Correction of broadband snow albedo measurements affected by unknown slope and sensor tilts

Abstract

Abstract. Geometric effects induced by the underlying terrain slope or by tilt errors of the radiation sensors lead to an erroneous measurement of snow or ice albedo. Consequently, artificial diurnal albedo variations in the order of 1-20 % are observed. The present paper proposes a general method to correct tilt errors of albedo measurements in cases where tilts of both the sensors and the slopes are not accurately measured or known. We demonstrate that atmospheric parameters for this correction model can either be taken from a nearby well-maintained and horizontally levelled measurement of global radiation or alternatively from a solar radiation model. In a next step the model is fitted to the measured data to determine tilts and directions of sensors and the underlying terrain slope. This then allows us to correct the measured albedo, the radiative balance and the energy balance. Depending on the direction of the slope and the sensors a comparison between measured and corrected albedo values reveals obvious over-or underestimations of albedo. It is also demonstrated that differences between measured and corrected albedo are generally highest for large solar zenith angles.

Characterization of Remote Sensing Albedo Over Sloped Surfaces Based on DART Simulations and In Situ Observations

“…An additional hour ahead of the local solar noon is observed for steeper slopes with the same aspect (Figure b). It is interesting to find that is not exactly equal to , as reported in a previous study (Weiser et al, ), especially at a large slope or in the early morning and late afternoon. These results can be explained by the different radiometer exposures and the low cosine response of the radiometer at large SZAs (Dirmhirn & Eaton, ).…”

“…Considering the distinct difference between α HHS and α IIS , a transformation relationship between these two albedos is important for determining the surface energy budget with the intrinsic surface albedo (Jonsell et al, ; Sicart et al, ; Weiser et al, ). For a clear‐sky day, a cosine equation ( ) is frequently used to correct α HHS to α IIS .…”

“…For example, the slope‐parallel measured outgoing irradiance peak at CS‐NF27 lags behind that measured horizontally, and a maximum absolute bias of 38 W/m 2 occurs at DYK. It should be emphasized that IISA is suggested to be adopted in the surface energy budget, as it is physically consistent with the surface intrinsic reflectivity (Matzinger et al, ; Weiser et al, ). It is also recommended as the standardized albedo in the remote sensing retrievals and in situ measurement.…”

“…For example, the geometry of a radiometer over glaciers or seasonal snow fields may change due to the slope movements (e.g., landslide, snow melting, refreezing, and riming). The general method proposed in this paper to correct broadband albedo affected by unknown slope and sensor tilts is promising (Weiser et al, ).…”

A daily, 1 km resolution data set of downscaled Greenland ice sheet surface mass balance (1958–2015)

“…To correct for this, downscaled melt and runoff are adjusted by estimating the missing amount of ice melt (ME add ) resulting from underestimated absorption of downward shortwave radiation (SW d ). In addition, as RACMO2.3 calculates radiative fluxes on a horizontal plane, the direct fraction of SW d is corrected for the slope and orientation of each 1 km glaciated grid cell, as described in Weiser et al (2016). For simplicity, we assume SW d to be equally partitioned between diffuse and direct radiation, and that the sun is exactly in the south at noon.…”