The Turbulent Structure of the Arctic Summer Boundary Layer During The Arctic Summer Cloud‐Ocean Study (original) (raw)
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Cloud and boundary layer interactions over the Arctic sea ice in late summer
Atmospheric Chemistry and Physics, 2013
Observations from the Arctic Summer Cloud Ocean Study (ASCOS), in the central Arctic sea-ice pack in late summer 2008, provide a detailed view of cloudatmosphere-surface interactions and vertical mixing processes over the sea-ice environment. Measurements from a suite of ground-based remote sensors, near-surface meteorological and aerosol instruments, and profiles from radiosondes and a helicopter are combined to characterize a weeklong period dominated by low-level, mixed-phase, stratocumulus clouds. Detailed case studies and statistical analyses are used to develop a conceptual model for the cloud and atmosphere structure and their interactions in this environment.
2012
Understanding the rapidly changing climate in the Arctic is limited by a lack of understanding of underlying strong feedback mechanisms that are specific to the Arctic. Progress in this field can only be obtained by process-level observations; this is the motivation for intensive ice-breaker-based campaigns such as that described in this paper: the Arctic Summer Cloud-Ocean Study (ASCOS). However, detailed field observations also have to be put in the context of the larger-scale meteorology, and short field campaigns have to be analysed within the context of the underlying climate state and temporal anomalies from this. To aid in the analysis of other parameters or processes observed during this campaign, this paper provides an overview of the synoptic-scale meteorology and its climatic anomaly during the ASCOS field deployment. It also provides a statistical analysis of key features during the campaign, such as some key meteorological variables, the vertical structure of the lower troposphere and clouds, and energy fluxes at the surface. In order to assess the representativity of the ASCOS results, we also compare these features to similar observations obtained during three earlier summer experiments in the Arctic Ocean, the AOE-96, SHEBA and AOE-2001 expeditions. We find that these expeditions share many key features of the summertime lower troposphere. Taking ASCOS and the previous expeditions together, a common picture emerges with a large amount of low-level cloud in a well-mixed shallow boundary layer, capped by a weak to moderately strong inversion where moisture, and sometimes also cloud top, penetrate into the lower parts of the inversion. Much of the boundary-layer mixing is due to cloud-top cooling and subsequent buoyant overturning of the cloud. The cloud layer may, or may not, be connected with surface processes depending on the depths of the cloud and surface-based boundary layers and on the relative strengths of surface-shear and cloud-buoyancy turbulence generation. The latter also implies a connection between the cloud layer and the free troposphere through entrainment at cloud top.
The thermodynamic structure of summer Arctic stratocumulus and the dynamic coupling to the surface
The vertical structure of Arctic low-level clouds and Arctic boundary layer is studied, using observations from ASCOS (Arctic Summer Cloud Ocean Study), in the central Arctic, in late summer 2008. Two general types of cloud structures are examined: the "neutrally stratified" and "stably stratified" clouds. Neutrally stratified are mixedphase clouds where radiative-cooling near cloud top produces turbulence that generates a cloud-driven mixed layer. When this layer mixes with the surface-generated turbulence, the cloud layer is coupled to the surface, whereas when such an interaction does not occur, it remains decoupled; the latter state is most frequently observed. The decoupled clouds are usually higher compared to the coupled; differences in thickness or cloud water properties between the two cases are however not found. The surface fluxes are also very similar for both states. The decoupled clouds exhibit a bimodal thermodynamic structure, depending on the depth of the sub-cloud mixed layer (SCML): clouds with shallower SCMLs are disconnected from the surface by weak inversions, whereas those that lay over a deeper SCML are associated with stronger inversions at the decoupling height. Neutrally stratified clouds generally precipitate; the evaporation/sublimation of precipitation often enhances the decoupling state. Finally, stably stratified clouds are usually lower, geometrically and optically thinner, non-precipitating liquidwater clouds, not containing enough liquid to drive efficient mixing through cloud-top cooling.
Atmospheric Chemistry and Physics Discussions
Impacts of aerosol on mixed-phase cloud evolution play a potentially important role in Arctic climate, but remain poorly understood. The way in which aerosol, clouds and turbulence interact, is speculated to significantly modify the cloud evolution. There has been an increasing number of field observations of the ice clouds in Arctic, however it has proven hard to gain insight into these complex interactions using measurements alone. This model study aims to help filling this gap in the current understanding of low-level Arctic clouds, by combining high resolution simulations with new field campaign data. The main focus is on the impact of the cloud condensation nuclei concentration (CCN) on the properties of cloud and mixed-layer turbulence in an evolving boundary layer. We configure semi-idealised model scenarios based on the weather situation observed over open ocean during two research flights of the ACLOUD campaign, which took place over Fram Strait northwest of Svalbard. A demi-Lagrangian frame of reference is adopted, with the model domain following low level air masses and the large-scale forcings derived from weather model analyses and short-range forecasts. Adjustments in the initial state are made based on comparison to dropsonde data. The simulations reproduce the observed general structure of the cloud-bearing Arctic mixed layer. Results further show that while the ice phase forms just a fraction of the mass of cloud water, it is responsible for most of the precipitation, in line with previous observational and LES studies. A lower initial CCN concentration generally results into a faster glaciation of the cloud, leading to a faster removal of the cloud water, and also affects the vertical structure of turbulence. Implications for radiative studies of clouds for the purpose of Arctic Amplification are discussed. 1 Introduction The Arctic has experienced since early 1990's more prominent warming than the rest of the world (Walsh and Crane, 1992) (Wendisch et al., 2013). This phenomenon of Arctic Amplification (Serreze and Francis, 2006) has generated a great amount of interest (Holland et al., 2003) (Overland et al., 2016) (Graversen., 2016). While low-level mixed-phase clouds are abundant in the Arctic climate, they significantly affect the surface radiative budget (Tsay et al., 1989) (Liu et al., 2017). Therefore, they are expected to play a very significant role in the Arctic Amplification (Kay et al., 2016). While the widespread melting of sea-ice and opening of leads is likely to lead to changes in the amount and the composition of mixed-phased clouds (Morrison et al., 2012) (Jun et al., 2016) (Chernokulsky et al., 2017), these changes are likely to create feedback loops due to changes in 1
Climate change is particularly noticeable in the Arctic. The most common type of cloud at these latitudes is mixed-phase stratocumulus. These clouds occur frequently and persistently during all seasons and play 15 a critical role in the Arctic energy budget. Previous observations in the central (north of 80 N) Arctic have shown a high occurrence of prolonged periods of a shallow, single-layer mixed-phase stratocumulus at the top of the boundary layer (BL; altitudes ~300 to 400 m). However, recent observations from the summer of 2018 instead showed a prevalence of a two-layer boundary-layer cloud system. Here we use large-eddy simulation to examine the maintenance of one of the cloud systems observed in the summer of 2018 as well as the sensitivity of the cloud 20 layers to different micro-and macro-scale parameters. We find that the model generally reproduces the observed thermodynamic structure well, with two near-neutrally stratified layers in the BL caused by a low cloud (located within the first few hundred meters) capped by a lower temperature inversion, and an upper cloud layer (based around one kilometer or slightly higher) capped by the main temperature inversion of the BL. The investigated cloud structure is persistent unless there are low aerosol number concentrations (≤ 5 cm-3), which cause the upper 25 cloud layer to dissipate, or high large-scale wind speeds ( 8.5 m s-1), which erode the lower inversion and the related cloud layer. These types of changes in cloud structure lead to a substantial reduction of the net longwave radiation at the surface due to a lower emissivity or higher altitude of the remaining cloud layer. The findings highlight the importance of better understanding and representing aerosol sources and sinks over the central Arctic Ocean. Furthermore, they underline the significance of meteorological parameters, such as the large-scale wind 30 speed, for maintaining the two-layer boundary-layer cloud structure encountered in the lower atmosphere of the central Arctic. 45 Arctic low-level, mixed-phase stratocumulus (MPS) clouds play a unique role in the surface energy budget of the region. In the central Arctic (north of 80 N), the longwave radiative effects dominate the annual average cloud radiative forcing at the surface due to the limited amount of solar radiation at these latitudes and the high surface albedo (Intrieri et al., 2002; Shupe and Intrieri, 2004; Sedlar et al., 2011). This leads to a net warming of the surface by the MPS clouds during most of the year. At the end of the summer season, however, MPS can have 50 a net cooling effect on the surface compared to clear-sky conditions due to higher insolation and a reduced surface albedo. The presence of MPS can also influence the timing of the autumn freeze-up period (Intrieri et al., 2002; Shupe and Intrieri, 2004; Tjernström et al., 2014). Arctic MPS clouds have a complex structure (e.g., Shupe et al., 2006, 2013). They are characterized by a liquid layer present at the cloud top within which ice crystals form and precipitate (
Arctic cloud macrophysical characteristics from CloudSat and CALIPSO
Remote Sensing of Environment, 2012
The lidar and radar profiling capabilities of the CloudSat and Cloud-Aerosol Lidar and Infrared Pathfinder (CALIPSO) satellites provide opportunities to improve the characterization of cloud properties. An Arctic cloud climatology based on their observations may be fundamentally different from earlier Arctic cloud climatologies based on passive satellite observations, which have limited contrast between the cloud and underlying surface. Specifically, the Radar-Lidar Geometrical Profile product (RL-GEOPROF) provides cloud vertical profiles from the combination of active lidar and radar. Based on this data product for the period July 2006 to March 2011, this paper presents a new cloud macrophysical property characteristic analysis for the Arctic, including cloud occurrence fraction (COF), vertical distributions, and probability density functions (PDF) of cloud base and top heights. Seasonal mean COF shows maximum values in autumn, minimum values in winter, and moderate values in spring and summer; this seasonality is more prominent over the Arctic Ocean on the Pacific side. The mean ratios of multi-layer cloud to total cloud over the ocean and land are between 24% and 28%. Low-level COFs are higher over ocean than over land. The ratio of low-level cloud to total cloud is also higher over ocean. Middle-level and high-level COFs are smaller over ocean than over land except in summer, and the ratios of middle-level and highlevel clouds to total cloud are also smaller over ocean. Over the central Arctic Ocean, PDFs of cloud top height and cloud bottom height show (1) two cloud top height PDF peaks, one for cloud top heights lower than 1200 m and another between 7 and 9 km; and (2) high frequency for cloud base below 1000 m with the majority of cloud base heights lower than 2000 m.
Atmospheric Chemistry and Physics, 2020
The combination of downward-looking airborne lidar, radar, microwave, and imaging spectrometer measurements was exploited to characterize the vertical and smallscale (down to 10 m) horizontal distribution of the thermodynamic phase of low-level Arctic mixed-layer clouds. Two cloud cases observed in a cold air outbreak and a warm air advection event observed during the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) campaign were investigated. Both cloud cases exhibited the typical vertical mixed-phase structure with mostly liquid water droplets at cloud top and ice crystals in lower layers. The horizontal, small-scale distribution of the thermodynamic phase as observed during the cold air outbreak is dominated by the liquid water close to the cloud top and shows no indication of ice in lower cloud layers. Contrastingly, the cloud top variability in the case observed during a warm air advection showed some ice in areas of low reflectivity or cloud holes. Radiative transfer simulations considering homogeneous mixtures of liquid water droplets and ice crystals were able to reproduce the horizontal variability in this warm air advection. Large eddy simulations (LESs) were performed to reconstruct the observed cloud properties, which were used subsequently as input for radiative transfer simulations. The LESs of the cloud case observed during the cold air outbreak, with mostly liquid water at cloud top, realistically reproduced the observations. For the warm air advection case, the simulated ice water content (IWC) was systematically lower than the measured IWC. Nevertheless, the LESs revealed the presence of ice particles close to the cloud top and confirmed the observed horizontal variability in the cloud field. It is concluded that the cloud top smallscale horizontal variability is directly linked to changes in the vertical distribution of the cloud thermodynamic phase. Passive satellite-borne imaging spectrometer observations with pixel sizes larger than 100 m miss the small-scale cloud top structures.
The Arctic Summer Cloud Ocean Study (ASCOS): overview and experimental design
Atmospheric Chemistry and Physics, 2014
The climate in the Arctic is changing faster than anywhere else on earth. Poorly understood feedback processes relating to Arctic clouds and aerosol-cloud interactions contribute to a poor understanding of the present changes in the Arctic climate system, and also to a large spread in projections of future climate in the Arctic. The problem is exacerbated by the paucity of research-quality observations in the central Arctic. Improved formulations in climate models require such observations, which can only come from measurements in situ in this difficult-to-reach region with logistically demanding environmental conditions. The Arctic Summer Cloud Ocean Study (ASCOS) was the most extensive central Arctic Ocean expedition with an atmospheric focus during the International Polar Year (IPY) 2007-2008. ASCOS focused on the study of the formation and life cycle of low-level Arctic clouds. ASCOS departed from Longyearbyen on Svalbard on 2 August and returned on 9 September 2008. In transit into and out of the pack ice, four short research stations were undertaken in the Fram Strait: two in open water and two in the marginal ice zone. After traversing the pack ice northward, an ice camp was set up on 12 August at 87 • 21 N, 01 • 29 W and remained in operation through 1 September, drifting with the ice. During this time, extensive measurements were taken of atmospheric gas and particle chemistry and physics, mesoscale and boundarylayer meteorology, marine biology and chemistry, and upper ocean physics.
Quarterly Journal of the Royal Meteorological Society, 2009
Results are presented from an intercomparison of single-column and cloud-resolving model simulations of a deep, multilayered, mixed-phase cloud system observed during the Atmospheric Radiation Measurement (ARM) Mixed-Phase Arctic Cloud Experiment. This cloud system was associated with strong surface turbulent sensible and latent heat fluxes as cold air flowed over the open Arctic Ocean, combined with a low pressure system that supplied moisture at mid-levels. The simulations, performed by 13 single-column and 4 cloud-resolving models, generally overestimate liquid water path and strongly underestimate ice water path, although there is a large spread among models. This finding is in contrast with results for the single-layer, low-level mixed-phase stratocumulus case in Part I, as well as previous studies of shallow mixed-phase Arctic clouds, that showed an underprediction of liquid water path. These results suggest important differences in the ability of models to simulate deeper Arctic mixed-phase clouds versus the shallow, single-layered mixedphase clouds in Part I. The observed liquid-ice mass ratios were much smaller than in Part I, despite the similarity of cloud temperatures. Thus, models employing microphysics schemes with temperature-based partitioning of cloud liquid and ice masses are not able to produce results consistent with observations for both cases. Models with more sophisticated, two-moment treatment of cloud microphysics produce a somewhat smaller liquid water path closer to observations. Cloudresolving models tend to produce a larger cloud fraction than single-column models. The liquid water path and cloud fraction have a large impact on the cloud radiative forcing at the surface, which is dominated by long-wave flux. Copyright
Journal of the Atmospheric Sciences, 2009
Macro- and microphysical properties of single-layer stratiform mixed-phase clouds are derived from multiple years of lidar, radar, and radiosonde observations. Measurements were made as part of the Mixed-Phase Arctic Clouds Experiment (MPACE) and the Study of Environmental Arctic Change (SEARCH) in Barrow, Alaska, and Eureka, Nunavut, Canada, respectively. Single-layer mixed-phase clouds occurred between 4% and 26% of the total time observed, varying with season and location. They had mean cloud-base heights between ∼700 and 2100 m and thicknesses between ∼200 and 700 m. Seasonal mean cloud optical depths ranged from 2.2 up. The clouds existed at temperatures of ∼242–271 K and occurred under different wind conditions, depending on season. Utilizing retrievals from a combination of lidar, radar, and microwave radiometer, mean cloud microphysical properties were derived, with mean liquid effective diameters estimated from 16 to 49 μm, mean liquid number densities on the order of 104–1...