Linking Sub-Tropical Evaporation and Extreme Precipitation Over East Antarctica: An Atmospheric River Case Study

We investigate an intense snowfall event between 15 and 18 February 2011 over the East Antarctic coastal region which contributed to roughly 24% of the annual snow accumulation. The event was previously associated with an atmospheric river, and here we use both Eulerian and Lagrangian analysis to gain an understanding of the processes contributing to the atmospheric river signature. The planetary-scale configuration during the event consisted of a persistent blocking situation resulting in a sustained meridional flow from the sub-tropics to the Antarctic ice sheet between 20 and 50°E. Within this configuration, synoptic-scale cyclogenesis contributed to slantwise ascent of moisture loaded air parcels toward Antarctica. Landfall of this cyclone’s warm sector coincided with the onset of Antarctic precipitation. Subsequently, a secondary cyclone developed along a pre-existing baroclinic zone. The rapid intensification and propagation speed of this mesoscale cyclone alongside the warm, moist air mass resulted in strong moisture flux convergence ahead of the cyclone, providing additional poleward moisture transport. The poleward progression of warm moist air and a corresponding decrease of sea-surface temperatures implied downward surface sensible and latent heat fluxes throughout the region of intense poleward moisture, roughly between 40 and 60°S. Hence, moisture uptake via surface evaporation was suppressed between the sub-tropics and the polar continent, favoring long-range transport. Identification of the surface moisture uptake region by tracing changes in moisture in air parcels confirmed the limited uptake of moisture during the poleward transport in this case study, with the primary moisture source for Antarctic precipitation located in the sub-tropics.publishedVersio

Characteristics of cold-air outbreak events and associated polar mesoscale cyclogenesis over the north Atlantic region

Equatorward excursions of cold polar air masses into ice-free regions, so-called cold-air outbreak (CAO) events, are frequently accompanied by the development of severe mesoscale weather features. Focusing on two key regions, the Labrador Sea and the Greenland–Norwegian Seas, we apply objective detection for both CAO events and polar mesoscale cyclones to outline the temporal evolution of CAO events and quantify associated mesoscale cyclogenesis. We introduce a novel metric, the CAO depth, which incorporates both the static stability and the temperature of the air mass. The large-scale atmospheric conditions during the onset of CAO events comprise a very cold upper-level trough over the CAO region and a surface cyclone downstream. As the CAO matures, the cold air mass extends southeastward, accompanied by lower static stability and enhanced surface fluxes. Despite the nearly 20° difference in latitude, CAO events over both regions exhibit similar evolution and characteristics including surface fluxes and thermodynamic structure. About two-thirds of the identified CAO events are accompanied by polar mesoscale cyclogenesis, with the majority of mesoscale cyclones originating inside the cold air masses. Neither the duration nor the maturity of the CAO event seems relevant for mesoscale cyclogenesis. Mesoscale cyclogenesis conditions during CAO events over the Labrador Sea are warmer, moister and exhibit stronger surface latent heat fluxes than their Norwegian Sea counterparts

An evaluation of surface meteorology and fluxes over the Iceland and Greenland Seas in ERA5 reanalysis: the impact of sea ice distribution

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Renfrew, I. A., Barrell, C., Elvidge, A. D., Brooke, J. K., Duscha, C., King, J. C., Kristiansen, J., Cope, T. L., Moore, G. W. K., Pickart, R. S., Reuder, J., Sandu, I., Sergeev, D., Terpstra, A., Vage, K., & Weiss, A. An evaluation of surface meteorology and fluxes over the Iceland and Greenland Seas in ERA5 reanalysis: the impact of sea ice distribution. Quarterly Journal of the Royal Meteorological Society, (2020): 1-22, doi:10.1002/qj.3941.The Iceland and Greenland Seas are a crucial region for the climate system, being the headwaters of the lower limb of the Atlantic Meridional Overturning Circulation. Investigating the atmosphere–ocean–ice processes in this region often necessitates the use of meteorological reanalyses—a representation of the atmospheric state based on the assimilation of observations into a numerical weather prediction system. Knowing the quality of reanalysis products is vital for their proper use. Here we evaluate the surface‐layer meteorology and surface turbulent fluxes in winter and spring for the latest reanalysis from the European Centre for Medium‐Range Weather Forecasts, i.e., ERA5. In situ observations from a meteorological buoy, a research vessel, and a research aircraft during the Iceland–Greenland Seas Project provide unparalleled coverage of this climatically important region. The observations are independent of ERA5. They allow a comprehensive evaluation of the surface meteorology and fluxes of these subpolar seas and, for the first time, a specific focus on the marginal ice zone. Over the ice‐free ocean, ERA5 generally compares well to the observations of surface‐layer meteorology and turbulent fluxes. However, over the marginal ice zone, the correspondence is noticeably less accurate: for example, the root‐mean‐square errors are significantly higher for surface temperature, wind speed, and surface sensible heat flux. The primary reason for the difference in reanalysis quality is an overly smooth sea‐ice distribution in the surface boundary conditions used in ERA5. Particularly over the marginal ice zone, unrepresented variability and uncertainties in how to parameterize surface exchange compromise the quality of the reanalyses. A parallel evaluation of higher‐resolution forecast fields from the Met Office's Unified Model corroborates these findings.This study was part of the Iceland Greenland Seas Project. Funding was from the NERC AFIS grant (NE/N009754/1), the ALERTNESS (Advanced models and weather prediction in the Arctic: enhanced capacity from observations and polar process representations) project (Research Council of Norway project number 280573), the Trond Mohn Foundation (BFS2016REK01), and the National Science Foundation grant OCE‐1558742. The Leosphere WindCube v2 and the Wavescan buoy are part of the OBLO (Offshore Boundary Layer Observatory) infrastructure funded by the Research Council of Norway (project number 227777)

Dynamical Perspectives on the Formation and Intensification of Polar Lows

This thesis consists of a collection of scientific publications addressing dynamical aspects of the formation and intensification of polar lows. Polar lows are small scale, intense, short-lived cyclones developing over ice-free oceans at high latitudes. In the first paper, environmental atmospheric conditions during polar low genesis are identified. These environmental conditions are classified based on the direction between the thermal wind and the mean flow in the lower troposphere. If the thermal wind and mean flow are in opposite direction the environment is classified as reverse shear, if they are in the same direction the environment is classified as forward shear. The two types of pre-polar low environments exhibit distinctly different features in terms of synoptic scale patterns, baroclinicity, configurations of sea surface temperature, depth and stratification of the troposphere, polar low propagation directions, and surface fluxes. The ambient polar low environment during forward shear conditions resembles typical mid-latitude baroclinic cyclogenesis, whereas the reverse shear conditions are characterized by the presence of a synoptic scale, occluded low over the genesis location, and a strong low-level jet. In the second paper, a method to initialize baroclinic channel models is devised. The initial conditions are hydrostatically and geostrophically balanced, which makes them particularly suitable for studying rapid cyclogenesis. Furthermore, the method allows the definition of an arbitrary windfield, hence the initialization method exhibits a large degree of freedom in defining the initial conditions. We performed an unperturbed simulation which features minimal gravity wave activity, illustrating the balanced initial conditions. We also performed a perturbed simulation to examplify the utility of the setup. In this simulation baroclinic instability is triggered resulting in perturbation growth with time- and length-scales in agreement with observed polar low development. In the third paper, an idealized baroclinic channel model is utilized to investigate the role of latent heating during polar low development in forward shear conditions. Within this idealized setup, a low-level, weak, cyclonic perturbation is able to amplify in absence of upper-level forcing, radiation, or surface fluxes. Crucial for rapid development is sufficient latent heat release in the north-eastern quadrant of the cyclone. The potential for latent heat release to occur depends on the environmental relative humidity, baroclinicity and static stability. Due to the Arctic configuration of the setup, the perturbation depth is relative shallow, which in turn improves the effectiveness of latent heat release on cyclone amplification

An Initialization Method for Idealized Channel Simulations

Abstract Idealized model simulations have long been established as valuable tools to gain insight into atmospheric phenomena by providing a simplified, easier to comprehend version of the complex atmospheric system. A specific subgroup of idealized simulations, such as baroclinic channel models, requires the initialization of the model with balanced atmospheric fields to investigate the evolution of an introduced perturbation. The quality of these simulations depends on the degree of balance of the initial state, as imbalances result in geostrophic and hydrostatic adjustment processes that potentially skew the results. In this paper, a general method to create geostrophically and hydrostatically balanced initial conditions is introduced. The major benefit of this method is the possibility to directly define a basic state wind field with the pertinent atmospheric fields being derived given appropriate boundary conditions. Application of the method is exemplified by constructing initial conditions for a baroclinic test case with WRF and analyzing a perturbed and unperturbed numerical simulation. The unperturbed simulation exhibits weak inertia–gravity wave activity and minimal adjustment of the initial state during a 5-day simulation, which confirms the high degree of initial balance provided by the initialization technique. In the perturbed simulation, baroclinic instability is initiated, resulting in a cyclogenesis event similar to previous idealized baroclinic channel simulations. The proposed method is compared with initial conditions formulated in a Boussinesq framework, illustrating the difference in imbalances and their effect on perturbation growth.</jats:p

Characteristics of Cold Air Outbreaks and associated Polar Mesoscale Cyclones in the North-Atlantic region

&amp;lt;p&amp;gt;Geographically confined, equatorward excursions of cold air masses into ice-free regions account for the majority of oceanic heat loss in key regions for deepwater formation in the North Atlantic. These cold-air outbreaks (CAO) are frequently accompanied by the development of severe mesoscale weather features, such as intense low-level jets and polar lows. Exchange of heat, moisture and momentum between the ocean and atmosphere in response to mesoscale features, either directly, or indirectly via modulating the longevity and intensity of the cold air mass modulates the wind-driven oceanic gyres. Yet, it remains unclear how often mesoscale cyclones accompany cold-air outbreaks, and how mesoscale features modify the air-sea interactions.&amp;amp;#160;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Focusing on two key regions, the Labrador Sea and the Greenland/Norwegian Sea, we outline the temporal evolution of CAO events and associated mesoscale cyclogenesis. We apply objective detection to both CAO events and mesoscale cyclones and introduce an alternative metric to characterize the cold air mass. Despite the nearly 20 degrees difference in latitude, CAOs over both regions exhibit rather similar evolution, surface fluxes, and thermodynamic structure. The large scale configuration during CAO onset comprises a very cold upper level through over the CAO region and a surface cyclone downstream. As the CAO matures the cold air mass extends towards the south-east, accompanied by enhanced surface fluxes and destabilization of the CAO airmass. About 2/3 of the CAO events are accompanied by mesoscale cyclogenesis, with the majority of mesoscale cyclones originating inside the cold air masses. Neither the duration nor the maturity of the CAO event is relevant for the initiation of mesoscale cyclogenesis. Genesis conditions for mesoscale cyclogenesis during CAOs over the Labrador Sea are moister and exhibit stronger surface fluxes compared to their Norwegian Sea counterparts.&amp;lt;/p&amp;gt; </jats:p

Forward and Reverse Shear Environments during Polar Low Genesis over the Northeast Atlantic

Abstract The synoptic and subsynoptic environments associated with polar low genesis are examined. Ambient pre–polar low environments are classified as forward or reverse shear conditions based on the angle between the thermal and mean wind. Forward shear environments are associated with a synoptic-scale ridge over Scandinavia, featuring a zonally oriented baroclinic zone extending throughout the troposphere with a wind speed maximum at the tropopause. Similar to typical midlatitude cyclogenesis, concurrent wavelike development occurs both in the lower and upper troposphere along the baroclinic zone and the mean propagation direction is eastward, parallel to isolines of sea surface temperature. Reverse shear environments exhibit a distinctly different structure and are characterized by a trough over Scandinavia, associated with a synoptic-scale, occluded cyclone. The genesis area exhibits strong cold air advection on its right-hand side and polar low development occurs on the warm side of an intense low-level jet. The environment resembles the characteristics conducive to secondary development associated with frontal instability. Polar lows developing in this configuration propagate mainly southward, perpendicular to isolines of sea surface temperature. The two genesis environments exhibit similar temperature differences between the sea surface and atmosphere near the surface, yet the magnitude of the surface fluxes is approximately double during reverse shear conditions due to stronger low-level winds. The ratio between surface sensible and latent heat fluxes is close to unity for both shear environments.</jats:p

Linking Sub-Tropical Evaporation and Extreme Precipitation Over East Antarctica: An Atmospheric River Case Study

We investigate an intense snowfall event between 15 and 18 February 2011 over the East Antarctic coastal region which contributed to roughly 24% of the annual snow accumulation. The event was previously associated with an atmospheric river, and here we use both Eulerian and Lagrangian analysis to gain an understanding of the processes contributing to the atmospheric river signature. The planetary-scale configuration during the event consisted of a persistent blocking situation resulting in a sustained meridional flow from the sub-tropics to the Antarctic ice sheet between 20 and 50°E. Within this configuration, synoptic-scale cyclogenesis contributed to slantwise ascent of moisture loaded air parcels toward Antarctica. Landfall of this cyclone’s warm sector coincided with the onset of Antarctic precipitation. Subsequently, a secondary cyclone developed along a pre-existing baroclinic zone. The rapid intensification and propagation speed of this mesoscale cyclone alongside the warm, moist air mass resulted in strong moisture flux convergence ahead of the cyclone, providing additional poleward moisture transport. The poleward progression of warm moist air and a corresponding decrease of sea-surface temperatures implied downward surface sensible and latent heat fluxes throughout the region of intense poleward moisture, roughly between 40 and 60°S. Hence, moisture uptake via surface evaporation was suppressed between the sub-tropics and the polar continent, favoring long-range transport. Identification of the surface moisture uptake region by tracing changes in moisture in air parcels confirmed the limited uptake of moisture during the poleward transport in this case study, with the primary moisture source for Antarctic precipitation located in the sub-tropics