Summertime cloud phase strongly influences surface melting on the Larsen C ice shelf, Antarctica

E Gilbert,A Orr,I A Renfrew,T Lachlan‐Cope,I A Boutle,P F Field,J C King

doi:10.1002/qj.3753

Abstract

AbstractSurface melting on Antarctic Peninsula ice shelves can influence ice shelf mass balance, and consequently sea level rise. We show that summertime cloud phase on the Larsen C ice shelf on the Antarctic Peninsula strongly influences the amount of radiation received at the surface and can determine whether or not melting occurs. While previous work has separately evaluated cloud phase and the surface energy balance (SEB) during summertime over Larsen C, no previous studies have examined this relationship quantitatively. Furthermore, regional climate models frequently produce surface radiation biases related to cloud ice and liquid water content. This study uses a high‐resolution regional configuration of the UK Met Office Unified Model (MetUM) to assess the influence of cloud ice and liquid properties on the SEB, and consequently melting, over the Larsen C ice shelf. Results from a case‐study show that simulations producing a vertical cloud phase structure more comparable to aircraft observations exhibit smaller surface radiative biases. A configuration of the MetUM adapted to improve the simulation of cloud phase reproduces the observed surface melt most closely. During a five‐week simulation of summertime conditions, model melt biases are reduced to <2 W·m−2: a four‐fold improvement on a previous study that used default MetUM settings. This demonstrates the importance of cloud phase in determining summertime melt rates on Larsen C.

Highlights

Despite their importance in the polar climate system, Antarctic clouds are among the most undersampled in the world because of the difficulties of in situ data collection in this harsh, remote environment (Lachlan-Cope, 2010; Bromwich et al, 2012)
Cloud impacts on surface energy balance (SEB) are most important in warmer regions like the Antarctic Peninsula, where surface temperatures can rise above freezing in summer and cause melting, and where fractional cloud cover is typically
Observed ice and liquid mass mixing ratios during case f152 (January 18, 2011) over AWS14 indicate that many thin cloud layers are present, with clouds below 2 km exhibiting the ‘water-overice’ structure typical of low-level polar mixed-phase clouds (Figure 2)

Summary

Introduction

Despite their importance in the polar climate system, Antarctic clouds are among the most undersampled in the world because of the difficulties of in situ data collection in this harsh, remote environment (Lachlan-Cope, 2010; Bromwich et al, 2012). The effect of Antarctic clouds on the amount of energy at the surface (the surface energy balance, SEB) can determine whether the ice surface remains frozen or melts (Kalesse et al, 2016; Nicholas et al, 2017), with consequent implications for ice sheet mass balance and, potentially, for global sea level rise. Cloud impacts on SEB are most important in warmer regions like the Antarctic Peninsula, where surface temperatures can rise above freezing in summer and cause melting, and where fractional cloud cover is typically. The recent surface temperature rise and the loss of ice mass on more than half of the Peninsula’s ice shelves further motivates investigation of cloud in this region (Turner et al, 2005; 2016; Cook & Vaughan, 2010). Neighbouring ice shelves, Larsen A and B, collapsed in 1995 and

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Conclusion