Chapter 4 - Atmospheric Influences

Chapter 4 Contents

4.3 The Planetary Boundary Layer

The atmosphere within 100m of the sea surface is influenced by the turbulent drag of the wind on the sea and the fluxes of heat through the surface. This is the atmospheric boundary layer. It's thickness Zi varies from a few tens of meters for weak winds blowing over water colder than the air to around a kilometer for stronger winds blowing over water warmer than the air. Above the planetary boundary layer, friction is not important, and the wind is calculated from the surface presure gradient plotted on section 4.2.

The lowest part of the atmospheric boundary layer is the surface layer. Within this layer, which has thickness of 0.1 Zi, vertical fluxes of heat and momentum are nearly constant.

Wind speed varies as the logarithm of height within the surface layer for neutral stability. See The Turbulent Boundary Layer Over a Flat Plate in Chapter 8. Hence, the height of a wind measurement is important. Usually, winds are reported as the value of wind at a height 10m above the sea U10.

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