Testing longwave radiation parameterizations under clear and overcast skies at Storglaciären, Sweden

J Sedlar,R Hock

doi:10.5194/tc-3-75-2009

Abstract

Abstract. Energy balance based glacier melt models require accurate estimates of incoming longwave radiation but direct measurements are often not available. Multi-year near-surface meteorological data from Storglaciären, Northern Sweden, were used to evaluate commonly used longwave radiation parameterizations in a glacier environment under clear-sky and all-sky conditions. Parameterizations depending solely on air temperature performed worse than those which include water vapor pressure. All models tended to overestimate incoming longwave radiation during periods of low longwave radiation, while incoming longwave was underestimated when radiation was high. Under all-sky conditions root mean square error (RMSE) and mean bias error (MBE) were 17 to 20 W m−2 and −5 to 1 W m−2, respectively. Two attempts were made to circumvent the need of cloud cover data. First cloud fraction was parameterized as a function of the ratio, τ, of measured incoming shortwave radiation and calculated top of atmosphere radiation. Second, τ was related directly to the cloud factor (i.e. the increase in sky emissivity due to clouds). Despite large scatter between τ and both cloud fraction and the cloud factor, resulting calculations of hourly incoming longwave radiation for both approaches were only slightly more variable with RMSE roughly 3 W m−2 larger compared to using cloud observations as input. This is promising for longwave radiation modeling in areas where shortwave radiation data are available but cloud observations are not.

Highlights

Energy balance studies on glaciers have shown that, on average, net radiation is usually the largest contributor to surface ice and snow melt
The inclusion of atmospheric moisture leads to an improved parameterization performance, suggesting that vapor pressure has a larger relevance than temperature for parameterizing clear-sky L↓
With coefficients fitted to the Storglaciaren data set, all tested longwave radiation parameterizations differ only slightly in their performance when compared to hourly observations

Summary

Introduction

Energy balance studies on glaciers have shown that, on average, net radiation is usually the largest contributor to surface ice and snow melt (see summary in Hock, 2005). Examination of the individual radiative components reveals that incoming longwave radiation ( referred to as downward or downwelling longwave radiation) is by far the largest source of energy for melt, followed by absorbed shortwave (or global) radiation (Ohmura, 2001), which accounts for roughly a quarter of the total energy source for melt. The relative importance of incoming longwave radiation as a source of surface energy input, compared to shortwave radiation or turbulent heat fluxes, may vary as climate changes (Philipona et al, 2004), but it will remain a large contributor, especially in the higher latitudes, due to the seasonal dependence of shortwave radiation. Unlike shortwave radiation, incoming longwave radiation is not readily measured at automated weather stations, often being derived through combination of global and net radiation measurements or parameterizations, the number of weather stations on glaciers equipped with longwave radiation instrumentation has increased during recent years (e.g. van den Broeke et al, 2004; Sicart et al, 2005, 2008; van de Wal et al, 2005; Hoch et al, 2007; Molg et al, 2009)

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