Melting over the East Antarctic Peninsula (1999–2009): evaluation of a high-resolution regional climate model (original) (raw)
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The Cryosphere, 2018
Surface melting over the Antarctic Peninsula (AP) may impact the stability of ice shelves and thus the rate at which grounded ice is discharged into the ocean. Energy and mass balance models are needed to understand how climatic change and atmospheric circulation variability drive current and future melting. In this study, we evaluate the regional climate model MAR over the AP at a 10 km spatial resolution between 1999 and 2009, a period when active microwave data from the QuikSCAT mission is available. This model has been validated extensively over Greenland, has is applied here to the AP at a high resolution and for a relatively long time period (full outputs are available to 2014). We find that melting in the northeastern AP, the focus area of this study, can be initiated both by sporadic westerly föhn flow over the AP mountains and by northerly winds advecting warm air from lower latitudes. A comparison of MAR with satellite and automatic weather station (AWS) data reveals that satellite estimates show greater melt frequency, a larger melt extent, and a quicker expansion to peak melt extent than MAR in the centre and east of the Larsen C ice shelf. These differences are reduced in the north and west of the ice shelf, where the comparison with satellite data suggests that MAR is accurately capturing melt produced by warm westerly winds. MAR shows an overall warm bias and a cool bias at temperatures above 0 • C as well as fewer warm, strong westerly winds than reported by AWS stations located on the eastern edge of the Larsen C ice shelf, suggesting that the underestimation of melt in this region may be the product of limited eastward flow. At higher resolutions (5 km), MAR shows a further increase in wind biases and a decrease in meltwater production. We conclude that non-hydrostatic models at spatial resolutions better than 5 km are needed to better-resolve the effects of föhn winds on the eastern edges of the Larsen C ice shelf.
2021
Atmospheric drivers of surface melting were implicated in the collapse of the Larsen A and B ice shelves that previously neighbored Larsen C-the largest remaining ice shelf on the eastern side of the Antarctic Peninsula and which extends north of the Antarctic circle-by increasing firn densification, meltwater ponding, and ultimately hydrofracturing and disintegration (Bell et al., 2018; Scambos et al., 2000). In particular, the large-scale circumpolar westerly circulation is known to have an important role in the Antarctic Peninsula region by influencing local atmospheric conditions via its effect on foehn winds. Foehn winds cause leeside warming and associated melting over these ice shelves (
2018
Surface melting over the Antarctic Peninsula (AP) may impact the stability of ice shelves and therefore the rate at which grounded ice is discharged into the ocean. Energy and mass balance models are needed to understand how climatic change and atmospheric circulation variability drive current and future melting. In this study, we evaluate the regional climate model MAR over the AP at a 10 km spatial resolution between 1999 and 2009, a period when active microwave data from the QuikSCAT mission is available. This model has been validated extensively over Greenland, has is applied here to the AP at a high resolution and for a relatively long time period (full outputs are available to 2014). We find that melting in the northeastern AP, the focus area of this study, can be initiated both by sporadic westerly föhn flow over the AP mountains and by northerly winds advecting warm air from lower latitudes. A comparison of MAR with satellite and automatic weather station (AWS) data reveals ...
Atmospheric drivers of surface melting on the Larsen C ice shelf, Antarctic Peninsula
2020
Observational data and high resolution (<4 km grid spacing) Met Office Unified Model (MetUM) output is used to investigate the dominant causes of surface melting on the Larsen C ice shelf. In the first two parts of the thesis, a case study approach is used to examine the role of wintertime foehn winds and summertime cloud phase on the surface energy balance (SEB) of Larsen C, and therefore surface melting. Firstly, wintertime foehn events are shown for the first time to drive significant and unseasonal surface melting by greatly enhancing surface sensible heat fluxes. Secondly, it is demonstrated that cloud phase, and particularly liquid water content, strongly influences the SEB and surface melting. More accurate model representations of cloud phase are shown to reduce biases in SEB terms and melt. As part of this work, an optimised MetUM configuration is developed for the Antarctic Peninsula. Thirdly, the final part of the thesis presents and analyses a novel, multi-decadal (19...
Journal of Climate
We calculate a regional surface “melt potential” index (MPI) over Antarctic ice shelves that describes the frequency (MPI-freq; %) and intensity (MPI-int; K) of daily maximum summer temperatures exceeding a melt threshold of 273.15 K. This is used to determine which ice shelves are vulnerable to melt-induced hydrofracture and is calculated using near-surface temperature output for each summer from 1979/80 to 2018/19 from two high-resolution regional atmospheric model hindcasts (using the MetUM and HIRHAM5). MPI is highest for Antarctic Peninsula ice shelves (MPI-freq 23%–35%, MPI-int 1.2–2.1 K), lowest (2%–3%, <0 K) for the Ronne–Filchner and Ross ice shelves, and around 10%–24% and 0.6–1.7 K for the other West and East Antarctic ice shelves. Hotspots of MPI are apparent over many ice shelves, and they also show a decreasing trend in MPI-freq. The regional circulation patterns associated with high MPI values over West and East Antarctic ice shelves are remarkably consistent for t...
The study of the recent variability and the future projections of the poles' climate currently relies on polar-oriented Regional Climate Models (RCMs). However, RCMs are subject to biases and systematic errors that impact the results of their simulations. Remote Sensing (RS) data can help to reduce these ambiguities by providing indirect observations to the modeled estimates. Using the behavior of radiofrequency signals with regard to the presence of water in a snowpack, passive and active microwave instruments such as AMSR2, ASCAT, and Sentinel-1 are used to detect melt at the surface of the snowpack. In this paper, we investigate the sensitivity of the RCM "Modèle Atmosphérique Régional" (MAR) to the assimilation of surface melt occurrence estimated by RS datasets. The assimilation is performed by nudging the MAR snowpack temperature to match the observed melt state by satellite. The sensitivity is tested by modifying parameters of the assimilation: (i) the depth to which MAR snowpack is warmed up or cooled down (corresponding to the penetration depth of the satellites) to match with satellite, and (ii) the quantity of water required into the snowpack to qualify a MAR pixel as melting or not, and (iii) by assimilating multiple RS datasets. The data assimilation is performed over the Antarctic Peninsula for the 2019-2021 period. The results show an increase in the melt production (+66.7 % on average, or +95 Gt) going along with a small decrease in surface mass balance (SMB) (-4.5 % on average, or-20 Gt) for the 2019-2020 melt season. The model is sensitive to the three parameters tested but with different orders of magnitude. The sensitivity to the assimilated dataset is reduced by using multiple datasets during the assimilation and discarding the remote observations that are not coherent. For the other two parameters, the penetration depth has more impact on the assimilation than the quantity of liquid water used as melt threshold. The first one is especially sensitive for the sensors with a shorter penetration depth. In the first centimeters, a densification due to a refreeze can impact the melt production and cause an overestimation of the melt production. For the second threshold, the impact is more important on the number of melt days rather than the melt production itself. The values tested for the quantity of liquid water required into the snowpack to qualify a MAR pixel as melting or not (0.1 or 0.2 % of the snowpack mass being water) are lower than during typical melt days (∼1.2 %) and impact results mainly at the beginning and end of the melt period when lower 1
Journal of Geophysical Research F: Earth Surface, 2013
Multidecadal meteorological station records and microwave backscatter time-series from the SeaWinds scatterometer onboard QuikSCAT (QSCAT) were used to calculate temporal and spatial trends in surface melting conditions on the Antarctic Peninsula (AP). Four of six long-term station records showed strongly positive and statistically significant trends in duration of melting conditions, including a 95% increase in the average annual positive degree day sum (PDD) at Faraday/Vernadsky, since 1948. A validated, threshold-based melt detection method was employed to derive detailed melt season onset, extent, and duration climatologies on the AP from enhanced resolution QSCAT data during 1999-2009. Austral summer melt on the AP was linked to regional-and synoptic-scale atmospheric variability by respectively correlating melt season onset and extent with November near-surface air temperatures and the October-January averaged index of the Southern Hemisphere Annular Mode (SAM). The spatial pattern, magnitude, and interannual variability of AP melt from observations was closely reproduced by simulations of the regional model RACMO2. Local discrepancies between observations and model simulations were likely a result of the QSCAT response to, and RACMO2 treatment of, ponded surface water, and the relatively crude representation of coastal climate in the 27 km RACMO2 grid.
Dynamics of surface melting over Amery and Ross ice shelf in Antarctic using OSCAT data
ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, 2014
Antarctic sea ice sheets play an important role in modulating the climate system. The present study investigates the dynamics of melt/freeze over Amery and Ross ice shelf located in Eastern and Southern part of continent using OSCAT, the microwave scatterometer data from OCEANSAT2. The study utilizes the sensitivity of backscatter coefficient values of scatterometer data to presence of liquid water in the snow caused due to melt conditions. The analysis carried out for four austral winters from 2010-2013 and five austral summer from 2009-2014 showed spatial and temporal variations in average backscatter coefficient over Amery and Ross shelf areas. A dynamic threshold based on the austral winter mean and standard deviation of HH polarization is considered for pixel by pixel analysis for the shelf area. There is significant spatio-temporal variability in melt extent, duration and melt index as observed in the analysis. Spatially, the melt over Amery shelf moves from South to North along coast and West towards inner shelf area. Maximum mean melt occurs on 9 th January with January1-15 fortnight accounting for 80% of the melt. Extreme low melt conditions were observed during summer 2010-11 and 2011-12 indicating cold summer. Summer 2012-13 and 2013-14 were warm summer. Year 2014 experienced melt only in the month of January with entire shelf under melt conditions. Practically no melt was observed over Ross ice shelf.
Downslope föhn winds over the Antarctic Peninsula and their effect on the Larsen ice shelves
Atmospheric Chemistry and Physics, 2014
Mesoscale model simulations are presented of a westerly föhn event over the Antarctic Peninsula mountain ridge and onto the Larsen C ice shelf, just south of the recently collapsed Larsen B ice shelf. Aircraft observations showed the presence of föhn jets descending near the ice shelf surface with maximum wind speeds at 250-350 m in height. Surface flux measurements suggested that melting was occurring. Simulated profiles of wind speed, temperature and wind direction were very similar to the observations. However, the good match only occurred at a model time corresponding to ∼ 9 h before the aircraft observations were made since the model föhn jets died down after this. This was despite the fact that the model was nudged towards analysis for heights greater than ∼1.15 km above the surface. Timing issues aside, the otherwise good comparison between the model and observations gave confidence that the model flow structure was similar to that in reality. Details of the model jet structure are explored and discussed and are found to have ramifications for the placement of automatic weather station (AWS) stations on the ice shelf in order to detect föhn flow. Cross sections of the flow are also examined and were found to compare well to the aircraft measurements. Gravity wave breaking above the mountain crest likely created a situation similar to hydraulic flow and allowed föhn flow and ice shelf surface warming to occur despite strong upwind blocking, which in previous studies of this region has generally not been considered. Our results therefore suggest that reduced upwind blocking, due to wind speed increases or stability decreases, might not result in an increased likelihood of föhn events over the Antarctic Peninsula, as previously suggested. The surface energy budget of the model during the melting periods showed that the net downwelling shortwave surface flux was the largest contributor to the melting energy, indicating that the cloud clearing effect of föhn events is likely to be the most important factor for increased melting relative to non-föhn days. The results also indicate that the warmth of the föhn jets through sensible heat flux ("SH") may not be critical in causing melting beyond boundary layer stabilisation effects (which may help to prevent cloud cover and suppress loss of heat by convection) and are actually cancelled by latent heat flux ("LH") effects (snow ablation). It was found that ground heat flux ("GRD") was likely to be an important factor when considering the changing surface energy budget for the southern regions of the ice shelf as the climate warms.
Journal of Geophysical Research: Atmospheres, 2015
We compare measurements of the turbulent and radiative surface energy fluxes from an automatic weather station (AWS) on Larsen C Ice Shelf, Antarctica with corresponding fluxes from three high-resolution atmospheric models over a 1 month period during austral summer. All three models produce a reasonable simulation of the (relatively small) turbulent energy fluxes at the AWS site. However, biases in the modeled radiative fluxes, which dominate the surface energy budget, are significant. There is a significant positive bias in net shortwave radiation in all three models, together with a corresponding negative bias in net longwave radiation. In two of the models, the longwave bias only partially offsets the positive shortwave bias, leading to an excessive amount of energy available for heating and melting the surface, while, in the third, the negative longwave bias exceeds the positive shortwave bias, leading to a deficiency in calculated surface melt. Biases in shortwave and longwave radiation are anticorrelated, suggesting that they both result from the models simulating too little cloud (or clouds that are too optically thin). We conclude that, while these models may be able to provide some useful information on surface energy fluxes, absolute values of modeled melt rate are significantly biased and should be used with caution. Efforts to improve model simulation of melt should initially focus on the radiative fluxes and, in particular, on the simulation of the clouds that control these fluxes.