daniela flocco, daniel feltham, david schr eder centre for polar observation and modelling...
TRANSCRIPT
Daniela Flocco, Daniel Feltham, David Schreder
Centre for Polar Observation and Modelling
University College London
MODELS VS OBSERVATIONS
Stroeve et al., 2007
ALBEDO AND ICE-ALBEDO FEEDBACK
Global warming is intensified in polar regions due to
the albedo feedback mechanism [IPCC, 2007]: since
ice and snow are highly reflective compared to other
surface types, a reduction in the ice or snow coverage
due to melting results in enhanced absorption of solar
radiation, which leads to further melt and local
warming (snow = 0.8, ocean = 0.1, grass = 0.3).
Using ERS satellite data, strong correlation is found between ice thickness and length of previous melt season (Laxon et al, Nature, 2003)
The fraction of first year ice is increasing in time
Pond area is larger over thin undeformed ice
SEA ICE MASS BALANCE
MELT PONDS
Photo Credit: ICESCAPE project - NASA/Kathryn Hansen
July 4, 2010: Arctic sea ice and melt ponds in the Chukchi Sea.
MELT PONDS FORMATION
Melt ponds form on Arctic sea ice during summer (rarely seen in Antarctic) when the snow/ice surface melts due to absorbed solar, short wave radiation and freeze in September
Pond coverage ranges from 5% to 50% of the sea ice cover
Albedo of pond-covered ice (0.15 - 0.45) is lower than the albedo of bare sea ice or snow covered ice (0.52 - 0.87)
In terms of the heat budget of the Arctic, a change in mean albedo of 0.1 (e.g. a change in pond fraction of 20%) is equivalent to a change in sea ice extent of 10%.
Underice refrozen ponds are insulated by an ice layer and take up to 2-3 months to freeze completely
GCM-COMPATIBLE MELT POND MODELRequirements of constructing a melt pond model for use in existing GCMs places strong constraints on the form the model can take: main difficulty is that GCMs do not determine the sea ice topography.
Modern GCMs contain a thickness distribution function g(h).
FractionalArea
Thickness
g1
g2
g3
g4
g5
€
gi =1∑
Flocco and Feltham, 2007
HEIGHT AND DEPTH DISTRIBUTIONFUNCTIONSNEED: information about the surface height to redistribute surface water.ANSWER: We introduce surface height α(h) and basal depth β(h) distributions, which give the relative area of ice of a given surface height or basal depth.
α
β
hsl
NOTE: α(h) and β(h) are derived from the thickness distribution g(h) but do not describe the topography.
Referenceheight
DELTA EDDINGTON and “TOPO” POND’S SCHEME
“Retained” melt water fraction: 0.15 + 0.7ai for hi > 1 cm
Refreezing: Vp = Vpe−0.005(−2−Tsfc) for Tsfc < −2◦
Area ap = (Vp/0.8)0.5 < 1
Depth hp = 0.8ap < 0.9hi
Volume Vp = aphp
Snow ap =(1−hs/0.03)ap
Rransport Vp, ap
Retained melt water fraction: 0.15 + 0.7ai for hi > 1 cm
Reduce Vp by fraction of ice
area that ridged Melt water fills thinnest
categories first, may overflow
Snow occupies space according to ρs. If the ice is permeable, pond can drain to sea level
Vip = refrozen “lid” on pond
newly forming: use Fsfc
thickening: use Stefan solution for
melting: use meltt
€
ρiLdH
dt=K i
∂T
∂z
(Elizabeth Hunke talk at NCAR)
MELT POND AREA AND DEPTH RETRIEVAL
fulli
nn
fullsurf h
Ice Bare of AreaSnow of Area.
)h( VolumeVolume Totalh +
++⋅
−=
∑=1
60
snowpond
Asnow
category i
category i-1
category i-2
A pond (i-1) A bare ice (i+1)
hfull
hsurfl
Flocco and Feltham (JGR, 2007)
ICE EXTENT ANOMALYIc
e e
xte
nt
anom
aly
(%
)
ICE VOLUME ANOMALYIc
e v
olu
me a
nom
aly
(10
3 k
m3)
DIFFERENTIAL ICE GROWTH
Sea ice growth difference between the two pond’s schemes runs
Cm
/month
RED: Sea ice growth greater in new scheme
POND AREA AND ICE FRACTION
Ice thickness category
Pondarea
I IIIII VIV
Pond area is larger over thin undeformed ice
Sea ice
conce
ntra
tion %
ALBEDO
May Jun July
POND AREA JUN-SEPTEMBER 2007
SEA ICE EXTENT: SEPTEMBER 2007
CONCLUSIONS
We study melt ponds to quantify their importance for the increased melting rate due to global warming
Global Circulation Models are sensitive to the presence of a melt pond routine
Initial runs suggest that incorporation of melt ponds might allow simulations of the 2007 minimum sea ice extent
The melt pond routine is being incorporated into the latest version of CICE, which is the sea ice model used also by the UK Met Office