An overview of the Community Earth System Model Version 2 (CESM2) is provided, including a discussion of the challenges encountered during its development and how they were addressed. In addition, an evaluation of a pair of CESM2 long preindustrial control and historical ensemble simulations is presented. These simulations were performed using the nominal 1° horizontal resolution configuration of the coupled model with both the “low-top” (40 km, with limited chemistry) and “high-top” (130 km, with comprehensive chemistry) versions of the atmospheric component. CESM2 contains many substantial science and infrastructure improvements and new capabilities since its previous major release, CESM1, resulting in improved historical simulations in comparison to CESM1 and available observations. These include major reductions in low-latitude precipitation and shortwave cloud forcing biases; better representation of the Madden-Julian Oscillation; better El Nino-Southern Oscillation-related teleconnections; and a global land carbon accumulation trend that agrees well with observationally based estimates. Most tropospheric and surface features of the low- and high-top simulations are very similar to each other, so these improvements are present in both configurations. CESM2 has an equilibrium climate sensitivity of 5.1–5.3 °C, larger than in CESM1, primarily due to a combination of relatively small changes to cloud microphysics and boundary layer parameters. In contrast, CESM2's transient climate response of 1.9–2.0 °C is comparable to that of CESM1. The model outputs from these and many other simulations are available to the research community, and they represent CESM2's contributions to the Coupled Model Intercomparison Project Phase 6.

The Community Earth System Model Version 2 (CESM2)

The Antarctic Ice Sheet is an important indicator of climate change and driver of sea-level rise. Here we combine satellite observations of its changing volume, flow and gravitational attraction with modelling of its surface mass balance to show that it lost 2,720 ± 1,390 billion tonnes of ice between 1992 and 2017, which corresponds to an increase in mean sea level of 7.6 ± 3.9 millimetres (errors are one standard deviation). Over this period, ocean-driven melting has caused rates of ice loss from West Antarctica to increase from 53 ± 29 billion to 159 ± 26 billion tonnes per year; ice-shelf collapse has increased the rate of ice loss from the Antarctic Peninsula from 7 ± 13 billion to 33 ± 16 billion tonnes per year. We find large variations in and among model estimates of surface mass balance and glacial isostatic adjustment for East Antarctica, with its average rate of mass gain over the period 1992–2017 (5 ± 46 billion tonnes per year) being the least certain.

/pdf/mass-balance-of-the-antarctic-ice-sheet-from-1992-to-2017-49s3lh63iz.pdf

Mass balance of the Antarctic ice sheet from 1992 to 2017.

We use updated drainage inventory, ice thickness, and ice velocity data to calculate the grounding line ice discharge of 176 basins draining the Antarctic Ice Sheet from 1979 to 2017. We compare the results with a surface mass balance model to deduce the ice sheet mass balance. The total mass loss increased from 40 ± 9 Gt/y in 1979–1990 to 50 ± 14 Gt/y in 1989–2000, 166 ± 18 Gt/y in 1999–2009, and 252 ± 26 Gt/y in 2009–2017. In 2009–2017, the mass loss was dominated by the Amundsen/Bellingshausen Sea sectors, in West Antarctica (159 ± 8 Gt/y), Wilkes Land, in East Antarctica (51 ± 13 Gt/y), and West and Northeast Peninsula (42 ± 5 Gt/y). The contribution to sea-level rise from Antarctica averaged 3.6 ± 0.5 mm per decade with a cumulative 14.0 ± 2.0 mm since 1979, including 6.9 ± 0.6 mm from West Antarctica, 4.4 ± 0.9 mm from East Antarctica, and 2.5 ± 0.4 mm from the Peninsula (i.e., East Antarctica is a major participant in the mass loss). During the entire period, the mass loss concentrated in areas closest to warm, salty, subsurface, circumpolar deep water (CDW), that is, consistent with enhanced polar westerlies pushing CDW toward Antarctica to melt its floating ice shelves, destabilize the glaciers, and raise sea level.

/pdf/four-decades-of-antarctic-ice-sheet-mass-balance-from-1979-4df6tvdif8.pdf

Four decades of Antarctic Ice Sheet mass balance from 1979-2017.

The Antarctic ice sheet has been losing mass over past decades through the accelerated flow of its glaciers, conditioned by ocean temperature and bed topography. Glaciers retreating along retrograde slopes (that is, the bed elevation drops in the inland direction) are potentially unstable, while subglacial ridges slow down the glacial retreat. Despite major advances in the mapping of subglacial bed topography, significant sectors of Antarctica remain poorly resolved and critical spatial details are missing. Here we present a novel, high-resolution and physically based description of Antarctic bed topography using mass conservation. Our results reveal previously unknown basal features with major implications for glacier response to climate change. For example, glaciers flowing across the Transantarctic Mountains are protected by broad, stabilizing ridges. Conversely, in the marine basin of Wilkes Land, East Antarctica, we find retrograde slopes along Ninnis and Denman glaciers, with stabilizing slopes beneath Moscow University, Totten and Lambert glacier system, despite corrections in bed elevation of up to 1 km for the latter. This transformative description of bed topography redefines the high- and lower-risk sectors for rapid sea level rise from Antarctica; it will also significantly impact model projections of sea level rise from Antarctica in the coming centuries.

/pdf/deep-glacial-troughs-and-stabilizing-ridges-unveiled-beneath-9imgtagipk.pdf

Deep glacial troughs and stabilizing ridges unveiled beneath the margins of the Antarctic ice sheet

Global mean sea level is an integral of changes occurring in the climate system in response to
unforced climate variability as well as natural and anthropogenic forcing factors. Its temporal
evolution allows detecting changes (e.g., acceleration) in one or more components. Study of
the sea level budget provides constraints on missing or poorly known contributions, such as
the unsurveyed deep ocean or the still uncertain land water component. In the context of the
World Climate Research Programme Grand Challenge entitled “Regional Sea Level and
Coastal Impacts”, an international effort involving the sea level community worldwide has
been recently initiated with the objective of assessing the various data sets used to estimate
components of the sea level budget during the altimetry era (1993 to present). These data sets
are based on the combination of a broad range of space-based and in situ observations, model
estimates and algorithms. Evaluating their quality, quantifying uncertainties and identifying
sources of discrepancies between component estimates is extremely useful for various
applications in climate research. This effort involves several tens of scientists from about fifty
research teams/institutions worldwide (www.wcrp-climate.org/grand-challenges/gc-sea-
level). The results presented in this paper are a synthesis of the first assessment performed
during 2017-2018. We present estimates of the altimetry-based global mean sea level (average
rate of 3.1 +/- 0.3 mm/yr and acceleration of 0.1 mm/yr2 over 1993-present), as well as of the
different components of the sea level budget (http://doi.org/10.17882/54854). We further
examine closure of the sea level budget, comparing the observed global mean sea level with
the sum of components. Ocean thermal expansion, glaciers, Greenland and Antarctica
contribute by 42%, 21%, 15% and 8% to the global mean sea level over the 1993-present. We
also study the sea level budget over 2005-present, using GRACE-based ocean mass estimates
instead of sum of individual mass components. Results show closure of the sea level budget
within 0.3 mm/yr. Substantial uncertainty remains for the land water storage component, as
shown in examining individual mass contributions to sea level.

/pdf/global-sea-level-budget-1993-present-31pxjlun8i.pdf

Global sea-level budget 1993 - present

. We evaluate modelled Antarctic ice sheet (AIS) near-surface climate, surface
mass balance (SMB) and surface energy balance (SEB) from the updated polar
version of the regional atmospheric climate model, RACMO2 (1979–2016). The
updated model, referred to as RACMO2.3p2, incorporates upper-air relaxation,
a revised topography, tuned parameters in the cloud scheme to generate more
precipitation towards the AIS interior and modified snow properties reducing
drifting snow sublimation and increasing surface snowmelt. Comparisons of RACMO2 model output with several independent observational
data show that the existing biases in AIS temperature, radiative fluxes and
SMB components are further reduced with respect to the previous model
version. The model-integrated annual average SMB for the ice sheet including
ice shelves (minus the Antarctic Peninsula, AP) now amounts to
2229 Gt y −1 , with an interannual variability of 109 Gt y −1 . The
largest improvement is found in modelled surface snowmelt, which now compares
well with satellite and weather station observations. For the high-resolution
( ∼  5.5 km) AP simulation, results remain comparable to earlier
studies. The updated model provides a new, high-resolution data set of the contemporary
near-surface climate and SMB of the AIS; this model version will be used for
future climate scenario projections in a forthcoming study.

https://tc.copernicus.org/articles/12/1479/2018/tc-12-1479-2018.pdf

Modelling the climate and surface mass balance of polar ice sheets using RACMO2 – Part 2: Antarctica (1979–2016)

Abstract. Numerical weather prediction (NWP) models are known to possess a distinct
moist bias in the mid-latitude lower stratosphere, which is expected to affect
the ability to accurately predict weather and climate. This paper
investigates the vertical structure of the moist bias in the European Centre
for Medium-Range Weather Forecasts (ECMWF) latest global reanalysis ERA5
using a unique multi-campaign data set of highly resolved water vapour
profiles observed with a differential absorption lidar (DIAL) on board the
High Altitude and LOng range research aircraft (HALO). In total, 41 flights
in the mid-latitudes from six field campaigns provide roughly 33 000 profiles
with humidity varying by 4 orders of magnitude. The observations cover
different synoptic situations and seasons and thus are suitable to
characterize the strong vertical gradients of moisture in the upper
troposphere and lower stratosphere (UTLS). The comparison to ERA5 indicates
high positive and negative deviations in the UT, which on average lead to a
slightly positive bias (15 %–20 %). In the LS, the moist bias rapidly
increases up to a maximum of 55 % at 1.3 km altitude above the thermal
tropopause (tTP) and decreases again to 15 %–20 % at 4 km altitude. Such a
vertical structure is frequently observed, although the magnitude varies
from flight to flight. The layer depth of increased moist bias is smaller at
high tropopause altitudes and larger when the tropopause is low. Our results
also suggest a seasonality of the moist bias, with the maximum in summer
exceeding autumn by up to a factor of 3. During one field campaign, collocated
ozone and water vapour profile observations enable a classification of
tropospheric, stratospheric, and mixed air using water vapour–ozone
correlations. It is revealed that the moist bias is high in the mixed air
while being small in tropospheric and stratospheric air, which highlights
that excessive transport of moisture into the LS plays a decisive role for
the formation of the moist bias. Our results suggest that a better
representation of mixing processes in NWP models could lead to a reduced LS
moist bias that, in turn, may lead to more accurate weather and climate
forecasts. The lower-stratospheric moist bias should be borne in mind for
climatological studies using reanalysis data.


/pdf/vertical-structure-of-the-lower-stratospheric-moist-bias-in-2kngl053.pdf

Konstantin Krüger

Papers

Modelling the climate and surface mass balance of polar ice sheets using RACMO2 – Part 2: Antarctica (1979–2016)

Vertical structure of the lower-stratospheric moist bias in the ERA5 reanalysis and its connection to mixing processes