Near‐surface meteorological observations and rawinsonde soundings from Arctic cruises with the German icebreaker RV Polarstern during August 1996, 2001, and 2007 are compared with each other and with ERA‐Interim reanalyses. Although the observations are usually applied in the reanalysis, they differ considerably from ERA data. ERA overestimates the relative humidity and temperature in the atmospheric boundary layer and the base height of the capping inversion. Warm biases of ERA near‐surface temperatures amount up to 2 K. The melting point of snow is the most frequent near‐surface temperature in ERA, while the observed value is the sea water freezing temperature. Both observations and ERA show that above 400 m, in the North Atlantic sector 0–90 E, the warmest August occurred in 2001, and August 2007 had the highest humidity. In the Eastern Siberian and Beaufort Sea region ERA temperatures along 80 and 85 N were highest in 2007.

Leads are elongated channels in sea ice which play an important role for the heat and moisture exchange between the polar ocean and atmosphere. The aircraft campaign STABLE aimed to improve our current understanding of the formation of convective plumes over leads and their impact on the polar atmospheric boundary layer. It was carried out over the pack ice in the northern Fram Strait in March 2013. We present case studies of the boundary layer modification and turbulent fluxes over four wide leads, which differed strongly with respect to lead characteristics and environmental conditions. The observed near-surface sensible heat fluxes ranged from 15 to 180 W m −2 . The leads also induced an increase of the near-surface temperature of up to 3.2 • C and a humidity increase of up to 0.2 g kg −1 . In one of the cases, large entrainment fluxes exceeding 30% of the surface fluxes were observed. Vertical profiles of turbulent sensible heat and momentum fluxes were nonlinear downstream of the leads with a distinct flux maximum in the core of the convective plumes. In two cases, the plumes also strongly affected the wind field within the atmospheric boundary layer. Low-level jets that existed in those cases in the region upstream of the leads disappeared in the plume region. Finally, it is shown that large errors can occur when flux measurements are derived from lead orthogonal flight legs only. Therefore, complex flight patterns, as presented in this study, are necessary to accurately determine the energy fluxes in the environment of leads.

Abstract. An analysis of Special Sensor Microwave/Imager (SSM/I) satellite data reveals that the Whaler's Bay polynya north of Svalbard was considerably larger in the three winters from 2012 to 2014 compared to the previous 20 years. This increased polynya size leads to strong atmospheric convection during cold air outbreaks in a region north of Svalbard that was typically ice-covered in the last decades. The change in ice cover can strongly influence local temperature conditions. Dropsonde measurements from March 2013 show that the unusual ice conditions generate extreme convective boundary layer heights that are larger than the regional values reported in previous studies.

A non‐eddy‐resolving microscale model is applied to simulate convection over three different leads (elongated channels in sea ice), which were observed by aircraft over the Arctic Marginal Ice Zone in 2013. The study aims to evaluate the quality of a local and a non‐local turbulence parametrization. The latter represents a lead‐width‐dependent approach for the turbulent fluxes designed for idealised conditions of a lead‐perpendicular, near‐neutral inflow in an atmospheric boundary layer (ABL) capped by a strong inversion at around 250 to 350 m height. The observed cases considered here are also characterised by an almost lead‐perpendicular flow but, in comparison to the idealised conditions, our analysis covers effects in stable inflow conditions and a much shallower ABL. The model simulations are initialised with observed surface parameters and upwind profiles, and the results are compared with measurements obtained above and downwind of the leads. The basic observed features related to the lead‐generated convection can be reproduced with both closures, but the observed plume inclination and vertical entrainment near the inversion layer by the penetrating plume are underestimated. The advantage of the non‐local closure becomes obvious by the more realistic representation of regions with observed vertical entrainment or where the observations hint at counter‐gradient transport. It is shown by comparison with the observations that results obtained with the non‐local closure can be further improved by including the determination of a fetch‐dependent inversion height and by specifying a parameter determining the plume inclination as a function of the upwind ABL stratification. Both effects improve the representation of fluxes, boundary‐layer warming, and vertical entrainment. The model is also able to reproduce the observed vanishing of a weak low‐level jet over the lead, but its downwind regeneration and related momentum transport are not always well captured, irrespective of the closure used.

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