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83
Katabatic winds and piteraq storms: observations from
the Greenland ice sheet
Dirk van As, Robert S. Fausto, Konrad Steffen and the PROMICE project team*
* Andreas P. Ahlstrøm, Signe B. Andersen, Morten L . Andersen, Jason E . Box, Charalampos Charalampidis, Michele Citterio, William T. Colgan, Karen
Edelvang , Signe H. Larsen, Søren Nielsen, Ma rtin Veicherts and Anker Weidick
In 2007 the Programme for Monitoring the Greenland Ice
Sheet (PROMICE) was initiated to observe and gain insight
into the mass budget of Greenland ice masses. By means of
in situ observations and remote sensing, PROMICE assesses
how much mass is gained as snow accumulation on the sur-
face versus how much is lost by iceberg calving and surface
ablation (Ahlstrøm et al. 2008). A key element of PROM-
ICE is a network of automatic weather stations (AWSs) de-
signed to quantify components of the surface mass balance,
including the energy exchanges contributing to surface abla-
tion (Van As et al. 2013).
e use of these AWS observations is not limited to studies
of ice-sheet mass balance. PROMICE contributes to Cryo-
Net (www.globalcr yospherewatch.org/cryonet), the core net-
work of surface measurement sites of the World Meteoro-
logical Organization (WMO) Global Cryosphere Watch.
By real-time delivery through WMO, PROMICE observa-
tions contribute to improve both operational forecasting
and climate analysis in the data-sparse Arctic. e Green-
landic population, highly dependent on accurate forecasting
of weather conditions, benets directly from these real-time
observations. For instance, extreme surface wind speeds are
a high-risk element in Greenland. e third-highest wind
speed observed at the surface of the Earth (93 m/s or 333
km/h), was recorded in a 8–9 March 1972 storm at ule in
North-West Greenland (Stanseld 1972).
In this paper, we discuss the extent to which the Green-
land ice sheet generates its own near-surface wind eld. We
use PROMICE data to gain insight into the interaction be-
tween air temperature, radiation and gravity-driven katabat-
ic winds. We focus on a particularly powerful spring storm in
2013 that contributed to a fatality on an ice-sheet ski traverse
attempt (Linden 2013).
Weather stations on the Greenland ice
sheet
e origina l PROMICE network consisted of fourteen AWSs
in seven ablation regions of the Greenland ice sheet, w ith each
region monitored by a lower (L) and an upper (U) elevation
station (Fig. 1; Ahlstrøm et al. 2008). PROMICE has collab-
orated logistically and nancially with other projects in the
regions of the TAS, QAS, NUK and KAN stations, leading
to the installation of eight additional AWSs. e PROMICE
© 2014 GEUS. Ge ological S urvey of Den mark and Gree nland Bull etin 31, 83–86 . Open access: w ww.geus.d k/publicati ons/bull
Fig 1. Map of Greenland showing the locations of automatic weather sta-
tions on the ice sheet a nd on local ice caps.
250 km
ZA
K
QAS
NUK
KAN
MAL
THU
UPE
KPC
SCO
MIT
TAS
Fig. 5
2250
Greenland Climate network
PROMICE network
Other GEUS stations
(co)funded by other projects
2250
3000
2750
2500
1000
1500
1750
2000
2500
2250
1750
1500
1250
2000
8484
AWS sites have been selected to complement the Greenland
Climate Network (GC-Net), which chiey monitors the ice-
sheet accumulation area (Steen et al. 1996).
Continuous PROMICE AWS observations include: air
temperature (c. 2.7 m above surface), barometric pressure, air
humidity, wind speed and direction (c. 3.1 m above surface)
as well as down- and upward solar (shortwave) and terrestrial
(longwave) radiation. e AWSs also record temperature
proles in the upper 10 m of the ice, GPS-derived location
and diagnostic parameters such as station tilt angles. A pres-
sure transducer and two sonic rangers measure snow and ice-
surface height change associated with ablation and accumu-
lation (Fausto et al. 2012). All data and metadata including
sensor specications are available at www.promice.org.
Here, we use averaged values of air temperature, wind
speed and direction, and radiation components. Single wind
measurements have an uncertainty of 0.3 m/s and 3° (Van
As 2011) and are not adjusted for shis in tilt, rotation or
measurement height as this does not impact the outcome
of this study. We also combine GC-Net and PROMICE
temperature data to give the most complete observed me-
teorological depiction of the Greenland ice sheet currently
possible. We calculated the daily average near-surface air
temperature across the ice sheet between 2008 and 2013 by
means of inverse-distance interpolation between as many as
32 AWSs that operated on a given day. We also determined
the daily average vertical near-surface air-temperature lapse
rate by means of a linear least-squares t to all available data.
Atmospheric temperature and stability
e average near-surface air temperature over the Greenland
ice sheet has a distinct annual cycle with minimum (winter)
values between –20°C and –40°C (Fig. 2A). During the rela-
tively short summer, temperatures are oen around –5°C and
are less variable due to (1) reduced cyclonic activity and (2)
surface melting over large parts of the ice sheet. e latter is a
moderating factor because near-surface temperatures are lim-
ited to near freezing. Since 2008, ice-sheet average air temper-
atures above 0°C have only been recorded on ve days (11–13
and 28–29 July 2012) during which surface melting occurred
over nearly the entire ice sheet (e.g. Nghiem et al. 2012).
e average near-surface air-temperature lapse rate over the
ice sheet exhibits a reversed cyclicity as compared to air tem-
perature with winter values oen exceeding a 10°C decrease
per vertical kilometre (Fig. 2B). Assuming a textbook value of
a 6.5°C/km free-atmospheric lapse rate to be representative
Year
20092008 2010 2011 2012
20092008 2010 2011 2012
A
B
Air temperature (°C)
0
–20
–10
–30
–40
Temperature lapse rate (°C/km)
15
10
5
Net surface radiation (W/m
2
)
0
–5
050100 150 200
1
Month of year
23
49
10 11
12
5678
A
B
Wind speed (m/s)Wind speed (m/s)
8
9
6
7
4
5
3
2
10
6
2
8
4
SCOKPC TAS QAS NUK KAN UPE THU
Fig. 2. A: Daily average (black) and 31-day average (red) air temperature
over the Greenla nd ice sheet as determined f rom interpolated weather sta-
tion observations from the GC-Net and PROMICE network. B: Same,
but vertical near-surface temperature lapse rates. The dashed line shows
a lapse rate of 6.5°C/km above which air masses are increasingly unstable .
Fig. 3. A: The average annual cycle in wind speed at the PROMICE sites.
Lines are drawn for each weather station, but only if three years of good
data are available. B: The monthly average wind speed versus the net
(shortwave + longwave) radiation budget. For locations of the stations see
Fig. 1.
85
of the threshold between stable and unstable conditions over
Greenland, this suggests that the near-surface atmosphere is
commonly less buoyant (denser) at higher elevations than air
at lower elevations. In a free atmosphere such a density dier-
ence over a few vertical kilometres would trigger an immedi-
ate adjustment through convection. Over the large horizontal
scale of the Greenland ice sheet, the actual density gradients
are roughly two orders of magnitude smaller, which adds in-
signicantly to the force balance. Figure 2B illustrates that
during winter, the high elevation interior of the ice sheet cools
more than lower elevation regions near the margin. As a re-
sult, the shallow (c. 10 0 m thick) stable atmospheric boundar y
layer that blankets the ice sheet atta ins an even larger tempera-
ture decit compared to the free atmosphere at high elevation
in winter. e larger this temperature decit relative to the
free atmosphere, the larger the density dierence relative to
the free atmosphere, and thus the larger the gravitational ac-
celeration of the shallow boundary layer. is katabatic force
increases linearly with increasing surface slope.
Katabatic winds
Winds over the Greenland ice sheet are strongest in winter
(e.g. Steen & Box 2001), as observed at every PROMICE
AWS (Fig. 3A). While part of this increase is due to lower
wintertime pressure and more frequent passage of cyclonic
systems, the primary cause of stronger winter winds is sur-
face radiative cooling. is well-known forcing mechanism
of katabatic wind is apparent from stronger winds at more
negative surface net radiation (Fig. 3B) and the strong corre-
lation between the directions in slope and wind (see below).
A negative radiation budget is common during winter due to
little or no solar radiation at high latitudes when the upward
emission of long-wave terrestrial radiation exceeds down-
ward atmospheric radiation at the surface.
e wind regimes over the ice sheet do dier between re-
gions. Winds are stronger at the higher-elevation AWSs due
to the larger radiative cooling of the surface (provided a sur-
face slope is present). e highest monthly-mean wind speed
values in Fig. 3B were recorded at KAN_M and KAN_U
(1270 and 1840 m a.s.l., red), and TAS_U and TAS_A (570
and 900 m a.s.l., blue).
Piteraq storms
e wind regimes at K AN and TAS are shown in a case study
of the 2012/2013 winter (Fig. 4). Figure 4A illustrates that
low-wind winter conditions are rare at KAN_U, PROM-
ICE station at highest elevation. Figure 4B shows the domi-
nant katabatic nature of winter winds. Nearly all measure-
ments from KAN_U show the wind to blow from upslope
direction (c. 90°, east), albeit deected to the right (c. 135°,
south-east) by the Coriolis eect due to the Earth’s rotation.
Typically, wind speeds at TAS_U are lower (but still non-
zero) due to the weaker radiative cooling at lower elevation.
Katabatic forcing also dominates here, given the persistent
non-zero winds originating from the upslope direction of c.
0° (north) and more westerly directions due to Coriolis forc-
ing. e major dierence between the two data series in Fig.
4 is the frequency of strong wind events exceeding c. 20 m/s,
which are more common in the TAS region. In the strongest
storms, the wind direction pivots towards the regional free-
atmospheric ow (Fig. 4B).
ese storms are known in Greenland as piteraqs, and
build up momentum due to the alignment of katabatic and
large-scale (geostrophic) forcing (Oltmanns et al. 2014).
ese notorious storms have repeatedly caused severe dam-
age to the towns such as Tasiilaq. e piteraq on 27 April
2013 (Fig. 4A), which jeopardised a sport expedition on the
ice sheet (Linden 2013), was exceptionally strong at TAS_U
in the context of the 2008 to 2013 PROMICE observa-
tional period, with 10-minute average wind speeds exceed-
ing 42 m/s (150 km/h). During this event, four persons (C.
Charalampidis, W.T. Colgan, H. Machguth and D. van As)
Wind direction (°)
90
0 180 270 36
0
Jan
Month of 2013
Feb Mar MayApr
A
B
Wind speed (m/s)
Wind speed (m/s)
40
30
10
20
0
40
20
0
30
10
EAST SOUTH WEST
TAS_U KAN_U 27 April 2013
Fig. 4. A: Hourly average wind speed at TAS_U and KAN_U weather
stations. The piteraq on 27 April 2013 is clearly visible in the TAS _U ob-
servations. B: Same, but wind speed plotted versus wind direction for the
period from October 2012 to May 2013.
8686
from the Geological Survey of Denmark and Greenland
were in the eld at KAN_U, and although they experienced
wind speeds approximately one third of those at TAS_U
(c. 300 km to the east) the white-out and heavy snowdri
yielded conditions too dangerous for them to leave shelter.
Satellite images from the 2013 piteraq event show that a
large region was a ected (Fig. 5). e striping on the ice sheet
in the top le corner of the lower image shows the wind di-
rection with snow transported toward and past the ice sheet
margin. Large areas of sea and ord ice disintegrated, a nd the
5–13 km wide Sermilik ord, into which Helheimgletscher
calves, was cleared of ice.
Clearly, katabatic winds and especially the piteraqs, have
a large impact on the ice sheet and its immediate surround-
ings. Given increasing commercial activity around the pe-
riphery of the Greenland ice sheet, there is a growing impetus
for understanding these winds and their response to climate
change. Regional atmospheric model projections until the
year 2100 suggests that while climate change will likely re-
sult in weaker winds in Greenland’s at interior, stronger
winds may occur in steeper regions around the ice sheet pe-
riphery (Gorter et al. 2013).
Acknowledgements
PROMICE is fund ed by the Danish M inistr y of Climate, E nergy and Bu ild-
ing and is operated by the Geological Survey of Denmark and Greenland.
Several we ather stations are (co)funded by the Green land Analo gue Project,
the RE FREEZE project and the Green land Climate Research Centre.
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24 April 2013
TAS_U MIT
Tasiilaq
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Sermilik
Helheimgletscher
Authors’ addresses
D.v.A. and others, except K. S., Geologica l Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mai l: dva@geus.dk
K.S ., Swiss Federal Institute for Forest, S now and Landscape Research (WSL), Zürcherstrasse 111, CH-8903 Birmensdorf, Switzerland.
Fig. 5. MODIS sate llite images of t he Tas iilaq region of South-E ast Green-
land on 24 and 27 April 2013, before and during a strong piteraq event.
For location, see Fig . 1.