Ocean / Sea Ice Surface - Earthsystemcog.org

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27 Οκτ 2013 (πριν από 4 χρόνια και 2 μήνες)

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Sea Ice

Xingren Wu and Robert Grumbine

Sea Ice

NEMS/GFS Modeling Summer School

2

Sea ice is a thin skin of frozen water covering the polar
oceans. It is a highly variable feature of the earth’s surface.

Nilas & Leads

First
-
Year Ice

Multi
-
Year Ice

Melt Pond

Snow
-
Ice

Rafting

Pancake Ice

Greece Ice

Sea ice affects climate and weather

related processes



Sea ice amplifies any change of climate due to its
“positive feedback” (coupled climate model concern):

Sea ice is white and reflects solar radiation back to space. More sea ice cools
the Earth, less of it warms the Earth. A cooler Earth means more sea ice and
vice versa.



Sea ice restricts the exchange of heat/water between
the air and ocean (NWP concern)


Sea ice modifies air/sea momentum transfer, ocean
fresh water balance and ocean circulation:

The formation of sea ice injects salt into the ocean which makes the water
heavier and causes it to flow downwards to the deep waters and drive a
massive ocean circulation



NEMS/GFS Modeling Summer School

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Arctic/Antarctic Sea Ice Concentration: The
Cryosphere

Today

(arctic.atmos.uiuc.edu/
cryosphere
)

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The Observed Arctic Sea Ice Extent (SIE):

(www.ijis.iarc.uaf.edu/en/home/seaice_extent.htm)

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Minimum Arctic Sea Ice (1982 and 2012)

The Cryosphere Today: (arctic.atmos.uiuc.edu/cryosphere)

NEMS/GFS Modeling Summer School

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Arctic SIE Anomalies for Sept: NSIDC

(from 1979
-
2000 mean)

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Arctic minimum SIE has decreased more than 45% since 1979

Issues related to sea ice and sea ice forecast


Data
assimilation


Sea ice concentration data are available but velocity data lack to
real time


Lack of sea ice and snow thickness data


Initial
conditions


Sea ice concentration data are available but velocity data lack to
real time


Sea ice and snow thickness data are based on model
spinup

values or climatology


Sea
ice models and
coupling


Ice thermodynamics/Ice dynamics


Ice model coupling to the atmosphere/ocean



NEMS/GFS Modeling Summer School

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Ice Model: Thermodynamics


Based on the principle of the
conservation of energy, determine:




Ice formation



Ice growth



Ice/snow melting



Ice/snow temperature structure


NEMS/GFS Modeling Summer School

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Ice Model: Dynamics


Based on the principle of the
conservation of momentum, determine:




Ice motions



Ice deformation



Leads (open water)


NEMS/GFS Modeling Summer School

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Sea Ice in the NCEP Global Forecast System


A three
-
layer thermodynamic sea ice model
(Winton 2000) was implemented into GFS in
May 2005.


It predicts sea ice/snow thickness, the surface
temperature and ice temperature structure.


In each model grid box, the heat and moisture
fluxes and albedo are treated separately for
ice and open water.

Winton
, M. 2000: A reformulated three
-
layer sea ice model,
J. of
Atmospheric and Oceanic Technology
,
17
, 525
-
531.


NEMS/GFS Modeling Summer School

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Sea Ice in the NCEP GFS

NEMS/GFS Modeling Summer School

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-
> +
----------
+ <
-

ts
-

diagnostic surface temperature ( <= 0C )


/ | |


hs | snow | <
-

0
-
heat capacity snow layer


\

| |


=> +
---------
+


/ | |


/ | | <
-

t1
-

upper 1/2 ice temperature; this layer has


/ | | a variable (T/S dependent) heat capacity


hi | ...ice... |


\

| |


\

| | <
-

t2
-

lower 1/2 ice temp. (fixed heat capacity)


\

| |


-
> +
----------
+ <
-

base of ice fixed at seawater freezing temp.



SEA ICE THERMODYNAMIC STRUCTURE (Winton 2000)

Sea Ice in the NCEP GFS

NEMS/GFS Modeling Summer School

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3
-
layer

thermodynamics

Ice Model

SW

Heat Flux

LW

Heat Flux

Turbulent

Heat Flux

Oceanic

Heat Flux

Salinity

Fresh Water

Atmospheric Model

[Ocean Model]

Ice

Temperature

Surface

Temperature

Ice/Snow

Thickness

Ice

Fraction

Snow

Rate

Ice

Temperature

surface

Temperature

Ice/Snow

Thickness

Ice

Fraction

Sea Ice in the NCEP GFS

NEMS/GFS Modeling Summer School

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The surface
T

is determined from the diagnostic balance of the
heat fluxes (Conductive, short
-

and longwave radiation, and
sensible and latent heat fluxes).


The ice
T

(upper and lower layers) is calculated based on
conservation of enthalpy.


If the calculated surface
T

is greater than the freezing
T

of snow
or sea ice the surface
T

is fixed at the melting T of snow or sea
ice, and the ice
T

is recomputed. The residual energy flux is
applied toward surface melting of snow or sea ice.


At the ice/water interface, the conservation of energy dictates
that the energy absorbed or released through melting (or
freezing) balances the oceanic heat flux to the ice bottom and
the conductive flux of heat upward from the bottom.

Sea Ice in the NCEP GFS

NEMS/GFS Modeling Summer School

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The analyzed fractional ice cover is kept unchanged during the
entire forecast period in GFS, but there is an option to use
climatology for daily sea ice concentration.


Sea ice analysis (0.5x0.5 grib file from
dump

for GDAS/CDAS):

FNACNA=“icegrb_file”


Sea ice climatology (from
fix
):

FNAISC=“/nwprod/fix/cfs_ice1x1monclim19822001.grb”

SST in the NCEP GFS

NEMS/GFS Modeling Summer School

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Current operation:


SST Analysis (1x1 in GFS; 0.25x0.25 in CFS from
dump

):

FNTSFA=“sstgrb_file”


SST climatology (from
fix
):

FNTSFC=“/nwprod/fix/cfs_oi2sst1x1monclim19822001.grb”


Future implementation:


Higher resolution SST will be tested in the new GFS

Temperature bias over 90
o
N
-
60
o
N for January 2004 (T62/L64)

NEMS/GFS Modeling Summer School

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Negative
bias
decreases

Sea Ice in the CFS (GFDL Sea Ice Simulator
-

SIS)


Hunke

and
Dukowicz

(1997) elastic
-
viscous
-
plastic (EVP) ice dynamics model


Improved numerical method for
Hibler’s

viscous
-
plastic
(VP) model


Computionally

more efficient than
Hibler’s

VP model


Winton (2000) 3
-
layer thermodynamic model
plus ice
thickness distribution


2
-
layer of sea ice and 1
-
layer of snow


Fully implicit time
-
stepping scheme, allowing
longer time steps


5 categories of sea ice


NEMS/GFS Modeling Summer School

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Sea
-
ice is one component of the CFS

NEMS/GFS Modeling Summer School

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Sea Ice

Time Step
Δ
i

GFS
(LAND)

Time Step
Δ
a

Ocean
(MOM4)

Time Step
Δ
o

Coupler

Time Step
Δ
c


Fluxes

Ts

Sea
-
Ice

X
-
grid

Slow loop
:
Δ
o

Fast loop:

Δ
a=
Δ
c=
Δ
i


Atmosphere
grid

Sea Ice “Assimilation” in CFSR/CFSv2

NEMS/GFS Modeling Summer School

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When sea ice concentration is greater than (or equal to)
15% from observation, sea ice concentration is reset to
the observed value in the guess field. New ice can occur
in this case.


In summer, the melt pond effect on ice albedo is
considered in the Arctic.


When sea ice concentration is less than 15% from
observation, sea ice in the guess field will be removed
(treated to melt).


Quality control has been used to prevent sea ice from the
warmer water.
(Ice analysis and SST analysis feedback)

Sea Ice “Assimilation” in CFSR/CFSv2

www.emc.ncep.noaa.gov/gc_wmb/wd20xw/cfsrr/index.html

NEMS/GFS Modeling Summer School

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Sept 16, 2012

Feb 28, 2013

NEMS/GFS Modeling Summer School

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Sea
-
ice concentration

198709

200709

Arctic

Antarctic

(monthly mean)

Near Record minimum
sea
-
ice in Sept 2007

over fixed area: 197902 sea
-
ice

over fixed area: 197909 sea
-
ice

Sea Ice in the CFS Reanalysis (CFSR)

NEMS/GFS Modeling Summer School

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Cold bias

in R1:

1.7K

than
obs

CFSR is:

1.8K

Warmer

than R1

Surface air temperature from CFSR (Sept) and

the difference amongst CFSR, R1, R2 and
ERA40

Sept sea ice concentration in CFSR and CFSv2
hindcast

NEMS/GFS Modeling Summer School

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Seasonal Sea Ice Forecast from CFSv2:

CPC (Wanqiu Wang)

NEMS/GFS Modeling Summer School

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www.cpc.ncep.noaa.gov/products/people/wwang/cfsv2fcst/



Extent

Concentration

Anom&Total


L0

L1

L2

L3

L4

L5

L6

L7

L8


Sea ice extent and concentration




Forecasts are from initial conditions of
the last 10 days, with 4 runs from each
day. Forecast ensembles consist of 40
members from initial a period of 10
days. Anomalies are with respect to
1982
-
2010 hindcast climatology.

Jan 2014 with Apr 2013 ICs

2012 Sea Ice Outlook for the Minimum

Arctic Sea Ice Extent:
August Report (SEARCH)

NEMS/GFS Modeling Summer School

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From
SEARCH
: Study of Environmental Arctic Change

2013 Sea Ice Outlook for the Minimum

Arctic Sea Ice Extent:
June Report (SEARCH)

NEMS/GFS Modeling Summer School

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From
SEARCH
: Study of Environmental Arctic Change

NEMS/GFS Modeling Summer School

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Thank you!




CFSv2 data:

http://cfs.ncep.noaa.gov


CFS Reanalysis data:

http://nomads.ncdc.noaa.gov/NOAAReanalysis/cfsrr