The atmospheric boundary layer

The log-wind profile

Surface roughness Zo Short grass 1m Long grass 3m Rural fields 20cm U = U*/ K ln(Z/Zo)

Prandtl theory (vertical wind profile)

-Assumes eddy magnitude increases with height -log wind speed profile increases linearly with height - Most useful within lowest 15% of PBL

Sea breezes (local circulation)

-Caused by differential heating between surfaces (sea and land) -Circulation develops and extends during day, and reverses at night. Cloud formation over coast; wind direction and speed also influenced by shape of coast

Effect of waves in the atmosphere (topographic forcing of waves)

-Flow over rough terrain can have non-local effects due to gravity wave propagation. -Mountain waves are gravity waves forced by topography - Gravity waves are also known as buoyancy waves - Gravity waves can induce clouds - Gravity waves influence the global circulation in the stratosphere -Lee wave induced polar stratospheric clouds are important in ozone destruction.

Ekmans solution (vertical wind profile)

-Spiral of winds (anti-clockwise in NH) towards surface -Ekmans spiral - winds change direction due to friction below the ABL. Stronger the friction the more it changes direction. -theory assumes constant eddy magnitude. Spiral of wind velocities: deceleration adn turning from geostrophic at the top of the mixed layer to zero at the ground.

Urban effect (local circulation)

Major urban areas have a large impact on ABL Effect of cities: increase surface roughness = strong drag less moisture different albedo different heat capacity pollutant and heat production Urban heat island will effect the temperature by a few degrees. City causes more overturning, causing more circulation creating its own boundary layer, called an urban plume.

Reynolds number

Shows how the behaviour of fluids can be similar even on vastly different scales.

The atmospheric boundary layer

The layer of air directly above the Earths surface in which the effects of the surface (heating and cooling, friction) are felt directly on time scales less than a day, and in which significant fluxes of momentum, heat or matter are carried by turbulent motions. ABL - thinnest layer in the atmosphere: hoe to biosphere Mixed layer during the course of the day. Surface air heating mixing deeper and deeper through the atmosphere during the day. Noon reaches maximum heating, then dies down. During night time the boundary layer is far more stable. Solar radiation heats surface. convection breaks up stable nocturnal layer and entrains air from above. Solar heating of surface stops. Rapid cooling of surface. stable nocturnal layer grows in from below.

Richardson number

The ratio of the stability of the air to the wind shear Ri often changes abruptly at the top of the atmospheric boundary layer Wavy motion turning into chaotic mixing.

The nature of turbulence

Turbulence may be: Mechanical, - due to surface roughness Thermal, - due to surface heating Inertial - due to shear flow