Clear-sky thermodynamic and radiative anomalies over a sea ice sensitive region of the Arctic

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27 Οκτ 2013 (πριν από 3 χρόνια και 11 μήνες)

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Clear
-
sky thermodynamic and radiative anomalies over
a sea ice sensitive region of the Arctic

Joseph
Sedlar

and
Abhay

Devasthale

joseph.sedlar@smhi.se

S
wedish Meteorological and Hydrological Institute

Norrköping
, Sweden

1.


Introduction


Rapid
Arctic sea
ice decline
over
the past decade has drawn community
-
wide awareness to extreme warming of the Arctic region. Prior to
2012, Arctic sea
-
ice reached a record minimum in September 2007. Since then, a number of studies have emerged identifying pote
ntial
mechanisms in atmospheric circulation, ocean heating and mixing, cloud and latent heat
anomalies,
and associated feedbacks that may have
contributed to the record ice
loss of 2007.
Recently,
Graversen

et al. (2011) examined thermodynamic advection anomalies from ERA
-
Interim over the Laptev and East Siberian Seas (Fig. 1), where they find anomalously large
heat and moisture advection and
cloud
longwave

radiation
could explain the large ice loss over that region.


In this study, we quantify the monthly
-
averaged vertical contribution of atmospheric thermodynamics on
radiative

fluxes over
the box region
in Fig. 1, the same
region as
Graversen

et al. (2011), however we use clear
-
sky profile observations from the AIRS sounder onboard the Aqua
satellite.
Radiative

fluxes are estimated from AIRS thermodynamic profiles and
radiative

transfer. This
allows
a quantification of the clear
-
sky
radiative

forcing over the Arctic and also provides
an
observational complement to studies that rely on model or reanalysis data. Since cloud
fields are prognostic and not directly assimilated into
reanalyses
, modeled clear
-

and cloudy fractions are not, to a 1
st

degree
, constrained by
reality.

2
.

Data



AIRS (Atmospheric Infrared Sounder) Aqua:


Daily Level3 1
°
x1
°

Version 5 Standard Product (
ascending+descending

overpasses)


Vertically
-
resolved temperature and water vapor mixing ratio



RRTM
-
LW/SW
radiative

transfer calculations (monthly mean SZA from hourly estimates for mean box latitude)



ERA
-
Interim monthly
-
averaged forecast surface
albedo



MODIS Level3 monthly
-
averaged cloud fraction

3a
.

Thermodynamic and
radiative

anomalies


All anomalies are
calculated
relative to the monthly averages from 2003
-
2010 (which are referred to as climatology).

5.

Conclusions


Fig
.

2
.

a
)

Monthly

climatology

of

clear
-
sky

fraction,

and

time
-
pressure

anomalies

of

b)

temperature

[K]

and

c)

water
-
vapor

mixing

ratio

(g/kg)
.

Fig
.

4
.

a)

Time

series

of

monthly

clear
-
sky

greenhouse

anomalies

(W/m
2
,

contours)
;

b)

the

GH

anomalies

contribution

of

holding

T

to

climatology

and

c)

the

contribution

holding

WV

to

climatology
.




Seasonal cycle in clear
-
sky
fraction

(
Fig. 2a
)




Positive
co
-
variablity

in T
-
WV anomalies
below 600
hPa

(Fig. 2
b,c
)




LWD
flux
anom

closely follow atm.
thermodynamic
anom

(Fig. 3 black
)




0.45 W/m
2
/
yr


trend
for 3
-
mon running
mean clear
-
sky LWD
anom

(
signif
. at 99%
)




Temperature contribution to LWD
anom

>
WV contribution (Fig. 3 blue, red)

Fig
.

3
.


Monthly

clear
-
sky

LWD

anomalies

(W/m
2
)

where

the

black

line

represent

the

total

LWD

anomaly,

the

blue

line

represents

LWD

anomaly

contribution

when

holding

the

T

profile

to

monthly

climatology

while

allow

WV

to

vary

as

observed,

the

red

line

is

the

LWD

anomaly

contribution

of

holding

the

WV

profile

to

monthly

climatology

while

allow

T

to

vary

as

observed
.

The

gray

line

is

the

monthly

precipitable

water

(mm)
.

4.

Clear
-
sky thermo. and
radiative

impact on 2007 evolution


Fig
.

6
.

Monthly

2007

evolution

(solid

lines)

and

climatology

(dashed)

of

a)

cloud

fraction

(back)

and

surface

albedo

(blue)
;

b)

net

SW

(red),

LW

(blue)

and

NET

(black)

surface

radiation
;

c)

anomalous

NET

radiation
;

d)

surface

temperature
;

e)

ice

melt

(positive)

or

freeze

(negative)

thickness

anomalies

due

to

anomalous

clear
-
sky

surface

radiation
.

CLIM

refers

to

the

climatological

surface

LWU

or

surface

temperature,

respectively
.

Fig
.

7
.

Monthly

clear
-
sky

temperature

anomalies

(K,

black)

and

clear
-
sky

LWD

(W/m
2
,

blue)
.

1.

AIRS clear
-
sky profiles indicate substantial variability in thermodynamic advection over the East
Siberian and Laptev Sea regions

2.
Monthly
downwelling

LW estimates show a clear response to thermodynamic anomalies, some as
large as +/
-

12 W/m
2

3.
The change from negative to positive greenhouse anomalies agrees with the 3
-
mon running mean
linear trend of LWD anomalies


suggesting the shift in GH anomaly is at least partially manifested on
the surface
downwelling

fluxes

4.
Rather than contributing to anomalous ice melt during the summer, the increased clear
-
sky
radiative

flux anomalies are shown to be significant during the winter, spring and late autumn seasons


atmospheric preconditioning of sea ice for the following melt season highlighted!

5.
We find a clear
-
sky anomalous retardation of ice growth of 0.3 m, on top of 0.7 m climatological ice
melt


in an region where ice thickness generally ranges 0.5
-
2 m


clear
-
sky melt contribution is
significant!

6.
Fig. 8 below shows
a

non
-
linear metric for ice melt/freeze potential as a function of cloud fraction,
surface albedo and
radiative

fluxes, which can be adapted to regions with similar solar and surface
properties (see
Sedlar

and
Devasthale

2012)


References

Graversen
, R.G., T.
Mauritsen
, S.
Drijfhout
, M.
Tjernström

and S.
Mårtensson

(2011), Warm winds from the Pacific
cuased

extensive Arctic sea
-
ice melt in summer 2007,
Clim
.
Dyn
., 36, 2103
-
2112,
doi
: 10.1007/s00382
-
010
-
0809
-
z.

Kwok, R. and D.A.
Rothrock

(2009), Decline in Arctic sea ice thickness from submarine and
ICESat

records: 1958
-
2008,
Geophys
. Res.
Lett
., 36, L15501, doi:10.1029/2009GL039035.

Sedlar
, J. and A.
Devasthale

(2012), Clear
-
sky thermodynamic and
radiative

anomalies over a sea ice sensitive region of the Arctic, J.
Geophys
. Res., 117, D19111,
doi
: 10.1029/2012JD017754.


A11K
-
0193

Fig
.

1
.

2003
-
2010

September

mean

sea
-
ice

concentation

(fraction)

from

AMSR
-
E

(
Spreen

et

al
.

2008
)
.

The

Laptev

Sea

and

East

Siberian

Sea

region

where

temporal

and

spatial

averages

are

examined

is

shown

in

black

(
74
-
82
°
N,

135
°
E
-
165
°
W
)
.

September

ice

area

(
a

x

10
5

km
2
)

with

the

box

for

ice

concentrations

>

15
%

shown

in

bottom

right

of

each

panel
.

Fig
.

5
.

a)

Clear
-
sky

GH

effect

(W/m
2
)

as

a

function

of

precipitable

water

(PW,

mm)

for

all

months

(black

dots)

and

climatological

monthly

means

(red

dots)
;

b)

Clear
-
sky

GH

anomalies

as

function

of

PW

anomalies
.

Clear
-
sky greenhouse (GH)

LWU(100
hPa
)
-
LWU(1000
hPa
)




2003
-
06 GH
anom

=
-
1.20

W/m
2 ;

2007
-
10 GH
anom

=
+1.18

W/m
2

(Fig. 4)




Double
-
sided t
-
test with null
hypothesis that means are equal
disproved at 99%




Monthly PW
anom

< +/
-

1 mm,
leads to linear GH
anom

response (Fig. 5)




Monthly WV averaging reduces
shorter time scale WV advection
events


Clausius
-
Clapyeron

controlled WV increases by
already large T anomalies

3b.
Greenhouse anomalies




Increased SWN corresponds to decreased
albedo

[order 20
-
40
W/m
2
] (Fig. 6
a,b
)




LWN deficit slightly greater than climatology [order 15 W/m
2
] (Fig.
6b)


surface temperature feedback; see large surface temperature
anomalies from July through year’s end (Fig. 6d)




Result is 20
-
40 W/m
2

anomalous clear
-
sky surface radiation available
for ice melt (Fig. 6c), but essentially only during summer when clear
-
sky
fraction is small (Fig. 6a)




Surface temperature anomalies positively correlated (
r

= 0.92) with
clear
-
sky LWD anomalies (Fig. 7); for Jan
-
May 2007, anomalous clear
-
sky
LWD enough to account for > 80% of temperature anomalies (
δ
LW/
δ
T
= 4
σ
T
3
)

C
lear
-
sky
radiative

ice melt:

(1)

(2)



Total clear
-
sky ice melt for May
-
August 2007 (0.713 m) approximately identical to
climatology (0.706 m) (Table 1)




Regional mean ice thickness: 1
-
2 m during spring; 0.5
-
1 m at end of melt season
(Kwok and
Rothrock

2009)


clear sky melt is significant
!




Anomalous LWD fluxes from warm and moist air masses important for
retardation of ice growth (~0.3 m) during winter, spring and late autumn


atmospheric preconditioning!


Fig
.

8
.

Temporal

evolution

of

monthly

clear
-
sky

ice

melt

(positive

contours)

and

freeze

(negative

contours)

anomalies

for

hypothetical

cloud

fraction

(ordinate)

and

surface

albedo

(abscissa)

anomalies
.

Contour

anomalies

are

shown

for

LWD

anomaly

of

+

9

W/m
2
,

the

mean

LWD

anomaly

for

the

same

months

of

2007
.

Solid

lines

represent

cloud

fraction

and

albedo

anomaly

relationships

needed

for

zero

change

in

ice

melt

(freeze)

relative

to

climatology

for

the

respective

LWD

anomalies
.


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