Meteorological conditions in the central Arctic summer during the Arctic Summer Cloud Ocean Study (ASCOS)

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-breakerbased 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 largerscale 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.

Advances in understanding and parameterization of small-scale physical processes in the marine Arctic climate system: a review

“…In other studies, however, humidity inversions have been found to usually coincide with temperature inversions Tjernström et al, 2012). Differences in the observations may at least partly originate from different seasons (early spring in Vihma et al, (2011) and late summer in Tjernström et al, 2012) (2012) include SHEBA and several years of data from Barrow, hence possibly indicating that there may also be regional differences. A nonlinear relationship between humidity and temperature inversion strength is clearly found in all seasons except during summer (Devasthale et al, 2011).…”

“…They concluded that this was mostly due to the role of the snow and sea ice surface as a sink for heat but not commonly for humidity (see also Persson et al, 2002). In other studies, however, humidity inversions have been found to usually coincide with temperature inversions Tjernström et al, 2012). Differences in the observations may at least partly originate from different seasons (early spring in Vihma et al, (2011) and late summer in Tjernström et al, 2012) (2012) include SHEBA and several years of data from Barrow, hence possibly indicating that there may also be regional differences.…”

“…This decoupling appears to be most common during the cold, dark months but also occurs during the transition and summer seasons (Kahl, 1990;Tjernström et al, 2004Tjernström et al, , 2012Sedlar et al, 2011Sedlar et al, , 2012Solomon et al, 2011;Shupe et al, 2013). Thus, the surface-based moisture source for Arctic MPS is often missing (Fig.…”

“…In summer, a slightly stable or near-neutral ABL prevails over sea ice with a capping inversion of variable depth. Tjernström et al (2012) analysed soundings from four summer expeditions in the central Arctic, including SHEBA, and found a very persistent picture of near-neutral boundary layer conditions with the layer depths ranging from ∼ 200 to ∼ 400 m. Tjernström and Graversen (2009) analysed all the soundings from SHEBA and concluded that virtually all temperature inversions fall into either surface-based inversions or elevated T. Vihma et al: Small-scale physical processes in the marine Arctic climate system 9407 inversions capping a near-neutral ABL, with no intermediary state. In winter, shifts between the two states are rapid, presumably depending on the presence of stratocumulus clouds, in which radiative processes and in-cloud turbulent dynamics together cause the shift of the inversion base from the surface to the air .…”

Modelling micro- and macrophysical contributors to the dissipation of an Arctic mixed-phase cloud during the Arctic Summer Cloud Ocean Study (ASCOS)

“…1) was dominated by a high-pressure system, with passages of a few weak fronts . A detailed description of the meteorological conditions can be found in Tjernström et al (2012). Observation from the vertically pointing Doppler millimeter cloud radar (MMCR) shows the cloud top at around 1 km during the morning hours, with a thinning and lowering cloud top during the af- ternoon ( Fig.…”