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See detailBrief communication: Evaluation of the near-surface climate in ERA5 over the Greenland Ice Sheet
Delhasse, Alison ULiege; Kittel, Christoph ULiege; Amory, Charles ULiege et al

in The Cryosphere (2020), 14

The ERA5 reanalysis, recently made available by the European Centre for Medium-Range Weather Forecasts (ECMWF), is a new reanalysis product at a high resolution replacing ERA-Interim and is considered to ... [more ▼]

The ERA5 reanalysis, recently made available by the European Centre for Medium-Range Weather Forecasts (ECMWF), is a new reanalysis product at a high resolution replacing ERA-Interim and is considered to provide the best climate reanalysis over Greenland to date. However, so far little is known about the performance of ERA5 over the Greenland Ice Sheet (GrIS). In this study, we compare the near-surface climate from the new ERA5 reanalysis to ERAInterim, the Arctic System Reanalysis (ASR) as well as to a state-of-the-art polar regional climate model (MAR). The results show (1) that ERA5 does not outperform ERA-Interim significantly when compared with near-surface climate observations over GrIS, but ASR better models the near-surface temperature than both ERA reanalyses. (2) Polar regional climate models (e.g., MAR) are still a useful tool to downscale the GrIS climate compared to ERA5, as in particular the near-surface temperature in summer has a key role for representing snow and ice processes such as the surface melt. However, assimilating satellite data and using a more recent radiative scheme enable both ERA and ASR reanalyses to represent more satisfactorily than MAR the downward solar and infrared fluxes. (3) MAR near-surface climate is not affected when forced at its lateral boundaries by either ERA5 or ERA-Interim. Therefore, forcing polar regional climate models with ERA5 starting from 1950 will enable long and homogeneous surface mass balance reconstructions. [less ▲]

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See detailEvaluation of summer Greenland blocking index of the CMIP6 models
Delhasse, Alison ULiege

Poster (2019, December 10)

The Greenland blocking index (GBI), an indicator of the synoptic-scale circulation over Greenland, has been anomalously positive during summer since the end of the 1990s. The GBI is negatively correlated ... [more ▼]

The Greenland blocking index (GBI), an indicator of the synoptic-scale circulation over Greenland, has been anomalously positive during summer since the end of the 1990s. The GBI is negatively correlated with the North Atlantic Oscillation (NAO), which has been consistently negative during the same period. These changes in atmospheric circulation have led to an increase in Greenland summer temperatures, a decrease in cloud cover and greater ice sheet surface melt. The GBI is therefore a key indicator of melting in Greenland. However, the fifth phase of the Coupled Model Intercomparison Project (CMIP5) models do not represent any increase in GBI as indicated by observations. Until 2100, no oscillation of the GBI, and therefore no circulation changes, are projected. In this study we evaluate the new generation CMIP6 models in order to analyze the evolution of the future GBI. Our initial analysis reveals a general trend towards a decrease of the GBI until 2100. Additionally, still no model reproduces the strong increase in GBI observed over the last few decades, similar to the CMIP5 models. It is therefore important to note that significant melting events related to a highly positive GBI, as observed this summer 2019 for example, are still not considered by CMIP6 models and therefore the projected mass balance decrease could be underestimated if such general circulation change will occur in future. [less ▲]

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See detailEvaluation of the near-surface climate of the Greenland ice sheet as modelled by the climate model MAR and the ERA-Interim, ERA5 and Arctic System reanalyses
Delhasse, Alison ULiege; Kittel, Christoph ULiege; Amory, Charles ULiege et al

Conference (2019, April 12)

The new reanalysis produced by the ECMWF, ERA5, is currently available over the period 2000-2017. Ultimately, it will cover the period 1950 to the present-time and will replace the ERA-Interim, which is ... [more ▼]

The new reanalysis produced by the ECMWF, ERA5, is currently available over the period 2000-2017. Ultimately, it will cover the period 1950 to the present-time and will replace the ERA-Interim, which is by many considered as one of the best reanalyses over the Greenland ice sheet (GrIS). We first compare the ERA5 reanalysis to ERA-Interim and ASR (Arctic System Reanalysis), which is a regional reanalysis specifically developed for the Arctic area at a finer resolution. We evaluate them against a set of near-surface climate observations from the AWS of the PROMICE network covering the GrIS. This observation data set is not assimilated in these reanalyses. We furthermore assess the ability of the state-of-the-art regional climate model (RCM) MAR, forced by the ECMWF reanalyses, ERA-Interim and ERA5, to represent the AWS observations. Finally, we demonstrate the advantages of using MAR compared to the forcing reanalyses alone. ERA5 improves ERA-Interim almost for radiative fluxes, but not significantly. ASR, which is more specific for Arctic region and has a finer spatial resolution, outclasses other reanalyses for wind speed and near-surface temperature. The comparison of results from MAR simulations forced by ERA-Interim and ERA5 reanalysis shows that the near-surface climate variables are closed to each other and then not significantly different according to the forcing used. ERA5 which should replace ERA-Interim after 2018, can be used to force a RCM such as MAR in the same way than ERA-Interim now. Although the reanalyses seem to be sufficient to study the surface climate of Greenland, the RCM MAR has the best representation of the near-surface temperature even without any data assimilation. This is mainly due to a better representation of the snowpack and interactions between surface and atmosphere by MAR, resulting in a better representation of the surface melt and the GrIS surface mass balance (SMB). Near-surface temperature and SMB are both very useful for constraining glacial dynamics models in order to represent the current and future evolution of the ice dynamics of the GrIS. [less ▲]

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See detailSensitivity of the current Antarctic surface mass balance to sea surface conditions using MAR
Kittel, Christoph ULiege; Amory, Charles ULiege; Agosta, Cécile ULiege et al

in Cryosphere (2018), 12

Estimates for the recent period and projections of the Antarctic surface mass balance (SMB) often rely on high-resolution polar-oriented regional climate models (RCMs). However, RCMs require large-scale ... [more ▼]

Estimates for the recent period and projections of the Antarctic surface mass balance (SMB) often rely on high-resolution polar-oriented regional climate models (RCMs). However, RCMs require large-scale boundary forcing fields prescribed by reanalyses or general circulation models (GCMs). Since the recent variability of sea surface conditions (SSCs, namely sea ice concentration, SIC, and sea surface temperature, SST) over the Southern Ocean is not reproduced by most GCMs from the 5th phase of the Coupled Model Intercomparison Project (CMIP5), RCMs are then subject to potential biases. We investigate here the direct sensitivity of the Antarctic SMB to SSC perturbations around the Antarctic. With the RCM “Modèle Atmosphérique Régional” (MAR), different sensitivity experiments are performed over 1979–2015 by modifying the ERA-Interim SSCs with (i) homogeneous perturbations and (ii) mean anomalies estimated from all CMIP5 models and two extreme ones, while atmospheric lateral boundary conditions remained unchanged. Results show increased (decreased) precipitation due to perturbations inducing warmer, i.e. higher SST and lower SIC (colder, i.e. lower SST and higher SIC), SSCs than ERA-Interim, significantly affecting the SMB of coastal areas, as precipitation is mainly related to cyclones that do not penetrate far into the continent. At the continental scale, significant SMB anomalies (i.e greater than the interannual variability) are found for the largest combined SST/SIC perturbations. This is notably due to moisture anomalies above the ocean, reaching sufficiently high atmospheric levels to influence accumulation rates further inland. Sensitivity experiments with warmer SSCs based on the CMIP5 biases reveal integrated SMB anomalies (+5 % to +13 %) over the present climate (1979–2015) in the lower range of the SMB increase projected for the end of the 21st century. [less ▲]

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See detailBrief communication: Impact of the recent atmospheric circulation change in summer on the future surface mass balance of the Greenland Ice Sheet
Delhasse, Alison ULiege; Fettweis, Xavier ULiege; Kittel, Christoph ULiege et al

in Cryosphere (2018)

Since the 2000s, a change in the atmospheric circulation over the North Atlantic resulting in more frequent blocking events has favoured warmer and sunnier weather conditions over the Greenland Ice Sheet ... [more ▼]

Since the 2000s, a change in the atmospheric circulation over the North Atlantic resulting in more frequent blocking events has favoured warmer and sunnier weather conditions over the Greenland Ice Sheet (GrIS) in summer, enhancing the melt increase. This circulation change is not represented by general circulation models (GCMs) of the Coupled Model Intercomparison Project Phase 5 (CMIP5), which do not predict any circulation change for the next century over the North Atlantic. The goal of this study is to evaluate the impact of an atmospheric circulation change (as currently observed) on projections of the future GrIS surface mass balance (SMB). We compare GrIS SMB estimates simulated by the regional climate model MAR forced by perturbed reanalysis (ERA-Interim with a temperature correction of +1, +1.5, and +2°C at the MAR lateral boundaries) over 1980–2016 to projections of the future GrIS SMB from MAR simulations forced by three GCMs over selected periods for which a similar temperature increase of +1, +1.5, and +2°C is projected by the GCMs in comparison to 1980–1999. Mean SMB anomalies produced with perturbed reanalysis over the climatologically stable period 1980–1999 are similar to those produced with MAR forced by GCMs over future periods characterised by a similar warming over Greenland. However, over the 2 last decades (2000–2016) when an increase in the frequency of blocking events has been observed in summer, MAR forced by perturbed reanalysis suggests that the SMB decrease could be amplified by a factor of 2 if such atmospheric conditions persist compared to projections forced by GCMs for the same temperature increase but without any circulation change. [less ▲]

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See detailInterest of a Regional Climate Model for doing future projections over the Greenland Ice Sheet
Fettweis, Xavier ULiege; Delhasse, Alison ULiege; Agosta, Cécile ULiege et al

Conference (2018, June 22)

With the aim of evaluating the added value of a regional climate model in downscaled future projections over the Greenland Ice Sheet, we have compared the surface fields (snowfall and summer near-surface ... [more ▼]

With the aim of evaluating the added value of a regional climate model in downscaled future projections over the Greenland Ice Sheet, we have compared the surface fields (snowfall and summer near-surface temperature) coming from the “best” CMIP5 and CMIP6 global models (GCMs) with these fields simulated by the MAR model forced by the same GCMs. These "best" GCMS were selected according to their ability to simulate the summer temperature at 700 hPa and the general circulation at 500 hPa over Greenland with respect to ERA-Interim over 1980-1999. However, despite their ability to correctly represent the free atmosphere, the selected GCMs present significant biases at the surface of the ice sheet. The comparison shows that MAR is however able to strongly reduce these GCM surface biases. We then forced the lateral boundaries of MAR with ERA-Interim to which we applied temperature corrections of +1°C and +2°C. The outputs were compared to MAR forced by GCM future projections corresponding to a climate about 1 and 2°C warmer than the current climate. The results of the different GCM-forced runs and sensitivity experiments are very similar to each other as the GCMs do not project general circulation changes. Moreover, the sensitivity experiments forced by modified ERA-Interim reveal that the projected SMB decrease is exponentially amplified if the increased occurrence of blocking events over Greenland in summer that has been observed since the 2000´s continues in the future. [less ▲]

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See detailInfluence of the recent circulation change in summer on future surface mass balance of Greenland ice sheet
Delhasse, Alison ULiege; Fettweis, Xavier ULiege; Kittel, Christoph ULiege et al

Conference (2018, April 11)

Regional Climate Models (RCM) driven by General Circulation Models (GCM) are often used to produce future projections of the surface climate and surface mass balance (SMB) of polar ice sheets. However ... [more ▼]

Regional Climate Models (RCM) driven by General Circulation Models (GCM) are often used to produce future projections of the surface climate and surface mass balance (SMB) of polar ice sheets. However, GCM do not represent the recent circulation change observed in summer over the Greenland Ice Sheet (GrIS) since the 2000’s and do not predict any circulation changes for the next century. The goal of this study is to evaluate the impact of an atmospheric circulation change (as currently observed) combined with a temperature increase on the future GrIS SMB. We compare here SMB results from the RCM MAR (Modèle atmosphérique régional) forced by warmer reanalyses (ERA-Interim with a temperature correction of +1, +1,5 and +2 C at the lateral boundaries) to SMB results from MAR future simulations forced with GCM during a period where there is a temperature increase of +1, +1,5 and +2 C compared to 1980-1999. Mean SMB produced with warmer reanalyses over 1980-1999 is similar to that obtained when forcing with GCM over a period characterized by a similarly warmer climate. During last years (2000-2016) when a circulation change has been observed in summer, MAR forced with warmer reanalyses shows a significant amplified SMB decrease compared to future simulations forced by GCM for the same temperature increase. [less ▲]

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See detailInterests of using the RCM MAR to downscale CMIP6 outputs
Amory, Charles ULiege; Kittel, Christoph ULiege; Delhasse, Alison ULiege et al

Conference (2017, December 10)

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