ATSC 5160                 extra Final                  20%                Fri May 14 2004

 

Please attempt all 6 questions. The first three questions are new, the last three are from the study list.

 

1. (3%) Undular bores and gravity waves:

a.       What are the three types of wave phenomena associated with a density current propagating into a stable environment?

b.       Under what conditions can a density current spawn an undular bore?

c.       What controls the propagation speed of an undular bore?

d.       How can you distinguish the passage of a gravity current from the passage of a gravity wave, from high-frequency surface pressure observations alone?

2. (3%) I showed an example in class of a spreading arc of clouds in northern Georgia. The visible satellite imagery and surface observations can be viewed again in the attached file (final_03.ppt). Discuss the dynamics of the observed development and spreading of this cloud arc.

 

3. (4%) Several tornadic storms were reported just south of Jackson MS (JAN) during the afternoon of 1 May 2004. (http://www.spc.noaa.gov/climo/reports/040501_rpts.html). The storms were observed to move at about 12 m/s towards 020 (NNW), according to radar reflectivity animations.

a)      On the hodograph attached at the end (JAN 20040502_00Z), draw the mean shear vector S0-6 between 1000-442 mb (assume this to be 0-6 km elevation – Larry’s hodographs don’t show km but mb for altitude). Consider both magnitude and orientation. Draw the vector in units of m/s per 6 km.

b)      Does the 0-6 km hodograph turn clockwise, counterclockwise, or is the shear straight?

c)      Locate the 0-6 km mean wind Vmean on the hodograph (you can eyeball this)

d)      Locate both the left-moving and right-moving storm motions on the hodograph [hint: use the ID method, i.e. , where D can be assumed to be 7.5 m/s. This equation applies to a right-mover (VRM); for a left-mover(VLM), change the plus + into a minus -][a graphical estimate is fine].

e)      Which one is more likely to survive after the split, or do they have equal survival chances? Assume that no shallow boundaries are present and the environment is horizontally uniform. Explain.

f)       By means of the definition of a cross product of vectors, prove mathematically that helicity, defined as , is proportional to an area on the hodograph. Here v is the ambient wind, c the storm motion, and S the ambient wind shear, and (a,b) are the lower and upper bounds over which helicity is calculated.

g)      Graphically, highlight (shade) the area on the hodograph that is proportional to the 0-3 km (1000-700 mb) helicity.

 

 

4. (3%) In the absence of any synoptic forcing, the west Texas dryline tends to move eastward and become better defined during the day (until the mid to late afternoon), and then it tends to retrograde to the west and fade into the evening. Explain.

 

5. (4%) Entrainment models have assumed that cumulus clouds behave either as thermals (bubbles) or as continuous plumes. Briefly discuss how these models differ, and what they have in common, both in terms of assumptions (i.e. how do they conceptualize a cumulus cloud) and outcome (i.e. how do they predict a cumulus cloud to evolve).

 

6. (3%) Discuss how, in a high-shear, sufficient-CAPE environment, the dynamically-induced perturbation pressure gradient acceleration (DPPGA) can explain:

  1. storm splitting
  2. the survival of the right-moving storm, in the case of clockwise-turning wind shear

[hint: two forcing terms in the dynamic source for  matter. You can show the equations for these terms, but it is sufficient to show a schematic for both. Be sure to indicate the location of the low(s) (and highs), and the direction of the dynamically-induced perturbation PGF]