Marine-Atmosphere Emitted Radiance Interferometer measurements during the Arctic Summer Cloud Ocean Study (ASCOS)

ABSTRACT

The Arctic Summer Cloud Ocean Study (ASCOS) is an interdisciplinary project focused on the study of the Arctic cloud formation. The ASCOS expedition to the high Arctic Ocean was supported by the Swedish Polar Research Secretariat and the ice-breaker Oden, and was conducted between 1 August and 9 September 2008. In an effort to improve our understanding of Arctic clouds, a multitude of measurements were taken during the expedition of processes involved in cloud formation.

This paper presents radiometric measurement of ocean skin temperature, surface snow temperature and the air – surface temperature difference. The ocean skin, the sub-millimeter thick topmost layer of the ocean, constitutes the thermal boundary between the bulk of the ocean and the overlying air. The skin is nearly always cooler then the bulk of the ocean due to heat losses by sensible and latent heat fluxes as well as the net outgoing longwave radiation. The exchange processes between the air and the ocean, such as heat, gas, and particle transfer, occur through this interface. In the Arctic, the particles originating in the ocean (sea algae, bacteria, viruses) and transferred into the atmosphere during the air-sea exchange processes are believed to constitute the main source of cloud condensation nuclei (CCN). The temperature gradients between the ocean skin and ocean bulk, and between the ocean skin and the air temperatures, and the wind speed regulate the rate of the exchange processes thus determine the influx of the CCN into the arctic atmosphere.

The M-AERI is a sea-going instrument that measures spectra of infrared radiation emitted from the sea (or snow or ice) surface and from the atmosphere in the wavelength range of 4 to 18 µm with ~10 min temporal resolution. These spectra are used to retrieve sea surface temperature (SST) with accuracy better than 0.05K, as well as profiles of temperature and humidity in the atmosphere up to 3km in altitude. The high accuracy the SST measurements and near surface air temperature makes M-AERI an important instrument air-sea temperature gradient measurement.

M-AERI operated almost continuously during the ASCOS expedition from Longyearbyen, Svalbard through the open water section and the marginal ice zone to the location above 87º N, during the three weeks ice drift when the Oden was moored to an ice flow in the High Arctic, and on the way back from the ice to Longyearbyen. The SST, snow, and air-sea temperature difference measured by M-AERI during the expedition are presented here. Additionally, the measurements of the ocean bulk temperature from the ship’s thermosalinograph are combined with M-AERI SST measurements so the skin-bulk temperature difference can be obtained.

Profiles of atmospheric temperature and humidity retrieved from M-AERI spectra provide information about the structure of the marine boundary layer. M-AERI retrievals are however limited to cloud free conditions that were rarely encountered during the ASCOS expedition. The retrievals obtained on the few cloud free days are also presented.