Observations of clouds over the Antarctic Peninsula during summer 2010 and 2011 are presented here. The peninsula is up to 2500 m high and acts as a barrier to weather systems approaching from the Pacific sector of the Southern Ocean. Observations of the number of ice and liquid particles as well as the ice water content and liquid water content in the clouds from both sides of the peninsula and from both years were compared. In 2011 there were significantly more water drops and ice crystals, particularly in the east, where there were approximately twice the number of drops and ice crystals in 2011.
Ice crystals observations as compared to ice nuclei parameterizations
suggest that secondary ice multiplication at temperatures around
There have been very few in situ measurements of cloud microphysical properties over the Antarctic continent (Bromwich et al., 2012; Lachlan-Cope, 2010). However, there is evidence, from surface radiation measurements, that clouds are poorly represented within numerical models over Antarctica (King et al., 2015; Bromwich et al., 2013) and over the surrounding oceans (Flato et al., 2013; Bodas-Salcedo et al., 2014). To correct these errors in climate (and forecast) models, a better understanding of the microphysical processes controlling these clouds is needed. In situ observations of cloud and aerosol properties over the Antarctic continent are required to develop and validate model parameterizations of these clouds. This paper presents observations that start to address this issue.
(Left panel) The Antarctic Peninsula and flight area in context, with the BAS Rothera station indicated (red cross). On the peninsula's eastern side is the ice-covered Weddell Sea. Solid lines indicate topographical features as well as ice shelves' boundaries. (Right panel) Close-up on the flight area showing the flight tracks of both campaigns of interest with black triangles (February 2010) and red circles (January 2011). Topography derived by Fretwell et al. (2013).
The main part of the Antarctic continent is an ice sheet that rises to over
4000 m above sea level (m a.s.l.). Coming off this continental mass and
heading north towards South America is the Antarctic Peninsula (see Fig. 1).
The Antarctic Peninsula is less than 100 km wide for the most part and rises
to over 3000 m in places. Although isolated measurements have been made over
the main continent (Belosi et al., 2014), more measurements have been made
over the peninsula. Measurements of ice-nucleating particles (INPs) have been
made at Palmer Station (64
As observations were made on both sides of the Antarctic Peninsula, there is an opportunity to see if the cloud microphysical properties vary from one side to the other. The western side of the peninsula is exposed to the Southern Ocean, which, in the summer at least, is relatively ice-free. However, on its eastern side, the peninsula is bordered by the western part of the Weddell Sea, which remains largely ice-covered for most of the year. If the main source of cloud condensation nuclei (CCN) were from the ocean surface, it would be expected that the total number of liquid droplets would be different from one side to the other. This hypothesis is supported by results from the Goddard Chemistry Aerosol Radiation and Transport model (GOCART) simulations (see Fig. 1 of Thompson and Eidhammer (2014)), which show sharp discontinuity in sea salt aerosols across the Antarctic Peninsula in February, with concentrations on the western side at least twice as large as on the eastern side. However the GOCART simulations do not include sources of aerosol within the sea ice pack that have been suggested by some authors (Yang et al., 2008). This paper is organized as follows: in Sect. 2 the observations obtained from the two aircraft campaigns are presented. The results from both years and both sides of the Antarctic Peninsula for liquid droplets, ice crystals, and aerosols are analysed in Sect. 3. In Sect. 4 the results are discussed and suggestions are made for the most plausible explanations of the temporal and regional differences observed in clouds and aerosols across the peninsula. Section 5 summarizes the findings and concludes on the possible implications of the results. Part 2 of this paper will look at the application of these observations to numerical modelling.
Two airborne field campaigns were performed during February and March 2010,
and January and February 2011, based at Rothera Research Station
(67
The observations were made with the British Antarctic Survey's instrumented Twin Otter aircraft (see King et al., 2008). This aircraft is fitted with a variety of instruments to measure temperature, humidity, radiation, turbulence, and surface temperature. The aircraft was also fitted with a Droplet Measurement Technology Cloud, Aerosol, and Precipitation Spectrometer (CAPS) (Baumgardner et al., 2001) carried on a wing-mounted pylon.
The CAPS instrument contains three discrete instruments: the Cloud and Aerosol Spectrometer (CAS), the Cloud Imaging Probe (CIP), and the Hotwire LWC Sensor. Data from the LWC sensor were only used in this study to help validate the CAS data. The CAS and CIP are described in turn below.
Latitudinal cross-section of flight tracks showing altitudes probed
on both sides of the peninsula. February 2010 (black triangles) and January
2011 (red circles). The grey shaded area delimits the maximum and minimum
topography height (at 5 km resolution) between 68 and 67
The CAS measures the diameter of particles between 0.5 and 50
The CIP images particles between a diameter of 25
The CIP instrument produces shadow images of the larger cloud particles and
small precipitation size particles onto a charge-coupled device (CCD) array. Data processing is
performed on these images to derive size-segregated ice crystal and large
liquid drop number concentrations. Particles that are imaged by the extreme
ends of the CCD array are rejected, and this means that the effective
collection volume, used to calculate the concentration, gets smaller as the
particles get larger. Further details concerning the data processing and
quality control of the CIP images can be found in Crosier et al. (2011).
Particles are separated into ice and liquid categories based on their
circularity,
Mean sea level pressure from the ERA-Interim reanalysis for the periods of the aircraft campaign in 2010 (left) and 2011 (right).
For this study it has been assumed that the clouds are mostly mixed-phase
clouds and that the particles observed by the CAS (less than 50
In this study the average cloud properties over all the flights shown in Fig. 1 are considered. Each flight did not go to every longitude bin, and even when a flight visited a longitude bin it did not necessarily enter a cloud. For example for the bin nearest Rothera the temperature and humidity show a large number of points as most flights will have data in this bin. For the cloud parameter graphs there are fewer points as it was normal to avoid clouds during take-off and landing and flying close to the mountains.
The CAS instrument has also been used to examine the aerosol concentrations
outside clouds. To improve statistics, all flights made in the study area
during 2010 and 2011 when the CAPS probe was operational have been used.
This includes an extra 31 flights that were conducted primarily to investigate the
boundary layer or the large-scale flow but still had clouds present in the
sky. To ensure that these measurements only include cloud-free conditions,
the data were filtered by removing periods when there were particles larger
than 1
Atmospheric temperature as measured by the aircraft (
Figure 3 shows the mean sea level pressure from the ERA-Interim reanalysis for the periods of the two campaigns. The flow during 2010 is generally slack, while in 2011 the Amundsen Sea low to the west of the Antarctic Peninsula has intensified and moved east. This brought a more northerly flow across western side of the peninsula, and this could be expected to bring warmer air. However, looking at the temperatures as a function of longitude from the aircraft flights in Fig. 4, we see that 2011 is actually colder in the west, and this is also seen in the temperature fields from the ERA-Interim reanalysis as well as in the radiosonde ascents performed daily at Rothera Research Station (not shown). The cold in the west is a result of air being pulled around the tip of the peninsula from the Weddell Sea, and this is confirmed by the back-trajectory analysis reported later in this paper. The relative humidity (RH) plotted as a function of longitude (Fig. 5) shows more variability than the temperature record, and there is no clear difference between the years except an increase of RH in the west in 2010 in spite of the larger temperatures (Fig. 4), indicating an increased amount of water vapour at that time.
Same as Fig. 4 but for the relative humidity.
Same as Fig. 4 but for the liquid droplet number concentration (cm
Same as Fig. 4 but for the liquid water content (LWC, g kg
Same as Fig. 4 but for the number concentration of ice crystals (L
The number concentration of liquid drops (cm
Average values for cloud measurements and out-of-cloud aerosols for both
years and both sides of the peninsula.
Statistical significance of the differences between either year on either
side of the peninsula as obtained from the
The LWC in clouds averaged by longitude for the 2
years is shown in Fig. 7. Again to get better statistics, the values of LWC
have been averaged on both sides of the peninsula using the same size
longitude bins that were used for liquid drop numbers. A significant
difference between the 2 years is found at the 90 % level in the east
and 95 % level in the west (see Table 2), although the east–west
difference in both years is not significant. The single point of high LWC in
2011 at 73
Same as Fig. 4, for the ice water content (IWC, g kg
Distribution of ice crystals (L
The ice particle numbers and ice water content, averaged in the same way as the drops in Fig. 6, are shown in Figs. 8 and 9 for areas in the cloud that were at least partially glaciated – that is, non-circular particles were observed in the CIP. The number of ice particles observed is roughly 5 to 6 orders of magnitude lower than the number of cloud droplets, and the relative amplitude of the variability higher (the standard deviation of liquid droplet number concentration is about 50 % of the average values, while it is as high as or higher than averages for ice crystal number concentrations). Also, not all the clouds investigated were glaciated to any extent, and so there are slightly fewer measurements (Table 1) for ice crystals than for the drops – except for the east of the peninsula in 2010 when there was an observation of a completely glaciated cloud. First looking at the number concentration of crystals, Fig. 8 and Table 1 show that in 2011 there were more crystals on both sides of the peninsula than in 2010. However, Table 1 shows the standard deviation of the crystal numbers is large. Table 2 shows that differences are not significant between the 2 years (< 80 %) in the west, but significant in the east (90 %). Differences are also significant between either side of the peninsula in 2011 (at the 95 % level), however not in 2010 (77 %).
The ice water content (Fig. 9) shows a similar trend to crystal numbers in Fig. 8. However, in this case using the averaged values on each side of the peninsula (Table 1), the difference between both sides in 2010 is significant at the 90 %level.
The distribution of crystals with atmospheric temperature for the 2 years
is shown in Fig. 10. Median ice crystal number concentrations have been
derived over 0.5
In the east there is no clear peak at the high temperatures (>
Same as Fig. 4 but for the number concentration of aerosols larger
than 0.5
It is to be expected that the number of drops and ice crystals will be
controlled by the number of aerosols acting as CCN and INPs, excluding for the
moment the role of secondary ice production. The instruments that were
fitted to the Twin Otter in 2010 and 2011 did not allow the full range of
aerosols to be measured. However the CAS,
which is part of the CAPS probe, measures aerosol size distribution down to
0.5
The variability of cloud droplets and ice concentration observed in this study is quite large, and the number of flights is small, compared to the number of measurements that have been made at mid-latitudes. This makes identifying statistically significant changes between geographical areas or between years difficult. This problem has been dealt with by first averaging the data into longitudinal bands (Figs. 6–9 and 11). Although this allows some of the differences of interest to be observed, there are still too few points to determine if these differences are significant. To help with this, averages were taken of all the data on each side of the peninsula (Table 1), and in this case clear statistical differences can be seen (Table 2).
Location and altitudes (colour-coded, in m) of low-altitude
(
The average number of cloud droplets in the clouds (see Fig. 6) varies over
a large range from just a few to almost 300 cm
Relative proportion of low-altitude (< 300 m) air masses passing over sea-ice-covered regions with respect to the total number of low-altitude air masses along all back trajectories derived from the HYSPLIT model for 55 flight tracks from both campaigns (see text for details). In parentheses, the same percentages but for the 24 flights only. Relative proportions are indicated for both years and both sides of the peninsula, with the side referring to the (start) ending point of the air mass (back) trajectory. Percentages are computed over different time ranges, prior to reaching a given point of a flight track on either side. A region is considered as covered by sea ice as long as the sea ice concentration from the NIMBUS-7 SMMR is larger than 1 % (see Sect. 4.1 for reference).
Averaged vertical profiles of number concentration of aerosols
(cm
To test this hypothesis, back-trajectory analysis using the Hybrid
Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model has been
performed (Stein et al., 2015). Seven-day back
trajectories were calculated using the National Centers for Environmental
Prediction (NCEP) reanalysis meteorological field with starting points
located at 60 s intervals along the track of each flight. Figure 12 shows
the position of the low-altitude (
Distribution of all measurements of ice crystals (g
Schematic summarizing main observations from aircraft measurements from both periods of interests (February 2010 and January 2011) and on both sides of the Antarctic Peninsula (blue rectangles), as well as hypotheses (grey-framed rectangles) proposed to explain observations, as presented in the Discussion (Sect. 4). PIP refers to primary ice production, while SIP refers to secondary ice production, as presented in Sect. 3.2.
Interestingly, the ice crystal number concentration (Fig. 8) does not
follow the same pattern as the liquid droplets. A significant difference (at
the 95 % level) is seen between the 2 years in the west, and a
similar significant difference is also seen between both sides of the peninsula
in 2011. However, this is not unexpected as, while aerosols are the source
of CCN and INPs so that a correlation between the numbers of liquid drops and
ice crystals might be expected, there is another process going on to create
ice crystals. This is the secondary ice production due to the Hallett–Mossop
process for temperatures warmer than
Figure 15 summarizes the results reported in this paper. Our observations show the significant differences in cloud properties between the two measurement periods, February 2010 and January 2011, and between measurements made to the east and to the west of the Antarctic Peninsula, and they present some possible explanation for those.
January 2011 showed almost twice as many aerosols on the eastern side of the
peninsula as on the western side, and more than in the previous year. The
larger number of droplets in 2011 can be explained by an increase of CCN
that can be inferred from the observed increase in large aerosols
(> 0.5
The Hallett–Mossop secondary ice multiplication (at temperatures warmer than
The number of liquid drops and primary ice crystals is correlated with the
sources of the air masses. These results indicate that the sea-ice-covered
Weddell Sea could be a more important source of CCN and INPs than the open
ocean. This may have more general implications for the microphysics of
clouds that cover the Southern Ocean. The Southern Ocean is an area in which
large errors have been identified in the simulated cloud cover, leading to
large radiation biases in global climate models (Flato et al., 2013;
Bodas-Salcedo et al., 2014). Given that, in winter, sea ice can extend up to
60
The present study is the very first of its kind attempting to depict cloud microphysics and aerosols across the Antarctic Peninsula from a small amount of flights; the scenario we suggest hopefully will stimulate other studies and measurements to better assess the plausibility of our interpretations.
The data are being formatted for inclusion in the Polar Data Centre and will be available soon.
This work would not have been possible without the help of the scientists and support staff who helped in the Antarctic. We thank the editor and two anonymous reviewers for their comments, which helped improve the manuscript. The work was funded by UK Natural Environment Research Council with core funding and under grant NE/K01305X/1 and under NERC grant NEB1134. Edited by: M. Krämer Reviewed by: two anonymous referees