ESSDEarth System Science DataESSDEarth Syst. Sci. Data1866-3516Copernicus PublicationsGöttingen, Germany10.5194/essd-8-159-2016Surface radiation during the total solar eclipse over Ny-Ålesund,
Svalbard, on 20 March 2015MaturilliMarionmarion.maturilli@awi.dehttps://orcid.org/0000-0001-6818-7383RitterChristophAlfred Wegener Institute, Helmholtz Centre for Polar and Marine
Research, Potsdam, 14473, GermanyMarion Maturilli (marion.maturilli@awi.de)22April20168115916411January201625January20167April201615April2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://essd.copernicus.org/articles/8/159/2016/essd-8-159-2016.htmlThe full text article is available as a PDF file from https://essd.copernicus.org/articles/8/159/2016/essd-8-159-2016.pdf
On 20 March 2015, a total solar eclipse occurred over
Ny-Ålesund (78.9∘ N, 11.9∘ E), Svalbard, in the high
Arctic. It was the first time that the surface radiation components
during the totality of a solar eclipse were measured by a Baseline
Surface Radiation Network (BSRN) station. With the Ny-Ålesund long-term
radiation data set as background (available at
10.1594/PANGAEA.150000), we present here the peculiarities
of the radiation components and basic meteorology observed during the
eclipse event. The supplementary data set contains the basic BSRN radiation
and surface meteorological data in 1 min resolution for March 2015, and
is available at 10.1594/PANGAEA.854326. The eclipse
radiation data will be a useful auxiliary data set for further studies on
micrometeorological surface–atmosphere exchange processes in the Svalbard
environment, and may serve as a test case for radiative transfer studies.
Introduction
At the high-Arctic site Ny-Ålesund (78.9∘ N, 11.9∘ E)
on the archipelago of Svalbard, surface radiation measurements operated by
the Alfred Wegener Institute since 1992 are contributed to the Baseline
Surface Radiation Network (BSRN). The measurements include the various
parameters related to solar radiation: global and reflected radiation
(downward and upward shortwave radiation, SWdown and SWup,
respectively), as well as the direct and diffuse shortwave radiation
(SWdirect and SWdiffuse, respectively). Furthermore, the
upward and downward thermal radiation components, LWup and
LWdown, are obtained. Detailed information about the instrumental
setup and the long-term radiation observations since 1992 is given in
Maturilli et al. (2015).
On 20 March 2015, the rare astronomical event of a total solar eclipse
occurred over Ny-Ålesund with about 2 min of totality, allowing, for the
first time, the measurement of the corresponding special radiation conditions by a
BSRN station. The eclipse was no. 61 in Saros cycle 120. Solar eclipses
have inspired several earlier investigations in the field of meteorology.
Surface atmospheric pressure fluctuations have been observed in connection
with solar eclipses caused by the cooling of the atmosphere during the eclipse
shadow period (Anderson et al., 1972; Anderson and Keefer, 1975). The
pressure perturbations and induced gravity waves generated by a solar eclipse
have been studied for several eclipse events (e.g. Schödel et al., 1973;
Goodwin and Hobson, 1978; Seykora et al., 1985; Marty et al., 2013). Gravity
wave forcing by a solar eclipse has also been found related to ozone photochemistry in the middle atmosphere
(Fritts and Luo, 1993). In the planetary
boundary layer, the observed effects of a solar eclipse include the
modulation of surface fluxes and a reduction in turbulence intensities. The
influence of the eclipse shadow on the surface energy budget has been
analysed in various climate zones, but so far never in the Arctic.
Micrometeorological measurements during total solar eclipse events in the
mid-latitudes revealed a decrease in the turbulent fluxes with reduced values
of turbulent kinetic energy caused by the eclipse shadow (Foken et al., 2001;
Founda et al., 2009). An effect on the vertical temperature structure towards
boundary layer stability was found during a total solar eclipse in the
tropical convective zone (Rao et al., 2013). Likewise, a transformation in
the stability of the near-surface air during an eclipse event producing
atmospheric conditions similar to the initiation of a nocturnal inversion was
observed in a desert zone (Eaton et al., 1997). Due to the sparseness of
observational sites in high northern latitudes, no Arctic observations of
radiation and micrometeorology in eclipse conditions exist so far.
Here, we present the complete set of surface radiation components and
meteorological mast observations with 1 min time resolution during the total
solar eclipse event over Ny-Ålesund on 20 March 2015. This high-resolution eclipse data set is intended as a baseline for, for example, further
micrometeorological studies analysing the sensitivity of the surface energy
balance on abrupt changes of the surface radiation budget. Moreover, a solar
eclipse is an excellent means to test radiative transfer models (Emde and
Mayer, 2007). The Ny-Ålesund solar eclipse data set is available at the
PANGAEA data repository under 10.1594/PANGAEA.854326.
Radiative and meteorological conditions on 20 March 2015
As Ny-Ålesund is located at 78.9∘ N, the site is affected by
polar day and polar night conditions from 18 April to 24 August and from
24 October to 18 February, respectively. On 20 March, the diurnal cycle is
characterized by regular day and night-time conditions, with the sun about
12 h above the horizon (Fig. 1). Yet, as Ny-Ålesund is situated on the
coastline with mountains to the south, the complex horizontal line
(10.1594/PANGAEA.669522) prevents direct sun visibility for some parts
of the day. The solar eclipse though occurred when the sun was above the
mountains, allowing observations of the partial and total solar eclipse
phases.
Solar elevation angle (black line) for Ny-Ålesund coordinates
(78.9∘ N, 11.9∘ E) on 20 March 2015, with local horizontal
line (grey shading). The totality of the solar eclipse is indicated (cyan
line).
Meteorological surface observations in Ny-Ålesund on
20 March 2015, with (a) cloud base height, (b) station
level pressure, (c) 10 m wind speed, (d) 10 m wind
direction, and (e) surface air temperature at 2 m height. Indicated
is the duration of the solar eclipse (dashed blue lines) and
the time of total eclipse (blue line).
On 20 March 2015, solar observations in Ny-Ålesund were undisturbed in
terms of cloudiness. The cloud base height measured by remote sensing with a
Vaisala CL51 ceilometer (10.1594/PANGAEA.854330) is shown in Fig. 2a.
Clearly, the early morning cloud cover on 20 March 2015 had vanished before
daylight and the beginning of the solar eclipse, allowing perfect clear-sky
observation conditions. Only at about 23:00 UTC were low clouds again
observed above the station. In fact, the synoptic situation on 20 March 2015
was very stable under the influence of a high-pressure system centred over
the Greenland Sea. The station level pressure was changing only marginal,
with a maximum of 1024.7 hPa at about 10:40 UTC slowly decreasing by about
3 hPa within the next 12 h. Without synoptic drag on the lower atmospheric
wind field, the local conditions dominated the near-surface wind. Throughout
the clear-sky phase of the day, the wind speed was very low below about
3 m s-1 (Fig. 2c). In the Ny Ålesund surroundings, these are
favourable conditions for prevailing katabatic winds from the glaciers
(Jocher et al., 2012). In these cases, the common south-east-oriented
near-surface atmospheric flow along the fjord axis (Beine et al., 2001;
Maturilli et al., 2013) is often interrupted by south-westerly katabatic
winds from the mountains and glaciers south of Ny-Ålesund, bringing cold
air to the station. On 20 March 2015, such cold air outflow events are
reflected in the surface air temperature (Fig. 2e). Being about 2 ∘C
higher in the presence of clouds in the early morning hours, the baseline
temperature is rather stable around -18 to -17 ∘C during the
rest of the day. Yet, short peaks of lower temperatures are observed. These
are correlated with associated changes in wind direction (Fig. 2d), pointing
to cold air advection to the station when the wind arrives from south-west,
the direction of the Brøggerbreen glacier. Overall, the meteorological
conditions for the observation of the total solar eclipse on 20 March 2015 in
Ny-Ålesund were excellent. The detailed surface radiation measurements
during the eclipse are described in the next section.
Surface radiation measurements during the solar eclipse
In Ny-Ålesund, the solar eclipse on 20 March 2015 started at 09:10 UTC
and lasted until 11:11 UTC, with the total eclipse phase between 10:09:53
and 10:12:11 UTC. The event was followed by instrumentation and by eye,
and its spectacular view is shown in Fig. 3.
Solar eclipse over Ny-Ålesund, Svalbard. (Photo: Nathalie
Grenzhaeuser)
The measured diurnal cycle of global radiation for 20 March 2015 is shown in
Fig. 4. Though clouds were not present during the sunlit part of the day,
broad reduction in global radiation is observed while the sun is shaded by
the mountains, as illustrated in Fig. 1 and indicated in Fig. 4. During the
total phase of the solar eclipse, the global radiation vanishes completely,
generating a unique signal in the measured downward radiation.
Ny-Ålesund global radiation (SWdown) on 20 March 2015,
measured by a Kipp & Zonen pyranometer CMP22. The colour bar indicates the
periods when the sun elevation was above the horizontal line (yellow) or the
instruments were shaded by mountains (grey).
The downward radiation components and the upward longwave radiation during the eclipse phase are
shown in Fig. 5. One quality control factor of BSRN sites is the availability
of independent measurements of direct and diffuse radiation in addition to
the global radiation. In Ny-Ålesund, the direct radiation is measured by
a Kipp & Zonen CHP1 pyrheliometer on a Schulz & Partner solar tracker
shared with the diffuse radiation measurement by a ball-shaded Kipp &
Zonen CMP22. Global and reflected radiation is also detected by CMP22
instruments, while upward and downward longwave radiation are obtained by
Eppley PIR pyrgeometers (Maturilli et al., 2015).
The direct radiation normalized to the horizontal plane, the diffuse
radiation, and the global radiation are shown in the upper panel of Fig. 5,
complemented by the downward thermal radiation. The eclipse event emerges
symmetrically in the shortwave radiation components, following the solar disk
shading by the moon. The largest contribution to the global radiation is
given by the direct radiation component, constituting about two-thirds of the
incoming shortwave radiation. The contribution by the diffuse radiation is, on
the one hand, limited by the mountain surroundings as indicated by the
horizontal line in Fig. 1. On the other hand, the mountains are snow-covered
and thus contribute to the incoming diffuse radiation by reflection.
The downward radiation
components' shortwave direct, diffuse, and global radiation as well as
downward thermal radiation (upper panel), and the upward thermal radiation
LWup and 2 m temperature (lower panel) between 09:10 and
11:10 UTC on 20 March 2015 in Ny-Ålesund.
On 20 March 2015, the ground underneath the radiation instrumentation setup
was also snow-covered, with a snow layer of about 30 cm height as measured
by a Jenoptik snow depth sensor SHM30. As low solar elevation angles limit
the reliability of the albedo retrieval, the albedo value
SWup/SWdown= 0.7 found for the highest solar
elevation on 20 March 2015 remains subject to high uncertainty. The resulting
emitted upward longwave radiation is shown in the lower panel of Fig. 5. As
the emission of thermal radiation is temperature-dependent, the course of the
upward longwave radiation during the solar eclipse is not symmetrical. With
the progression of the solar eclipse and the consequent reduction in downward
shortwave radiation, the thermal cooling of the snow surface becomes predominant. As a
consequence, a decrease in upward longwave radiation related to surface
and/or near-surface cooling was observed. The longwave radiation measurements
are temperature-corrected with three internal temperature sensors that
confirm the radiative cooling and indicate that the decrease in upward
longwave radiation is not caused by changes in air temperature due to
advection. Instead, a decrease in the 2 m temperature is observed with about
10 min delay to the decreasing LWup. This cooling, however, is
potentially related to the advection of colder air from the glacier since it
occurs simultaneous to a change in wind direction (see Fig. 2d). As periods
with prevailing katabatic winds from the glaciers were observed throughout
the day, the changing wind and air temperature conditions cannot
unambiguously be attributed to the solar eclipse. With the returning sunlight
after the total eclipse phase, the upward longwave radiation also gently
inclines to reach its original level.
The downward shortwave radiation components' direct, diffuse, and
global radiation during the total eclipse phase.
Vertical temperature profiles measured by RS92 radiosondes launched
from Ny-Ålesund on 20 March 2015. The radiosonde launched at 11:14 UTC has
been processed as a GRUAN reference sonde, and the corresponding temperature
profile (purple line) is provided with measurement uncertainty in each
altitude level (grey lines).
Close to the total eclipse phase, more measurement details of the shortwave
radiation components become apparent (Fig. 6). It turns out that the
independent global radiation measurement falls to 0 W m-2 in minutes
10:09 to 10:12 UTC, i.e., shortly before and shortly after the actual
totality when considering minute mean values. In fact, for the small
irradiance values during the eclipse, the accuracy of the instruments needs
to be taken into account. For the CMP22 used for the global radiation
measurement, the zero-offset type A due to the inner dome having a different
temperature from the cold junctions of the sensor is given
at < 3 W m-2 by the manufacturer, as well as a directional response
< 5 W m-2. These effects may also affect the measurement of diffuse
radiation with a similar CMP22. The values for the diffuse radiation drop to
0 W m-2 even earlier and then rise later, which is probably also related to the
fact that the pyranometer for the detection of diffuse radiation is shaded by
a ball slightly larger than the sun disk, thus covering a small fraction of
the sky that is still visible for the global radiation measurement. The
signal of direct shortwave radiation detected by pyrheliometer vanishes for
the shortest period.
Atmospheric profiles
Three Vaisala RS92 radiosondes were launched from Ny-Ålesund during the
eclipse period on 20 March 2015, at 08:39, 10:10, and 11:14 UTC. The
Ny-Ålesund radiosonde programme is certified by the Global Climate
Observing Systems (GCOS) Reference Upper-Air Network (GRUAN), providing
radiosonde data in reference quality. The radiosonde launched at 11:14 UTC
has therefore been processed by GRUAN, and the data are available
with quantitative uncertainty values for every measurement point
(10.5676/GRUAN/RS92-GDP.2).
The temperature profiles in the lowermost 300 m of the atmosphere are shown
in Fig. 7. As the balloons have an ascent rate of 5 m s-1, the vertical
profiles should be considered snapshots of the atmospheric state. In all
profiles, a surface-based inversion below 100 m altitude indicates stable
conditions in the planetary boundary layer. Above the inversion, the
temperature decreases more or less following the adiabatic temperature
gradient, slightly differing from one profile to the next. Considering the
measurement accuracy and the temperature uncertainty shown for the GRUAN
processed profile (Fig. 7), potential changes in the altitude of the surface-based inversion top fall within the detection limit of the instrumentation.
Overall, no temperature changes in the vertical column can be attributed to
the solar eclipse. Before, during, and after the eclipse, the boundary layer
was characterized by very stable conditions.
Summary
Adding to the long-term surface radiation observations from Ny-Ålesund
(Maturilli et al., 2015), the rare event of a total solar eclipse was
measured by BSRN instrumentation on 20 March 2015. Under favourable
meteorological conditions, the eclipse left its imprint on the surface
radiation diurnal cycle. The associated data set
(10.1594/PANGAEA.854326) provides highest quality measurement data of
direct, diffuse, global, and shortwave reflected radiation; upward and
downward longwave radiation; and 2 m temperature, relative humidity, and
station-level pressure in 1 min resolution. Embedded in the larger frame of
long-term surface radiation monitoring data, the solar eclipse data presented
here provide the unique occasion to study the atmospheric response to rapid
cut-off and reappearance
of shortwave radiation under low solar elevation angles in an Arctic
environment. In a larger frame, the data may provide potential benefit to
radiative transfer model evaluation or satellite data validation. On a more
local scale, the eclipse radiation data complement in situ and remote sensing
experiments operated in Ny-Ålesund. As one of the main concerns of
current Arctic research is the energy budget at the Earth's surface, the
presented surface radiation measurements contribute as a baseline to the
various Ny-Ålesund boundary layer studies, including turbulent flux
measurements of sensible and latent energy. The surface radiation data may
also support the discussion of, for example, a photochemical response in
measured atmospheric trace gases, or radiation- and temperature-related
changes in the fjord's sea surface layer. Overall, the diversity of research
at the Arctic research centre Ny-Ålesund offers various possibilities to
apply the solar eclipse radiation data for process studies under
free-atmospheric laboratory conditions switched to abrupt night-time
conditions.
Acknowledgements
The authors thank the station personnel of the AWIPEV research base in
Ny-Ålesund for maintaining the instrumentation and operating the
measurements. Particular thanks to S. Debatin for data validation. We thank
Nathalie Grenzhaeuser for providing the photo (Fig. 3, copyright
Nathalie Grenzhaeuser). Edited by:
A. Kokhanovsky
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