Abstract. As surface melt affects larger areas of the Greenland Ice Sheet, quantifying the energetic processes governing the near-surface firn becomes increasingly important. This work characterizes subsurface temperature and its spatiotemporal variability in the upper meter of firn in the percolation zone in southwest Greenland from two months of observations in summer 2024. We provide novel methods for identifying the snow surface height from high resolution (2 cm and 15 minute) temperature string measurements and further correct the observations for apparent biases from solar heating. Using these observations, we identify several thermodynamic layers relative to the surface. The rapid-response layer is the upper few centimeters of firn or snow where subsurface temperature is highly correlated (>0.9) with skin temperature due to coupling with the atmosphere and absorption of incoming solar radiation. In the diurnally-responsive layer, temperature still responds to atmospheric variability with large positive and negative vertical and temporal temperature gradients, down to approximately 35 cm below the surface. Below the diurnally-responsive layer, the firn response to seasonal warming becomes decoupled from diurnal- and synoptic-scale atmospheric variability with depth; beneath 65 cm below the surface, correlations are less than 0.1 between subsurface temperature and skin temperature. While conduction slowly transports energy below the diurnally-responsive layer, surface melt and the advection of meltwater or latent heat can move relatively large amounts of energy that cause complex temperature gradients. Our results highlight both the value of high-resolution observations for understanding energy transfer in the near-surface firn and the need for additional observations.

Summertime surface melt over sea ice was observed in situ by the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) on May 25, 2020. The melt was initiated by longwave radiative forcing from a warm, cloudy air mass advected from lower latitudes, similar to that observed in 1998 during the Surface Heat Budget of the Arctic Ocean in the Beaufort Sea. In addition to triggering melt onset, research suggests that springtime atmospheric advection may also precondition Arctic sea ice for melt onset by rapidly increasing its internal energy. In keeping with this, we find that an advection event in April 2020 delivered 42% of the springtime increase in internal energy the MOSAiC ice floes underwent prior to melt onset. However, we argue that the potential for this preconditioning to affect the melt onset date is limited. We test whether the energy retained within the snow/ice column from the event was significant relative to the hypothetical scenario that the event did not reach MOSAiC. We apply both observed surface temperatures and counterfactual (simulated) ones characterized by the event’s absence to a diffusion model representing the snow and ice. We find that the energy storage anomaly in the ice from the event decays rapidly relative to the scenario that omits advection (e-folding time 6.9 days). This diminishing impact is because the initial warming reduces the temperature gradient in the ice column, creating a negative feedback that suppresses heating thereafter. The influence of the April advection dissipated in the ice several weeks prior to melt onset. A more important precursor to melt onset may be the date of persistently positive atmospheric forcing, which causes convergence of flux within the ice. These results imply that negative feedbacks associated with heat conduction are a moderating factor to the beginning of the melt season.

We document the isotopic evolution of near-surface snow at the East Greenland Ice Core Project (EastGRIP) ice core site in northeast Greenland using a time-resolved array of 1 m deep isotope (δ18O, δD) profiles. The snow profiles were taken from May–August during the 2017–2019 summer seasons. An age–depth model was developed and applied to each profile, mitigating the impacts of stratigraphic noise on isotope signals. Significant changes in deuterium excess (d) are observed in surface snow and near-surface snow as the snow ages. Decreases in d of up to 5 ‰ occur during summer seasons after deposition during two of the three summer seasons observed. The d always experiences a 3 ‰–5 ‰ increase after aging 1 year in the snow due to a broadening of the autumn d maximum. Models of idealized scenarios coupled with prior work indicate that the summertime post-depositional changes in d (Δd) can be explained by a combination of surface sublimation, forced ventilation of the near-surface snow down to 20–30 cm, and isotope-gradient-driven diffusion throughout the column. The interannual Δd is also partly explained with isotope-gradient-driven diffusion, but other mechanisms are at work that leave a bias in the d record. Thus, d does not just carry information about source-region conditions and transport history as is commonly assumed, but also integrates local conditions into summer snow layers as the snow ages through metamorphic processes. Finally, we observe a dramatic increase in the seasonal isotope-to-temperature sensitivity, which can be explained solely by isotope-gradient-driven diffusion. Our results are dependent on the site characteristics (e.g., wind, temperature, accumulation rate, snow properties) but indicate that more process-based research is necessary to understand water isotopes as climate proxies. Recommendations for monitoring and physical modeling are given, with special attention to the d parameter.

Abstract Surface processes alter the water stable isotope signal of the surface snow after deposition. However, it remains an open question to which extent surface post‐depositional processes should be considered when inferring past climate information from ice core records. Here, we present simulations for the Greenland Ice Sheet, combining outputs from two climate models with an isotope‐enabled snowpack model. We show that surface vapor exchange and associated fractionation imprint a climate signal into the firn, resulting in an increase in the annual mean value of δ18O by +2.3‰ and a reduction in d‐excess by −6.3‰. Further, implementing isotopic fractionation during surface vapor exchange improves the representation of the observed seasonal amplitude in δ18O from 65.0% to 100.2%. Our results stress that surface vapor exchange is important in the climate proxy signal formation and needs consideration when interpreting ice core climate records.

Deuterium-excess (d) decreases up to 5 o /oo in near-surface snow during some summers at EastGRIP, likely due to net sublimation.• After one-to-two years in the snowpack, the peak d shifts from Autumn snow layers towards Summer snow layers.• Isotope-gradient diusion explains some but not all of the d seasonality changes in the near-surface snow.

The chemistry and aerosols in ice core records are used as proxy data for the past climate. Traditional interpretation of this recorded climate signal is that during formation snow captures a snapshot of the atmosphere. In recent years, observations have documented that the snow surface’s chemistry and isotopic composition change during the post-depositional interaction with the surface-near atmosphere. To more accurately interpret the climate signal in ice cores it is necessary to understand thesource of the water vapor in the planetary boundary layer (PBL), as well as the vertical mixing and transportation in the polar atmosphere. However, the dynamics in the polar PBL are poorly constrained in most climate models due to a lack of observations. Here we present insights from the first Arctic in-situ water-vapor isotope record both within and above the PBL up to 1500 meters above the Greenland Ice Sheet (GrIS) from the EastGRIP ice core camp 2022 field campaign. Flights were performed with a fixed-wing uncrewed aircraft recording high resolution atmospheric profiles. Moreover, air is sampled in glass flasks and brought to the surface for determination of δ18O and δDof water vapor. The observational set-up has been proven to guarantee reliable measurements of the isotopic composition of the atmospheric water vapor in remote locations and under extremely cold temperatures. Based on 105 observed temperature, humidity and isotopic profiles we identify different types of atmospheric structure above the GrIS. We evaluate the vertical atmospheric representation of the polar regional climate model MAR and the isotope-enabled global climate model ECHAM-wiso. Finally, from observations we estimate the height up to which the surface-near δ18O and δD isotopic values are affected by the atmosphere above.

Clouds have a large effect on the radiation budget and represent a major source of uncertainty in climate models. Supercooled liquid clouds can exist at temperatures as low as 235 K, and the radiative effect of these clouds depends on the complex refractive index (CRI) of liquid water. Laboratory measurements have demonstrated that the liquid‐water CRI is temperature‐dependent, but corroboration with field measurements is difficult. Here we present measurements of the downwelling infrared radiance and in‐situ measurements of supercooled liquid water in a cloud at temperatures as low as 240 K, made at South Pole Station in 2001. These results demonstrate that including the temperature dependence of the liquid‐water CRI is essential for accurate calculations of radiative transfer through supercooled liquid clouds. Furthermore, we show that when cloud properties are retrieved from infrared radiances (using the spectral range 500–1,200 cm −1 ) spurious ice may be retrieved if the 300 K CRI is used for cold liquid clouds (∼240 K). These results have implications for radiative transfer in climate models as well as for retrievals of cloud properties from infrared radiance spectra.

The Concordiasi project is making innovative observations of the atmosphere above Antarctica. The most important goals of the Concordiasi are as follows: To enhance the accuracy of weather prediction and climate records in Antarctica through the assimilation of in situ and satellite data, with an emphasis on data provided by hyperspectral infrared sounders. The focus is on clouds, precipitation, and the mass budget of the ice sheets. The improvements in dynamical model analyses and forecasts will be used in chemical-transport models that describe the links between the polar vortex dynamics and ozone depletion, and to advance the under understanding of the Earth system by examining the interactions between Antarctica and lower latitudes. To improve our understanding of microphysical and dynamical processes controlling the polar ozone, by providing the first quasi-Lagrangian observations of stratospheric ozone and particles, in addition to an improved characterization of the 3D polar vortex dynamics. Techni...

Six levels of meteorological sensors have been deployed along a 45 m tower at the French‐Italian Concordia station, Dome C, Antarctic. We present measurements of vertical profiles, the diurnal cycle, and interdiurnal variability of temperature, humidity, and wind speed and direction for 3 weeks during the southern summer of 2008. These measurements are compared to 6‐hourly European Center for Medium‐Range Forecasts (ECMWF) analyses and daily radiosoundings. The ECMWF analyses show a 3–4°C warm bias relative to the tower observations. They reproduce the diurnal cycle of temperature with slightly weaker amplitude and weaker vertical gradients. The amplitude of the diurnal cycle of relative humidity is overestimated by ECMWF because the amplitude of the absolute humidity diurnal cycle is too small. The nighttime surface‐based wind shear and Ekman spiral is also not reproduced in the ECMWF analyses. Radiosonde temperatures are biased low relative to the tower observations in the lowest 30 m but approach agreement at the top of the tower. Prior to bias correction for age‐related contamination, radiosonde relative humidities are biased low relative to the tower observations in the lowest 10 m but agree with tower observations above this height. After correction for the age‐related bias, the radiosonde relative humidity agrees with tower observations below 10 m but is biased high above this height. Tower temperature observations may also be biased by solar heating, despite radiation shielding and natural ventilation.

Measurements of pan evaporation were made during the summers of 1957 and 1958 on an ice station drifting between 80° and 86°N. Using weather reports, measurements were either screened for absence of precipitation (to obtain evaporation, E) or not screened (to obtain P‐E). Applying the screened data either to the entire month or only to the days without precipitation results in upper and lower limits to E. Monthly average values of E are positive in June and July, 3–5 and 5–8 mm/month, within the range of prior estimates, but are negative in August and September, indicating net deposition of frost or dew, at variance with prior estimates. The monthly averages of latent heat flux are small, 2–10 W m−2, by comparison to the individual components of net radiation, each on the order of 100–300 W m−2.

Abstract 26 Measurements,of pan evaporation were made on a drifting ice station between 80° 27 and 86°N during the summers of 1957 and 1958. Using weather reports, measurements 28 were either screened for absence of precipitation(to obtain evaporation, E) or not 29 screened (to obtain P-E). Applying the screened data eitherto the entire month or only to 30 the days without precipitation results in upper and lower limits to E. Monthly average 31 values of Eare positive in June and July, 3-5 and 5-8 mm/month, within the range of 32 prior estimates, but are negative in August and September, indicating net deposition of 33 frost or dew, at variance with prior estimates. The monthly averages of latent heat flux 34 are small, 2-10 W m,, by comparison to longwave and shortwave radiative heat fluxes of 35

Routine radiation and meteorological data at South Pole Station are used to investigate historical discrepancies of up to 50 W m −2 in the monthly mean surface energy budget and to investigate the behavior of turbulent heat fluxes under stable atmospheric temperature conditions. The seasonal cycles of monthly mean net radiation and turbulent heat fluxes are approximately equal, with a difference of 40 W m −2 between summer and winter, while the seasonal cycle of subsurface heat fluxes is only a few W m −2 . For an 8‐month period (the winter of 2001), we calculate two estimates of turbulent heat fluxes, one from Monin‐Obukhov (MO) similarity theory and one as the residual of the surface energy budget (i.e., subsurface heat fluxes minus net radiation, where all fluxes toward the snow surface are positive). The turbulent fluxes from MO theory agree well with the residual of the energy budget under lapse conditions. However, under stable conditions MO theory underestimates turbulent fluxes by approximately 40–60%. The relationship between turbulent heat fluxes as a residual of the energy budget, temperature inversion strength, and wind shear as a function of the bulk Richardson number ( Ri b ) is examined under stable conditions (i.e., positive Ri b ). The Ri b used here is calculated from 10‐m wind speeds and 0‐ to 2‐m temperature inversion strength. No critical value of Ri b is found where the turbulent heat fluxes drop to zero. However, a threshold ( Ri b = 0.05) exists below which 70% of the turbulent energy fluxes can be explained by only the temperature inversion strength. For Ri b > 0.05, the relationship between turbulent heat fluxes and temperature inversion strength decreases, while the importance of wind shear to turbulent heat transfer increases. Above Ri b = 0.05, a growing linear correlation also exists between atmospheric temperature inversion strength and wind shear. Thus, inversion strength and wind shear are not independent predictors of turbulent heat flux for extremely stable conditions. The exact values of the correlation coefficients and Ri b threshold are likely specific to the experimental conditions; however, their implications are probably valid for all stable flows. Knowledge of the time‐varying surface characteristics would help to generalize these parameters.

The News Focus story “Reshuffling graduate training” (J. Mervis, 31 July, p. [528][1]) details Roald Hoffmann's proposal to improve the U.S. science system by making students more independent and empowered through an increase in government fellowships granted directly to students. Responses to this story included suggestions to increase funding for fellowships (“Increase grants, too,” M. J. Castellano and K. E. Mueller, Letters, 18 September, p. [1498][2]) and to provide more stable funding for students (“Stable funding is key,” R. J. Butera, Letters, 18 September, p. [1499][3]). These suggestions are all related to the theme of financially supporting and maintaining the most important resource to modern scientific research: graduate students. However, although academic culture recognizes the importance of graduate students, it currently does almost nothing to train current and future principal investigators (PIs) to effectively manage this resource. Few Ph.D.s have substantial hands-on experience managing others before they land a faculty position, and even fewer have any formal training in management. Faculty are left to learn this skill on the job at the expense of productivity and the well-being of the people they are managing. Furthermore, there is an emphasis on student independence in this discussion, which is natural; independence is an essential quality in a career researcher. The unfortunate implication is that ideal students are independent from the start. In fact, independence is a skill that can be taught and nurtured, just like the other skills that are explicitly taught in graduate school. To improve the efficiency of the science industry, I suggest improving the management of its most important resource. Unproductive students are a consequence of student inexperience and poor advising. Better management of students can be achieved through a range of mechanisms that involve both faculty and students (such as regular mutual evaluations and human resources training for current and future PIs). Such efforts may cost time initially, but will certainly pay off in the long run. [1]: /lookup/doi/10.1126/science.325_528 [2]: /lookup/doi/10.1126/science.325_1498 [3]: /lookup/doi/10.1126/science.325_1499a