During cold-air outbreaks in 2001 winter, three cases of air mass transformation over the Sea of Japan were observed by a Rusian aircraft. The modification from the Siberian coast to 300 km offshore depends decisively on the coldness of air mass at the coast. The downstream temperature increase in the boundary layer is mainly controlled by the turbulent heat fluxes from the sea surface. The sensible and latent heat fluxes near sea surface amounted to 600 and 300 W/m^2 in the upstream region, respectively. The sensible heat flux decreased downstream which accompanied the decrease in air-sea temperature difference, on the other hand, the latent heat flux was almost constant. The relationships between the turbulent heat fluxes and the development of boundary layer and convective clouds are presented.