Radio sounding and in-situ observations at land-based (26 January-11 February 1998) and ship stations (4-11 February 1998) 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 during cold-air outbreaks. Surface heat flux estimations were carried out using two indirect methods; an atmospheric heat budget analysis using the radio sounding data at three stations and a bulk method utilizing meteorological and sea ice thickness data obtained by the ship observations. The estimated total upward turbulent heat flux over the analyzed area was approximately 100 W m(-2) even within the intense cold-air outbreak period. The height of the mixing layer associated with this small amount of the heat flux is only about 1 km, which implies that sea ice acts as an insulating material between the ocean and atmosphere and thus significantly reduces the turbulent heat fluxes. Nevertheless, the upward turbulent heat flux from the ice surface was found to be non-negligible during cold-air outbreaks and it is suggested that the sea ice grows through the sea or ice surface cooling.