Broadband radiation data in the Antarctic Peninsula and estimation of cloud cover from two different methods

In the past years different methods have been developed to infer cloud cover from radiation data. Since surface cloud observations are scarce in Antarctica but broadband radiation instruments are more common and widespread, such methods could be very useful to reconstruct information whenever and wherever radiation measurements are available. The present work is centred on data from Marambio (64°14’50.6’’S - 56°37’39.3’’W) and Professor Julio Escudero (62°12’57’’S - 58°57’35’’W) stations, which are situated on opposite sides of the Antarctic Peninsula, respectively on Seymour and King George islands. At Marambio, from July 2019, a SPN1 (DeltaT Devices) sunshine pyranometer provides global shortwave and diffuse radiation data, and a NR01 (Hukseflux) net radiometer measures, for both shortwave and longwave radiation, the upward and downward components. At Escudero, both shortwave and longwave downward radiation observations are made, respectively, by a SMP21/22 pyranometer (from December 2016) and a SGR4 pyrgeometer (from December 2017), both by Kipp&Zonen, although with some interruptions in winter. Quality check of the data sets is made, when possible, applying the tests recommended by the Baseline Surface Radiation Network, even if the stations are not part of it. The control procedure shows a good quality of the measurements: generally, and for most variables, over 99% of the data pass the physically possible and extremely rare limits tests. At Marambio, in the worst case, 6% of the data fail the tests, whereas for Escudero only 2%. A first analysis of the radiation data does not show unusual results but it is noteworthy that, despite the lower solar elevation angles, irradiance is comparable to that of mid-latitudes, thanks to the greater transparency of the atmosphere in the Antarctic region. To infer cloud cover and estimate its effect on radiation measurements, two methods are chosen, each exploiting a particular broadband radiation component. The first is the Long et al.[1] method, which exploits measured shortwave downward and diffuse radiation components; the second is APCADA[2], based on longwave downward radiation measurements. Alongside them, meteorological parameters are also required and they are provided by the respective national meteorological services. As both sites are located in the peninsula, frequent cloudy sky conditions are expected from the results, which will depend on the performance of the methods in a peculiar environment such as Antarctica, different for solar elevation, humidity and surface cover from where the methods were developed and mainly tested.

Bibliography
[1] Long C. N.,  Ackerman T. P., Gaustad K. L., and Cole J. N. S. (2006): Estimation of fractional sky cover from broadband shortwave radiometer measurements, J. Geophys. Res., 111, doi: 10.1029/2005JD006475
[2] Dürr B. and Philipona R. (2004): Automatic cloud amount detection by surface longwave downward radiation measurements, J. Geophys. Res., 109, doi.org/10.1029/2003JD004182