Why is wind speed generally considered at 10m above the sea surface? Is there any specific reason for this? For example, if we say wind speed is 8 m/s which implies that wind speed at a height of 10 m above the ocean surface is 8 m/s, U10 = 8 m/s. Why it is like that?
The nature of the surface of oceans varies according the weather conditions and other factors. During periods of no wind seas can be calm and the surface of the water is "flat". As wind speed increases so can the turbulence (roughness) of the surface of the water which affects things such as wave height and periodicity of waves, swells and general sea roughness.
The boundary conditions of seas affects the boundary conditions of the air in contact with the sea. A calm sea has a calm air flow and a rough sea will have a rough air flow over it. With rough seas, localized wind speeds can vary, due to turbulence. The degree of air turbulence decrease with height above the sea surface. At a height of 10 m air flow is considered to be uniform and thus wind speed will also be uniform.