| 1. |
- Brooks, Ian M., et al.
(författare)
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The Turbulent Structure of the Arctic Summer Boundary Layer During The Arctic Summer Cloud-Ocean Study
- 2017
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Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 122:18, s. 9685-9704
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Tidskriftsartikel (refereegranskat)abstract
- The mostly ice covered Arctic Ocean is dominated by low-level liquid-or mixed-phase clouds. Turbulence within stratocumulus is primarily driven by cloud top cooling that induces convective instability. Using a suite of in situ and remote sensing instruments we characterize turbulent mixing in Arctic stratocumulus, and for the first time we estimate profiles of the gradient Richardson number at relatively high resolution in both time (10 min) and altitude (10 m). It is found that the mixing occurs both within the cloud, as expected, and by wind shear instability near the surface. About 75% of the time these two layers are separated by a stably stratified inversion at 100-200 m altitude. Exceptions are associated with low cloud bases that allow the cloud-driven turbulence to reach the surface. The results imply that turbulent coupling between the surface and the cloud is sporadic or intermittent.Plain Language Summary: The lower atmosphere over the summertime Arctic Ocean often consists of two well-mixed layers-a surface mixed layer and a cloud mixed layer-that are separated by a weak decoupling layer at about 100 to 300 m above the surface. In these cases, the cloud cannot interact directly with the surface. Large-scale forecast and climate models consistently fail to reproduce this observed structure and may thus fail to correctly reproduce the cloud properties and the amount of energy absorbed by or emitted from the surface as solar and infrared radiation. This contributes to errors in reproducing changes in sea ice concentration over time. Here we use measurements made in the central Arctic to study the processes controlling whether or not the cloud is coupled to the surface. The effect of wind at the surface is found not to be a controlling factor. The depth of the cloud mixed layer is critical, but the multiple processes influencing it cannot be separated using the data available here. However, cooling at cloud top by infrared radiation is key, as is the extension of cloud into the temperature inversion-a unique feature of Arctic clouds.
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| 2. |
- Devasthale, Abhay, et al.
(författare)
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A DECADE OF SPACEBORNE OBSERVATIONS OF THE ARCTIC ATMOSPHERE : Novel Insights from NASA's AIRS Instrument
- 2016
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Ingår i: Bulletin of The American Meteorological Society - (BAMS). - 0003-0007 .- 1520-0477. ; 97:11, s. 2163-2176
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Tidskriftsartikel (refereegranskat)abstract
- Arctic sea ice is declining rapidly and its annual ice extent minima reached record lows twice during the last decade. Large environmental and socioeconomic implications related to sea ice reduction in a warming world necessitate realistic simulations of the Arctic climate system, not least to formulate relevant environmental policies on an international scale. However, despite considerable progress in the last few decades, future climate projections from numerical models still exhibit the largest uncertainties over the polar regions. The lack of sufficient observations of essential climate variables is partly to blame for the poor representation of key atmospheric processes, and their coupling to the surface, in climate models. Observations from the hyper spectral Atmospheric Infrared Sounder (AIRS) instrument on board National Aeronautics and Space Administration (NASA)'s Aqua satellite are contributing toward improved understanding of the vertical structure of the atmosphere over the poles since 2002, including the lower troposphere. This part of the atmosphere is especially important in the Arctic, as it directly impacts sea ice and its short-term variability. Although in situ measurements provide invaluable ground truth, they are spatially and temporally inhomogeneous and sporadic over the Arctic. A growing number of studies are exploiting AIRS data to investigate the thermodynamic structure of the Arctic atmosphere, with applications ranging from understanding processes to deriving climatologies; all of which are also useful to test and improve parameterizations in climate models. As the AIRS data record now extends more than a decade, a select few of many such noteworthy applications of AIRS data over this challenging and rapidly changing landscape are highlighted here.
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| 3. |
- Devasthale, Abhay, et al.
(författare)
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The vertical distribution of thin features over the Arctic analysed from CALIPSO observations : Part 1: Optically thin clouds
- 2011
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Ingår i: Tellus. Series B, Chemical and physical meteorology. - : Stockholm University Press. - 0280-6509 .- 1600-0889. ; 63:1, s. 77-85
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Tidskriftsartikel (refereegranskat)abstract
- Clouds play a crucial role in the Arctic climate system. Therefore, it is essential to accurately and reliably quantify and understand cloud properties over the Arctic. It is also important to monitor and attribute changes in Arctic clouds. Here, we exploit the capability of the CALIPSO-CALIOP instrument and provide comprehensive statistics of tropospheric thin clouds, otherwise extremely difficult to monitor from passive satellite sensors. We use 4 yr of data (June 2006-May 2010) over the circumpolar Arctic, here defined as 67-82 degrees N, and characterize probability density functions of cloud base and top heights, geometrical thickness and zonal distribution of such cloud layers, separately for water and ice phases, and discuss seasonal variability of these properties. When computed for the entire study area, probability density functions of cloud base and top heights and geometrical thickness peak at 200-400, 1000-2000 and 400-800 m, respectively, for thin water clouds, while for ice clouds they peak at 6-8, 7-9 and 400-1000 m, respectively. In general, liquid clouds were often identified below 2 km during all seasons, whereas ice clouds were sensed throughout the majority of the upper troposphere and also, but to a smaller extent, below 2 km for all seasons.
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| 4. |
- Igel, Adele L., et al.
(författare)
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The free troposphere as a potential source of arctic boundary layer aerosol particles
- 2017
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 44:13, s. 7053-7060
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Tidskriftsartikel (refereegranskat)abstract
- This study investigates aerosol particle transport from the free troposphere to the boundary layer in the summertime high Arctic. Observations from the Arctic Summer Cloud Ocean Study field campaign show several occurrences of high aerosol particle concentrations above the boundary layer top. Large-eddy simulations suggest that when these enhanced aerosol concentrations are present, they can be an important source of aerosol particles for the boundary layer. Most particles are transported to the boundary layer by entrainment. However, it is found that mixed-phase stratocumulus clouds, which often extend into the inversion layer, also can mediate the transport of particles into the boundary layer by activation at cloud top and evaporation below cloud base. Finally, the simulations also suggest that aerosol properties at the surface sometimes may not be good indicators of aerosol properties in the cloud layer.
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| 5. |
- Loewe, Katharina, et al.
(författare)
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Modelling micro- and macrophysical contributors to the dissipation of an Arctic mixed-phase cloud during the Arctic Summer Cloud Ocean Study (ASCOS)
- 2017
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 17:11, s. 6693-6704
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Tidskriftsartikel (refereegranskat)abstract
- The Arctic climate is changing; temperature changes in the Arctic are greater than at midlatitudes, and changing atmospheric conditions influence Arctic mixed-phase clouds, which are important for the Arctic surface energy budget. These low-level clouds are frequently observed across the Arctic. They impact the turbulent and radiative heating of the open water, snow, and sea-ice-covered surfaces and influence the boundary layer structure. Therefore the processes that affect mixed-phase cloud life cycles are extremely important, yet relatively poorly understood. In this study, we present sensitivity studies using semi-idealized large eddy simulations (LESs) to identify processes contributing to the dissipation of Arctic mixed-phase clouds. We found that one potential main contributor to the dissipation of an observed Arctic mixed-phase cloud, during the Arctic Summer Cloud Ocean Study (ASCOS) field campaign, was a low cloud droplet number concentration (CDNC) of about 2 cm(-3). Introducing a high ice crystal concentration of 10 L-1 also resulted in cloud dissipation, but such high ice crystal concentrations were deemed unlikely for the present case. Sensitivity studies simulating the advection of dry air above the boundary layer inversion, as well as a modest increase in ice crystal concentration of 1 L-1, did not lead to cloud dissipation. As a requirement for small droplet numbers, pristine aerosol conditions in the Arctic environment are therefore considered an important factor determining the lifetime of Arctic mixed-phase clouds.
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| 6. |
- Mauritsen, T., et al.
(författare)
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An Arctic CCN-limited cloud-aerosol regime
- 2011
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 11:1, s. 165-173
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Tidskriftsartikel (refereegranskat)abstract
- On average, airborne aerosol particles cool the Earth's surface directly by absorbing and scattering sunlight and indirectly by influencing cloud reflectivity, life time, thickness or extent. Here we show that over the central Arctic Ocean, where there is frequently a lack of aerosol particles upon which clouds may form, a small increase in aerosol loading may enhance cloudiness thereby likely causing a climatologically significant warming at the ice-covered Arctic surface. Under these low concentration conditions cloud droplets grow to drizzle sizes and fall, even in the absence of collisions and coalescence, thereby diminishing cloud water. Evidence from a case study suggests that interactions between aerosol, clouds and precipitation could be responsible for attaining the observed low aerosol concentrations.
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| 7. |
- Naakka, Tuomas, et al.
(författare)
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Atmospheric moisture transport between mid-latitudes and the Arctic : Regional, seasonal and vertical distributions
- 2019
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Ingår i: International Journal of Climatology. - : Wiley. - 0899-8418 .- 1097-0088. ; 39:6, s. 2862-2879
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Tidskriftsartikel (refereegranskat)abstract
- Horizontal moisture transport has a manifold role in the Arctic climate system as it distributes atmospheric water vapour and thereby shapes the radiative and hydrological conditions. Moisture transport between the Arctic and the mid-latitudes was examined based on ERA-Interim reanalysis. The meridional net transport is only a small part of the water vapour exchange between the Arctic and mid-latitudes and does not give a complete view of temporal and spatial variations in the transport. Especially near the surface, most of the northwards moisture transport is balanced by the southwards transport, and therefore the meridional net moisture transport at 60 degrees-70 degrees N peaks approximately at 100 hPa higher altitude than the northwards and southwards moisture transports. The total moisture transport (sum of absolute northwards and southwards moisture transports) has a much larger seasonal variation than the net transport (mean meridional transport), and the strength of the total transport is related to atmospheric humidity rather than the wind field. Strong individual moisture transport events contribute to a large part of the northwards moisture transport. This is consistent with the result that the net moisture transport is essentially generated by temporal variations of moisture fluxes. The moisture transport due to stationary zonal variation in the mass flux mostly defines the spatial distribution of the meridional moisture transport. The seasonal cycle of the net moisture transport is related to the seasonal cycle of transient eddy moisture transport but inter-annual variations of the net moisture transport are largely influenced by the stationary eddy moisture transport.
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| 8. |
- Sedlar, Joseph, et al.
(författare)
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A Process-Based Climatological Evaluation of AIRS Level 3 Tropospheric Thermodynamics over the High-Latitude Arctic
- 2019
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Ingår i: Journal of Applied Meteorology and Climatology. - 1558-8424 .- 1558-8432. ; 58:8, s. 1867-1886
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Tidskriftsartikel (refereegranskat)abstract
- Measurements from spaceborne sensors have the unique capacity to fill spatial and temporal gaps in ground-based atmospheric observing systems, especially over the Arctic, where long-term observing stations are limited to pan-Arctic landmasses and infrequent field campaigns. The AIRS level 3 (L3) daily averaged thermodynamic profile product is widely used for process understanding across the sparsely observed Arctic atmosphere. However, detailed investigations into the accuracy of the AIRS L3 thermodynamic profiles product using in situ observations over the high-latitude Arctic are lacking. To address this void, we compiled a wealth of radiosounding profiles from long-term Arctic land stations and included soundings from intensive icebreaker-based field campaigns. These are used to evaluate daily mean thermodynamic profiles from the AIRS L3 product so that the community can understand to what extent such data records can be applied in scientific studies. Results indicate that, while the mid- to upper-troposphere temperature and specific humidity are captured relatively well by AIRS, the lower troposphere is susceptible to specific seasonal, and even monthly, biases. These differences have a critical influence on the lower-tropospheric stability structure. The relatively coarse vertical resolution of the AIRS L3 product, together with infrared radiation through persistent low Arctic cloud layers, leads to artificial thermodynamic structures that fail to accurately represent the lower Arctic atmosphere. These thermodynamic errors are likely to introduce artificial errors in the boundary layer structure and analysis of associated physical processes.
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| 9. |
- Sedlar, Joseph, et al.
(författare)
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A transitioning Arctic surface energy budget : the impacts of solar zenith angle, surface albedo and cloud radiative forcing
- 2011
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 37:7-8, s. 1643-1660
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Tidskriftsartikel (refereegranskat)abstract
- Snow surface and sea-ice energy budgets were measured near 87.5A degrees N during the Arctic Summer Cloud Ocean Study (ASCOS), from August to early September 2008. Surface temperature indicated four distinct temperature regimes, characterized by varying cloud, thermodynamic and solar properties. An initial warm, melt-season regime was interrupted by a 3-day cold regime where temperatures dropped from near zero to -7A degrees C. Subsequently mean energy budget residuals remained small and near zero for 1 week until once again temperatures dropped rapidly and the energy budget residuals became negative. Energy budget transitions were dominated by the net radiative fluxes, largely controlled by the cloudiness. Variable heat, moisture and cloud distributions were associated with changing air-masses. Surface cloud radiative forcing, the net radiative effect of clouds on the surface relative to clear skies, is estimated. Shortwave cloud forcing ranged between -50 W m(-2) and zero and varied significantly with surface albedo, solar zenith angle and cloud liquid water. Longwave cloud forcing was larger and generally ranged between 65 and 85 W m(-2), except when the cloud fraction was tenuous or contained little liquid water; thus the net effect of the clouds was to warm the surface. Both cold periods occurred under tenuous, or altogether absent, low-level clouds containing little liquid water, effectively reducing the cloud greenhouse effect. Freeze-up progression was enhanced by a combination of increasing solar zenith angles and surface albedo, while inhibited by a large, positive surface cloud forcing until a new air-mass with considerably less cloudiness advected over the experiment area.
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| 10. |
- Sedlar, Joseph, 1981-
(författare)
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Arctic clouds - interactions with radiation and thermodynamic structure
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Clouds play in important role in the climate system through their interaction with radiation. Globally, clouds tend to cool the Earth by reflecting solar radiation and shading the surface. Over the Arctic, clouds tend to have the opposite impact, where they instead warm the surface through the cloud greenhouse effect because the surface is generally quite reflective. The magnitude and overall effect of clouds on the surface varies significantly with the surface, cloud and thermodynamic characteristics and can have large impacts on the energy budget at the surface. Low-level central-Arctic stratus clouds interact with the thermodynamics in a manner differently than sub-tropical stratus. Observations from several Arctic observatories indicate that these clouds penetrate and persist within stable temperature inversion structures, rather than being limited to the base of the stable layer as observed in the subtropics. It is hypothesized that such interactions with the thermodynamics can impact for example the cloud phase, lifetime, and their relationship with the sub-cloud layer and surface. Analysis indicates both the thermodynamic setting and the cloud properties affect the vertical location of the cloud top relative to inversion base. Hypothetical longwave radiative impacts resulting from liquid water redistributions are identified and discussed. Clouds primarily influence the energy at the surface via interactions with radiation. Measurements from the central Arctic suggest that the transition of season from melting to freezing was largely determined by the presence, or absence, of liquid-containing clouds and the incumbent cloud longwave warming effect. The components affecting the cloud-radiative forcing are described with relation to the energy budget and the change of season. Additionally, the influence of altering cloud condensation nuclei as a mechanism for limiting cloud liquid water is shown to have strong influences on surface temperature and lower atmospheric stability. Finally, regional climate models, RCMs, are evaluated against an annual dataset to assess the ability of RCMs to represent cloud and radiation processes in the Arctic. It is shown that both inter-model and model-observation spread are rather significant. Biases in the cloud representations yield distinct biases in the radiative fluxes, and can result in significant local climate variations solely through these parameters.
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