Aircraft Observations of Air-mass Modification Over the Sea of Okhotsk during Sea-ice Growth

Abstract

In order to quantitatively investigate the role of leads and sea-ice in air-mass modification, aircraft observations were conducted over the partially ice-covered Sea of Okhotsk. We investigated two cold-air outbreak events with different sea-ice concentrations. In both cases, the difference between the temperatures of surface air and the sea surface (ΔT) dropped rapidly with the accumulated fetch-width of leads up to about 35-40 km, and then decreased very slowly. The surface sensible heat flux originating from open water was about 300 W m⁻² within a few kilometres from the coast and decreased with increasing accumulated fetch-width. The sensible heat flux was about 100 W m⁻² on average. These results indicate that the downwind air-mass modification depends mainly on the total (accumulated) extent of open water. The total buoyancy flux $\left(\overline {w'T \, '_v}\right)$ calculated by the joint frequency distribution method correlated very well with ice concentration. Such a relationship was not clear in the case of the moisture flux $(\overline {w'q'})$. The ratio between rising thermals $\left(\overline {w^+T^+_v}\right)$ and cold downdrafts $\left(\overline {w^-T^-_v}\right)$ differed significantly between upwind and downwind regions; that is, the buoyancy flux was dominated by $\overline {w^+T^+_v}$ in the developing stage of the boundary layer, while $\overline {w^-T^-_v}$ also became important after the development of the boundary layer.