Radiative measurements at Thule, Greenland: factors affecting the cloud-free shortwave and longwave radiative budget in the Arctic

Abstract

The Arctic region plays a central role in the global climate system. Modifications in the Arctic radiative budget may strongly influence large scale atmospheric and oceanic circulation. The evaluation of the surface energy balance sensitivity to variations in several parameters, such as surface temperature, water vapour content, surface albedo, and atmospheric aerosols, is one of the main issues in assessing how the Arctic will respond to future climate changes.

The NDACC station at Thule Air Base (76.5°N, 68.8°W) is equipped with a variety of instruments for the measurement of the radiative fluxes at the surface, aerosol optical properties, water vapour atmospheric content, and meteorological parameters. A Yankee Environmental System Total Solar Pyranometer (YES-TSP) and an Eppley pyrgeometer (PIR) are installed at Thule for the measurement of the global shortwave and longwave downward irradiances at the surface. The TSP was installed in 2002, while the PIR in 2009. A Cimel Sunphotometer measures aerosol optical properties and water vapour columnar content; the Cimel is part of the Aerosol Robotic Network and was installed in 2007. In winter, the water vapour columnar content is also measured at Thule with a millimeter-wave spectrometer (GBMS) operating in the 230-280 GHz range. GBMS measurements have been carried out during several winters between 2002 and 2011. A meteorological station, which measures surface temperature and pressure, relative humidity, wind speed and direction is also continuously operational at Thule. Satellite observations of the surface shortwave albedo obtained from MODIS have been used together with ground-based measurements.

Four years (2007 to 2010) of surface shortwave irradiance at the surface, aerosol optical properties, and water vapour have been combined with satellite observations of the surface albedo. Radiative transfer model calculations are used to reproduce the observed shortwave fluxes and to separate the effects of the different parameters in modulating the cloud-free downward shortwave radiation at the ground. Water vapour is the main factor affecting the cloud-free shortwave irradiance at the surface. Its column value varies between 0.1 and 1.4 cm during the period spring to early autumn. Water vapour produces a reduction of the surface shortwave flux by -(212%). The surface albedo varies between 0.05 and 0.66 in the period March to September, with values larger than 0.5 in spring and smaller than 0.1 in summer. In spring the surface albedo induces an increase by +(2-4.5%) in the downward shortwave radiation. The aerosol optical depth at 500 nm is generally lower than 0.2; atmospheric aerosols produce a reduction in the shortwave radiation down to -5%. On annual base, the mean effects of water vapour and surface albedo are estimated to be –(10-11) Wm-2 and +(2-3) Wm-2, respectively.

The temperature and humidity profiles in the troposphere have the strongest influence on the cloud-free downwelling longwave irradiance. In wintertime, in absence of solar radiation, the longwave fluxes dominate the surface radiation budget. GBMS water vapour measurements from winters 2009 to 2011 have been used, together with surface humidity and temperature, to investigate the relative influence of these factors in affecting the downwelling longwave irradiance.