The boundary layer is not frozen in time but instead changes dramatically during the course of the day. Let’s start with the midday when the boundary looks like the hazy scene over Maryland (figure in 11.1). The boundary layer consists of a mixed layer that is stirred by solar heating of the surface and convection of warm moist air that pops up sporadically from place-to-place and time-to-time, and, as a result, mixes the air within the boundary layer. This convective stirring takes about ten to twenty minutes to go from bottom to top. As the air bubbles up, it mixes with the air surrounding it and with the air from the free troposphere at the top, thus creating an entrainment zone, which is where the clouds are.
Check Your Understanding
Does ten to twenty minutes for boundary layer vertical stirring make sense?
You learned in Lesson 2 that buoyant acceleration equaled the gravity times the difference between the air parcel virtual temperature and the virtual temperature of its surroundings divided by the virtual temperature of its surroundings. Let's assume that temperature difference between an air parcel above a heated surface and its surroundings is 0.1 K, which seems pretty reasonable, and that the temperature is 300 K. The buoyant acceleration, B, is just 9.8 m s–2 times 0.1/300, or 0.0033 m s–2. So, if the initial air parcel velocity is 0 m s–1 and the top of the PBL, z0, = 1 km, then since z0 = 1/2 Bt2, where t is time, then t is the square root of 2z0/B ~ 13 minutes. So now you can see that it takes a very small virtual temperature difference to stir the planetary boundary layer.
As the sun sets, the solar heating of the surface and the convection and associated turbulent eddies cease. Air from the surface no longer mixes with air throughout the convective boundary layer, and the air that was mixed during the day stays above the much lower nighttime stable boundary layer in a layer called the residual layer. Any gaseous or particle emissions from the surface are mixed within this nocturnal boundary layer. Because convection ceases at night, the winds in the residual layer are no longer affected by the friction caused by convection and they accelerate in the presence of a horizontal pressure gradient. So, the residual layer winds accelerate, blowing harder across the top of the more stagnant nocturnal boundary layer and a shear develops. This shearing is unstable and creates turbulence that mixes the boundary layer air and the residual layer air near the interface, so the nocturnal boundary layer grows a little during the night.
In the morning, the sun returns to heat the surface and to start driving convection and mixing again. This convection bubbles up, bumping into and entraining air from the residual layer. As the solar heating increases, the convection has more energy and can rise higher and entrain more air from the residual layer. Eventually, the air driven by convection reaches its maximum energy level and this maximum energy limits how high the boundary layer will grow into the stable free troposphere above it.
The following video explains the variation of the planetary boundary layer over the course of a typical day:
Let’s summarize the diurnal behavior of the boundary layer with a bulleted list of technical terms:
Mixed Layer (Convective Boundary Layer):
- turbulence driven by convection (large eddies or thermals)
- heat transfer from solar heating of the ground to the atmosphere
- mixed layer grows by entrainment of air from above it
- virtual temperature nearly adiabatic in middle; superadiabatic (i.e., potential temperature decreases with height) near surface; subadiabatic (i.e., potential temperature increases with height) at top, where exchange of air between the ABL and the free troposphere occurs
- wind speeds are sub-geostrophic in mixed layer, crossing isobars because of turbulent drag
- directly in contact with Earth’s surface
- usually has vertical gradients in potential temperature, water vapor, and other quantities
- logarithmic wind speed profile with height, with low wind speed near ground
- typically is ~10% of the mixed layer
- disconnected from boundary layer and Earth’s surface
- neutrally stratified, with small but near-equal turbulence in all directions
- contains moisture and trace atmospheric constituents from the day before
Stable Boundary Layer
- statically stable with weaker turbulence that occurs sporadically
- winds aloft may increase to supergeostrophic speeds (low-level jet or nocturnal jet)
- stability tends to suppress turbulence, except for occasional shear-generated turbulence caused by the low-level jet