Trend analysis of surface cloud-free downwelling long-wave radiation from four Swiss sites

We present a trend analysis of surface cloud‐free downwelling long‐wave radiation provided by pyrgeometer measurements of four stations of the Alpine surface radiation budget network in Switzerland. The stations cover an altitude range between 370 and 3580 meters above sea level. Cloud‐free downwelling long‐wave radiation, screen‐level temperature, and relative humidity were selected from 10 min measurements, and monthly means were calculated. We performed two distinct trend analyses: the annual overall trend was determined applying least squares fitting, whereas nonparametric statistical methods were used to calculate the monthly trends. The cloud‐free downwelling long‐wave radiation time series shows a consistent and significant increase of 3.5 W m−2 per decade in the last 12 years at all four stations. The monthly trend analysis of the downwelling long‐wave radiation revealed trend estimates exceeding the overall trend by a factor of 4 and partly with opposite signs. The monthly trends of the downwelling long‐wave radiation are in agreement with the trends observed in screen‐level temperature and specific humidity which have been determined using the same statistical methods. By applying a parameterization of cloud‐free downwelling long‐wave radiation, we quantitatively inferred the causes for the observed cloud‐free trends. More than 50% of the downwelling long‐wave radiation trends can be explained with the observed variations of temperature and humidity. There is some indication that the radiative effect of high‐level clouds has changed and considerably contributed to the downwelling long‐wave radiation trends that are not induced by screen‐level temperature and humidity. Key Points Significant increase of cloud‐free downwelling long‐wave radiation Consistency of corresponding temperature and humidity trends Indication for a long‐term change in high‐level clouds