A Ship-Based Characterization of Coherent Boundary-Layer Structures Over the Lifecycle of a Marine Cold-Air Outbreak

Convective coherent structures shape the atmospheric boundary layer over the lifecycle of marine cold-air outbreaks (CAOs). Aircraft measurements have been used to characterize such structures in past CAOs. Yet, aircraft case studies are limited to snapshots of a few hours and do not capture how coherent structures, and the associated boundary-layer characteristics, change over the CAO time scale, which can be on the order of several days. We present a novel ship-based approach to determine the evolution of the coherent-structure characteristics, based on profiling lidar observations. Over the lifecycle of a multi-day CAO we show how these structures interact with boundary-layer characteristics, simultaneously obtained by a multi-sensor set-up. Observations are taken during the Iceland Greenland Seas Project’s wintertime cruise in February and March 2018. For the evaluated CAO event, we successfully identify cellular coherent structures of varying size in the order of 4 × 102 m to 104 m and velocity amplitudes of up to 0.5 m s−1 in the vertical and 1 m s−1 in the horizontal. The structures’ characteristics are sensitive to the near-surface stability and the Richardson number. We observe the largest coherent structures most frequently for conditions when turbulence generation is weakly buoyancy dominated. Structures of increasing size contribute efficiently to the overturning of the boundary layer and are linked to the growth of the convective boundary-layer depth. The new approach provides robust statistics for organized convection, which would be easy to extend by additional observations during convective events from vessels of opportunity operating in relevant areas.publishedVersio

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)

Functional and structural readouts for early detection of retinal involvement in multiple sclerosis

IntroductionThe retina, a window into the brain, allows for the investigation of many disease-associated inflammatory and neurodegenerative changes affecting the central nervous system (CNS). Multiple sclerosis (MS), an autoimmune disease targeting the CNS, typically impacts on the visual system including the retina. Hence, we aimed to establish innovative functional retinal measures of MS-related damage, e.g., spatially resolved non-invasive retinal electrophysiology, backed by established morphological retinal imaging markers, i.e., optical coherence tomography (OCT).Methods20 healthy controls (HC) and 37 people with MS [17 without history of optic neuritis (NON) and 20 with (HON) history of optic neuritis] were included. In this work, we differentially assessed photoreceptor/bipolar cells (distal retina) and retinal ganglion cell (RGC, proximal retina) function besides structural assessment (OCT). We compared two multifocal electroretinography-based approaches, i.e., the multifocal pattern electroretinogram (mfPERG) and the multifocal electroretinogram to record photopic negative response (mfERGPhNR). Structural assessment utilized peripapillary retinal nerve fiber layer thickness (pRNFL) and macular scans to calculate outer nuclear thickness (ONL) and macular ganglion cell inner plexiform layer thickness (GCIPL). One eye was randomly selected per subject.ResultsIn NON, photoreceptor/bipolar cell layer had dysfunctional responses evidenced by reduced mfERGPhNR-N1 peak time of the summed response, but preserved structural integrity. Further, both NON and HON demonstrated abnormal RGC responses as evidenced by the photopic negative response of mfERGPhNR (mfPhNR) and mfPERG indices (P < 0.05). Structurally, only HON had thinned retina at the level of RGCs in the macula (GCIPL, P < 0.01) and the peripapillary area (pRNFL, P < 0.01). All three modalities showed good performance to differentiate MS-related damage from HC, 71–81% area under curve.ConclusionIn conclusion, while structural damage was evident mainly for HON, functional measures were the only retinal read-outs of MS-related retinal damage that were independent of optic neuritis, observed for NON. These results indicate retinal MS-related inflammatory processes in the retina prior to optic neuritis. They highlight the importance of retinal electrophysiology in MS diagnostics and its potential as a sensitive biomarker for follow-up in innovative interventions

Statistic and coherence response of ship-based lidar observations to motion compensation

Ship-based profiling Lidar systems experience a strong influence of rotational and translational motion on beam direction and hence the line-of-sight velocity. This motion error is inherited by the retrieved 3-dimensional wind vector and is especially visible in the velocity spectra and cross-spectra of velocities at different measurement heights (coherence). Applying motion compensation on the line-of-sight velocity observations was found to have a strong impact on the statistical properties of the retrieved wind vector and successfully improved the corresponding velocity distributions. The impact of motion correction on the spectra of the horizontal wind speed components was found to be neglectable. The Lidar measurement principle, in particular the effect of cross-contamination at higher frequencies, was found to have a larger impact in shaping the horizontal spectra than motion correction. Vertical velocity spectra were strongly affected by ship motion and the motion correction was only partly successful. Precisely, this effect was present at frequencies larger than the resonance frequency of the ship

A ship-based characterization of coherent boundary-layer structures over the lifecycle of a marine cold-air outbreak

Convective coherent structures shape the atmospheric boundary layer over the lifecycle of marine cold-air outbreaks (CAOs). Aircraft measurements have been used to characterize such structures in past CAOs. Yet, aircraft case studies are limited to snapshots of a few hours and do not capture how coherent structures, and the associated boundary-layer characteristics, change over the CAO time scale, which can be on the order of several days. We present a novel ship-based approach to determine the evolution of the coherent-structure characteristics, based on profiling lidar observations. Over the lifecycle of a multi-day CAO we show how these structures interact with boundary-layer characteristics, simultaneously obtained by a multi-sensor set-up. Observations are taken during the Iceland Greenland Seas Project’s wintertime cruise in February and March 2018. For the evaluated CAO event, we successfully identify cellular coherent structures of varying size in the order of 4 × 102 m to 104 m and velocity amplitudes of up to 0.5 m s−1 in the vertical and 1 m s−1 in the horizontal. The structures’ characteristics are sensitive to the near-surface stability and the Richardson number. We observe the largest coherent structures most frequently for conditions when turbulence generation is weakly buoyancy dominated. Structures of increasing size contribute efficiently to the overturning of the boundary layer and are linked to the growth of the convective boundary-layer depth. The new approach provides robust statistics for organized convection, which would be easy to extend by additional observations during convective events from vessels of opportunity operating in relevant areas

The COTUR project: remote sensing of offshore turbulence for wind energy application

The paper presents the measurement strategy and data set collected during the COTUR (COherence of TURbulence with lidars) campaign. This field experiment took place from February 2019 to April 2020 on the southwestern coast of Norway. The coherence quantifies the spatial correlation of eddies and is little known in the marine atmospheric boundary layer. The study was motivated by the need to better characterize the lateral coherence, which partly governs the dynamic wind load on multi-megawatt offshore wind turbines. During the COTUR campaign, the coherence was studied using land-based remote sensing technology. The instrument setup consisted of three long-range scanning Doppler wind lidars, one Doppler wind lidar profiler and one passive microwave radiometer. Both the WindScanner software and LidarPlanner software were used jointly to simultaneously orient the three scanner heads into the mean wind direction, which was provided by the lidar wind profiler. The radiometer instrument complemented these measurements by providing temperature and humidity profiles in the atmospheric boundary layer. The scanning beams were pointed slightly upwards to record turbulence characteristics both within and above the surface layer, providing further insight on the applicability of surface-layer scaling to model the turbulent wind load on offshore wind turbines. The preliminary results show limited variations of the lateral coherence with the scanning distance. A slight increase in the identified Davenport decay coefficient with the height is partly due to the limited pointing accuracy of the instruments. These results underline the importance of achieving pointing errors under 0.1∘ to study properly the lateral coherence of turbulence at scanning distances of several kilometres

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

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

The Iceland Greenland seas project

The Iceland Greenland Seas Project (IGP) is a coordinated atmosphere–ocean research program investigating climate processes in the source region of the densest waters of the Atlantic meridional overturning circulation. During February and March 2018, a field campaign was executed over the Iceland and southern Greenland Seas that utilized a range of observing platforms to investigate critical processes in the region, including a research vessel, a research aircraft, moorings, sea gliders, floats, and a meteorological buoy. A remarkable feature of the field campaign was the highly coordinated deployment of the observing platforms, whereby the research vessel and aircraft tracks were planned in concert to allow simultaneous sampling of the atmosphere, the ocean, and their interactions. This joint planning was supported by tailor-made convection-permitting weather forecasts and novel diagnostics from an ensemble prediction system. The scientific aims of the IGP are to characterize the atmospheric forcing and the ocean response of coupled processes; in particular, cold-air outbreaks in the vicinity of the marginal ice zone and their triggering of oceanic heat loss, and the role of freshwater in the generation of dense water masses. The campaign observed the life cycle of a long-lasting cold-air outbreak over the Iceland Sea and the development of a cold-air outbreak over the Greenland Sea. Repeated profiling revealed the immediate impact on the ocean, while a comprehensive hydrographic survey provided a rare picture of these subpolar seas in winter. A joint atmosphere–ocean approach is also being used in the analysis phase, with coupled observational analysis and coordinated numerical modeling activities underway