ATSC 5160 Midterm 22% Thursday, March 27, 2003

2. In the lee of the Rockies, cyclogenesis is clearly due to subsidence (adiabatic warming and vortex stretching). In the lee of the Alps, this is not necessarily the case, depending on the strength and stability of the upstream flow. Discuss briefly. (1%)

1. List all physical mechanisms responsible for vertical motion, anywhere between the earth surface and the tropopause? Indicate for each process how rising motion is achieved, and the dimensions of the ascent (horizontal scale, depth, and strength). Distinguish between (a) convective-type processes, (b) quasigeostrophic processes, (c) frictional processes, and (d) other processes. (10%)

2. Draw a schematic of the ageostrophic flow and con/divergence in the entrance and exit regions of a zonal upper-level jet streak (show a map, or an xy plane). Also show the location of the IPV maximum (+) and IPV minimum (-). Then draw two meridional cross sections (yz planes), one across the jet entrance region, another across the jet exit region. In these sections, plot some isentropes (including a hint of the low-level frontal zone), the tropopause, and ageostrophic flow vectors. (3%)

3. (a) Give two reasons why the semistationary upper-level wave troughs in the northern hemisphere jet stream are deeper than in the southern hemisphere.
(b) Indicate the approximate longitude or geographic locations of the semistationary wave troughs in the northern hemisphere.
(c) In what season or these semistationary wave troughs stronger [in terms of minimum height or relative vorticity or potential vorticity, whatever], and why? (3%)

4. Assume a midlatitude region with a anomalously low surface temperatures. Assume that the PV anomaly in this cold pool is sufficiently strong to induce flow, extending to the upper troposphere. Assume that westerly winds increase with height aloft. And assume that the westerly flow, in the upper troposphere, advects a positive PV anomaly of about the same magnitude and size as the low-level one into the region, from the west.

(a) draw a transect, from west to east, showing some isentropes and isotaches (x means into the page, . is out of the page). Show both the surface PV anomaly (- or + ??), and the upper level PV anomaly (+), at the time that the latter (UL) is a quarter wavelength to the west of the former (LL) anomaly. (3%)

(b) Draw two maps, one aloft and one at low levels, with typical basic-state gradients of P aloft and q at low levels, to show how in this relative position the UL anomaly will act to weaken the LL anomaly, and vice versa. There is no need to provide quasi-geostrophic arguments. (3%)