Meteorologists are often so busy looking at weather maps for 850 mb and above that they don’t pay attention to what is happening near Earth’s surface. Yet we walk on Earth’s surface and we live in the air near Earth’s surface, which is called the planetary boundary layer (PBL). The PBL has repeatable cycles from day-to-day, but it also has chaotic, turbulent behavior that cannot easily be described by mathematics and certainly cannot be predicted. These repeatable cycles and chaotic behavior have a huge impact on the quality of the air that we breathe and on the sources of sensible heat and water vapor that are essential for the weather above the PBL. In this lesson, we will show how the PBL rises during the day and drops down at night. This behavior is driven by turbulent motion, so we will see how all motion can be separated into the mean flow and the turbulent flow using some simple concepts, including Reynolds averaging. We will see that while the mean wind is most important for moving water vapor and momentum horizontally, turbulent transport is most important for moving these quantities and others vertically throughout the PBL to the base of the free atmosphere above it. We will see how turbulent eddies of different sizes are related and which ones have the most energy. Finally, we will take a look at the energy budget for different types of locations and different times of day. Hopefully you will understand that while weather forecast models treat the planetary boundary layer as a boundary condition, the atmosphere treats it as an integral part of weather.
By the end of this lesson, you should be able to:
- draw the PBL and its diurnal variation
- perform Reynolds averaging on an equation and derive an equation for the turbulent parts
- explain kinematic fluxes
- show vertical motion using eddy fluxes
- explain turbulent kinetic energy (TKE) and its behavior
- sketch the surface energy budget for different conditions
If you have any questions, please post them to the Course Questions discussion forum. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.