Abstract
Abstract. 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 the Arctic Summer Cloud-Ocean Study (ASCOS), described here. 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 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-generated turbulence. The latter also implies a connection between the cloud layer and the free troposphere through entrainment at cloud top.
Highlights
The rapidly changing Arctic climate (ACIA, 2005; IPCC, 2007; Richter-Menge and Jeffries, 2011) has focused scientific attention on this region
Many aspects of the Arctic climate show an “Arctic amplification” (Serreze and Francis, 2006) and no consensus exists about primary reasons for this, it is likely related to feedbacks in the Arctic climate system, some of which are related to clouds and surface albedo
While the AOE-96, Surface Heat Budget of the Arctic Ocean (SHEBA) and AOE-2001 summers had distinctly different large-scale circulation over the Arctic Ocean compared to Arctic Summer Cloud-Ocean Study (ASCOS), all the expedition tracks were located in regions between the mean low- and high-pressure centers, but with more high-pressure influence during AOE-96 and more low-pressure conditions during SHEBA
Summary
The rapidly changing Arctic climate (ACIA, 2005; IPCC, 2007; Richter-Menge and Jeffries, 2011) has focused scientific attention on this region. Low-level clouds are ubiquitous in the Arctic, especially during the summer half of the year with monthly averaged cloud fraction as large as 80–90 % (Curry and Ebert, 1992; Wang and Key, 2005; Tjernstrom, 2005; Shupe et al, 2005, 2011) These clouds have a substantial effect on the surface energy budget While short observation campaigns cannot be used to detect climate trends, the utility of shorter expeditions lies in the detailed studies of important processes that are possible with extensive short-term observations Such data can be used to inform the development of better models and while simultaneously studying several detailed processes, it can increase the understanding of the Arctic climate system.
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