trans
.
Corr.

Stat.
Corr.

Trans.
Corr.

1.048
2

0.998
1

0.962
1

0.996
1

0.9526

1.06
76

1.047
1

0.998
2

0.962
7

0.996
1

0.9558

1.07
95

1.08
99

0.997
6

0.964
5

0.99
65

0.949
4

ZONAL WIND (850 MBAR)
SEA+LAND

SEA

LAND

Tran
Stat.
s.
Corr Corr
.
.

Stat.
Var.
Rati
o

Tran
s.
Var.
Rati
o

Stat.
Corr.

Trans
.
Corr.

Tot
al

0.99
36

0.93
36

1.140 1.111
7
6

0.995
1

0.9343 0.992

0.924

LW

0.99
4

0.93
44

1.141 1.112
1
1

0.995
5

0.992
0.9354
6

0.9263

SW

0.98
60

0.92
02

1.13
26

0.98
85

0.927
8

0.911
3

1.11
37

Department of Earth and Environmental
Sciences
Columbia University
New York, NY

Trans.
Corr.

LBC

Small Limited-area
High Resolution
model

l

Stat.
Var.
Ratio

*Department of Applied Physics and Applied
Mathematics
Columbia University
New York, NY

International Research Institute For Climate
Prediction
LAND
Columbia University
Stat.
Corr Trans.
Palisades, NY
.
Corr.

SEA

SEA+LAND

All of the statistical computations were performed excluding
the first day in order to reject any short time-scale spin-up
phenomenon. A ten grid point lateral boundary zone was also
excluded.
To mimic the resolution of the operational GCM (ECHAM at T42 resolution), a cutoff wavelength of ~900 km was used in the
filtering of the Big Brother output before using it to drive the
Little Brother.

Stat.
Corr.

0.7841

SEA+LAND

IC

LBC = Lateral
Boundary
Condition Mode

One month simulation
completed for April, 1994
Both domains centered at
37.5W and 5S
Horizontal resolution of
60 km
Big Brother domain at
181 x 182 grid points
Little Brother domain at
91 x 92 grid points
18 vertical layers

ECHAM4.5 data, archived every 6 hours was used to drive the
Big Brother simulation.
The Big Brother simulation data was archived every 6 hours
for the nesting of the Little Brother simulation, as well.
All experiments were executed for a month.

Trans.
Corr.

SPECIFIC HUMIDITY (850 MBAR)

Large regional
High Resolution
Reference
Simulation

DOMAIN:

Experimental Details:

Stat.
Corr.

LAND

IC and
LBC

5.Since the same model is used for both the
Big Brother and Little Brother experiments,
the climate of the two should in principle be
Our identical.
Experiment
Any: Domains
difference in the solutions
can only be attributed to the nesting
strategy, and not to TROPICAL
model error.

BIG
BROTHER

Tot
al

Large Limited Area
High Resolution
Model

4.We then compare the climate statistics of
the Little Brother with those of the Big
Brother on the Little Brother Domain

LITTLE
BROTHER

Deborah Herceg*
Adam Sobel*
Liqiang Sun
Stephen Zebiak

Stat. Trans.
Corr. Corr.

0.98
23

Conclusions:

[email protected]

In most fields, good agreement
between Big and Little Brothers was
obtained, for both small and large
scales, after a brief initial spin-up
period.
Precipitation was the exception. For
our tropical domain experiment, the
small-scale features of the precipitation
field in the Big Brother were not
reproduced by the Little Brother. The
spatial and temporal variations of the
large-scale component of the
precipitation were well reproduced.
The reason for the poor reproduction
of the small-scale component of the
precipitation is that this component is
inherently unpredictable, at least in this
model, rather than because of any
inherent flaw in the nesting strategy.
This was shown by an ensemble of
identically forced Little Brother
simulations which were initialized at
different times differing by 6-24 hours,
in which the correlations of the smallscale precipitation between these
different simulations was nearly as low
as that between the Big and Little
Brothers. Related to this, the
agreement between Big and Little
Brothers is fairly insensitive to the
choice of the cutoff wave number used
for the filtering.
Table
Legend: of the small-scale
Good reproduction
was obtained for a
precipitation
Total Field (TOTAL) is separated into a small-scale
wintertime
midlatitude
component
(SW),
and large-scalesimulation.
component (LW);
defined
therelatively
wave numbers
abovereproduction
and below the
Thus, as
the
poor
cutoff wave number.
of this field for the tropical case
Stationary Correlation (Stat. Corr.) = spatial
appears of
totime-mean
be due fields.
to specifically
correlation
tropical
precipitation
Stationary (convective)
Variance Ratio (Stat.
Var. Ratio)=ratio
of
variances of time-mean fields.
processes.
Transient Correlation (Trans. Corr.), Transient
Variance Ratio (Trans. Var. Ratio) = spatial
correlation, variance ratio of transient fields (totalstationary).
Statistics provided for sea and land separately, as
well as total.

Annual Climate Diagnostics and Prediction Workshop; October 18-22,

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