PNNL

Abstract

The surface downwelling longwave radiation component (LW?) is crucial for the determination of the surface energy budget and has significant implications for the resilience of ice surfaces in the polar regions. Accurate model evaluation of this radiation component requires knowledge about the phase, vertical distribution, and associated temperature of water in the atmosphere, all of which control the LW? signal measured at the surface. In this study, we examine the LW? model errors found in the Antarctic Mesoscale Prediction System (AMPS) operational forecast model and the ERA5 reanalysis model relative to observations from the AWARE campaign at McMurdo Station and the West Antarctic Ice Sheet (WAIS) Divide. The errors are calculated separately for observed clear sky conditions, ice cloud occurrences, and liquid-bearing cloud layer (LBCL) occurrences. The analysis results show a tendency (in both models at each site) to underestimate the LW? during clear sky conditions, high error variability (standard deviations > 20 W/m2) during any type of cloud occurrence, and negative LW? biases when LBCLs are observed (bias magnitudes > 15 W/m2 in tenuous LBCL cases; > 43 W/m2 in optically thick/opaque LBCLs instances). We suggest that a generally dry and liquid water-deficient atmosphere responsible for the identified LW? biases in both models is the result of excessive ice formation and growth, which could stem from model initial and lateral boundary conditions, microphysics scheme, aerosol representation, and/or limited vertical resolution.