Planetary Boundary Layer and Turbulence

wind shear

Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal wind shear.

Mechanical turbulence

forced convection, can form if there is shear in the mean wind (main wind direction). Wind shear can be caused by frictional drag (slower winds near ground than aloft), by wake turbulence (wind swirls near ground than aloft), by wake turbulence (wind swirls behind obstacles as trees, buildings) and by free shear (regions away from solid surface).

Describe the typical development of the planetary boundary layer (and its various subdomains) over the course of 24 hours.

*Day 1:* ☼ Heating and drag at Earth's surface causes turbulence (/rising of air parcels) resulting in a layer of air with a fairly uniform temperature and mixed composition (The Mixed Layer). Above the Mixed Layer is an Entrainment Zone, which is a layer with no turbulence characterized by a sudden jump in temperature. *Sunset:* ☼Surface of Earth cools as longwave radiation radiates form the ground into the atmosphere *During the Night:* ☼Thermals cease, turbulence dies down in the former Mixed layer, now called the Residual Layer. ☼As air near the ground cools, the bottom of the residual layer transforms into a gradually deepening nocturnal stable boundary layer. *Day 2:* ☼ Process from Day 1 starts again

Thermal turbulence

Convective turbulence=free convection; consists of plumes of thermal of warm air that rises and cold air that sinks due to buoyancy forces. For air containing sufficient moisture, the tops of these thermals contain cumulus clouds.

Be able to name and briefly describe the three main mechanisms that generate turbulence in the boundary layer.

Mechanical turbulence, intertial turbulence and thermal turbulence

Intertial turbulence

Special form of wind shear turbulence, where the wind shear is generated by larger eddies → large eddies generate internal wind shear, which in turn produce new smaller eddies