During the early melt season, when sea-ice extent begins to retreat due to increasing heat absorption from the atmosphere and ocean, the sea ice becomes porous and weakens. This induces the break-up of ice floes and accelerates the melting process substantially. Determining how sea-ice structure evolves during this season is important for understanding the melting process on a global scale. We investigated this issue using field observations, laboratory experiments and numerical modeling, focusing on the effect of the C-shaped temperature profile on the internal structure of sea ice. Field observations were conducted to examine this effect on ~0.3 m thick ice on Lake Saroma, located at the coast of Hokkaido, Japan. To test the generalization from these observational case studies, laboratory experiments were conducted using a tank with 0.12 m thick ice. Additionally, evolution of ice structure from winter to early spring was numerically analyzed using meteorological data. From these studies, we find that the heat convergence, caused by the C-shape temperature profile, is essential to the internal melting particularly in the upper layer. This increases the porosity of the sea ice, leading to a rapid decrease in its flexural strength during the season.