This exercise focuses on orographic lifting. This process explains precipitation patterns over much of the western United States as well as many other regions in the world. The purpose of this exercise is to provide you with a chance to work with the concepts of lapse rates, atmospheric stability, condensation, and temperature by following conditions in a parcel of air as it is forced by wind to rise over a topographic barrier, and then descends again down the leeward slope.
The following figure represents a mountain 4 km in height with a base level at 0 km above sea level. The letters indicate different elevations on the windward and leeward sides of the mountain.
For this exercise we will use cooling rates of 10°C/km for the DAR and 5°C/km for the MAR to simplify the calculations. For questions 1-7, suppose that a parcel of air at point A is 25°C, the ELR is 7°C/km, and the dew point is 5°C.
What atmospheric conditions are represented by this situation (stable, unstable, or conditionally unstable)? Explain your answer.
Given these conditions, at point A, would rain be likely to occur if the mountain did not exist? Explain your answer?
At what rate will the parcel of air cool as it initially begins to rise? Why?
At what point (indicate the letter) will clouds begin to form?
Explain why a rising parcel of air cools more slowly when it reaches its dew point.
What will be the temperature of the parcel of air when it reaches the top of the mountain?
What will be the temperature of the parcel of air when it reaches the bottom of the leeward slope?
Dew points can be quite variable from place to place and over time. What would the dew point need to be in this example to produce a temperature of 0°C at the top of the mountain?
Why does sinking air get warmer as it descends?
What is the term for the dry areas leeward of mountains such as this one?
Would you expect to find more vegetation on the windward or leeward side of this mountain? Why?
Now suppose the parcel of air at point A is 15°C rather than 25°C. All other conditions are the same. Again indicate where clouds would begin to form, what the temperature of the parcel would be at the top of the mountain (Point E), and what the temperature of the parcel would be at the bottom of the leeward side.