Abstract
Abstract. Snow plays an important role in the Arctic climate system, modulating heat transfer in terrestrial and marine environments and controlling feedbacks. Changes in snow depth over Arctic sea ice, particularly in spring, have a strong impact on the surface energy budget, influencing ocean heat loss, ice growth and surface ponding. Snow conditions are sensitive to the phase (solid or liquid) of deposited precipitation. However, variability and potential trends of rain-on-snow events over Arctic sea ice and their role in sea-ice losses are poorly understood. Time series of surface observations at Utqiaġvik, Alaska, reveal rapid reduction in snow depth linked to late-spring rain-on-snow events. Liquid precipitation is key in preconditioning and triggering snow ablation through reduction in surface albedo as well as latent heat release determined by rainfall amount, supported by field observations beginning in 2000 and model results. Rainfall was found to accelerate warming and ripening of the snowpack, with even small amounts (such as 0.3 mm recorded on 24 May 2017) triggering the transition from the warming phase into the ripening phase. Subsequently, direct heat input drives snowmelt, with water content of the snowpack increasing until meltwater output occurs, with an associated rapid decrease in snow depth. Rainfall during the ripening phase can further raise water content in the snow layer, prompting onset of the meltwater output phase in the snowpack. First spring rainfall in Utqiaġvik has been observed to shift to earlier dates since the 1970s, in particular after the mid-1990s. Early melt season rainfall and its fraction of total annual precipitation also exhibit an increasing trend. These changes of precipitation over sea ice may have profound impacts on ice melt through feedbacks involving earlier onset of surface melt.
Highlights
Arctic sea ice has been experiencing rapid decline in both extent and thickness in recent decades (Stroeve et al, 2007, 2012; Comiso and Nishio, 2008)
In order to determine how liquid precipitation affects the surface ablation of sea ice and to assess its quantitative contribution to the reduction in snow depth over sea ice, here, we investigate the role of liquid precipitation in initiating snowmelt and the sea ice ablation season based on field measurements in the coastal Chukchi Sea
While the role of snow depth and snowfall variations is well understood, this study demonstrated that rain-on-snow events are a critical factor in initiating the onset of surface melt over Arctic sea ice, primarily through reduction in surface albedo as well as latent heat release
Summary
Arctic sea ice has been experiencing rapid decline in both extent and thickness in recent decades (Stroeve et al, 2007, 2012; Comiso and Nishio, 2008). Sea-ice thinning trends (Kwok et al, 2009; Kwok and Untersteiner, 2011) have been associated with first-year sea ice replacing thicker multi-year sea ice (Maslanik et al, 2007, 2011; Giles et al, 2008) These changes make Arctic sea ice more susceptible to variations in thermodynamic forcings, increasing interannual variability Screen and Simmonds (2012) showed that the fraction of Arctic summer precipitation occurring as snow has declined in recent decades due to lower-atmosphere warming, and this change of precipitation has likely contributed to the decrease in sea ice extent by reducing the area of snow-covered ice and the resulting surface albedo during summer. The variability of rain-on-snow events over sea ice and the timing of first spring rain are analysed using long-term meteorological records available at Utqiagvik, Alaska
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