Greenland Summit Experiment
is not an official web page of the project, but a personal overview
over some of the work that has been done in the project.
The Greenland Summit Environmental Observatory (GEO-Summit,
Summit Camp) lies in the
center of the Dry Snow Zone of the Greenland Ice Sheet at
72°35' N, 34°30' W at an elevation of 3230 m a.s.l.
Due to the importance
of the Dry Snow Zone for the mass balance of the entire
Greenland Ice Sheet, detailed information on the energy
balance is needed to understand the role of Greenland in a
changing global climate.
Besides the energy
balance study at this representative site, fundamental research
was conducted on longwave radiation exchange within the boundary
layer and on the spectral reflectivity of snow.
All components of the
surface energy balance were investigated during an intensive
project season which lasted from May 2001 to July 2002.
Radiation is determined from the individual
measurements of the components of the radiation balance
measured on the radiation monitoring tower (Figure 1).
1: Radiation monitoring at Summit,
The annual cycle of the radiation balance at Summit,
Greenland, for 2001 is shown in Figure 2.
2: Annual cycle of radiative quantities
observed at Summit, Greenland.
measured on a
installed at a
are shown in
3: Tautochrones observed at Summit,
system at 4
measured at 8
levels of the
Heat Flux is directly measured or
determined from measurements at different
levels. With the use of snow lysimeters
sublimation and resublimation is
monitored regularly. A comparison of the
latent heat flux determined from the
lysimeter observation with calculations
using the profile method is shown in
As an example, the monthly mean
diurnal cycles of the different
components of the energy balance are
shown in Figure 6.
6: Diurnal cycles of the components of the energy
balance at Summit, Greenland, for February 2002 (left)
and June 2002 (right).
From the observations during the ETH
Summit Project we can conclude:
- During the winter months, the surface is cooled due to a
negative longwave radiation balance. This cooling effect is mainly
balanced by a positive sensible heat flux, and to approximately
1/3 by the subsurface heat flux. Latent heat flux is slightly
positive. As the sun is below the horizon between mid November and
late January, no significant diurnal variation of energy fluxes
- In summer, monthly mean net radiation is positive, and net
radiation is positive throughout the larger part of the day.
Monthly mean sensible heat flux is close to zero, but a transport
of sensible heat away from the surface can be seen around solar
noon as early as in April. Negative sensible flux dominates for 10
hours in June, indicating unstable conditions close to the
- Positive net radiation is balanced in equal parts by the cooling
due to sublimation (loss of latent heat) and by a negative
subsurface heat flux. Warming of the surface by a positive latent
heat flux (resublimation) can be observed during clear-sky summer
nights, but in the monthly mean diurnal cycle, latent heat flux
remains directed away from the surface.
- The residuals of the seasonal energy balance are relatively
large, ranging from -6 Wm-2 in fall to 4 Wm-2
occurrence of strong temperature inversions makes Summit
a field laboratory for studying the stable boundary
layer (SBL). Although recognized as an important
component of the thermodynamics of the boundary layer
(Robinson, 1950; Garratt and Brost, 1981), the
divergence of the longwave radiative flux has never been
investigated experimentally across several layers within
the lower boundary layer. The few available observations
are either limited to a single layer near the ground, or
cover only a limited period of time. Especially the
shape of the radiative heating/cooling rate profile has
been under debate (Stull, 1988). It has so far only been
determined from radiative transfer calculations. The
results of such calculations, however, have been
contradictory for the near surface layers, as they are
highly sensitive to vertical resolution (Räisänen,
ETH Greenland Summit Experiment (June 2001
to July 2002) the profile of longwave
fluxes was measured on up to 6 levels of the
50 m meteorological tower (Figure 4). A
relative calibration of the pyrgeometers
(Eppley Precision Infrared Radiometer)
reduced the uncertainty of net flux
difference measurements to ± 0.75 Wm-2
and a correction for tower‘s influence on
the longwave flux measurements was applied
(Hoch et al., 2007). As a first example, the
longwave fluxes measured at 2 m and 50 m are
shown for 21 January 2002 (Figure 7, left
panel). The divergence of the net longwave
flux of (0.35 Wm-3) corresponds
to a longwave radiative cooling of about -30
K per day (K d-1) . The
divergence of the longwave radiative fluxes
is shown in the right panel of Figure 7.
7: Longwave radiative fluxes measured on 21
January , 2002, at Summit, Greenland, at 2 m and 50 m
above the surface (left panel), and the corresponding
radiative flux divergence (right panel).
In the following, the observations made
during the summer months during near clear
sky conditions are presented. These
conditions include situations with a thin
and high cirrus cloud cover.
8: Diurnal cycle of longwave radiative heating rate and
the observed temperature tendency or "full" heating rate during
summer (near) clear sky days at Summit, Greenland.
radiative flux divergence leads to radiative
heating/cooling of the same order of
magnitude as the observed temperature
tendency (Figure. 8). The diurnal variation
of longwave radiative flux divergence is
shown for 4 layers and the incoming,
outgoing, and net flux components in Figures
9 a-d. Close to the surface
(0.5 -2 m), the individual
components of the longwave radiative heating
rates are very large (~ 200 Kd-1).
Similar magnitudes have previously been
observed by Eliseev et al. (2002). A
decrease in the absolute magnitude of
longwave radiative heating and cooling is
seen with height.
7: Diurnal cycles of longwave radiative flux divergence
(incoming, outgoing and net component) for 4 layers during
summer near clear sky conditions at, at Summit, Greenland.
The divergence of the outgoing flux is
usually stronger than the divergence of the
incoming flux. The divergence of the
incoming flux usually opposes the divergence
of the outgoing flux. The importance of the
divergence of the incoming flux component
increases towards the surface.
The characteristic heating rate profile
Figure 8 shows the daytime and nighttime
profiles of longwave radiative flux
divergence for near clear sky conditions.
Close to the surface, daytime cooling and
nighttime heating is observed. During the
day, a dominating incoming flux
divergence, during the night a dominating
convergence of the outgoing flux leads to
this result. Above 2 m, the sign
changes, a nighttime cooling and a daytime
heating results - controlled by the
divergence of the outgoing flux. Above
10 m, radiative cooling is observed
throughout the entire day.
8: Profile of the longwave radiative heating rate for
near near sky conditions during summer at Summit, Greenland.
correlation is seen between the temperature
gradient within a layer and the net longwave
radiative heating rate. In the near surface
layer (0.5-2 m) an increased radiative
heating is seen with stronger gradients
(Figure 9). Above 2 m, the opposite is
seen, stronger cooling within increasingly
stable layers (Figure 10).
heating and the fine structure of the
During near clear sky daytime conditions
during summer, an elevated surface inversion
is seen in the temperature profile (Figure
11). A similar feature has previously been
observed in Antarctica (Sodemann and Foken,
2004). This inversion lies at the height
were a change is observed from strong
longwave radiative cooling below to strong
radiative heating above (Figure 11).
Radiative flux divergence is thus suggested
to play an important role in the formation
and maintenance of this feature. At
nighttime, a characteristic feature is
observed in the near surface air layers as
well. A layer with a reduced stability
between 0.3 m and 5 m is found. It
is less stably stratified than the air
layers above and below (Figure 12). Again,
this pattern is suggested to reflect the
variation of longwave radiative flux
divergence with height. Near surface
radiative heating reduces the stable
Longwave radiative flux divergence shows a
sign change in the first few meters above
the surface. Under stable nighttime
(unstable daytime) conditions, a shallow
layer of radiative heating (cooling) is
observed at the surface. Above about 2 m,
radiative cooling (heating) is found. The
vertical variation of radiative flux
divergence is reflected in the fine
structure of the temperature profile near
the surface. Nevertheless, the strong
evening cooling can not be attributed to
radiative flux divergence alone (Fig.
8) - contrasting the findings of Ha
and Mahrt (2003).
A. A. et. al.,2002: Izvestiya,
Atmospheric and Oceanic Physics, 38 (5),
Garratt J. R. and R. A. Brost,
1981: J. Atmos. Sci. 38 (12), 1307-1316
Ha, K. J. and L. Mahrt, 2003: Tellus 55A
Hoch et al. 2007: J. Appl. Meteorol.,
Räisänen P.: 1996: Tellus 48A (3),
Robinson, G.D.,1950: Q. J. Roy. Meteor.
Soc. 76, 37-51
Sodemann H. and T. Foken, 2004: Theor.
Appl. Climatol. 80 (2-4), 81-89
Stull, R. B., 1988. Kluver Academic
State of the art instrumentation was developed by C.
Saskia Bourgeois to measure spectral and directional
reflected solar radiation. The main purpose is the examination
of the Hemispherical Directional Reflectance Factor, HDRF, for
various snow surfaces and solar zenith angles. Unfortunately
Saskia's web pages have been removed. >>