Despite rapid melting in the coastal regions of the Greenland Ice Sheet, a significant area (~40%) of the ice sheet rarely experiences surface melting. In these regions, the controls on annual accumulation are poorly constrained owing to surface conditions (for example, surface clouds, blowing snow, and surface inversions), which render moisture flux estimates from myriad approaches (that is, eddy covariance, remote sensing, and direct observations) highly uncertain. Accumulation is partially determined by the temperature dependence of saturation vapor pressure, which influences the maximum humidity of air parcels reaching the ice sheet interior. However, independent proxies for surface temperature and accumulation from ice cores show that the response of accumulation to temperature is variable and not generally consistent with a purely thermodynamic control. Using three years of stable water vapor isotope profiles from a high altitude site on the Greenland Ice Sheet, we show that as the boundary layer becomes increasingly stable, a decoupling between the ice sheet and atmosphere occurs. The limited interaction between the ice sheet surface and free tropospheric air reduces the capacity for surface condensation to achieve the rate set by the humidity of the air parcels reaching interior Greenland. The isolation of the surface also acts to recycle sublimated moisture by recondensing it onto fog particles, which returns the moisture back to the surface through gravitational settling. The observations highlight a unique mechanism by which ice sheet mass is conserved, which has implications for understanding both past and future changes in accumulation rate and the isotopic signal in ice cores from Greenland.

Keywords

The boundary layer dynamics over the ice sheet, which mediate the coupling between synoptic climate and surface processes, are distinct from terrestrial environments in nonpolar domains in a number of significant ways. The surface has a high albedo, low friction, an essentially infinite source of moisture, and muted diurnal cycles of radiative inputs for much of the year ( 12 ). As a consequence of these particular conditions, surface temperature inversions occur almost every day and can last without interruption for days to weeks (fig. S1) ( 13 ). Under these stable atmospheric conditions, the ability for the surface and free tropospheric air to mix is significantly reduced. Therefore, gas species produced at the surface or within the upper firn have a tendency to accumulate in the near-surface atmospheric layer ( 14 ). Similarly, water vapor sublimated from the ice can remain near the surface and influence accumulation by recondensing on the surface and through facilitating the growth of near-surface cloud and fog particles ( 15 – 17 ). The importance of these processes is poorly understood because observing latent heat fluxes in polar environments using classical methods (for example, flux-gradient and eddy covariance) is highly uncertain due to the failure of critical assumptions in similarity theory ( 18 , 19 ). Here, we use observations of the vertical profiles of water vapor isotope ratios, which do not require the same assumptions, to assess the net exchange of mass associated with boundary layer processing of water vapor. The results show that under the stable conditions that typically persist during the winter, the surface is isolated from free-tropospheric moisture sources, which impedes both accumulation associated with surface condensation and mass loss through sublimation.

Air temperature influences accumulation through its effect on the humidity of air parcels reaching the ice sheet, which determines the maximum amount of condensation/precipitation that can occur ( 4 ). Therefore, rising temperatures are anticipated to generate an increase in accumulation in regions where summer temperatures remain below the 0° isotherm ( 5 ). To understand the aggregated influence of temperature on ice accumulation, previous studies have relied on analysis of ice core records ( 6 ), which confirm the presence of a strong positive correlation between temperature (from stable isotope ratios in the ice) and accumulation (from layer thickness) ( 7 ). However, the response of accumulation to temperature is inconsistent with an exclusively thermodynamic process ( 8 ), which has been interpreted to mean that large-scale shifts in atmospheric circulation have occurred coincident with past temperature changes ( 9 – 11 ). Previous work, however, has largely neglected the role that boundary layer and surface processes play in modulating ice-atmosphere moisture exchange. In arid glaciated landscapes where sublimation and condensation are occurring persistently between the limited number of storm events, the role of these processes may influence the response of accumulation to temperature.

The rate of ice accumulation on the Greenland Ice Sheet (GrIS) emerges from a balance between mass loss from coastal calving associated with melting and sublimation and additions from precipitation and condensation. In coastal regions, the former now significantly exceeds the latter, leading to a receding ice edge and a contribution to rising sea levels ( 1 ). However, at high altitudes (~≥2500 m) where melting is rare and runoff is negligible ( 2 ), snow accumulation continues to occur at a rate between 2 and 10 cm liquid-equivalent year –1 , implying a tendency for the balance of surface fluxes (precipitation and condensation minus sublimation and evaporation) to exceed the rate of ice flow divergence ( 3 ).

RESULTS

The exchange of water between the ice sheet and the atmosphere is evaluated here using the vertical profiles of the isotope ratios of water vapor [18O/16O and 2H/1H, referred to hereafter in common “δ notation” (20)] above the ice sheet. Between July 2012 and July 2014, the isotopic composition of water vapor and the water vapor mixing ratio were measured using a laser absorption spectrometer installed at the base of a 46-m research tower at Summit Camp within the accumulation zone of GrIS (72°35′46.4″N, 38°25′19.1″W and 3210 amsl) (21). Air was continuously sampled from inlets ranging in height from 30 cm below the surface of the snow (firn air) to 40 m above the ice sheet surface, generating more than 12,000 water vapor isotopic profiles (Materials and Methods and fig. S2).

We observe significant vertical variations in humidity near the ice sheet surface, indicating that the exchange of water between the ice sheet and atmosphere has a strong influence on the atmosphere through sublimation and condensation processes. The water vapor mixing ratio gradient (the difference between the water vapor mixing ratio measured at the lowermost and uppermost inlets) is characterized by a bimodal distribution (Fig. 1A). One population emerges during unstable atmospheric conditions when mixing leads to vertical homogeneity of the water vapor concentration and its isotopic ratio from the surface to 40 m (Fig. 2 and Materials and Methods; see fig. S3 for definition of stability). Using observations from the same site, Box and Steffen (16) found an overall tendency for the surface to be a moisture sink under these unstable conditions. Their results suggest that the surface is, on average, a condensing environment, implying that the radiative balance produces a condition where free tropospheric air is supersaturated, with respect to the surface. The lack of a measurable humidity gradient is maintained in spite of surface condensation by rapid mixing between the surface and free tropospheric air masses.

Fig. 1 Humidity profiles over Summit Camp. (A) Distribution of the ratio between the water vapor mixing ratio at the surface (~0.1 m) and at ~40 m taken from hourly averaged profiles between 2012 and 2014. (B) Saturation mixing ratios as a function of temperature (47). Theoretical summer and winter mixing lines with schematic depictions of where the air inlets in Fig. 2 would be sampling.

Fig. 2 Vertical profiles of the isotopic ratio (δ18O) and humidity at Summit Camp. (Top) Isotopic ratio profile. (Bottom) Water vapor mixing ratio profile. The seasonal profiles were generated as the average of all hourly profiles from the respective time window and reported relative to the value at the top inlet. Red (green) lines are generated from profiles during stable (unstable) periods (fig. S3), and the error envelope represents the 66th and 90th percentiles. The dotted line is the average of all profiles. Blue dots below the profiles are the average isotopic ratios measured from the firn interstitial vapor. The error envelop for the firn vapor is 1 SD.

Under stable conditions, the predominant condition all winter and during summertime nights, surface temperature inversions lead to cold and saturated air at the surface. Because of the strength of the temperature inversion, the water vapor mixing ratio at the surface is often less than 20% of that at 40 m (Figs. 1B and 2, bottom row). These remarkably steep humidity gradients are conducive to phase changes when mixing brings these proximal, yet thermodynamically distinct, air masses in contact. However, the processes involved in controlling moisture exchange under stable conditions have not been studied, and thus, the influence that changes in atmospheric stability have on accumulation is unknown.

Water vapor isotopic profiles above the ice sheet The vertical profiles of the stable water isotopic ratio are used to trace the presence of condensation or sublimation at or near the surface based on the understanding that condensation depletes the heavy isotopes from the water vapor remaining in the air. The most prominent feature in the water vapor isotopic profiles is the presence of a pronounced isotopic minimum of 2 to 6‰ between 2 and 8 m above the ice surface during stable periods (Fig. 2, top row). To our knowledge, isotopic profiles of this shape have not been observed elsewhere because previous studies only made summertime measurements and/or did not have sufficient vertical resolution to capture this feature (22, 23). This isotopic pattern is distinct from the water vapor mixing ratio profiles (Fig. 2, bottom row), which increase with height. The distinction between the shape of the water vapor mixing ratio and the isotopic ratio profiles arises because the former follows temperature, a consequence of the temperature dependence of water vapor saturation, whereas the latter is sensitive to the amount of condensation relative to the amount of water vapor at that height (Fig. 1B). Condensation preferentially removes heavy water isotopes from the air, which results in the observed isotopic depletion that occurs a few meters above the surface. Observations of persistent low-level fogs from a pair of particle probes at 2 and 10 m support this assertion. To understand the physical origin of this isotopic feature, we must consider the processes driving the exchange of vapor and ice between the ice sheet and atmosphere. During spring and early summer, the firn is cooler than the air below the inversion, and if the latter is saturated (or near-saturated), it has a higher water vapor mixing ratio than the air in the pore spaces (24, 25). Under these circumstances, there is a tendency for the latent heat flux to be toward the surface. During stable periods in the winter months, the air below the inversion is cooler than both the air above and the firn air below, which retains heat from the previous summer. Mixing across the inversion, a product of inversion-wind shear (12), generates condensation of moisture originating from the free troposphere and would produce low-level clouds and fog. Because the firn pore spaces are warmer than the air below the inversion, the air in the pore spaces remains at saturation with respect to the snow temperature, resulting in a higher water vapor mixing ratio than in the air below the inversion (26). The isotopic minimum in the vapor above the surface during the winter therefore reflects a combination of two coexisting processes: one involving the upward mixing of firn air and the second involving downward shear mixing between the air parcels below and above the inversion (12). The condensation above the surface can be conceptualized as the atmospheric analog to the mechanism associated with the formation of “depth hoar,” which occurs through vapor diffusion and deposition toward the cold point in the snowpack (27). The temperature minimum above the surface of the ice sheet effectively acts as a trap of moisture by condensing vapor from the free troposphere and retaining water that has sublimated from the surface. Under these stable conditions, the mass added to the ice sheet from surface condensation and settling of fog particles should follow free atmospheric temperatures because the only net exchange of moisture would be through condensation of moisture whose humidity would be set by the temperature of the overlying air mass. However, with increasing atmospheric stability, the near-surface atmosphere becomes isolated, which limits interaction between the surface and the reservoir of moisture available from air masses reaching the ice sheet from marine origins. We hypothesize that as decoupling between the surface and free tropospheric air increases, the ice sheet approaches a condition of zero net accumulation irrespective of the temperature.