Katabatic diminish contribution to the Antarctic ice mass balance Jacopo Graziolia,1, Jean-Baptiste Madeleineb, Hubert Gallee´ c, Richard M. Forbesd, Christophe Genthonc, Gerhard Krinnerc, and Alexis Bernea,2

aEnvironmental Remote Sensing Laboratory, Environmental Engineering Institute, School of Architecture, Civil and Environmental Engineering, Ecole´ Polytechnique Fed´ erale´ de Lausanne, CH-1015 Lausanne, Switzerland; bLaboratoire de Met´ eorologie´ Dynamique/Institut Pierre-Simon Laplace, Sorbonne Universites,´ Universite´ Pierre et Marie Curie Paris 06, Paris Sciences and Lettres Research University, Ecole´ Normale Superieure,´ Universite´ Paris-Saclay, Ecole´ Polytechnique, CNRS, F-75005 Paris, France; cUniversite´ Grenoble Alpes, CNRS, Institut des Geosciences´ de l’Environnement, F-38000 Grenoble, France; and dResearch Department, European Centre for Medium-Range Weather Forecasts, Shinfield Park, Reading, RG2 9AX, United Kingdom

Edited by Dennis L. Hartmann, University of Washington, Seattle, WA, and approved August 25, 2017 (received for review May 8, 2017) Snowfall in is a key term of the ice sheet mass budget data allowed us to unveil the typical spatial structure of pre- that influences the sea level at global scale. Over the continental cipitation in the vertical dimension in particular. Because of its margins, persistent katabatic winds blow all year long and sup- strong katabatic regime, Ad´elie Land is particularly well- ply the lower troposphere with unsaturated air. We show that suited for studies of katabatic winds and their interactions with this dry air leads to significant low-level sublimation of snowfall. precipitation. We found using unprecedented data collected over 1 year on the coast of Adelie´ Land and simulations from different atmospheric Results models that low-level sublimation accounts for a 17% reduction Low-Level Sublimation of Precipitation. Our observations at DDU of total snowfall over the continent and up to 35% on the mar- consistently show that snowfall is frequently either not reaching gins of East Antarctica, significantly affecting satellite-based esti- the ground (virga) or strongly decreasing in intensity in the lower mations close to the ground. Our findings suggest that, as climate kilometer of the atmosphere (Fig. 1). This suggests that the pre- warming progresses, this process will be enhanced and will limit cipitation undergoes a significant sublimation process close to expected precipitation increases at the ground level. the ground. Daily losses of precipitation mass over the vertical column caused by sublimation can get close to 100% when snow- Antarctica | precipitation sublimation | katabatic wind fall intensities at the ground level are low and decrease to about 20% in the most intense snowfall cases (Fig. S1). The available recipitation in Antarctica falls almost exclusively as snowfall remote sensing measurements over a full year exhibit a clear sig- P(1). As the dominant input term, precipitation is a key com- nal of low-level sublimation: the maximum snowfall accumula- ponent in the ice sheet mass balance, and changes to this bal- tion occurs ∼0.7 ± 0.05 km above the surface, just above the ance can directly affect the sea level at the global scale (2–5). typical top height of the katabatic flow coming from the inner Over Antarctica, snowfall is still not well-quantified and less doc- continent, while near the surface, yearly accumulation values are umented than elsewhere. This is partly because of the lack of about 25% lower than the maximum ones (Fig. 2). measurements covering the processes from precipitation forma- These observations give us insight into the origin of this pro- tion to ground deposition. After the recent momentum of atmo- cess of low-level sublimation of precipitation (LSP) observed in spheric research in the polar regions (6) and thanks to the latest the data, which is driven by the wind regime and atmospheric technological advances in the field of remote sensing, such mea- surements are starting to be regularly collected in various loca- Significance tions of the continent (7–9). The challenges affecting in situ snowfall observations are the extremely low and snowfall rates in the interior Precipitation over Antarctica remains largely unknown, de- and the wind regime close to the coasts. Over the continental spite its crucial role in the surface mass balance of the Antarc- margins, persistent katabatic winds blow all year long (10, 11). tic ice sheet. Using unprecedented observations covering an They originate from the cold and dry inner continent and there- entire year, this work describes a previously unknown mecha- fore, supply the lower troposphere with large masses of unsatu- nism that leads to the sublimation of a large fraction of snow- rated air. Surface inversion and the absence of oro- fall in the lower atmosphere, resulting from the interaction of graphic barriers allow katabatic winds to develop into some of precipitation and katabatic winds. Snowfall sublimation in the the strongest, most persistent, and most directional near-surface atmosphere, caused by katabatic winds, is in the order of 35% winds on Earth (10–12). The effect of these winds on the trans- in the margins of East Antarctica. This process critically affects port and sublimation of snow after deposition at the surface the interpretation of satellite-based remote sensing observa- has been studied, modeled, and quantified (13, 14). This pro- tions close to the ground and suggests that snowfall sublima- cess turned out to be of primary importance for the ice sheet tion in a warming climate may counterbalance the expected mass balance (3), in particular in the eastern part of the conti- increase of precipitation. nent. The interaction of katabatic winds with snowfall, however, Author contributions: J.G., C.G., and A.B. designed research; J.G. and A.B. performed remains unknown, despite the importance of precipitation for research; J.G., J.-B.M., H.G., R.M.F., C.G., G.K., and A.B. analyzed data; J.G., C.G., and A.B. the ice sheet mass balance. conducted field work; J.-B.M. ran the LMDZ model; H.G. ran the MAR model; R.M.F. ran From November of 2015 to November of 2016, we conducted the ECMWF IFS model; and J.G. wrote the paper. a field campaign dedicated to the monitoring of precipitation The authors declare no conflict of interest. (9) at the Dumont d’Urville (DDU) station on the coast of East This article is a PNAS Direct Submission. Antarctica (66.6628 S, 140.0014 E). We obtained unprecedented 1Present address: Radar Satellite and Nowcasting Division, MeteoSwiss, CH-6605 Locarno- weather radar measurements of precipitation in Ad´elie Land by Monti, Switzerland. means of a scanning X-band polarimetric radar (deployed from 2To whom correspondence should be addressed. Email: alexis.berne@epfl.ch. December of 2015 to January of 2016) and a K-band vertically This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. profiling radar (operating since November of 2015). These radar 1073/pnas.1707633114/-/DCSupplemental.

10858–10863 | PNAS | October 10, 2017 | vol. 114 | no. 41 www.pnas.org/cgi/doi/10.1073/pnas.1707633114 Downloaded by guest on September 23, 2021 Downloaded by guest on September 23, 2021 nAtrtc,w,teeoe s uptfo he different Meth- [Mod R model three and research far from (Materials high-resolution a so models output ods): collected atmospheric use datasets complementary therefore, and precipitation we, few scale Ad Antarctica, continental very than in Antarctic the the other at given regions mechanism and LSP Antarctic the in study mechanism To LSP exist in the statistics do of of sounding occurrence hence margins the the the for in conditions of shown Favorable majority as the Antarctica over East avail- low-level observed unsaturated data an are radiosounding and of troposphere winds records katabatic Antarctica: long investigate in the given able to is of indication used analysis important the be first A by cannot scale. continental approach the loca- same at three vertical LSP the at DDU. of far), (collected so at Measurements rare very tions Antarctica? precipitation being of precipitation total of rest on profiles the mechanism about LSP What the of tance Scale. Continental the at LSP ytm(CW F),adagoa lmt oe [Labora- model climate global a and Forecast IFS)], Integrated (ECMWF Forecasts System Weather Medium-Range for tre is sublimation ( snowfall important where most areas the the on the of than are higher margins plateau, much elevated the the is per- air, precipitation the dry where of continent, of Because Antarctic accumulation (18). and ocean outflow the near over sistent and dissipated escarpment is the the coasts over until observed the layer saturates dry gradually the and of of accumulates slope structure air slows the airflow dry where katabatic The ocean, the down. zero, the to descending of decreases the abruptly proximity terrain of very the the humidity the In relative in mass. the air subsaturation 18). decreasing terrain by enhances (17, the layer further warming of dry warming adiabatic slope adiabatic com- the with air This as associated The accelerates precipitate. steeper, continent to becomes required the of size growth from the and ing to nucleation up the oversatura- for crystals or condition ice saturation necessary m), the (>1,200 is roughly Aloft tion near-saturation (below DDU). unsaturated this for and of m dry Because 1,200 top remains 3). above the (Fig. (but air at the to (16) layer, inversion up surface temperature lifted the the be above of can of m sublimation that 300 the 15) exceeds) by (14, rarely snow saturated snow, blowing often deposited and and recirculate drifting moistened and is lift air to and enough wind the strong surface, usually the continent to is the Close level. over ground clutter the ground near by stratification corrupted are measurements radar where areas The profile. 203 individual of an gray. azimuth as in shown true shadowed is a are location at radar plateau the Antarctic over column the of direction reflectivity the radar in conducted is cross-section 1. Fig. rzoie al. et Grazioli ´ goa MR] noeainlgoa oe Erpa Cen- [European model global operational an (MAR)], egional vdneo o-ee ulmto rmavria rs-eto fwahrrdrmaueet efre nDUo eebr2,21.The 2015. 29, December on DDU in performed measurements radar weather of cross-section vertical a from sublimation low-level of Evidence Z H epesdi B)lne osofl aetruhapwrlw(eal r in are (details law power a through rate snowfall to linked dBZ) in (expressed i.S2 Fig. ). h nlssaoesosteimpor- the shows above analysis The ` l Atmosph ele ´ leLand. elie i.S3. Fig. ´ erique etrmthstelcto ftemxmmadteamplitude the and maximum the and of precipitation location of the resolution, structure matches 5-km vertical better full at the MAR, reproduces processes. better not microphysical parameter- therefore, models’ of the is, a izations between which to differences points process, by the This affected the and strongly observed. of surface than origin the lower dynamical maxi- above is robust km losses snowfall the 1.2 the of and of amplitude 0.9 height between respect the varies with although mum precipitation mod- observations, of atmospheric used the structure three to Model similar all a of System reproduced simulations Earth els The an (19). CMIP5 of in component atmospheric the M de toire A,adLD)rna ifrn eouin,adwt ifrn physical different with and sublimation. resolutions, low-level different show at all (ECMWF, parametrizations, models run three LMDZ) The and value. 1/2y). MAR, maximum MRR respective and their while (MAR by curves, 2016 normalized period of snowfall half-year May cumulative the to or absolute IFS) 2015 ECMWF of and November 2015 LMDZ, 1y, of from November (MRR from 2015 period of 1-y October the to over curves the atmospheric obtained numerical We and models. measurements radar ground-based by obtained 2. Fig. uuaiesofl safnto fhih bv eri nDUas DDU in terrain above height of function a as snowfall Cumulative ´ et ◦ ´ oooi yaiu omd(MZ] hc is which (LMDZ)], Zoomed Dynamique eorologie h aibeta sclrcddi h rs-eto sthe is cross-section the in color-coded is that variable The . PNAS ,wiesofl nest na in intensity snowfall while Methods), and Materials | coe 0 2017 10, October Right | o.114 vol. hw h aecurves same the shows | o 41 no. Left | shows 10859

EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES Fig. 3. Mechanisms leading to sublimation enhancement over the coastal locations at the margin of the Antarctic ice sheet. The spatial scales are approx- imated from the section crossing DDU in the direction of maximum slope toward the plateau. Accumulation of air with low relative humidity near the Antarctic margins leads to an enhancement of sublimation. The curves of relative humidity (RH), temperature (T), and wind speed (W) correspond to the median profiles of these variables obtained from the daily radiosonde measurements conducted at 00 UTC for the period from November of 2015 to November of 2016 conditioned on precipitation occurrence. They highlight the typical stratification of the atmosphere during snowfall events at this location.

of sublimation, while ECMWF IFS (≈10 km) and LMDZ (≈70 of measurements (November of 2015 to November of 2016), at km) locate the maximum higher and reproduce a smoother least 651 Gt water equivalent snowfall sublimated because of the cumulative precipitation profile, probably because of their lower LSP process described above. This corresponds to 17% of the spatial resolution. Blowing snow in MAR is assigned to the total snowfall on the entire Antarctic continent never reaching same water category as snowflakes. This explains the appar- the ground. It is worth noting that this percentage has roughly ent increase of precipitation below 300 m of altitude, because the same magnitude (10–15%) as the estimated proportion of blowing snow and precipitation particles cannot be separated in snow sublimating after deposition (13, 22). On the margins of the model. The ECMWF IFS model does not consider blow- East Antarctica (defined here from 30 ◦W to 150 ◦E), where ing snow and its feedback on air humidity, but it assimilates the snowfall accumulation is larger than in the interior, the contribu- atmospheric soundings data over several locations in Antarc- tion of snowfall sublimation is larger: 35% for all of the regions tica. Differences between the three models above the level of where the surface is lower than 1 km above sea level. maximum snowfall (Fig. 2) result from the various microphysi- cal parametrizations and cloud-to-snow conversion rates used by Discussion and Conclusion the models. The influence of LSP on the vertical structure of precipitation By looking at the simulations of the ECMWF IFS model over raises the question of the interpretation of satellite-borne radar the entire continent, we observe that, in several regions form- precipitation measurements over the Antarctica continent (23, ing a belt around the margin of East Antarctica, more than 40% 24) that are currently the only model-free source of informa- of the snowfall sublimates in the lower kilometer of the atmo- tion able to cover the entire continent. These measurements sphere before reaching the ground (Fig. 4 and Fig. S3). Subli- must be discarded close to the surface because of ground clutter mation larger than 30% in relative terms is also observed over contamination of the radar signal (24), and the measurements many ice shelves, but the absolute amounts sublimated at these at a certain height (1.2 km for CloudSat and around 0.8 km locations are at least an order of magnitude smaller than the val- expected for EarthCare) above the surface are used as a proxy ues observed on the margins of East Antarctica. Given the prox- for near-ground precipitation. The vertical structure in Fig. 2 imity to the mountain ranges of the Antarctic Peninsula, foehn and the sublimation belt observed in Fig. 4 suggest that satel- winds may play a role in the case of the Larsen ice shelf (20), lite retrievals overestimate precipitation in the areas where sub- and the accumulation of air originating on the plateau and cross- limation dominates the low-level structure of precipitation in the ing the trans-Antarctic mountains may affect the Ross ice shelf satellite blind range. As shown in Fig. S4, the expected biases (21) as well. Finally, the contribution of sublimation is low both of satellite estimates are indeed nonuniformly distributed over in relative and in absolute terms on the elevated part of the con- the continent, with significant overestimations that can be on tinent, and a clear link with topography through wind channeling the order of 50% in the areas of katabatic channeling as a con- is observed, especially on the margins of East Antarctica (Fig. 4). sequence of the nonuniform distribution of the height of LSP We estimate from ECMWF IFS simulations that, over the year (Fig. S5).

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EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES the year corresponding with the observations from November of 2015 to Quantification of Total Sublimation. The quantification of sublimation was November of 2016. The effects of spin up of precipitation in the first obtained from the vertical profiles of snowfall intensity. The difference hours of the forecast have been verified to be, for the domain of inter- between the maximum snowfall accumulation over the vertical column at est, negligible. The data are interpolated to a 0.25◦ latitude/longitude a daily timescale and the near-ground accumulation is taken as a proxy for grid for faster processing. For the analysis in Fig. 4 and Figs. S2 and S3, the total sublimation. we used ECMWF IFS, because data were available on the entire Antarc- Fig. 2, for what concerns the models, is obtained by accumulating snow- tic domain, and they include the assimilation of available measurements. fall over the same period as when MRR data were available. The total sub- At DDU, where reference observations have been collected by the MRR limation ratio, shown in Fig. 4A, was obtained by accumulation over 1 y down to 300 m above ground (minimum observable height), the total of data aggregated at the daily scale as explained below. For each day sublimation from ECMWF IFS, down to 300 m above ground as well, is i and at each grid point, we calculated the maximum snowfall accumula- about 27%, while it is 32% from MRR data. ECMWF IFS sublimation down i tion over the vertical column Cmax (in millimeters) and the total near-ground to 10 m above ground (as shown in Fig. 4C and Fig. S3) is 36%. It is i accumulation Cgr (in millimeters). The height levels vary slightly from pro- worth noting that ECMWF IFS does not directly simulate blowing snow file to profile at the subdaily scale, as they result from the conversion of that leads to resaturation of the lowest layer of the atmosphere (37) but model levels to heights above ground that depend on the temperature and assimilates atmospheric soundings where this resaturation appears (Fig. 3 pressure at these levels. For this reason, they are linearly interpolated into and Fig. S3). a regular grid ranging from 10 to 4,500 m above ground for the sake of MAR is a limited area atmosphere, blowing snow, and snow pack coupled homogeneity. The analysis excludes all profiles that have a positive temper- research model. Atmospheric dynamics is based on the hydrostatic approx- ature in degrees Celsius to avoid the process of snow to rain conversion imation of the primitive equations (38), and the parameterization of tur- being confused with snowfall sublimation; 0.42% of the profiles over conti- bulence in the surface boundary layer takes into account the stabilization nental Antarctica are removed in this way, with maximum values becoming effect of the blowing snow flux (39). The blowing snow model used in MAR significant (up to 30%) at the lowest latitude of West Antarctica on the is described in refs. 40 and 41. MAR was set up over Adelie´ Land (Antarctica), Antarctic Peninsula. The total sublimation ratio in Fig. 4C is obtained over N with a horizontal resolution of 5 km over 136 × 136 grid points. Lateral forc- days as ing and sea surface conditions were taken from the global reanalysis (called N N ERA-Interim) provided by the ECMWF (42). There were 26 levels in the ver- P i P i Cmax − Cgr tical, with a higher vertical resolution in the low troposphere. The first level i i SR = . [1] is situated very close to the ground (0.15 m), as blowing snow is explicitly N P i Cmax modeled. The simulation shown in Fig. 3 was performed from the November i 1, 2015 to May 31, 2016. The total sublimation in Fig. 4B for the period under analysis is given by The LMDZ global climate model was run using a zoom over the region the numerator of Eq. 1. Therefore, the estimation of sublimation is based of the DDU station and a 6-h relaxation of the winds to ECMWF reanalysis on daily data. At subdaily scales, the contribution of sublimation can be data. This relaxation was active outside the stretched domain to represent even larger. the global synoptic conditions and progressively reduced inside the domain. The model was run with a horizontal resolution of 70 km in the vicinity of the station and 79 vertical levels, with a first level at 8 m. Cloud and ACKNOWLEDGMENTS. We thank the French Polar Institute, which logisti- cally supported the Antarctic Precipitation: Remote Sensing from Surface precipitation physics of the model are described in ref. 43. and Space (APRES3) measurement campaign. We also thank the Univer- With the first model elevation above the surface ranging from less than sity of Wyoming Department of Atmospheric Science for their service in 1 m to up to 10 m, different vertical resolutions in the different models providing atmospheric sounding data and Met´ eo´ France for the sound- can also contribute to different reproduction of the vertical structure of ings at Dumont d’Urville. We thank Florentin Lemonnier (Laboratoire de precipitation. Met´ eorologie´ Dynamique) for the LMDZ runs that he performed and the development team of LMDZ for their major contribution to the improve- Radiosounding Data. We used atmospheric sounding data in Fig. 3 and Fig. ments of the code: Jean-Yves Grandpeix, Fred´ eric´ Hourdin, Catherine Rio, and Etienne Vignon for their contributions to the parametrization of S3. The data are made freely available by the University of Wyoming Depart- clouds and winds and all of the engineers who allowed LMDZ to run. We ment of Atmospheric Science (weather.uwyo.edu/upperair/sounding.html) are also grateful to T. Battin and A. Rinaldo for their help on an earlier for several sounding stations. The relative humidity with respect to water version of this manuscript. J.G. thanks the Swiss National Science Founda- RHW was converted to relative humidity with respect to ice RHI taking into tion (Grant 200021 163287) for financing his participation in the project. account the temperature of air (44, 45). The curves in Fig. 3 represent the We also acknowledge the support of the French National Research Agency same measurement period as the MRR radar data, and they are conditioned to the APRES3 project. The project Expecting Earth-Care, Learning from A- on precipitation occurrence using the MRR as a reference. The statistics Train (EECLAT) funded by the Centre National d’Etudes Spatiales (CNES) also displayed in Fig. S3 cover the period from 2005 to 2016. In this case, as no contributed support to this work. MAR simulations were performed using high performance computing resources from the French Big National Equip- in situ data are available, they are conditioned on precipitation occurrence ment Intensive Computing (GENCI) from the Institut du Developpement´ et from ERA-Interim, which is thought to provide the best reconstruction of des Ressources en Informatique Scientifique (IDRIS) (Grant 2017-1523) and precipitation (46). The threshold on precipitation accumulation used to dis- the High-Performance Computing/Modeling/Numerical and Technological criminate precipitation occurrence was 0.07 mm in 6 h as suggested for this experimentation infrastructure (CIMENT), which is supported by the Rhone-ˆ reanalysis (23). Alpes region (Grant CPER07 13 CIRA).

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