wind energy resource evaluation in a site of central italy ... - WindSim

CHAPTER 1. Atmospheric Boundary Layer.1.1 The Atmospheric Boundary LayerThe Atmospheric Boundary Layer (ABL) is the lowest part of the atmosphere and itsbehavior is directly influenced by the interaction with a planetary surface, accordingto Stull, R. B. (1988) [12].It’s known that the ABL responds to surface forcing in a timescale of one hour orless, while the typical space scale is few kilometers. In this layer physical quantitiessuch as flow velocity, temperature, moisture, etc. display relatively rapidfluctuations.Above the ABL there is the “free atmosphere”, where the wind is consideredgeostrophic (parallel to isobars), while inside the ABL the wind is affected by surfaceeffects and turns across the isobars.1.2 Description of the ABLThe ABL is a part of the troposphere which is the lowest portion of Earth'satmosphere. It contains approximately 75% of the atmosphere's mass and almost allof its water vapor and aerosols.The average depth of the troposphere is about 11 km in the middle latitudes. It isdeeper in the tropical regions (up to 20 km) and shallower near the poles (about 7km).As already said, the lowest part of the troposphere is the ABL and this layer is about2 km deep, depending on the landform and time of day.In a neutral ABL, where heat transfer is not taken into account, several sub-layerscan be identified.The canopy layer is the first sub-layer near the terrain, where obstacles are present;above it, the surface layer can be found, in which Coriolis effects are negligible;finally there is the so called Ekman layer, where Coriolis effects are dominant.4