Part 3:
Factors Influencing the Planetary Boundary Layer
Several factors have an affect on properties of the PBL. We will cover 6 of these: energy budgets, moisture, diurnal variations, buoyancy, shear, and roughness length.
In Session 2, we introduced atmospheric energy budgets.
The amount of energy entering the PBL and the amount leaving the PBL are very important. The more energy, the more significant the surface heating. The amount of surface heating determines the magnitude of convective turbulence that develops in the PBL. A day with lots of incoming energy will have a higher mixing height than a day with little surface heating. During the night, the amount of radiative cooling (energy loss) determines the height of the stable boundary layer. The more energy in the layer, the more turbulence and more well-mixed is the air in the layer.
Because of its importance to heat transfer, moisture also plays an important role in the planetary boundary layer. Moisture in the air has the ability to retain large amounts of heat energy, which can then be transferred throughout the layer via turbulent mixing. Therefore, air with high humidities will tend to have a thinner mixing layer than air with little moisture present, due to the fact that it takes longer to heat the air because the moisture absorbs so much energy. Because air with high moisture content also prevents heat energy from escaping during the night, the height of the stable boundary layer will be greater that that of air with low humidities. Moisture tends to decrease variability in air temperature. One interesting note is how moisture is involved in the residual layer. Moisture can be transferred into the mixed layer via turbulence and remain in the air after the turbulence decays. Because it mixes with neither the stable layer nor the capping inversion, more moisture can be added the following day. This aids in the formation of clouds. Another factor, which is often forgotten, is soil moisture. Lakes and oceans are not the only sources of water vapor in the atmosphere. Water in the soil will evaporate into air and affect the mixing in the PBL. Soil moisture can have a significant effect on the PBL if the relative difference between soil and air humidities is enough. Unfortunately, many models do not take into account the moisture the air can gain from the soil on the earth.
Because the height of the PBL is so dependent on the energy budget of the layer, the diurnal change in surface temperature is one of the most significant factors influencing the PBL. The surface heating of the daytime increases the convective turbulence within the mixed layer. At night, the dramatic decrease in incoming energy allows the mixed layer to decay into the nocturnal boundary layer.
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