Surface radiation and climate at high latitudes. Homogenization and analyses of data from Arctic and Antarctic BSRN sites

Warming of the lower troposphere associated with increasing greenhouse gases is expected to be more pronounced at high latitudes. Surface-air temperature measurements of the last 30 years exhibit increased warming rates for the Arctic and reveal that the high latitudes are more sensitive to climate change. The Baseline Surface Radiation Network (BSRN) was initiated to measure shortwave and longwave surface radiation fluxes and also standard meteorological variables with a high sampling rate and the best possible quality at sites in different climatic zones. Five sites of the BSRN are located in high latitudes, two in the Arctic and three in the Antarctic. Four of these sites, which are Barrow (Alaska), Ny Ålesund (Svalbard), Neumayer (antarctic coast) and South Pole, provide currently about 10 years of data. Data from these high latitude BSRN sites is qualified for analysis of radiation and radiation related climatological parameters and, in the context of global warming and an enhanced greenhouse effect, it can be searched for possible signals in relation with climate change. Radiation measurements from the four BSRN sites in high latitudes were first processed with different methods for quality control and data homogenization. An applicable approach to accomplish the stipulated control strategy of the BSRN with the longwave data was developed. It revealed significant inhomogeneities caused by inadequate calibration methods, unfavorable measurement strategies and technical problems. Also measurements of shortwave radiation and other climatological variables were inspected and these examinations revealed the necessity of monitoring and partly improving the quality of the data. Time series of the radiation fluxes and the main climatological variables over a period of about 10 years were analyzed on the one hand in regard to presenting an overview of the radiation budget and climatological conditions at these high latitude sites. On the other hand, observable trends were searched in the measurements to provide insights into possible relations to the known enhanced warming rates during the cold season, which are also observable at the arctic sites and at Neumayer. Further results from these analyses are: variations in the humidity measurements occur parallel to the variations in surface-air temperature; direct shortwave radiation reveals increasing trends at all four sites; furthermore, albedo is clearly decreasing in autumn at Barrow, indicating that the surface gets snow-covered later in the year. This is not observable at the other sites, where albedo is rather increasing. Other measured radiation time series reveal no comparable trends over the observed period. An automatic cloud detection algorithm called APCADA, based on processing longwave downward radiation, temperature and humidity measurements, was applied to the data. It allowed continuous determination of cloud cover and cloud related variables as well as day and night detection of cloud free skies. Time series of APCADA computed cloud amount were determined and analyzed. They show a decreasing trend at all sites in spring, and at Ny Ålesund and Neumayer during all seasons, which result in a decrease of the annual mean computed cloud cover of about -1 octa over the observed period at these two sites. The cloud information from APCADA was compared with weather service cloud amount observations and data from the International Satellite Cloud Climatology Project (ISCCP). Compared to APCADA and observations, the ISCCP provides significantly higher cloud amounts during winter and spring, whereas agreement is much better during summer and autumn. Furthermore, detection of cloud free skies with APCADA was compared with another automatic method for cloud free sky detection, based on evaluation of shortwave radiation measurements. The performance of the two methods is comparable with the exception of APCADA misjudging the cloud cover during very strong surface-based temperature inversions and not detecting thin high clouds occasionally. Time series of monthly and annual mean cloud free radiation were derived and analyzed. According to the calculations, the cloud free global shortwave downward radiation (i.e. the transmittance of the cloud free atmosphere) is increasing towards the end of the 1990s and then leveling off or even starting to decrease in the beginning of the 2000s at the high latitude sites. Cloud free longwave downward radiation (i.e. the apparent emittance of the cloud free sky) is rather decreasing at the arctic sites and increasing at the antarctic sites. Methods for portioning changes in the cloud free longwave radiation to temperature and water vapor forcing were applied. The results reveal that the assumptions, that variations in surface-air temperature and surface humidity measurements are representative for variations of temperature and humidity in the lower atmosphere, are possibly not appropriate for the high latitude regions, because of the often very stable stratification of the lower atmosphere with very strong surface-based temperature inversions.