Cold Firn-Meteorology (CoFi-Met)
Coldest Firn – Meteorology is a joint measurement campaign performed by the Leipzig Institute for Meteorology (LIM), University of Leipzig and the Alfred-Wegener-Institute (AWI) at Kohnen Station, Antarctica (75°00’ S, 00°04’ E, 2892 m a.s.l.) in austral summer 2013/14. Observations are an integral part of the LIM-AWI- project “Coupling of airborne and in situ ground based measurements of surface albedo, bidirectional reflectivity BRDF and snow properties in Antarctica to improve prognostic snow models” funded by German Research Foundation. Measurements are also embedded in the AWI-project “Coldest Firn”, which aims to improve knowledge on microstructure as well as physical / chemical characteristics and isotopic composition of snow and firn in key regions of the Antarctic Plateau.
Although surface albedo and BRDF strongly depend on snow grain size, snow surface roughness and atmospheric parameters, they are mostly parameterized in a simple way in regional and global climate models. Therefore, our overall goal is to improve such parameterizations by combining our measurements with prognostic snow models. Two work packages (WP), based on measurements and simulations have been designed to reach these goals:
WP1: Airborne measurements of the spatial variability of surface albedo, BRDF, surface roughness and snow grain size
WP2: Ground-based measurements and numerical simulations of the temporal variability of surface albedo and BRDF as influenced by variations in snow and atmospheric properties
The goal of CoFi-Met is to improve the current knowledge on radiative properties of snow in the Antarctic by using broadband, spectral and multi-spectral measurements of surface reflectivity. To obtain a full view of the surface properties, ground based measurements at Kohnen Station and airborne measurements over Dronning Maud Land are coupled as illustrated in Fig. 1. Identical radiometric calibration of both instrument sets and detailed airborne sampling of the ground site allow linking both data sets.
Ground based observation programme
The ground based observation programme at Kohnen within CoFi-Met consisted of the following elements:
1) The continuous operation of upward- and downward-looking pyranometers and pyrgeometers provided time series of the broadband surface albedo and of the surface radiation balance. Simultaneously, the compact radiation measurement system (CORAS) was operated to derive a time series of spectral surface albedo in the wavelength range between 350-2100 nm (Fig. 2).
2) Surface structures like small ripples and dunes have a non-negligible impact on the surface albedo. To estimate the quantitative influence of these effects, we installed two digital cameras and use photogrammetric methods to estimate the dimensions of macroscopic roughness structures.
3) To estimate snow BRDF, a downward-looking digital camera with a 180° field of view fish-eye lens was operated (Fig. 3).
4) The continuous operation of an automatic weather station provided time series of surface pressure, temperature, relative humidity, wind speed and direction.
5) Hourly, synoptic observations on cloud cover and types and significant weather conditions (e.g., precipitation, drifting or blowing snow) were performed.
6) Although the content of dust and black carbon in the snow at Kohnen is expected to be negligible low, we took snow samples from shallow snow pits (50 cm deep) five times during the 2-month measurement period to check this assumption.
7) The operation of a sun-photometer will allow retrieving aerosol optical thickness.
8) Radio soundings were performed four times a day at 00, 06, 12 and 18 UTC to derive vertical profiles of pressure, temperature, relative humidity as well as wind speed and direction. Those profiles are important input parameters for simulations of the radiative transfer in the atmosphere. Furthermore, the radio soundings will be used to validate so-called reanalysis data sets provided by weather centres like the European Center for Medium Range Weather Forecast (ECMWF). Since the number of manned and automatic weather stations is very limited in the inner part of Antarctica, meteorological forecasts and reanalyses often differ considerably from reality.
Measurements of the grain size and the density of snow as well as the vertical profile of temperature in snow and firn were performed by cooperation partners in the framework of the project “Coldest Firn”.
Airborne measurements were performed on joint flights with AWI-glaciology between 22th December, 2014 and 7th January, 2015, when Polar 6 was based at Kohnen Station (Fig. 4 and Fig. 5).
Fig. 4: Polar 6 at Kohnen (Picture by G. Birnbaum), Fig. 5: Flights performed over Dronning Maud Land (Illustration by D. Steinhage, AWI)