Posted on 15/12/2016 by Honora
Antarctica, DOMEX and diamond dust
This year’s DOMEX campaign to support ESA’s SMOS satellite mission has started at Dome-C in Antarctica. It’s a multiyear campaign that we have been carrying out at the Italian–French Concordia station since 2013.
The aim is to build up a collection of a well-calibrated, multiyear, time series of L-band brightness temperature and infrared brightness temperature measurements over several years. These data are compared with those of SMOS to verify that the satellite instrument is calibrated.
The experiment, led by the Institute of Applied Physics ‘Nello Carrara’ is supported by ESA and by the Italian Programme of National Research in Antarctica (PNRA).
This field report is from Francesco Montomoli (IFAC-CNR) and Vito Stanzione (CNR-ISAFOM):
On Thursday 17 November after a long and exhausting trip I landed in Dome-C for the first time. It’s on the Antarctic Plateau, which is 75.1°S, 123.3°E, and 3233 m above sea level. It’s better known as White Mars for good reason. To me it seemed the most hostile place on the planet, where, besides the low temperatures (i.e. lower than –40°C you have to cope with extremely low pressure – almost half that at sea level.
Wind is the public enemy when it blows higher than 5 m/s, changing your temperature perception dramatically. In these challenging and unique conditions, scientist set up many experiments to understand the past and present conditions of our planet, covering different branches of glaciology, physics of the atmosphere and astrophysics.
Here, I am responsible of DOMEX-3 project together with Vito Stanzione who is in charge of the project and also the station leader for the winter period.
At the core of the project is an L-band (1.4 GHz) microwave radiometer that measures the brightness temperature of deep snow. The instrument is placed 20 m high on the American Tower and observes over a clean area, where facilities and people are strictly forbidden.
In past DOMEX campaigns, continuous observations revealed, for the first time, different properties in electromagnetic emission from the Antarctic snow. As a primary feature, long-term DOMEX experiments demonstrated that the brightness temperature of the Antarctic Plateau has been very stable over the years and the site therefore selected as a possible target for L-band spaceborne sensor calibration like SMOS, SMAP and Aquarius. Although the snow is stable, the instrument also revealed that L-band can be used to detect changes in snow density, in particular, resulting from meteorological conditions. For instance, a big change of brightness temperature was observed in March 2015 as a consequence of strong wind around Dome-C. The anomaly was also observed by other sensors and opens new discussions to understand the L-band sensitivities.
As part of the same project, a GPS receiver mounted on the top of the tower to study the properties of deep snow from the reflected signal originating from GPS constellations. The experiment is the ESA–GRAIS (Long Term GNSS Reflectometry over Antarctica Ice Sheet) led by the Institut d’Estudis Espacials de Catalunya in Spain. The instrument has monitored continuously over the year and is now at the end of operations.
Vito and I are in charge of replacing the DOMEX-3 receiver, which was damaged because of an electric blackout in August, and routine maintenance of snow temperature probes. Also, we are removing the GRAIS instrument from the top of American Tower and taking some some GPR measurements to infer information about the snow stratification in the area observed by GRAIS. Everything has to be done outside in the most hostile of weather conditions.
Working outside in these extreme conditions is not easy. The day must be planned at least one or two days in advance, and planning must take temperature and wind chill into account. Everyday life can be tough with wind chill of –50°C even if you are well wearing Antarctic gear.
After a few days of hard work, the DOMEX-3 receiver was replaced. A picture was taken at the end.
The new acquisitions started at the end of November looking at the snow 42° from nadir. The temperature brightness signal is now nominal, maintaining very stable on V-polarisation (small fluctuations are because of thermal variations of the cables) while H-polarisation is more sensitive to snow density of the first layers. Peaks of H-polarisation on the right side of figure are the reflected radiation of the Sun by the snow layers.
As well as the set-up of the new receiver, some special experiments are in progress to improve the absolute calibration of the receiver and the antenna’s deconvolution. The picture below illustrate the different targets observed by DOMEX, namely clear sky pointing at 160° on the left and the snow, at 42° in the centre and at 20° to the right.
Snow accumulation can be very high when there is an obstacle in the direction of the wind. Southeast of the American Tower temperature probes are positioned from 5 cm to 10 m below the surface; here, in a few years the has wind buried all the cables and the supports.
Lifting the cables up and replacing snow at was necessary and we hope such accumulation will decrease in the next few years.
Sometimes it happens that nature gives you a gift with something unique that never happens in other places. The moment was captured in the picture clearly showing a halo around DOMEX in diamond dust conditions – beautiful!
Post from: Francesco Montomoli (IFAC-CNR) and Vito Stanzione (CNR-ISAFOM)