ATSC 5160 Midterm 18% Tuesday, March 29, 2005

1. Plot a meridional section, pole to equator, surface to 20 km, depicting the annual-mean distribution of zonal-mean potential vorticity P (in standard P units) and q (K). Please label your contours. (1%)

2. (a) Define P on isentropic surfaces.

(b) Discuss how IPV can be changed.

(c) Discuss how the initial phase of cyclogenesis in the lee of the Rockies can be explained well by means of potential vorticity conservation, while potential vorticity generation is the more common cause of cyclogenesis in the lee of the Alps. (2%)

3. (a) Sketch a zonal transect of the general circulation of the atmosphere at about 25ºN in July. Your transect does not need to cover all longitudes, but it should show an ocean (e.g. the Atlantic or the Pacific) and an adjacent continent (e.g. the Sahel or S. Asia, resp.) Include the mean circulation, some isentropes (and label temperature anomalies as W and C at the surface, at ~5 km and at ~15 km), the tropopause, any subsidence inversions, surface pressures (H and L), deep convection (latent heating), any jet streams (2%)

4. Imagine an east-west oriented mountain range in the northern hemisphere (e.g. the Alps). The air to the north of the range is stable stratified, as shown below.

Now imagine a Foehn event blowing over the mountains from the south, pushing down isentropes on the north side, somewhere in the middle of the range.

(a) schematically draw the resulting low-level PV anomaly

(b) indicate in which direction it will move (2%)

5. What are the differences between the polar-front jet and the subtropical jet, in terms of location, intensity, formation mechanism, and effect on surface weather? (2%)

6. Banded precipitation has been observed, and you are to assess whether symmetric instability occurred. List three different methods of analysis. (2%)

7. Consider the height (solid) and temperature (dashed) field at 700 mb, below. Assume that . Is the atmosphere at point P symmetrically unstable? Show your work. (3%)

8. Thermal vorticity is defined as the vorticity of the thermal wind, for instance between 1000 and 500 mb:

Show mathematically that an increase in thermal vorticity must be accompanied by a decrease in thickness, i.e. cooling in the 1000-500 mb layer. (2%)

9. In the Fig below, solid lines represent gp height, and dashed lines are temperature, say at 1000 mb. Assume that the 850 mb temperature variation is in phase with that at 1000 mb, but the meridional temperature gradient is weaker. For what values of x is the height tendency positive? Negative? Use QG theory, and consider both vorticity and temperature advection terms. Briefly explain your answer. (2%)