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  • 1.
    Axelsson, Per
    et al.
    Stockholm Univ, Dept Meteorol, S-10691 Stockholm, Sweden.;Stockholm Univ, Bert Bolin Ctr Climate Res, S-10691 Stockholm, Sweden..
    Tjernström, Michael
    Stockholm Univ, Dept Meteorol, S-10691 Stockholm, Sweden.;Stockholm Univ, Bert Bolin Ctr Climate Res, S-10691 Stockholm, Sweden..
    Söderberg, Stefan
    WeatherTech Scandinavia Inc, S-75310 Uppsala, Sweden..
    Svensson, Gunilla
    Stockholm Univ, Dept Meteorol, S-10691 Stockholm, Sweden.;Stockholm Univ, Bert Bolin Ctr Climate Res, S-10691 Stockholm, Sweden..
    An Ensemble of Arctic Simulations of the AOE-2001 Field Experiment2011In: Atmosphere, ISSN 2073-4433, E-ISSN 2073-4433, Vol. 2, no 2, p. 146-170Article in journal (Refereed)
    Abstract [en]

    An ensemble of model runs with the COAMPS (c) regional model is compared to observations in the central Arctic for August 2001 from the Arctic Ocean Experiment 2001 (AOE-2001). The results are from a 6-km horizontal resolution 2nd, inner, nest of the model while the outermost model domain covers the pan-Arctic region, including the marginal ice zone and some of the land areas around the Arctic Ocean. Sea surface temperature and ice cover were prescribed from satellite data while sea-ice surface properties were modeled with an energy balance model, assuming a constant ice thickness. Five ensemble members were generated by altering the initialization time for the innermost nest, the surface roughness and the turbulent mixing scheme for clouds. The large size of the outer domain means that the model simulations have substantial deviations from the observations at synoptic-scale time scales. Therefore the evaluation focuses on statistical measures, rather than in details of individual ensemble member performance as compared directly to observations. In this context, the ensemble members are surprisingly similar even though details differ significantly. The ensemble average results features two main systematic problems: a consistent temperature bias, with too low temperatures below 2-3 km and slightly high temperatures through the rest of the troposphere, and a significant underestimation of the lowest clouds. In terms of total cloud cover, however, the model produces a realistic result; it is the very lowest clouds that are essentially missing. The temperature bias initially appears to be related to an interaction between clouds and radiation; the shape of the mean radiative heating-rate profile is very similar to that of the temperature bias. The lack of the lowest clouds could be due to the too low temperatures in conjunction with a cloud scheme that overestimates the transfer of cloud droplets to ice particles that precipitate. The different terms in the surface energy balance as well as the surface stress has only small systematic errors and are surprisingly consistent between the members.

  • 2. Naakka, T.
    et al.
    Nygård, T.
    Tjernström, Michael
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Vihma, T.
    Pirazzini, R.
    Brooks, I. M.
    The Impact of Radiosounding Observations on Numerical Weather Prediction Analyses in the Arctic2019In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 14, p. 8527-8535Article in journal (Refereed)
    Abstract [en]

    The radiosounding network in the Arctic, despite being sparse, is a crucial part of the atmospheric observing system for weather prediction and reanalysis. The spatial coverage of the network was evaluated using a numerical weather prediction model, comparing radiosonde observations from Arctic land stations and expeditions in the central Arctic Ocean with operational analyses and background fields (12-hr forecasts) from European Centre for Medium-Range Weather Forecasts for January 2016 to September 2018. The results show that the impact of radiosonde observations on analyses has large geographical variation. In data-sparse areas, such as the central Arctic Ocean, high-quality radiosonde observations substantially improve the analyses, while satellite observations are not able to compensate for the large spatial gap in the radiosounding network. In areas where the network is reasonably dense, the quality of background field is more related to how radiosonde observations are utilized in the assimilation and to the quality of those observations.

  • 3. Sedlar, Joseph
    et al.
    Tjernström, Michael
    Stockholms universitet, Meteorologiska institutionen (MISU).
    A Process-Based Climatological Evaluation of AIRS Level 3 Tropospheric Thermodynamics over the High-Latitude Arctic2019In: Journal of Applied Meteorology and Climatology, ISSN 1558-8424, E-ISSN 1558-8432, Vol. 58, no 8, p. 1867-1886Article in journal (Refereed)
    Abstract [en]

    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.

  • 4.
    Sedlar, Joseph
    et al.
    Stockholm Univ, Dept Meteorol, S-10691 Stockholm, Sweden..
    Tjernström, Michael
    Stockholm Univ, Dept Meteorol, S-10691 Stockholm, Sweden..
    Mauritsen, Thorsten
    Max Planck Inst Meteorol, Hamburg, Germany..
    Shupe, Matthew D.
    Univ Colorado, Boulder, CO 80309 USA.;NOAA ESRL PSD, Boulder, CO USA..
    Brooks, Ian M.
    Univ Leeds, Sch Earth & Environm, Leeds, W Yorkshire, England..
    Persson, P. Ola G.
    Univ Colorado, Boulder, CO 80309 USA.;NOAA ESRL PSD, Boulder, CO USA..
    Birch, Cathryn E.
    Univ Leeds, Sch Earth & Environm, Leeds, W Yorkshire, England..
    Leck, Caroline
    Stockholm Univ, Dept Meteorol, S-10691 Stockholm, Sweden..
    Sirevaag, Anders
    Univ Bergen, Bergen, Norway.;Bjerknes Ctr Climate Res, Bergen, Norway..
    Nicolaus, Marcel
    Norwegian Polar Res Inst, Tromso, Norway.;Alfred Wegener Inst Polar & Marine Res, D-2850 Bremerhaven, Germany..
    A transitioning Arctic surface energy budget: the impacts of solar zenith angle, surface albedo and cloud radiative forcing2011In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 37, no 7-8, p. 1643-1660Article in journal (Refereed)
    Abstract [en]

    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.

  • 5. Tjernstrom, Michael
    Is there a diurnal cycle in the summer cloud-capped arctic boundary layer?2007In: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 64, no 11, p. 3970-3986Article in journal (Refereed)
    Abstract [en]

    Data from the Arctic Ocean Experiment 2001 (AOE-2001) are used to study the vertical structure and diurnal cycle of the summertime central Arctic cloud-capped boundary layer. Mean conditions show a shallow stratocumulus-capped boundary layer, with a nearly moist neutrally stratified cloud layer, although cloud tops often penetrated into the stable inversion. The subcloud layer was more often stably stratified. Conditions near the surface were relatively steady, with a strong control on temperature and moisture by the melting ice surface. A statistically significant diurnal cycle was found in many parameters, although weak in near-surface temperature and moisture. Near-surface wind speed and direction and friction velocity had a pronounced cycle, while turbulent kinetic energy showed no significant diurnal variability. The cloud layer had the most pronounced diurnal variability, with lowest cloud-base height midday followed by enhanced drizzle and temporarily higher cloud-top heights in the afternoon. This is opposite to the cycle found in midlatitude or subtropical marine stratocumulus. The cloud layer was warmest (coolest) and more (less) stably stratified midafternoon (midmorning), coinciding with the coolest (warmest) but least (most) stably stratified capping inversion layer. It is speculated that drizzle is important in regulating the diurnal variability in the cloud layer, facilitated by enhanced midday mixing due to a differential diurnal variability in cloud and subcloud layer stability. Changing the Arctic aerosol climate could change these clouds to a more typical "marine stratocumulus structure," which could act as a negative feedback on Arctic warming.

  • 6.
    Tjernström, Michael
    et al.
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Shupe, Matthew D.
    Brooks, Ian M.
    Achtert, Peggy
    Prytherch, John
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Sedlar, Joseph
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Arctic Summer Airmass Transformation, Surface Inversions, and the Surface Energy Budget2019In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 32, no 3, p. 769-789Article in journal (Refereed)
    Abstract [en]

    During the Arctic Clouds in Summer Experiment (ACSE) in summer 2014 a weeklong period of warm-air advection over melting sea ice, with the formation of a strong surface temperature inversion and dense fog, was observed. Based on an analysis of the surface energy budget, we formulated the hypothesis that, because of the airmass transformation, additional surface heating occurs during warm-air intrusions in a zone near the ice edge. To test this hypothesis, we explore all cases with surface inversions occurring during ACSE and then characterize the inversions in detail. We find that they always occur with advection from the south and are associated with subsidence. Analyzing only inversion cases over sea ice, we find two categories: one with increasing moisture in the inversion and one with constant or decreasing moisture with height. During surface inversions with increasing moisture with height, an extra 10-25 W m(-2) of surface heating was observed, compared to cases without surface inversions; the surface turbulent heat flux was the largest single term. Cases with less moisture in the inversion were often cloud free and the extra solar radiation plus the turbulent surface heat flux caused by the inversion was roughly balanced by the loss of net longwave radiation.

1 - 6 of 6
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