Pan-Antarctic map of near-surface permafrost temperatures at 1 km 2 scale

Permafrost is present under almost all of the Antarctic’s ice-free areas but little is known about spatial variations of permafrost 20 temperatures outside a few areas with established ground temperature measurements. We modelled a temperature at the top of the permafrost (TTOP) for all the ice-free areas of Antarctic mainland and Antarctic Islands at 1 km resolution during 2000– 2017. The model was driven by remotely-sensed land surface temperatures and down-scaled ERA-Interim climate reanalysis data and subgrid permafrost variability was simulated by variable snow cover. The results were validated against in-situ measured ground temperatures from 40 permafrost boreholes and the resulting root mean square error was 1.9 °C. The lowest 25 near-surface permafrost temperature of -33 °C was modelled at Mount Markham in Queen Elizabeth Range in the Transantarctic Mountains. This is the lowest permafrost temperature on Earth according to the modelling results on global scale. The temperatures were most commonly modelled between -23 and -18 °C for mountainous areas rising above the Antarctic Ice Sheet and between -14 and -8 °C for coastal areas. The model performance was good where snow conditions were modelled realistically but errors of up to 4 °C can occur at sites with strong wind-driven redistribution of snow. 30 https://doi.org/10.5194/tc-2019-148 Preprint. Discussion started: 25 June 2019 c © Author(s) 2019. CC BY 4.0 License.


Introduction
Permafrost in the Antarctic is present beneath all ice-free terrain, except for the lowest elevations of the maritime Antarctic and sub-Antarctic islands (Vieira et al., 2010), unlike the northern hemisphere, where permafrost underlies approximately 15 % of the unglaciated land.The ice and snow-free land occupies 0.22% (30 900 km 2 ) of the whole Antarctic (Burton-Johnson et al., 2016;Hrbáček et al., 2018).The major ice-free areas include the Queen Maud Land, Enderby Land, the Vestfold Hills, Wilkes Land, the Transantarctic Mountains, the Ellsworth Mountains, Marie Byrd Land and the Antarctic Peninsula (Green et al., 1967;Fig. 1).Despite the relatively small area, in comparison to glaciated areas, permafrost is one of the major factors controlling terrestrial ecosystem dynamics in the Antarctic (Bockheim et al., 2008).
Compared with the Northern Hemisphere, where the first permafrost investigations date back to 19th century (Shiklomanov, 2005;Humlum et al., 2016), the ground temperatures in the Antarctic have been systematically studied only during the last two decades.Permafrost was studied in relation to patterned ground since the 1960s and during the Dry Valley Drilling Project in the 1970s but temperatures have been measured only occasionally (Decker and Bucher, 1977;Guglielmin, 2012).A first Antarctic permafrost borehole network was implemented in 1999 in the Victoria Land (Transantarctic Mountains) and was extended during the International Polar Year 2007-2009 to cover all eight major ice-free regions (Vieira, 2010).
Permafrost distribution was estimated and mapped on the Antarctic Peninsula by Bockheim et al. (2013) based on mean annual temperature, periglacial features, shallow excavations, borehole measurements, geophysical surveys, and existing permafrost models.Bockheim et al. (2007) characterised permafrost in the McMurdo Dry Valleys based on ground ice properties and active layer thickness from more than 800 shallow excavations.
Antarctic permafrost modelling efforts were limited to small areas in the Antarctic Peninsula region and sub-Antarctic Islands.Ferreira et al. (2017) used freezing indexes and the temperature at the top of the permafrost (TTOP) modelling for eight monitored sites on Hurd Peninsula, Livingston Island for the 2007 and 2009 seasons to study the controlling factors of ground temperatures.Rocha et al., (2010) ran the H-TESSEL scheme forced by ERA-Interim reanalysis to simulate ground temperatures at Reina Sofia Peak on Livingston Island.Ground temperature measurements and permafrost modelling efforts have been limited to point sites and little is known about spatial variability of ground temperatures at the regional and continentwide scales.
In this study we employed the TTOP modelling scheme based on The Moderate Resolution Imaging Spectroradiometers (MODIS) land surface temperatures (LST) and ERA-Interim reanalysis to model the spatial distribution of temperatures at the top of permafrost on all ice-free areas of Antarctic and Antarctic Islands.We adapt the existing modelling scheme from the https://doi.org/10.5194/tc-2019-148Preprint.Discussion started: 25 June 2019 c Author(s) 2019.CC BY 4.0 License.
Northern Hemisphere (Westermann et al., 2015;Obu et al., 2019) according to the available input data and their characteristics for the Antarctic.

The Cryogrid 1 model
The CryoGrid 1 model (Gisnås et al., 2013) calculates mean annual ground temperature (MAGT) and is based on the TTOP equilibrium approach (Smith and Riseborough, 1996): Where: FDD and TDD represent freezing and thawing degree days, respectively, of the surface meteorological forcing accumulated over the model period τ in days.The influence of the seasonal snow cover, vegetation, and ground thermal properties were taken into account by the semi-empirical adjustment factors rk (ratio of thermal conductivity of the active layer in thawed and frozen state), nf (scaling factor between average winter surface and ground surface temperature) and nt (scaling factor between average summer surface and ground surface temperature).We use land surface temperature (LST) to compute FDD and TDD (see following section) instead of the air temperature that was used initially by Smith and Riseborough (2002) and hence omit thawing nt-factors.

Freezing and thawing degree days
Spatially distributed data sets of TDD and FDD were compiled from remotely sensed land surface temperature products and climate reanalysis data following the procedure of Obu et al. (2019).We used LST data (level 3 product in processing version 6) from MODIS on board the Terra and Aqua satellites, which contain up to two daytime and two night-time measurements per day at a spatial resolution of 1 km since the year 2000 (Wan, 2014).For this reason, the study extends from 2000 and to the end of 2017.Data gaps in the MODIS LST time series due to cloud cover can result in a systematic cold bias in seasonal averages (Westerman et al., 2012;Soliman et al., 2012;Østby et al., 2014), so a gap filling with near-surface air temperatures from the ERA-interim and ERA-5 reanalysis was applied (Westermann et al., 2015).
The ERA-interim reanalysis provides gap-free meteorological data from 1979 onwards at a spatial resolution of 0.75° × 0.75° (Dee et al., 2011).The ERA-5 reanalysis is ERA-Interim upgrade and provides the data at improved 0.28125° × 0. 0.28125° Data 2010 (GMTED2010) (Danielson and Gesch, 2011).The downscaling methodology is described in detail by Fiddes and Gruber (2014), Westermann et al. (2015) and Obu et al. (2019).The gap-filled MODIS LST time series were averaged to eight-day periods from which FDD and TDD were finally accumulated for the 2000-2017 study period.

Average annual snowfall and nf-factors
Spatially distributed data sets of nf-factors were generated from average annual snowfall forced by ERA-Interim and ERA-5 reanalysis data.Snow cover in the majority of the Antarctic is dominated by sublimation and processes related to blowing snow (Gallée, 1998;Bintanja and Reijmer, 2001), which were not taken in to account by the snowfall and degree-day model that Obu et al. (2019) used to estimate nf-factor for the Northern Hemisphere.For this reason, only mean annual snowfall was calculated using downscaled ERA-Interim precipitation before 2008 and ERA-5 precipitation after.Precipitation was downscaled based on the difference between reanalysis elevation and GMTED2010 using a precipitation gradient, found in drier areas (Hevesi et al., 1992), of 2 % per 100 m up to 1000 m and 1% per 100 m for elevations above 1000 m.Snowfall was defined as precipitation at air temperatures below 0 °C, using the downscaled ERA-Interim air temperatures as employed for the gap filling of MODIS LST (see above).For a detailed downscaling procedure description see Obu et al. (2019).
Nf-factors were defined based on average annual snowfall, however the reported nf-factors in Antarctic were calculated in respect to snow depth.Oliva et al. (2017) identified nf-factor values around 0.3 for snow accumulations of 80 cm on Livingston Island, although the nf-factors can reach up to 0.55 in the thick snow cover due to its temporal variability (de Pablo et al., 2017).Nf-factors close to, or even greater than, one were measured in areas with little or no snow cover on Veleskarvet nunatak (Kotzé andMeiklejohn, 2017), in the McMurdo Dry Valleys (Lacelle et al., 2016) and on James Ross Island (Hrbáček et al., 2016).This range was used to constrain the nf-factor ranges in relation to average annual snowfall (Table 1).Since the snow model was not able to simulate snow free-sites nor snowdrifts due to strong wind distribution, we used a maximum nf-factor of 0.95 and minimum of 0.3.However, smaller snow-depth variations on a local scale were taken into account with an ensemble of different values of average annual snowfall for each 1 km 2 pixel (e.g.Gisnås et al., 2014).

rk-factors
The rk-factor is defined as the ratio of thawed and frozen thermal conductivities of the active layer material (Romanovsky and Osterkamp, 1995) and is related to water and organic contents (e.g.Gisnås et al., 2013).Soil moisture properties were mapped on a regional scale (Bockheim et al., 2007) but no pan-Antarctic datasets related to soil water or organic contents were available.The ESA CCI Landcover that Obu et al. (2019) used for Northern Hemisphere contains only "permanent snow and ice" class on the Antarctic mainland, therefore, a rock outcrops dataset (Burton-Johnson et al., 2016) was used to constrain non-glaciated areas.An rk-factor of 0.85 was used for the whole Antarctic, representing an average value between very dry sites on continental Antarctic and moderately moist sites on the Antarctic Peninsula.

Ensemble-based modelling of subpixel heterogeneity
Ground temperatures can vary considerably at short distances due to heterogeneous snow cover, vegetation, topography and soil properties (Beer, 2016;Gisnås et al., 2014;2016;Zhang et al., 2014).We ran an ensemble of 200 model realisations with different combinations of nf-and rk-factors to simulate the variability.Rk-factor values were set to randomly vary by ±0.1 between 0.75 and 0.95 to represent both very dry sites and locations with higher soil moisture.The distribution of snowfall within the 1 km pixel was simulated using a log-normal distribution function where mean annual snowfall determined the mean of the distribution.The coefficient of variation of the distribution for the open areas (0.9) according to Liston (2004) was assigned to all modelled areas.An nf-factor was assigned to the estimated average annual snowfall according to pixels not overlapping with rock outcrops were masked out.A fraction of model runs with MAGT < 0 °C was used to derive permafrost type (zones) on Antarctic Islands (see Obu et al. (2019) for detailed description).

Model validation
We  2012) and ( 2014).The data found in publications were not necessarily calculated for the top of the permafrost and do not completely overlap with the modelling period and are, thus, less reliable than the author-supplied validation data.For instance, the data from Vieira et al. (2010) represents MAGT for the periods before 2010 usually lasting only few years (Table 2).

Comparison to borehole measurements
Ground temperatures can vary significantly inside a 1 km 2 model pixel, which is, to a certain extent, represented by the TTOP model ensemble runs.Average MAGT derived from the ensemble runs was compared to the measured site ground temperatures, which might limit representativeness for sites with locally specific ground and snow properties.The comparison yielded a RMSE of 1.94 °C and a mean absolute error for all boreholes of 0.06 °C (Fig. 2).The small mean absolute error is partly achieved with fine adjustment of the nf-factor class limits.For 50 % of the boreholes, the agreement between borehole temperatures and modelled MAGT was better than 1 °C, while it is better than 2 °C for 75%, and better than 3 °C for 85 % of Antarctic boreholes, 18 (45 %) boreholes were contained within one, 29 (73 %) within two, and 31 (78 %) within three standard deviations from the mean, which is comparable to the results for the Northern Hemisphere (Obu et al., 2019).

Queen Maud Land
There are 2430 km 2 of ice free areas in Queen Maud Land according to the rock outcrop map (Burton-Johnson et al., 2016;Hrbáček et al., 2018).The average MAGT is -18.2 °C and it ranges from -26.2 °C in the highest parts of Fimbulheimen Range to -6.3 °C in Prince Olav Coast, where MAGTs down to -10 °C were modelled at elevations exceeding 200 m (Fig. 3).MAGTs above -10 °C can be found also at Schirmacher Oasis (Hills) and reach above -8 °C.MAGTs in Sør Rondane Mountains and in Fimbulheimen Range range from -12 °C at elevations of around 800 m a.s.l. to -24 °C at elevations exceeding 3000 m a.s.l..

Vestfold Hills
The ice-free area of the Vestfold Hills region is 2750 km 2 .The modelling showed an average MAGT of -17.4 °C in this region.5 The MAGT ranged from -6.6 °C on the islands of the Ingrid Christensen Coast to -28.3 °C in the highest parts of the Prince Charles Mountains (Fig. 5).The MAGT in Amery Oasis, which is a part of the Prince Charles Mountains, was modelled as -13 °C, in the lowest-lying areas, down to -18 °C at elevations approaching 1000 m a.s.l.At the similar elevations on the Mawson Escarpment significantly lower MAGTs, from -19 °C at 200 m a.s.l down to -24 °C at 1500 m a.s.l were recognised.
In the coastal lowland areas of the Larsemann and Vestfold Hills the MAGT ranged between -10 to -7 °C. 10

Wilkes Land
The majority of the 400 km 2 of ice free area in Wilkes Land lies in the area surrounding the Bunger Hills, where MAGTs of around -9°C were modelled close to the Shackleton Ice Shelf.The lowest MAGT of -15.9 °C was modelled on the adjacent mountains at 1300 m elevation (Fig. 6).Modelled MAGTs were -8 to -6 °C at the Budd Coast, -8 °C at the Adélie Coast, and -10 to -11 °C at the George V Coast.Due to the prevalence of low-lying regions the mean MAGT of the region was only -8.9 °C.

Transantarctic Mountains
The Transantarctic Mountains are the largest ice-free region, comprising 19 750 km 2 and extending from Cape Adare to Coats Land.The part west of the 90° meridian (Fig. 7) consists of mountain ranges mostly lower than 2000 m (except for the Thiel

McMurdo Dry Valleys
The McMurdo Dry Valleys are a part of Transantarctic Mountains and include a large ice-free area (6700 km 2 ) and are one of the most extensively studied permafrost regions in Antarctica (Bockheim et al., 2007).The lowest MAGT among the dry valleys is modelled in the Victoria Valley falling below -24 °C at the lowest part (Fig. 9).A winter ground temperature 5 inversion is pronounced with MAGTs in the surrounding valleys modelled at around -21 °C.The MAGT also increases up the

Marie Byrd Land
The ice free areas in Marie Byrd Land occupy only 210 km 2 and consist mostly of rock outcrops at lower elevations close to 5 the coast and volcanos protruding through the ice sheet.In the Ford Ranges, MAGT was modelled as from -10 °C close to sea level to -16 °C at elevations exceeding 1000 m a.s.l (Fig. 11).MAGTs between -11 and -8 °C were modelled at the Hobbs, Walgreen, Eights and Ruppert Coasts, reaching -6 °C on the islands surrounded by open sea.The volcano-mountain ranges reaching 3000 m a.s.l.(Flood and Kohler Range), had MAGTs typically ranging between -16 to -14 °C at their peaks.Modelled MAGTs were between -25 and -21 °C on the Executive Committee Range, where rock outcrops occur above 2000 m a.s.l. and 10 the highest peaks extend to over 4000 m a.s.l.

Antarctic Peninsula 5
The ice-free areas of the Antarctic Peninsula cover 3 800 km 2 including the South Shetland islands where modelled MAGT was slightly below 0°C and the mountains of the south-eastern Antarctic Peninsula with modelled MAGT of around -19 °C.
The modelled near-surface permafrost temperatures in the Antarctic Peninsula were the warmest among all Antarctic regions with an average modelled MAGT of -7.3 °C (Fig. 12).

Palmer Land 5
The mountains of Palmer Land show considerable differences between the eastern and western parts of the peninsula.Modelled MAGTs at the mountains of the Orville Coast were around -17 °C, increasing to about -12 to -15 °C at the Black Coast and eventually rising above -10 °C at the Wilkins Coast.On the west side at the Fallieres Coast MAGTs of up to -4 °C were modelled.Temperatures decreased to -6 to -8 °C at the Rymill Coast, around -10 °C at 1000 m a.s.l., and approached -12°C at 1500 m a.s.l.. Similar MAGT ranges were modelled on Alexander Island where MAGTs close to the coast were around -5 °C, 10 decreasing to between -9 and -7 °C at 1000 m a.s.l. and falling below -10 °C at elevations above 2000 m a.s.l.

Graham Land
A considerable increase in modelled MAGT from the east to the west of Graham Land was observed, where the south part of the east coast is protected by the Larsen Ice Shelf.Ground temperatures gradually increased in the northward direction on both sides of the peninsula.MAGTs at the Bowman and Foyn Coasts ranged between -8 and -6 °C and slowly increased from -6 °C at the Oscar II Coast to around -4 °C at the northern-most part of the mainland although still falling below -8 °C at higher elevations.On the west side, MAGT gradually decreased from around -2 °C in the north Davis Coast to -5 °C at the south of the Danco Coast and Anvers Island, where MAGT also fell below -6°C at higher elevations.Similar MAGT ranges were observed at the Graham and Loubet Coasts and at Adelaide Island but were lower than -7 °C at higher elevations.Small Islands along the west coast of the Antarctic Peninsula had a modelled MAGT of between -3 and -1 °C.

South Shetland Islands and James Ross Island
Another frequently studied area in the Antarctic is the Northern Antarctic Peninsula, especially the South Shetland Islands.At sites close to sea level, the modelled MAGT was usually above -1 °C on the South Shetland Islands (Fig. 13).MAGT decreased below -2 °C on sites that are not adjacent to the coast but still low-lying such as Byers and Hurd Peninsulas on Livingston Island and Fildes Peninsula on King George Island.The MAGT was below -4 °C on the highest unglaciated peaks of Livingston and Smith Islands and reached -3 °C on Deception Island.
Modelled MAGTs were significantly colder on James Ross Island than on the South Shetland Islands, and, at sites adjacent to the sea, ranging from -3 °C in the north down to -5 °C in the south.Lower lying ice-free areas, including Seymour Island, had MAGTs typically between -6 and -5 °C.The MAGTs on the highest rock outcrops were modelled as down to -7 °C.

Other Antarctic and sub-Antarctic Islands
The modelling results were derived for the Antarctic Islands and sub-Antarctic Islands where the size and ice-free area of the 5 island is sufficient that MODIS LST data were available.MAGT on Signy Island ranged from -1.5 °C at the coast to -4 °C in the interior.Permafrost was modelled on all South Sandwich Islands with similar MAGT ranges to those on Signy Island (Fig. 14).Less permafrost is modelled on South Georgia Island, where MAGTs at the coast increase to 1 °C and sporadic permafrost starts to occur at elevations above 100 m.The MAGTs decreased below -2 °C at the highest-lying rock outcrops.No permafrost was modelled on the Crozet Islands.MAGTs below 0 °C were modelled at the highest elevations of Kerguelen Island and in 10 isolated permafrost patches occurring above 500 m a.s.l.Permafrost is present also on the southern-most ice-free area of Heard Island.

Regional MAGT distribution
The MAGT distribution of Antarctic is bimodal with the most pronounced peak at -21 °C and the second peak at -7 °C (Fig. 15).The peaks correspond to the two largest ice-free areas of the Transantarctic Mountains and the Antarctic Peninsula, however the temperatures around the -20 °C peak also occur in other regions such as Queen Maud Land, the Vestfold Hills and the Ellsworth Mountains.The most commonly modelled temperatures were between -23 and -18 °C and usually occurred 5 in the mountains rising above the Antarctic Ice-sheet and glaciers.MAGTs of between -10 and -6 °C occurred in the coastal areas of Wilkes Land, Marie Byrd Land, Queen Maud Land, Enderby Land and the Vestfold Hills, but the peak of temperature distribution at coastal sites shifted towards -7 °C because of MAGTs on the Antarctic Peninsula.The peak of coastal areas would be at -9 °C if the Antarctic Peninsula was excluded.The lapse rates are is indicating significant regional differences in the modelled MAGT patterns in relation to elevation (Fig. 16).The warmest among all regions was the Antarctic Peninsula which had MAGTs similar to Marie Byrd Land at around -12 °C between 1300 m a.s.l., which is again colder at higher elevations.Marie Byrd Land, Queen Maud Land, Enderby Land and Wilkes Land had MAGTs of -9 °C at the coast but show varying characteristics of temperature decrease with elevation.
Marie Byrd Land was the warmest with a decrease in MAGT up to 1500 m a.s.l., and a faster decrease to -25 °C at 3000 m a.s.l.The slower decrease in temperature with altitude to the 1500m elevation was also modelled in Enderby Land but the MAGT was colder.The MAGT decreased rapidly to -13 °C at 700 m a.s.l. in Queen Maud Land, where it slightly increased with elevation and then dropped steadily to -22 °C at 2800 m a.s.l.
The modelled MAGT in the Vestfold Hills dropped rapidly from -10 °C at the coast to -17 °C at 200 m a. s. l. and then gradually decreased to -24 °C at 2000 m a.s.l.There are no rock outcrops close to sea level in the Ellsworth Mountains and the MAGT at 100 m a.s.l. was -20 °C.In the Ellsworth Mountains there was a slow decrease of the modelled MAGT, to -25 °C at 3000 m a. s. l.The coldest MAGTs (below -30 °C) were modelled in the Transantarctic Mountains.In the part west of the 90 ° meridian, the MAGT dropped from -20 °C at 500 m a.s.l to -27 °C at 2000 m a.s.l.but the absolute temperatures were lower east of the 90 ° meridian where elevations exceed 4000 m a.s.l.According to the gradients for sub-regions of the Northern Antarctic Peninsula, the warmest were the South Shetland Islands followed by Palmer Archipelago (Fig. 17).The altitudinal MAGT profiles showed clear differences between the western and 5 eastern parts of the Northern Antarctic Peninsula mainland, although James Ross Island had a similar altitudinal profile to the West Antarctic Peninsula mainland.A significant decrease in MAGT from sea level to 100 m elevation was observed on the South Shetland Islands, Palmer Archipelago and the West Antarctic Peninsula mainland.

Queen Maud Land
The modelled MAGT is according to validation data, was over-estimated in Queen Maud Land.Over-estimation in Schirmacher Hills and Troll Station was below 1 °C but was higher than 4 °C in Flarjuven Bluff and Vesleskarvet.Both stations are located on nunataks, which are exposed to wind that blows away the snow.The estimated 120 mm of annual snowfall resulted in too low nf-factors for snow-free conditions.However, the minimum MAGT of the ensemble spread 10 approaches the measured MAGT at both stations (Appendix A).

Enderby Land, Vestfold Hills and Wilkes Land
The borehole data in Enderby Land were from the Molodejnaya station (Thala Hills), where modelled MAGT was overestimated by 1.6 °C.The over-estimation can be explained by rather thin snow cover due to strong winds and snow redistribution at the borehole site, which is confirmed by small differences between measured winter air and ground surface temperatures.The MAGT is accurately modelled in the coastal parts of the Vestfold Hills region, where validation data are available.The difference between modelled and measured MAGT is small at the Larsemann Hills borehole but slight MAGT over-estimation (between 0.6 and 0.9 °C) was observed in comparison to the Larsemann and Landing Nunatak borehole measurements.Similarly, the MAGT was accurately modelled at the Bunger Hills station in the Wilkes Land region, with only small differences in comparison to measured ground temperature.

Transantarctic Mountains
The °C too cold.The difference can be explained by the micro-location of the both sites, which are sheltered from katabatic winds, are above the winter inversion layer, and receive abundant summer solar radiation.The MAGT was over-estimated by up to 1 °C at Mt. Fleming, Scott Base and the Marble Point Borehole, which lie outside the valleys.The MAGT at the Marble Point site, characterised by glacial till, was over-estimated by 2.2 °C, however, the Marble Point Borehole, which was drilled in granite bedrock approx. 1 km away, showed smaller over-estimation.The MAGT was under-estimated by more than 4 °C at the Granite Harbour site which has a warm microclimate as it is situated on north-facing moraine that receives a lot of meltwater from up-slope.
Outside the McMurdo Dry Valleys, MAGT measurements from the Zucchelli Station and Baker Rocks are available.The MAGT is over-estimated by 2.3 °C at the Boulder clay borehole, which was drilled in a glacial till, exposed to katabatic winds, and characterised by numerous snow drifts (Guglielmin, 2006) which causes high local variability in permafrost conditions.
On the other hand, the MAGT was over-estimated by only 0.2 °C at the Oasi New borehole, which is located in a granitic outcrop.MAGT at the Baker Rocks site, which is situated in littoral deposits (Raffi and Stenni, 2011), was under-estimated by 0.9 °C.The modelled MAGT lapse rate of 0.40 °C 100 m -1 for the whole Antarctic is lower than the average air temperature lapse rate of 0.65 °C 100 m -1 in the International Standard Atmosphere (ISO 2533(ISO :1975) ) but is, however, the same as the mean air temperature lapse rate measured for ice-free sites on James Ross Island from 2013-2016(Ambrozova et al., 2019)).Small increases of MAGTs with elevation were observed in many regions.This might be attributed to the nature of the analysis rather than a presence of temperature inversions.The rock outcrops on the scale of the regions are often present at different elevations far from each other, which results in occurrence of higher MAGTs at higher elevations.Additionally, some elevation bins have only a few rock outcrop pixels, which makes the averaged MAGT less representative for the whole region.
The lowest MAGT of -33.5 °C was modelled in the Transantarctic Mountains at the Queen Elizabeth Range, where the highest peak reaches 4350 m.The MAGT there could fall below -36 °C according to the coldest ensemble member.This is the lowest MAGT modelled on Earth according to the modelling in the other parts of the globe (Obu et al., 2019).On the highest Antarctic peak Mount Vinson in the Ellsworth Mountains reaching 4892 m a MAGT of only -26.1 °C was modelled.The Queen Elizabeth Range lies approximately 5° further south than Mount Vinson, illustrating the effect of latitude on permafrost temperatures.
The winter air temperature inversions that occur in the McMurdo Dry Valleys result in air temperatures that are approx.10 °C lower in the valleys than in the surroundings, but inversions are occasionally disrupted by katabatic winds from the polar ice sheet (Nylen et al., 2004).The winter inversions reflect in MAGTs being approx.3 °C colder on the floor than in surroundings in Victoria Valley, where the inversions are particularly intense in comparison to other valleys.No MAGT inversions were modelled in the Wright Valley, which lies only 50 m a.s.l (though winter temperature inversions do occur there), or in the Taylor Valley, which is opened to the coast and can drain cold air.

Model performance and limitations
As the LST data are primarily derived from satellite data, their availability and accuracy depend on cloud cover.On one hand the frequent cloud cover might be a reason for general under-estimation of modelled MAGT at Antarctic Peninsula.On the The absence of vegetation in the Antarctic results in high snow redistribution by wind and highly spatially variable snow cover, which influences ground temperatures in many parts of Antarctic (Guglielmin et al., 2014;Ramos et al., 2017;Ferreira et al. 2017).Neither snow redistribution nor sublimation are simulated by our snowfall model and the average snow depths could not be estimated and nf-factors derived as for the Northern Hemisphere by Obu et al. (2019).Although an ensemble of 200 model runs with varying annual snowfall is used, the mean of the ensemble runs does not always represent the borehole site microclimate and ground properties.In the case of wind-exposed nunataks where the snow presence is over-estimated on larger areas, such as Flarjuven Bluff and Vesleskarvet, the MAGTs can be over-estimated up to 4 °C, but the modelled ensemble minimum still approaches the measured MAGT.
Several permafrost and active-layer studies in the Antarctic have noted occurrence of nf-factors above 1 (Lacelle et al., 2016;Kotzé and Meiklejohn, 2017), which indicates that average air temperatures are higher than ground surface temperatures.The likely explanation could be a presence of snow during the warmer part of the year, which is insulating ground from heat, unlike from cold in the winter.However Kotzé and Meiklejohn (2017) mention also a presence of blocky deposits at the Vesleskarvet site, which could result in ground cooling due to cold air advection.The concept of nf-factors was introduced for the Northern Hemisphere for snowcover effect during freezing conditions (Smith and Riseborough, 1996), which challenges the derivation of nf-factors for TTOP modelling on sites with highly temporarily variable snow cover.
Elevation is one of the major permafrost controlling factors in the Antarctic (Vieira et al., 2010).In steep terrain, the model input datasets are less likely to be representative for the micro-locations present within the modelled pixel or the borehole site as for example shown on the boreholes sites on the walls of the Wright Valley.A number of the Antarctic boreholes are situated in mountainous environments, which might explain some of the discrepancies between measured and modelled MAGT.
Elevation uncertainties in DEM are inherited by the model and reflected in the estimated average annual snowfall and downscaled ERA reanalysis temperatures.The elevations of some peaks might also not be well represented at the spatial resolution of 1 km, therefore the modelled MAGT might appear warmer than MAGT found on the top a peak.

Conclusions
Near-surface permafrost temperatures in the Antarctic were most commonly modelled as between -23 and -18 °C for mountainous areas rising above the Antarctic Ice Sheet.The Earth's lowest permafrost temperature of -33 °C was modelled at Mount Markham in Queen Elizabeth Range in the Transantarctic Mountains.Coastal regions were usually characterised with ground temperatures of between -14 and -8 °C, approaching 0 °C in the coastal areas of the Antarctic Peninsula and rising Comparison of modelled temperatures to 40 permafrost boreholes and soil climate stations yielded root mean square error of 1.9 °C but the accuracy varied significantly among borehole sites.The difference was smaller than 1 °C for more than 50 % of the sites, but can exceed 4 °C.The greatest differences between the modelled and measured permafrost temperatures occurred where snow conditions were not successfully represented in the model.These sites are generally exposed to a strong wind-driven redistribution of snow as for example at nunataks in Queen Maud Land, on the Hobs Coast, and in Marie Byrd Land.Considerable differences between modelled and measured MAGTs also occurred at sites with microclimate and ground properties that are not representative for the respective modelled 1 km 2 pixel.Permafrost temperatures on the walls of Wright Valley and in Granite Harbour were under-estimated by up to 4 °C, which can be explained by warm microclimates of the borehole sites compared to surroundings.The model performed well in areas with frequent cloud-free conditions such as the McMurdo Dry Valleys, where even winter air temperature inversions are reflected in the modelled permafrost temperatures.
Frequent cloudy conditions in the north-western Antarctic Peninsula can to some extent explain the systematic underestimation of modelled permafrost temperatures in this area.
This study is the first continent-wide modelling of permafrost temperatures for the Antarctic.It reports near-surface permafrost temperatures for remote regions without observations, which is highly valuable for research fields, such as climate change, terrestrial ecology, microbiology or astrobiology.Our study suggests that extended networks of currently sparse borehole temperature measurements and spatially distributed information on snow cover and ground properties are crucial for improving future permafrost modelling results in the Antarctic.

Figure 1 :
Figure 1: Overview map of the ice-free Antarctic regions and extents of the maps presented in the paper.

(
31 km) spatial resolution but was at the time of the study available only from 2008 onwards (Hersbach and Dee, 2016).ERA-https://doi.org/10.5194/tc-2019-148Preprint.Discussion started: 25 June 2019 c Author(s) 2019.CC BY 4.0 License.Interim data were for this reason used before 2008 and ERA-5 data afterwards.The reanalysis data were downscaled to the 1 km resolution of individual MODIS pixels using atmospheric lapse rates and The Global Multi-resolution Terrain Elevation compared our results to available in-situ measurements in 40 permafrost boreholes and shallow boreholes at soil-climate stations.The ensemble mean of modelled MAGTs was compared to the borehole measurements to take the simulated spatial variability, provided by the ensemble spread, into account.The accuracy of the model was estimated with root mean square error (RMSE) and mean absolute error between the modelled and measured MAGT.The validation data were provided by the authors for the McMurdo Dry Valleys, ice-free areas near Russian stations (Bunger Hills, Schirmacher Hills, Larsemann Hills, Thala Hills, King George Island and Hobs coast) and the Northern Antarctic Peninsula.Ground temperatures from these locations represent mean MAGT at the top of the permafrost and usually overlap well with the modelling period 2000-2017 (Appendix A).Validation data for Queen Maud Land (Troll Station, Flarjuven Bluff, Vesleskarvet) and the Baker Rocks site were obtained from Hrbáček et al., (2018) and MAGTs from Terra Nova Bay (Oasi New and Boulder Clay) were obtained from Vieira et al., (2010).MAGTs from Hope Bay, Mt.Dolence, Marble Point Borehole and Limnopolar Lake were obtained from Schaefer et al. (2017a), Schaefer et al. (2017b), Guglielmin et al. (2011) and de Pablo et al. (2014), respectively.The borehole data from Signy Island and Rothera Point were extracted from Guglielmin et al., ( the boreholes.Assuming a Gaussian distribution of standard deviation MAGT, 68% of borehole comparisons should fall within one MAGT, while 95% within two MAGT, and 99% should be within three MAGT.For the comparison with https://doi.org/10.5194/tc-2019-148Preprint.Discussion started: 25 June 2019 c Author(s) 2019.CC BY 4.0 License.

Figure 2 :
Figure 2: Measured vs. modelled permafrost temperatures for all boreholes.The dashed lines represent ±2 °C intervals around the 1:1 solid line.

Figure 3 :
Figure 3: Permafrost temperature maps of Queen Maud Land and differences between borehole and modelled MAGT.See Fig. 1 for location of panels (a) and (b).

Figure 4 :
Figure 4: Permafrost temperature maps of Enderby Land and differences between borehole and modelled MAGT.See Fig. 1 for location of panels (a) and (b).

Figure 5 :
Figure 5: Permafrost temperature map of the Vestfold Hills and differences between borehole and modelled MAGT.

Figure 6 :
Figure 6: Permafrost temperature maps of Wilkes Land and differences between borehole and modelled MAGT.See Fig. 1 for location.
Mountains) that don't extend to the sea level.The highest MAGT of -17.0 °C was modelled at the foot of the Shackleton Range and the Pensacola Mountains.The MAGTs decreased down to -29 °C in the high mountains.The lowest MAGT of -29.8 °C was modelled in the Thiel Mountains.https://doi.org/10.5194/tc-2019-148Preprint.Discussion started: 25 June 2019 c Author(s) 2019.CC BY 4.0 License.

Figure 7 :
Figure 7: Permafrost temperature map of the Transantarctic Mountains west of the 90° meridian.See Fig. 1 for location.

Figure 8 :
Figure 8: Permafrost temperature maps of the Transantarctic Mountains east of the 90° meridian and differences between borehole and modelled MAGT.See Fig. 1 for location of panels (a), (b) and (c).
valleys to -21 °C in the McKelvey, Balham and Barwick Valleys.No MAGT inversion was modelled in the Wright Valley, where MAGTs ranged between -21 and -19 °C and in the Taylor Valley, which is the warmest with MAGT of -17 °C in the lower-lying parts, and -20 °C in the upper part, of the valley.At the Olympus and Asgard Ranges and the Kukri Hills, which surround the valleys, MAGTs of between -23 and -20°C were modelled.In the surrounding mountains, close to the ice sheet, 10 modelled MAGTs were around -25 °C and reached -27 °C at the highest and the most east-lying mountains.MAGTs at the coast of McMurdo Sound ranged from -13 to -17 °C.On Ross Island, the MAGT was modelled to -16 °C at the Scott and McMurdo Bases which are near sea level and down to -24 °C on Mount Erebus.https://doi.org/10.5194/tc-2019-148Preprint.Discussion started: 25 June 2019 c Author(s) 2019.CC BY 4.0 License.

Figure 10 :
Figure 10: Permafrost temperature map of the Ellsworth Mountains and differences between borehole and modelled MAGT.See Fig. 1 for location.

Figure 11 :
Figure 11: Permafrost temperature maps of Marie Byrd Land and differences between borehole and modelled MAGT.See Fig. 1 for location of panels (a) and (b).

Figure 12 :
Figure 12: Permafrost temperature maps of the Antarctic Peninsula and differences between borehole and modelled MAGT.See Fig. 1 for location of panels (a) and (b).

Figure 13 :
Figure 13: Permafrost temperature map of the Northern Antarctic Peninsula and differences between borehole and modelled MAGT.See Fig. 1 for location.Note: the MAGT colour ramp is different from other figures.1: Hurd Peninsula; 2: Fildes Peninsula.

Figure 14 :
Figure 14: Permafrost temperature maps of Antarctic Islands and difference between borehole and modelled MAGT.See Fig. 1 for location of panels (a), (b), (c), (d) and (e).Note: the MAGT colour ramp is different from other figures. 10

Figure 15 :
Figure 15: Histogram of temperature distribution across the Antarctic for 1 °C bins.Note: The ice-free areas were often smaller than the analysed pixel size.

Figure 16 :
Figure 16: Altitudinal MAGT gradients for Antarctic Soil Regions calculated for 100 m elevation bins.

Figure 17 :
Figure 17: Altitudinal MAGT gradients for the Northern Antarctic Peninsula calculated for 100 m elevation bins.
majority of the validation data in the Transantarctic Mountains region is available in the vicinity of the McMurdo Dry Valleys.The sites on the floors of the McMurdo Dry Valleys (Victoria Valley, Wright Valley Floor and Bull Pass) were modelled well with slight over-estimation in Victoria Valley and under-estimation of around 1 °C in the Wright Valley.Two observation sites on small terraces on the walls of the Wright Valley (WV south wall and WV north wall) were modelled 3-4 https://doi.org/10.5194/tc-2019-148Preprint.Discussion started: 25 June 2019 c Author(s) 2019.CC BY 4.0 License.elevation, which was observed especially in the Vestfold Hills, the Transantarctic Mountains east of 90° meridian and in Marie Byrd Land where the MAGT dropped significantly with the first elevation increase of 100 or 200 m.The same phenomenon was observed in the North Antarctic Peninsula on the South Shetland Islands, Palmer Archipelago and on the West Antarctic Peninsula mainland where there is less sea ice presence in comparison to James Ross Island and the West Antarctic Peninsula mainland where this MAGT decrease was not observed.The continental mountainous regions of the Ellsworth Mountains and the Transantarctic Mountains west of the 90° meridian have, on the other hand, no open sea in the vicinity and had a significantly slower MAGT drop with the altitude.
https://doi.org/10.5194/tc-2019-148Preprint.Discussion started: 25 June 2019 c Author(s) 2019.CC BY 4.0 License.other hand the clear sky conditions in the McMurdo Dry Valleys might explain the relatively successful modelling results in this area.