1. Exam 2 Review AOS 121 November 16 2009

2. Geostrophic Balance and Geostrophic Winds Balance between the pressure gradient force and Coriolis force Will the wind direction forever turn to right (left) in Northern (Southern) Hemisphere? The answer is no. It will keep turning until a balance between the pressure gradient force and Coriolis force is achieved. This balance happens when the wind direction becomes parallel to the isobars. This is called the geostrophic balance. The wind satisfying the geostrophic balance condition is called the geostrophic wind. Geostrophic winds are very closed to (but not exactly the same as) the actually observed winds. Actual Wind Converges toward Low Pressure, and Diverges outward from High Pressure due to Friction. Northern Hemisphere P. K. Wang

3. What happen when air moves up and down? Remember that the atmosphere is stratified—density and pressure change with height. Adiabatic Process – a process during which no heat is withdrawn from or added to the system involved. This means that the total energy of the system is conserved. So the total energy of the system undergoing an adiabatic process becomes a zero-sum game. If something becomes more, others have to become less. When air expands adiabatically, it must do expansion work at the expense of other things. The most common ‘other thing’ is cooling (so the ‘internal energy’ of the system becomes less). The “system” we are considering here is an air parcel. The figure to the right shows the adiabatic ascent and descend of an unsaturated air parcel and the important concept of lapse rate. Air parcel – An air parcel is a convenient conceptual system for the purpose of understanding what will happen to the atmosphere under certain conditions. It doesn’t have a specific size. You can imagine it as a balloon without a skin. It can expand or shrink (be compressed) like a balloon and but, unlike a balloon, it certainly can mix with the environmental air to exchange mass and energy. Air parcel Environmental air P. K. Wang

4. When the air parcel is saturated Saturation means that the parcel contains a maximum amount of water vapor with respect to the temperature—warm air can hold more water vapor per volume than cold air. When an air parcel containing moisture rises adiabatically, it will cool just like a dry air parcel initially. However, when it rises to a level where its temperature becomes cold enough to saturate the moisture, its cooling rate will change to smaller values when the parcel continues to rise. Why? Because when the parcel becomes saturated, further cooling will cause excess moisture to condense to form liquid drops. During condensation, latent heat is released so as to buffer the expansion cooling rate. This rate is called the moist adiabatic lapse rate—it is not a constant (it depends on the amount of condensation) but is always smaller than the dry adiabatic lapse rate. The level at which the parcel becomes saturated is called the lifting condensation level (LCL). Traditionally, LCL is taken as the cloud base.

5. Convective Cloud Formation The formation of a convective cloud can be approximated as a moist adiabatic process. A moist air parcel is lifted upward adiabatically. It expands and cools at the dry adiabatic lapse rate initially. When it reaches its LCL, condensation begins and cloud appears. Further ascend of the parcel continues to cool the parcel but at a smaller moist adiabatic lapse rate. If there is enough moisture in the parcel, the cloud will continue to ‘grow’ upward and, when the temperature is cold enough, ice may start to appear in the cloud. In reality, the ascent of an air parcel is not adiabatic because mixing with its environmental air (usually drier and colder) always occurs. This mixing process is called the entrainment. Therefore the moist adiabatic process should be regarded only as a convenient approximation. P. K. Wang

6. Example: Imagine two parcels of air, A and B, both with surface temperatures of 25 o C. Compare the temperatures at 4km above the ground, if parcel A has an LCL at 2km, but parcel B remains unsaturated. Assume the dry adiabatic lapse rate is -10 o C/km, and the moist adiabatic lapse rate is -7 o C/km. a). They will have the same temperature. b). Parcel A will be warmer. (-9 o C) c). Parcel B will be warmer. (-15 o C)

7. Example: An Unsaturated parcel of air located 3km above the ground has a temperature of -13 o C. What will its temperature be if it is brought down adiabatically to the surface. Assume the dry adiabatic lapse rate is -10 o C/km, and the moist adiabatic lapse rate is -7 o C/km. a). 8 o C b). 20 o C c). 17 o C d). -43 o C

8. Example: Imagine a parcel of air with a surface temperature of 25 o C, and an LCL at 2km. Lift the parcel adiabatically to 4km above the ground. If the surrounding environment at 4km has a temperature of -5 o C, would the air parcel warmer or cooler than its environment. Assume the dry adiabatic lapse rate is -10 o C/km, and the moist adiabatic lapse rate is -7 o C/km. a). They will have the same temperature. b). The air parcel will be warmer. c). The air parcel will be cooler. (Parcel temp = -9 o C at 4km)