Geophysical Research Letters
Supporting Information for
Warm-air advection, air mass transformation and fog causes rapid ice
melt
Michael Tjernström1, Matthew D. Shupe2,3, Ian M. Brooks4, P. Ola. G. Persson2,3, John
Prytherch4, Dominic J. Salisbury4, Joseph Sedlar1, Peggy Achtert4, Barbara J. Brooks5,
Paul E. Johnston2,3, Georgia Sotiropoulou1, Dan Wolfe2,3
(1) Department of Meteorology and Bolin Centre for Climate Research, Stockholm
University, Stockholm Sweden
(2) Cooperative Institute for the Research in Environmental Sciences, University of
Colorado, Colorado, Boulder, Colorado, USA
(3) Earth System Research Laboratory, National Oceanic and Atmospheric
Administration, Boulder, Colorado, USA
(4) School of Earth and Environment, University of Leeds, Leeds, UK
(5) National Centre for Atmospheric Science, University of Leeds, Leeds, UK
Contents of this file
Additional information on the large scale setting and the vertical structure of the event
described in the main text, and high-temporal resolution representation of the surface net
radiation; Figures S1 to S4.
Introduction
In the main paper we argue that a combination of sustained advection of warm and moist
air, the transformation of this air mass as it passes in over the pack ice, formation of low
clouds and fog in the resulting surface-based inversion, and a downward turbulent
sensible heat flux, all come together to generate an enhanced heat flux to the ice. This in
Supporting Online Material for
Warm-air advection and fog causes rapid ice melt
turn triggers a rapid ice melt that rapidly reduces the ice concentration in the affected
area.
In this supplementary text we provide some additional information on the larger-scale
setting of the episode from a synoptic-scale perspective, using weather maps and images
from weather satellite. We also provide a statistical illustration of the vertical structure of
the lower troposphere, and the 10-minute resolution representation of surface net
radiation to supplement the diurnally-averaged surface-energy balance used in the main
text.
Large scale atmospheric setting
Figure S1 shows satellite images while Figure S2 shows European Centre for Medium
range Weather Forecast (ECMWF) surface analyses for representative times during the
episode discussed in the main text; note that the satellite images are not at exactly the
same times as the analyses since the satellite overpasses are not synchronized to the main
UTC synoptic times.
At first, during DoY 211-212, there is a transient warm-air advection sub-episode
associated with a low pressure, which in Figure S1a and S2a was located south of Oden
moving northwestward. During this period, the wind direction at Oden’s location (not
shown) rotates the full cycle from south to north and back to south again. After this
system passed a weak synoptic wave follows, moving eastward on DoY 213, see Figures
S1b and S2b, and after this the main episode, with persistent southerly or southeasterly
winds, is established. This is also the main fog episode and is illustrated here by Figure
S1c and S2c, from late in this period. The warm front on DoY 216 can be seen to the
northeast of Oden’s location, having already passed at the time of the satellite passage.
Finally, the episode ends with the passing of the cold front on DoY 219; in Figure S1d
and S2d, from early this day, the approaching cold front is seen as a sharp band of low
clouds well northeast of Oden. Also, here the sky is already partly clear, or clouds are
optically thin, and the ice can thus be seen in the satellite image (Figure S1d). After the
passing of this front, the event has ended as the air comes from inside the ice pack.
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Supporting Online Material for
Warm-air advection and fog causes rapid ice melt
Vertical structure
Figure S3 illustrates the air-mass transformation structure evident in the vertical profiles
of temperature, specific humidity, equivalent potential temperature and wind speed.
Included here are all radiosoundings for the core of the advection episode, DoY 213-217;
the plots show the probability in shading and the mean and median profiles as solid and
dashed lines. The vertical structure is signified by the deep inversion of > 10 ºC, almost
20 ºC at maximum, extending to ~500 m with increasing humidity through a slightly
more shallow layer, ~300 m, with more than a doubling of the moisture from the surface.
Decreasing moisture and temperatures aloft contribute to a nearly moist-neutrally
stratified layer across a relatively deep layer above the inversion. The wind speed profile
shows a clear low-level jet, with winds speeds at their highest reaching 18 m s-1 at ~200
m. This jet is likely an inertial oscillation, propagating from the ice edge, where the
momentum flux is drastically reduced as the warm air enters over the cool melting ice
and the low-level static stability increases abruptly.
High-temporal resolution surface net radiation
In the main text, the surface energy balance (SEB) is presented in diurnal averages, since
the diurnal range of the net solar radiation is so large that it becomes difficult to
graphically compare this signal to any of the other SEB terms that do not exhibit this
strong diurnal signal. Figure S3 shows the 1-minute averaged surface net radiation. The
important conclusion here is that while the diurnal cycle typically has a large maximum
when the sun is the highest above the horizon and a negative minimum at local midnight
before and after the warm advection event, the episode discussed in the main text features
minima that remain above zero all the time.
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Supporting Online Material for
Warm-air advection and fog causes rapid ice melt
Figure S1. Satellite images of the broader area of interest for a selection of days. The
images are false-colored so that land is green and ocean dark blue. Clouds appear in
white (high clouds), yellow (dense low clouds or fog) and light blue (thinner clouds or
fog). Sea ice, when visible through the clouds, is also light blue or magenta and is
distinguishable from thin low clouds by a distinct, quasi-stationary structure; e.g. 7
August, 2014. Oden’s position is marked by a red star.
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Warm-air advection and fog causes rapid ice melt
Figure S2. ECMWF analyses for times near those in the satellite images (Figure S1)
showing surface pressure isobars (solid lines), wind direction (barbs) and wind speed
(wind barb flags and shading; yellow is 3-8 ms-1 and green is 8-14 m s-1). Oden’s
approximate position is marked by a red star.
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Supporting Online Material for
Warm-air advection and fog causes rapid ice melt
Figure S3. Probability (color scale) as a function of height (note the logarithmic scale),
of (a) temperature (ºC), (b) specific humidity (g kg-1), (c) equivalent potential temperature
(ºC) and wind speed (m s-1), for the core of the warm-air advection period DoY 213
through 219. Also included are the mean (solid red) and median (dashed red) profiles.
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Supporting Online Material for
Warm-air advection and fog causes rapid ice melt
Figure S4. Time series of surface net radiation (W m-2) for (red) 70% and (blue) 50%
albedo at 1-minute time resolution.
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