Polar föhn winds and warming over the Larsen C Ice Shelf, Antarctica

Recent hypotheses that the foehn effect is partly responsible for warming to the east of the Antarctic Peninsula (AP) and enhanced melt rates on the Larsen C Ice Shelf are supported in a study combining the analysis of observational and high resolution model data. Leeside warming and drying during foehn events is observed in new aircraft, radiosonde and automatic weather station data and simulated by the UK Met Office Unified Model at ~1.5 km grid spacing (MetUM 1.5 km). Three contrasting cases are investigated. In Case A relatively weak southwesterly flow induces a nonlinear foehn event. Strongly accelerated flow above and a hydraulic jump immediately downwind of the lee slopes lead to high amplitude warming in the immediate lee of the AP, downwind of which the warming effect diminishes rapidly due to the upward ‘rebound’ of the foehn flow. Case C defines a relatively linear case associated with strong northwesterly winds. The lack of a hydraulic jump enables foehn flow to flood across the entire ice shelf at low levels. Melt rates are high due to a combination of large radiative heat flux, due to dry, clear leeside conditions, and sensible heat flux downward from the warm, well-mixed foehn flow. Climatological work suggests that such strong northwesterly cases are often responsible for high Larsen C melt rates. Case B describes a weak, relatively non-linear foehn event associated with insignificant daytime melt rates. Previously unknown jets – named polar foehn jets – emanating from the mouths of leeside inlets are identified as a type of gap flow. They are cool and moist relative to adjacent calmer regions, due to lower-altitude upwind source regions, and are characterised by larger turbulent heat fluxes both within the air column and at the surface. The relative importance of the three mechanisms deemed to induce leeside foehn warming (isentropic drawdown, latent heating and sensible heating) are quantified using a novel method analysing back trajectories and MetUM 1.5 km model output. It is shown that, depending on the linearity of the flow regime and the humidity of the air mass, each mechanism can dominate. This implies that there is no dominant foehn warming mechanism, contrary to the conclusions of previous work

Earth’s polar night boundary layer as an analogue for dark side inversions on synchronously rotating terrestrial exoplanets

A key factor in determining the potential habitability of synchronously rotating planets is the strength of the atmospheric boundary layer inversion between the dark side surface and the free atmosphere. Here we analyse data obtained from polar night measurements at the South Pole and Alert Canada, which are the closest analogues on Earth to conditions on the dark sides of synchronously rotating exoplanets without and with a maritime influence, respectively. On Earth, such inversions rarely exceed 30 K in strength, because of the effect of turbulent mixing induced by phenomena such as so-called mesoscale slope winds, which have horizontal scales of 10s to 100s of km, suggesting a similar constraint to near-surface dark side inversions. We discuss the sensitivity of inversion strength to factors such as orography and the global-scale circulation, and compare them to a simulation of the planet Proxima Centauri b. Our results demonstrate the importance of comparisons with Earth data in exoplanet research, and highlight the need for further studies of the exoplanet atmospheric collapse problem using mesoscale and eddy-resolving models

Foehn warming distributions in nonlinear and linear flow regimes: a focus on the Antarctic Peninsula

The structure of lee-side warming during foehn events is investigated as a function of cross-barrier flow regime linearity. Two contrasting cases of westerly flow over the Antarctic Peninsula (AP) are considered – one highly nonlinear, the other relatively linear. Westerly flow impinging on the AP provides one of the best natural laboratories in the world for the study of foehn, owing to its maritime setting and the Larsen C Ice Shelf (LCIS) providing an expansive, homogeneous and smooth surface on its east side. Numerical simulations with the Met Office Unified Model (at 1.5 km grid size) and aircraft observations are utilized. In case A, relatively weak southwesterly cross-Peninsula flow and an elevated upwind inversion dictate a highly nonlinear foehn event, with mountain wave breaking observed. The consequent strongly accelerated downslope flow leads to high-amplitude warming and ice-shelf melt in the immediate lee of the AP. However this foehn warming diminishes rapidly downwind due to upward ascent of the foehn flow via a hydraulic jump. In case C, strong northwesterly winds dictate a relatively linear flow regime. There is no hydraulic jump and strong foehn winds are able to flow at low levels across the entire ice shelf, mechanically mixing the near-surface flow, preventing the development of a strong surface inversion and delivering large fluxes of sensible heat to the ice shelf. Consequently, in case C ice-melt rates are considerably greater over the LCIS as a whole than in case A. Our results imply that although nonlinear foehn events cause intense warming in the immediate lee of mountains, linear foehn events will commonly cause more extensive lee-side warming and, over an ice surface, higher melt rates. This has major implications for the AP, where recent east-coast warming has led to the collapse of two ice shelves immediately north of the LCIS

Orographic drag in the Met Office Unified Model: Sensitivity to parameterisation and insights from inter‐model variability in drag partition

Global model experiments investigating the sensitivity of MetUM performance at both NWP and climate time-scales to individual changes in orographic drag parameterization indicate that forecast improvements may be possible via the retuning of orographic drag scheme parameters, reassuringly towards more physically realistic values. The experiments reveal considerable model sensitivity to drag parameterization configuration. The most beneficial changes are found to arrive via decreases in the low level drag associated with orographic flow blocking and an increase in the higher-altitude drag associated with gravity wave breaking. The former has the effect of reducing high latitude high pressure biases in the MetUM, whilst the latter improves in particular temperature distribution, and to a lesser extent circulation, in the stratosphere. These tendencies in stress components may be brought about via changes to each of the five tuneable orographic drag scheme parameters. For four of the five parameters, these changes yield values which are closer to those recommended in the literature (whilst the fifth parameter, being poorly constrained, is relatively flexibly tuneable). In a month-long global model comparison between the MetUM and ECMWF IFS, considerable differences in drag partition are identified, highlighting the considerable uncertainty in the representation of orographic drag that remains. This disparity is linked to differences in the diurnal and spatial variability in surface stress over high mountain ranges. The MetUM displays marginally higher amplitude diurnal variability – arguably the opposite of that which would be intuitively expected. The beneficial tendencies in drag components found in the MetUM sensitivity experiments – an increase in gravity wave drag and a decrease in flow blocking drag (plus an ensuing compensating increase in boundary layer drag) – would bring the MetUM into closer agreement with the partition of drag components seen in the IFS

Surface melt and ponding on Larsen C Ice Shelf and the impact of föhn winds

A common precursor to ice shelf disintegration, most notably that of Larsen B Ice Shelf, is unusually intense or prolonged surface melt and the presence of surface standing water. However, there has been little research into detailed patterns of melt on ice shelves or the nature of summer melt ponds. We investigated surface melt on Larsen C Ice Shelf at high resolution using Envisat advanced synthetic aperture radar (ASAR) data and explored melt ponds in a range of satellite images. The improved spatial resolution of SAR over alternative approaches revealed anomalously long melt duration in western inlets. Meteorological modelling explained this pattern by föhn winds which were common in this region.Melt ponds are difficult to detect using optical imagery because cloud-free conditions are rare in this region and ponds quickly freeze over, but can be monitored using SAR in all weather conditions. Melt ponds up to tens of kilometres in length were common in Cabinet Inlet, where melt duration was most prolonged. The pattern of melt explains the previously observed distribution of ice shelf densification, which in parts had reached levels that preceded the collapse of Larsen B Ice Shelf, suggesting a potential role for föhn winds in promoting unstable conditions on ice shelves

Atmospheric sensitivity to marginal-ice-zone drag: local and global responses

The impact of a physically-based parameterization of atmospheric drag over the marginal-icezone (MIZ) is evaluated through a series of regional and global atmospheric model simulations. The sea-ice drag parameterization has recently been validated and tuned based on a large set of observations of surface momentum flux from the Barents Sea and Fram Strait. The regional simulations are from March 2013 and make use of a collection of cold-air outbreak observations in the vicinity of the MIZ for validation. The global model analysis uses multiple 48-hour forecasts taken from a standard test suite of simulations. Our focus is on the response of the modelled atmosphere to changes in the drag coefficient over the MIZ. We find that the parameterization of drag has a significant impact on the simulated atmospheric boundary layer: for example, changing the surface momentum flux by typically 0.1-0.2 N m-2 (comparable to the mean) and low-level temperatures by 2-3 K in the vicinity of the MIZ. Comparisons against aircraft observations over and downwind of the MIZ show that simulations with the new sea ice drag scheme generally have the lowest bias and lowest rootmean-square errors. The wind speed and temperature biases are reduced by up to 0.5 m s-1 and 2 K respectively, compared to simulations with two settings of the previous drag scheme. In the global simulations the atmospheric response is widespread – impacting most of the Arctic and Antarctic sea-ice areas – with the largest changes in the vicinity of the MIZ and affecting the entire atmospheric boundary layer

Atmospheric drivers of melt on Larsen C Ice Shelf: Surface energy budget regimes and the impact of foehn

Recent ice shelf retreat on the east coast of the Antarctic Peninsula has been principally attributed to atmospherically driven melt. However, previous studies on the largest of these ice shelves—Larsen C—have struggled to reconcile atmospheric forcing with observed melt. This study provides the first comprehensive quantification and explanation of the atmospheric drivers of melt across Larsen C, using 31-months' worth of observations from Cabinet Inlet, a 6-month, high-resolution atmospheric model simulation and a novel approach to ascertain the surface energy budget (SEB) regime. The dominant meteorological controls on melt are shown to be the occurrence, strength, and warmth of mountain winds called foehn. At Cabinet Inlet, foehn occurs 15% of the time and causes 45% of melt. The primary effect of foehn on the SEB is elevated turbulent heat fluxes. Under typical, warm foehn conditions, this means elevated surface heating and melting, the intensity of which increases as foehn wind speed increases. Less commonly—due to cooler-than-normal foehn winds and/or radiatively warmed ice—the relationship between wind speed and net surface heat flux reverses. This explains the seemingly contradictory results of previous studies. In the model, spatial variability in cumulative melt across Larsen C is largely explained by foehn, with melt maxima in inlets reflecting maxima in foehn wind strength. However, most accumulated melt (58%) occurs due to solar radiation in the absence of foehn. A broad north-south gradient in melt is explained by the combined influence of foehn and non-foehn conditions

The impact of wintertime sea‑ice anomalies on high surface heat flux events in the Iceland and Greenland Seas

The gyres of the Iceland and Greenland Seas are regions of deep-water formation, driven by large ocean-to-atmosphere heat fluxes that have local maxima adjacent to the sea-ice edge. Recently these regions have experienced a dramatic loss of sea ice, including in winter, which begs the question have surface heat fluxes in the adjacent ocean gyres been affected? To address this a set of regional atmospheric climate model simulations has been run with prescribed sea ice and sea surface temperature fields. Three 20-year model experiments have been examined: Ice max, Ice med and Ice min, where the surface fields are set as the year with maximum, median and minimum sea-ice extents respectively. Under conditions of reduced sea-ice extent there is a 15% (19 W m −2) decrease in total wintertime heat fluxes in the Iceland Sea. In contrast, there is an 8% (9 W m −2) increase in heat fluxes in the Greenland Sea primarily due to higher local SSTs. These differences are manifest as changes in the magnitude of high heat flux events (such as cold air outbreaks). In the Iceland Sea, 76% of these events are lower in magnitude during reduced sea-ice conditions. In the Greenland Sea, 93% of these events are higher in magnitude during reduced sea-ice conditions as a result of higher SSTs coincident with retreating sea ice. So, in these experiments, the reduced wintertime sea-ice conditions force a different response in the two seas. In both gyres, large-scale atmospheric circulation patterns are key drivers of high heat flux events

Foehn jets over the Larsen C Ice Shelf, Antarctica

Previously unknown foehn jets have been identified to the east of the Antarctic Peninsula (AP) above the Larsen C Ice Shelf. These jets have major implications for the east coast of the AP, a region of rapid climatic warming and where two large sections of ice shelf have collapsed in recent years. During three foehn events across the AP, leeside warming and drying is seen in new aircraft observations and simulated well by the Met Office Unified Model (MetUM) at ∼1.5 km grid spacing. In case A, weak southwesterly flow and an elevated upwind inversion characterise a highly nonlinear flow regime with upwind flow blocking. In case C strong northwesterly winds characterise a relatively linear case with little upwind flow blocking. Case B resides somewhere between the two in flow regime linearity. The foehn jets – apparent in aircraft observations where available and MetUM simulations of all three cases – are mesoscale features (up to 60 km in width) originating from the mouths of leeside inlets. Through back trajectory analysis they are identified as a type of gap flow. In cases A and B the jets are distinct, being strongly accelerated relative to the background flow, and confined to low levels above the Larsen C Ice Shelf. They resemble the ‘shallow foehn’ of the Alps. Case C resembles a case of ‘deep foehn’, with the jets less distinct. The foehn jets are considerably cooler and moister relative to adjacent regions of calmer foehn air. This is due to a dampened foehn effect in the jet regions: in case A the jets have lower upwind source regions, and in the more linear case C there is less diabatic warming and precipitation along jet trajectories due to the reduced orographic uplift across the mountain passes

Moving towards a wave-resolved approach to forecasting mountain wave induced clear air turbulence

Mountain wave breaking in the lower stratosphere is one of the major causes of atmospheric turbulence encountered in commercial aviation, which in turn is the cause of most weather-related aircraft incidents. In the case of clear air turbulence (CAT), there are no visual clues and pilots are reliant on operational forecasts and reports from other aircraft. Traditionally mountain waves have been sub-grid-scale in global numerical weather prediction (NWP) models, but recent developments in NWP mean that some forecast centres (e.g. the UK Met Office) are now producing operational global forecasts that resolve mountain wave activity explicitly, allowing predictions of mountain wave induced turbulence with greater accuracy and confidence than previously possible. Using a bespoke turbulent kinetic energy diagnostic, the Met Office Unified Model (MetUM) is shown to produce useful forecasts of mountain CAT during three case studies over Greenland, and to outperform the current operational Met Office CAT prediction product (the World Area Forecast Centre (WAFC) London gridded CAT product) in doing so. In a long term, 17-month, verification, MetUM forecasts yield a turbulence prediction hit rate of 80% with an accompanying false alarm rate of under 40%. These skill scores are a considerable improvement on those reported for the mountain wave component of the WAFC product, although no direct comparison is available. The major implication of this work is that sophisticated global NWP models are now sufficiently advanced to provide skilful forecasts of mountain wave turbulence