Role of sea-ice in the air mass transformation over the southwestern region of the Sea of Okhotsk during cold air outbreaks

Radio sounding and in-situ observations at land-based and ship stations during cold-air outbreaks were performed over the southwestern Sea of Okhotsk covered with sea ice to estimate turbulent heat flux and investigate characteristics of thermodynamic air mass transformation processes. Surface heat flux estimations were carried out using two indirect methods, an atmospheric heat budget analysis using three-point radio sounding data and a bulk method utilizing meteorological and sea ice thickness data on a ship. The estimated total upward sensible and latent heat fluxes over the analyzed area were approximately 100 W/m^2 even under the intense cold air outbreak condition.

Development of the mixing layer associated with the small amount of the heat flux is below only about 1 km height, which implies that sea ice acts as an insulating material and significantly reduces the turbulent heat fluxes. However, the result of the bulk estimation suggests that a non-negligible amount of upward heat flux from the ice surface exists under the cold-air outbreak condition, which contributes to an increase of total ice mass through ice growth at the bottom of the sea ice.