Comment on amt-2021-23

This paper investigates the influence of a snow cover on near-surface air temperature measurements which are generally subject to solar radiation-induced errors (even when sensors are protected in a shield), Such errors are amplified by a contribution of surfacereflected shortwave radiation, additionally heating sensor shields and sensors. To quantify this so-called albedo effect, several pairs of identical sensor/shield combinations are deployed at two neighboring locations, experiencing the same meteorological conditions. At one site the seasonal snow cover was unchanged, at the other snow has been removed after every snowfall such that these sensors were mounted above bare ground. Prior to the field experiment, the sensor pairs have been tested and characterized in controlled laboratory environments to determine relative errors and biases. Air temperature, shortwave radiation and wind speed data have been collected and analyzed to quantify the albedo effect on air temperature measurements by comparing temperature observation at the two sites. Observed temperature differences were related to the magnitude of incident solar radiation and wind speed to assess their relative importance. Albedo-induce errors were found to be as large as 3.8°C. The study follows certain WMO guidelines and proposes recommendations for measurement protocols to be applied on reference stations, e.g., in the context of GCOS.