A Lagrangian analysis of the dynamical and thermodynamic drivers of large-scale Greenland melt events during 1979–2017

Abstract. In this study, we systematically investigate the dynamical and thermodynamic processes that lead to 77 large-scale melt events affecting high-elevation regions of the Greenland Ice Sheet (GrIS) in June–August (JJA) 1979–2017. For that purpose, we compute 8 d kinematic backward trajectories from the lowermost ∼500 m above the GrIS during these events. The key synoptic feature accompanying the melt events is an upper-tropospheric ridge southeast of the GrIS associated with a surface high-pressure system. This circulation pattern is favorable to induce rapid poleward transport (up to 40∘ latitude) of warm (∼15 K warmer than climatological air masses arriving on the GrIS) and moist air masses from the lower troposphere to the western GrIS and subsequently to distribute them in the anticyclonic flow over north and east Greenland. During transport to the GrIS, the melt event air masses cool by ∼15 K due to ascent and radiation, which keeps them just above the critical threshold to induce melting. The thermodynamic analyses reveal that the final warm anomaly of the air masses is primarily owed to anomalous horizontal transport from a climatologically warm region of origin. However, before being transported to the GrIS, i.e., in their region of origin, these air masses were not anomalously warm. Latent heating from condensation of water vapor, occurring as the airstreams are forced to ascend orographically or dynamically, is of secondary importance. These characteristics were particularly pronounced during the most extensive melt event in early July 2012, where, importantly, the warm anomaly was not preserved from anomalously warm source regions such as North America experiencing a record heat wave. The mechanisms identified here are in contrast to melt events in the low-elevation high Arctic and to midlatitude heat waves, where adiabatic warming by large-scale subsidence is essential. Considering the impact of moisture on the surface energy balance, we find that radiative effects are closely linked to the air mass trajectories and enhance melt over the entire GrIS accumulation zone due to (i) enhanced downward longwave radiation related to poleward moisture transport and a shift in the cloud phase from ice to liquid primarily west of the ice divide and (ii) increased shortwave radiation in clear-sky regions east of the ice divide. Given the ongoing increase in the frequency and the melt extent of large-scale melt events, the understanding of upper-tropospheric ridges over the North Atlantic, i.e., also Greenland blocking, and its representation in climate models is crucial in determining future GrIS accumulation zone melt and thus global sea level rise.

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Lagrangian analysis of the dynamical and thermodynamic drivers of large-scale Greenland melt events during 1979-2017

10.5194/egusphere-egu21-365 ◽

2021 ◽

Author(s):

Mauro Hermann ◽

Lukas Papritz ◽

Heini Wernli

Keyword(s):

Large Scale ◽

Heat Waves ◽

Lower Troposphere ◽

Greenland Ice Sheet ◽

Region Of Origin ◽

Air Masses ◽

Accumulation Zone ◽

Warm Anomaly ◽

Downward Longwave Radiation ◽

The Impact

We systematically investigate the dynamical and thermodynamic processes that lead to 77 large-scale melt events affecting high-elevation regions of the Greenland Ice Sheet (GrIS) in June-August (JJA) 1979-2017. For that purpose, we compute 8&#8201;day kinematic backward trajectories from the lowermost ~500&#8201;m above the GrIS. The key synoptic feature accompanying the melt events is an upper-tropospheric ridge over Southeast Greenland associated with a surface high-pressure system. This circulation pattern is favorable to induce rapid poleward transport (up to 40&#176; latitude) of warm (~15&#8201;K warmer than climatological air masses arriving on the GrIS) and moist air masses from the lower troposphere to the western GrIS and subsequently to distribute them in the anticyclonic flow over north and east Greenland. During transport to the GrIS, the melt event air masses cool by ~15&#8201;K due to ascent and radiation, which keeps them just above the critical threshold to induce melting.The thermodynamic analyses reveal that the final warm anomaly of the air masses is primarily owed to anomalous horizontal transport from a climatologically warm region of origin. However, before being transported to the GrIS, i.e., in their region of origin, these air masses were not anomalously warm. Latent heating from condensation of water vapor, occurring as the airstreams are forced to ascend orographically or dynamically, is of secondary importance. These characteristics were particularly pronounced during the most extensive melt event in early July 2012. In this event, importantly, the warm anomaly was not preserved from anomalously warm source regions such as North America experiencing a record heat wave. Considering the impact of moisture on the surface energy balance, we find that radiative effects are closely linked to the air mass trajectories and enhance melt over the entire GrIS accumulation zone due to (i) enhanced downward longwave radiation related to poleward moisture transport and a shift in the cloud phase from ice to liquid primarily west of the ice divide and (ii) increased shortwave radiation in clear-sky regions east of the ice divide.The temporal evolution, positioning, and intensity of synoptic scale weather systems deserve further attention as they are responsible for strong and partly opposing atmospheric forcing of the GrIS surface mass balance. Also, the mechanisms identified here are in contrast to melt events in the low-elevation high Arctic and to midlatitude heat waves, where the upper-tropospheric ridge is essential to induce adiabatic warming by large-scale subsidence. Given the ongoing increase in the frequency and the melt extent of large-scale melt events, the understanding of upper-tropospheric ridges over the North Atlantic, i.e., also Greenland blocking, and its representation in climate models is crucial in determining future GrIS accumulation zone melt and thus global sea level rise.&#160;

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A Lagrangian Analysis of the Dynamical and Thermodynamic Drivers of Greenland Melt Events during 1979–2017

10.5194/wcd-2020-16 ◽

2020 ◽

Author(s):

Mauro Hermann ◽

Lukas Papritz ◽

Heini Wernli

Keyword(s):

Heat Waves ◽

Lower Troposphere ◽

Greenland Ice Sheet ◽

Shortwave Radiation ◽

Critical Threshold ◽

Region Of Origin ◽

Air Masses ◽

Warm Anomaly ◽

Downward Longwave Radiation ◽

The Impact

Abstract. In this study, we systematically investigate the dynamical and thermodynamic processes that lead to 77 Greenland melt events affecting high-elevated regions of the Greenland Ice Sheet (GrIS) in June–August (JJA) 1979–2017. For that purpose, we compute 8-day kinematic backward trajectories from the lowermost ~ 500 m above the GrIS during these events. The key synoptic feature accompanying the melt events is an upper-tropospheric ridge southeast of the GrIS associated with a surface high pressure system. This circulation pattern is favourable to induce rapid poleward transport (up to 40° latitude) of warm (~ 15 K warmer than climatological air masses arriving on the GrIS) and moist air masses from the lower troposphere to the western GrIS and subsequently to distribute them in the anticyclonic flow over North and East Greenland. During transport to the GrIS, the melt event air masses cool by ~ 15 K due to ascent and radiation, which keeps them just above the critical threshold to induce melting. The thermodynamic analyses reveal that the final warm anomaly of the air masses is primarily owed to anomalous horizontal transport from a climatologically warm region of origin. However, before being transported to the GrIS, i.e., in their region of origin, these air masses were not anomalously warm. Latent heating from condensation of water vapour, occurring as the airstreams are forced to ascend orographically or dynamically, is of secondary importance. These characteristics were particularly pronounced during the most extensive melt event in early July 2012, where, importantly, the warm anomaly was not preserved from anomalously warm source regions such as North America experiencing a record heat wave. The mechanisms identified here are in contrast to melt events in the low-elevation high Arctic and to midlatitude heat waves, where adiabatic warming by large-scale subsidence is essential. Considering the impact of moisture on the surface energy balance, we find that radiative effects are closely linked to the air mass trajectories and enhance melt over the entire GrIS due to (i) enhanced downward longwave radiation related to poleward moisture transport and a shift in the cloud phase from ice to liquid primarily west of the ice divide, and (ii) increased shortwave radiation in clear-sky regions east of the ice divide. Given the identified mechanisms that cause extensive melt over the GrIS, the understanding of upper-tropospheric ridges over the North Atlantic, i.e., also Greenland blocking, and its representation in climate models is crucial in determining future GrIS melt and so global sea-level rise.

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Lagrangian Analysis of the Dynamical and Thermodynamic Drivers of Greenland Melt Events during 1979-2017

10.5194/egusphere-egu2020-13492 ◽

2020 ◽

Author(s):

Mauro Hermann ◽

Lukas Papritz ◽

Heini Wernli

Keyword(s):

High Pressure ◽

Lower Troposphere ◽

Greenland Ice Sheet ◽

Surface Energy Budget ◽

Simultaneous Occurrence ◽

Temperature Pattern ◽

Near Surface ◽

Air Mass ◽

Air Masses ◽

Warm Anomaly

Specific atmospheric circulation patterns can lead to strongly positive near-surface temperature anomalies over Greenland, fostering the occurrence of extensive surface melt events. In this study, we objectively identify 77 Greenland melt events in June-August 1979-2017, which also affect high-elevated regions of the Greenland ice sheet (GrIS), from ERA-Interim reanalysis data. Eight-day backward trajectories from the lowermost 500 m above the GrIS are used to investigate the air mass history and the synoptic, dynamical, and thermodynamic drivers of Greenland melt events. The key synoptic feature is a high-pressure system, in 65% of the events classified as atmospheric blocking, southeast of the GrIS. It is favorably located to induce rapid and long-range poleward transport of anomalously warm air masses (compared to climatology) from the lower troposphere to the GrIS. Due to orographic and dynamical lifting, latent heating from condensation of water vapor contributes additionally to the air mass&#8217; warm anomaly - most important for melt events on top of the GrIS. Adiabatic warming by subsidence, however, is insignificant, in contrast to warm events in the central Arctic. Exemplarily, the warm anomaly of air masses arriving in the Summit area during the most extensive melt event in early July 2012 arose due to strong meridional transport, mainly from the western North Atlantic, and latent heat release during ascent to Greenland. The simultaneous occurrence of a North American record heat wave did not play any direct role for the Greenland melt event. Further, regionally varying short- and longwave radiative effects induced by the warm-moist air masses enhance melt all over the GrIS. The identified mechanisms that cause Greenland melt events imply that the understanding of the formation of high-pressure systems and their representation in climate models is crucial in determining future GrIS melt. More generally, we highlight the importance of atmospheric dynamics and air flow patterns for Greenland melt events as they eventually determine the temperature pattern and surface energy budget over the GrIS with consequences for global sea-level rise.

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Economics of the disintegration of the Greenland ice sheet

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1814990116 ◽

2019 ◽

Vol 116 (25) ◽

pp. 12261-12269 ◽

Cited By ~ 5

Author(s):

William Nordhaus

Keyword(s):

Climate Change ◽

Large Scale ◽

Social Cost ◽

Climate Models ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Reduced Form ◽

Small Contribution ◽

Wide Range ◽

The Impact

Concerns about the impact on large-scale earth systems have taken center stage in the scientific and economic analysis of climate change. The present study analyzes the economic impact of a potential disintegration of the Greenland ice sheet (GIS). The study introduces an approach that combines long-run economic growth models, climate models, and reduced-form GIS models. The study demonstrates that social cost–benefit analysis and damage-limiting strategies can be usefully extended to illuminate issues with major long-term consequences, as well as concerns such as potential tipping points, irreversibility, and hysteresis. A key finding is that, under a wide range of assumptions, the risk of GIS disintegration makes a small contribution to the optimal stringency of current policy or to the overall social cost of climate change. It finds that the cost of GIS disintegration adds less than 5% to the social cost of carbon (SCC) under alternative discount rates and estimates of the GIS dynamics.

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Evaluating the impact of climate on snow- and ice-melt dynamics in the Taillon basin, French Pyrénées

Journal of Glaciology ◽

10.3189/s0022143000035176 ◽

1997 ◽

Vol 43 (145) ◽

pp. 563-568 ◽

Cited By ~ 1

Author(s):

David M. Hannah ◽

Glenn R. McGregor

Keyword(s):

Large Scale ◽

Energy Budgets ◽

Computer Assisted ◽

Air Masses ◽

Atmospheric Circulation Patterns ◽

Ice Melt ◽

Circulation Types ◽

The Impact ◽

Snow And Ice Melt ◽

Snow And Ice

AbstractThis pilot study adopts a computer-assisted synoptic typing methodology to evaluate the totality of climatic influences on snow- and ice-melt dynamics within a small cirque basin in the French Pyrénées. The synoptic categories identified possess contrasting large-scale atmospheric circulation patterns and surface energy budgets which generate differential ablation responses. Continental air masses yield consistently high melt. Advection of moist maritime air also produces elevated but more variable ablation due to air-mass transitions. The two observed local valley circulation types show melt to be higher under nocturnal katabatic drainage than for anabatic wind flows associated with development of daytime ridge-top cumulus.

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Closed Anticyclones of the Subtropics and Midlatitudes: A 54-Yr Climatology (1950–2003) and Three Case Studies

Meteorological Monographs ◽

10.1175/0065-9401-33.55.349 ◽

2008 ◽

Vol 55 ◽

pp. 349-392 ◽

Cited By ~ 1

Author(s):

Thomas J. Galarneau ◽

Lance F. Bosart ◽

Anantha R. Aiyyer

Keyword(s):

Case Studies ◽

Large Scale ◽

Heat Waves ◽

The United States ◽

Hemispheric Differences ◽

Air Masses ◽

Hot Air ◽

Upper Level ◽

Height Fields ◽

Subtropical Oceans

Abstract The pioneering large-scale studies of cyclone frequency, location, and intensity conducted by Fred Sanders prompt similar questions about lesser-studied anticyclone development. The results of a climatology of closed anticyclones (CAs) at 200, 500, and 850 hPa, with an emphasis on the subtropics and midlatitudes, is presented to assess the seasonally varying distribution and hemispheric differences of these features. To construct the CA climatology, a counting program was applied to twice-daily 2.5° NCEP–NCAR reanalysis 200-, 500-, and 850-hPa geopotential height fields for the period 1950–2003. Stationary CAs, defined as those CAs that were located at a particular location for consecutive time periods, were counted only once. The climatology results show that 200-hPa CAs occur preferentially during summer over subtropical continental regions, while 500-hPa CAs occur preferentially over subtropical oceans in all seasons and over subtropical continents in summer. Conversely, 850-hPa CAs occur preferentially over oceanic regions beneath upper-level midocean troughs, and are most prominent in the Northern Hemisphere, and over midlatitude continents in winter. Three case studies of objectively identified CAs that produced heal waves over the United States, Europe, and Australia in 1995, 2003, and 2004, respectively, are presented to supplement the climatological results. The case studies, examining the subset of CAs than can produce heat waves, illustrate how climatologically hot continental tropical air masses produced over arid and semiarid regions of the subtropics and lower midlatitudes can become abnormally hot in conjunction with dynamically driven upper-level ridge amplification. Subsequently, these abnormally hot air masses are advected downstream away from their source regions in conjunction with transient disturbances embedded in anomalously strong westerly jets.

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Air mass origins influencing TTL chemical composition over West Africa during 2006 summer monsoon

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-10753-2010 ◽

2010 ◽

Vol 10 (22) ◽

pp. 10753-10770 ◽

Cited By ~ 23

Author(s):

K. S. Law ◽

F. Fierli ◽

F. Cairo ◽

H. Schlager ◽

S. Borrmann ◽

...

Keyword(s):

West Africa ◽

Summer Monsoon ◽

Large Scale ◽

Lower Stratosphere ◽

Deep Convection ◽

Lower Troposphere ◽

Trace Gas ◽

Air Mass ◽

Back Trajectories ◽

Air Masses

Abstract. Trace gas and aerosol data collected in the tropical tropopause layer (TTL) between 12–18.5 km by the M55 Geophysica aircraft as part of the SCOUT-AMMA campaign over West Africa during the summer monsoon in August 2006 have been analysed in terms of their air mass origins. Analysis of domain filling back trajectories arriving over West Africa, and in the specific region of the flights, showed that the M55 flights were generally representative of air masses arriving over West Africa during the first 2 weeks of August, 2006. Air originating from the mid-latitude lower stratosphere was under-sampled (in the mid-upper TTL) whilst air masses uplifted from central Africa (into the lower TTL) were over-sampled in the latter part of the campaign. Signatures of recent (previous 10 days) origins were superimposed on the large-scale westward flow over West Africa. In the lower TTL, air masses were impacted by recent local deep convection over Africa at the level of main convective outflow (350 K, 200 hPa) and on certain days up to 370 K (100 hPa). Estimates of the fraction of air masses influenced by local convection vary from 10 to 50% depending on the method applied and from day to day during the campaign. The analysis shows that flights on 7, 8 and 11 August were more influenced by local convection than on 4 and 13 August allowing separation of trace gas and aerosol measurements into "convective" and "non-convective" flights. Strong signatures, particularly in species with short lifetimes (relative to CO2) like CO, NO and fine-mode aerosols were seen during flights most influenced by convection up to 350–365 K. Observed profiles were also constantly perturbed by uplift (as high as 39%) of air masses from the mid to lower troposphere over Asia, India, and oceanic regions resulting in import of clean oceanic (e.g. O3-poor) or polluted air masses from Asia (high O3, CO, CO2) into West Africa. Thus, recent uplift of CO2 over Asia may contribute to the observed positive CO2 gradients in the TTL over West Africa. This suggests a more significant fraction of younger air masses in the TTL and needs to taken into consideration in derivations of mean age of air. Transport of air masses from the mid-latitude lower stratosphere had an impact from the mid-TTL upwards (20–40% above 370 K) during the campaign period importing air masses with high O3 and NOy. Ozone profiles show a less pronounced lower TTL minimum than observed previously by regular ozonesondes at other tropical locations. Concentrations are less than 100 ppbv in the lower TTL and vertical gradients less steep than in the upper TTL. The air mass origin analysis and simulations of in-situ net photochemical O3 production, initialised with observations, suggest that the lower TTL is significantly impacted by uplift of O3 precursors (over Africa and Asia) leading to positive production rates (up to 2 ppbv per day) in the lower and mid TTL even at moderate NOx levels. Photochemical O3 production increases with higher NOx and H2O in air masses with O3 less than 150 ppbv.

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Summertime low clouds mediate the impact of the large-scale circulation on Arctic sea ice

Communications Earth & Environment ◽

10.1038/s43247-021-00114-w ◽

2021 ◽

Vol 2 (1) ◽

Author(s):

Yiyi Huang ◽

Qinghua Ding ◽

Xiquan Dong ◽

Baike Xi ◽

Ian Baxter

Keyword(s):

Sea Ice ◽

Large Scale ◽

Climate Models ◽

Lower Troposphere ◽

Arctic Sea Ice ◽

Albedo Feedback ◽

Ice Melt ◽

Low Clouds ◽

Arctic Sea ◽

The Impact

AbstractThe rapid Arctic sea ice retreat in the early 21st century is believed to be driven by several dynamic and thermodynamic feedbacks, such as ice-albedo feedback and water vapor feedback. However, the role of clouds in these feedbacks remains unclear since the causality between clouds and these processes is complex. Here, we use NASA CERES satellite products and NCAR CESM model simulations to suggest that summertime low clouds have played an important role in driving sea ice melt by amplifying the adiabatic warming induced by a stronger anticyclonic circulation aloft. The upper-level high pressure regulates low clouds through stronger downward motion and increasing lower troposphere relative humidity. The increased low clouds favor more sea ice melt via emitting stronger longwave radiation. Then decreased surface albedo triggers a positive ice-albedo feedback, which further enhances sea ice melt. Considering the importance of summertime low clouds, accurate simulation of this process is a prerequisite for climate models to produce reliable future projections of Arctic sea ice.

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Air mass origins influencing TTL chemical composition over West Africa during 2006 summer monsoon

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-10-15485-2010 ◽

2010 ◽

Vol 10 (6) ◽

pp. 15485-15536 ◽

Cited By ~ 11

Author(s):

K. S. Law ◽

F. Fierli ◽

F. Cairo ◽

H. Schlager ◽

S. Borrmann ◽

...

Keyword(s):

West Africa ◽

Summer Monsoon ◽

Large Scale ◽

Lower Stratosphere ◽

Deep Convection ◽

Lower Troposphere ◽

Trace Gas ◽

Air Mass ◽

Back Trajectories ◽

Air Masses

Abstract. Trace gas and aerosol data collected in the tropical tropopause layer (TTL) between 12–18.5 km by the M55 Geophysica aircraft as part of the SCOUT-AMMA campaign over West Africa during the summer monsoon in August 2006 have been analysed in terms of their air mass origins. Analysis of domain filling back trajectories arriving over West Africa, and in the specific region of the flights, showed that the M55 flights were generally representative of air masses arriving over West Africa during the first 2 weeks of August, 2006. Air originating from the mid-latitude lower stratosphere was under-sampled (in the mid-upper TTL) whilst air masses uplifted from central Africa (into the lower TTL) were over-sampled in the latter part of the campaign. Signatures of recent (previous 10 days) origins were superimposed on the large-scale westerly flow over West Africa. In the lower TTL, air masses were impacted by recent local deep convection over Africa at the level of main convective outflow (350 K, 200 hPa) and on certain days up to 370 K (100 hPa). Estimates of the fraction of air masses influenced by local convection vary from 10 to 50% depending on the method applied and from day to day during the campaign. The analysis shows that flights on 7, 8 and 11 August were more influenced by local convection than on 4 and 13 August allowing separation of trace gas and aerosol measurements into ''convective'' and ''non-convective'' flights. Strong signatures, particularly in short-lived species like CO, NO and fine-mode aerosols were seen during flights most influenced by convection up to 350–365 K. Observed profiles were also constantly perturbed by uplift (as high as 39%) of air masses from the mid to lower troposphere over Asia, India, and oceanic regions resulting in import of clean oceanic (e.g., O3-poor) or polluted air masses from Asia (high O3, CO, CO2) into West Africa. Thus, recent uplift of CO2 over Asia may contribute to the observed positive CO2 gradients in the TTL over West Africa. This suggests a more significant fraction of younger air masses in the TTL making it difficult to derive mean age of air from average gradients. Transport of air masses from the mid-latitude lower stratosphere had an impact from the mid-TTL upwards (20–40% above 370 K) during the campaign period importing air masses with high O3 and NOy. Ozone profiles show a less pronounced lower TTL minimum than observed previously by regular ozonesondes at other tropical locations. Concentrations are less than 100 ppbv in the lower TTL and vertical gradients less steep than in the upper TTL. The air mass origin analysis and simulations of in-situ net photochemical O3 production, initialised with observations, suggest that the lower TTL is significantly impacted by uplift of O3 precursors (over Africa and Asia) leading to positive production rates (up to 2 ppbv per day) in the lower and mid TTL even at moderate NOx levels. Photochemical O3 production increases with higher NOx and H2O in air masses with O3 less than 150 ppbv.

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Atmospheric conditions associated with high and low ozone concentrations in the lower troposphere over the Aegean sea

Global NEST Journal ◽

10.30955/gnj.001032 ◽

2013 ◽

Vol 15 (2) ◽

pp. 143-151

Keyword(s):

Large Scale ◽

Eastern Mediterranean ◽

Lower Troposphere ◽

Aegean Sea ◽

Weather Conditions ◽

Atmospheric Conditions ◽

Air Masses ◽

Synoptic Weather ◽

Major Factors ◽

Sea Area

In this study the vertical ozone profiles during summertime (June to August) of the MOZAIC (Measurement of Ozone and Water Vapor by Airbus in Service Aircraft) Project over the eastern Mediterranean airports of Heraklion and Rhodes in the Aegean Sea, have been analyzed in order to identify the major factors determining the ozone variability in the lower troposphere over this area. In total 42 ozone profiles have been examined, which have been collected during a 10-year period (1996-2006). In addition, the corresponding vertical profiles of temperature, relative humidity, carbon monoxide and wind speed have been also examined in parallel. The vertical summer ozone profiles have been classified into groups of highest and lowest ozone levels in the free troposphere (at the 3000-5000m and 1500-3000m layers) and the corresponding composite weather maps of geopotential heights have been plotted and their examination was focused on the Aegean Sea area. From the data analysis it comes out that for the examined area, in the lower troposphere but also within the boundary layer the role of the synoptic weather conditions and the associated large-scale transport of air masses, especially anticyclonic subsidence, seem to be more important in understanding the ozone variability than the local or regional short-term ozone photochemical production.

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