Print11.2 A Day in the Life of the Boundary Layer
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 trapped within this nocturnal boundary layer. Because convection ceases at night, the winds in the residual layer are no longer affected by turbulent drag at Earth's surface and they accelerate in the presence of a horizontal pressure gradient, sometimes forming a supergeostrophic flow called the nocturnal jet.
Turbulent drag at Earth's surface is the primary source of mixing in the shallow nighttime boundary layer. Turbulent eddies in the nighttime stable boundary layer often have about 1/10th the velocity of the eddies that occur in the daytime convective boundary layer. The impact of these weak turbulent motions is countered by the tendency of radiation to cool Earth's surface at night. Sometimes the wind shear at the top of the nighttime boundary layer creates some instability and turbulence within the nighttime boundary layer. Unlike the vigorous deep covective boundary layer found during the day, the nighttime stable boundary layer is shallow, about 1/10th the depth of the daytime boundary layer and has weak intermittent turbulence and temperatures that fall during the night due to radiative cooling.
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 daytime convection warms the boundary layer to its maximum temperature and this maximum temperature determines the height to which the convection will penetrate into the stable free atmosphere above the boundary layer.
The following video explains the variation of the planetary boundary layer over the course of a typical day:
Let's look at the variation of the planetary boundary layer over the course of a typical day. We'll start midday when the sun is out and solar heating of the surface is causing buoyant air parcels to rise until the virtual potential temperature matches that of the overlying air. These air parcels have momentum, and they overshoot the level of neutral buoyancy. In the process, they entrain air from the free troposphere. Clouds form in this layer. The rising air parcels collide with the air above them and rub against the air around them, producing a whole range of different eddie sizes in mixing. These large, buoyant eddies cycle in tens of minutes, mixing the air. As the sun sets later in the day, there's less solar energy to power the convection that stirs the mixed layer, and the boundary layer collapses, leaving behind a residual layer that contains the mixed layer air that was leftover. Emissions from the surface keep pouring into the boundary layer, but the boundary layer height is much lower. With less turbulence in the residual layer, the air can speed up. The faster moving air above the slower moving air in the boundary layer causes a shear to develop between the two air masses. And sporadically, turbulence is generated when the shear breaks down, mixing air and increasing the boundary layer height. At sunrise, solar heating again begins to warm the surface, and the warm parcels rise up entraining residual layer air until eventually, the mixed layer reaches its maximum height again.
Summing Up
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
Surface Layer
- 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
Residual 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 just above the stable boundary layer (in the residual layer) may increase to supergeostrophic speeds (low-level jet or nocturnal jet)
- surface cooling increases stability and suppresses turbulence
- turbulence is generated primarily by turbluent drag at Earth's surface and sometimes through shear-generated turbluene caused by the low-level-jet.