Summer Atmosphere Boundary-Layer Vertical Structure Downwind of the Ice Edge in on-Ice Advection

The climate in the Arctic is changing fast. Annual temperatures increase at least twice as fast as the global averaged temperatures and the sea ice is disappearing fast, especially in summer. Over the last decade or more, the influence of advection of heat and moisture into the Arctic from the south has been discussed and argued. The focus has mainly been on winter and spring conditions and very few studies have been based on observations, because there are very few. In summer the situation is slightly different as there are more observations available from ship-borne expeditions. Also the physical processes are different. As the ice retreats, and the marginal ice zone (MIZ) has widened, processes at the MIZ has become more and more a focus for research. Areas of open water are exposed to solar radiation heating the ocean and potentially delaying the autumn freeze up, with effects for the upcoming winter.

In summer a very special situation is set up by the fact that the ice, and snow on surface of the ice, is melting. This constrains the surface temperature to at or slightly below the melting point of fresh water. As relatively warm and moist air is advected from south in over the ice, this may lead to the formation of very strong surface inversions; in these inversions the relatively high moisture often leads to formation of fog while partial turbulent decoupling at the MIZ often leads to formation of low-level jets. In this zone there is a competition between several factors: radiative cooling at the fog top potentially leading to turbulent overturning; strong static stability leading to reduction turbulence; increased wind shear in the jet promoting more shear-generated turbulence, and; the maintenance of the fog by moisture advection and the stability by temperature advection. The vertical structure of the PBL and hence the thermodynamic forcing from the fog and the inversion by turbulence and radiation is determined by the balance of these factors.

In this presentation we will present an analysis of these processes using field observations from the Arctic Clouds in Summer Experiment (ACSE), conducted as a part of the SWERUS-C3 expedition on board the Swedish icebreaker Oden. The experiment was conducted in summer/autumn 2014 traversing across the Arctic from Tromsö, Norway, to Barrow, Alaska, in July and August staying mostly staying on the Siberian Shelf and shelf break, and back again in late August and September, following a similar path but slightly more northward, arriving back in Tromsö early October. The observations include a suite of remote sensing instruments (W-band cloud radar, 449 MHz wind profiler, scanning microwave radiometers as well as a scanning 3D lidar and several ceilometer lidars) as well as in-situ instruments (radiosoundings, weather station, IR surface temperature and broad-band incoming short- and longwave radiation as well as eddy-covariance turbulent fluxes).