ATSC 5160 Final 22% due: Fri 10 May 2002, 5 pm

Open book – take home

Use any resource you like, including the Anticipating Convective Storm Structure CD held in the black laptop. Your answers are an individual effort.

1. Question 3.1 on p. 577 in the textbook, Bluestein 1993 (4%) (pls limit your answer to 1 p typed or 2 p handwritten)

2. Compute the Richardson’s number for an environment where the lapse rate is 6.5 K/km and the meridional temperature gradient -3 K/100 km, ie it is colder to the north. (3%)

a) Determine Ri (hint: use the expression on p. 281 in Holton 1992; assume that the wind is in thermal balance with f=10^-4s^-1) (2%)

b) Is this environment symmetrically unstable? (0.5%)

c) If it was symmetrically unstable, and the instability was released, what would be the orientation of the resulting rainbands? (0.5%)

3. A tornadic storm occurred near Melbourne Australia on 3 Oct 2001 (http://www.stormchasers.au.com/3_10_01.htm). The storm moved at 12 m/s towards 025 (NNW). Perhaps the skew T and hodograph in the storm vicinity a few hours after the event (12 Z or 10 pm LT, shown below) are not very representative, but assume that the environment was indeed sufficiently unstable and sheared to support supercell storms. (5%)

a) On the hodograph below, draw the mean shear vector between 1000-400 mb (assume this to be 0-6 km elevation). Consider both magnitude and orientation. Draw the vectors in units of m/s per 6 km. (0.5%)

b) Does the 0-6 km hodograph turn clockwise, counterclockwise, or is the shear straight? (0.5%)

c) Locate the 0-6 km mean wind on the hodograph below (you can eyeball this) (0.5%)

d) How would the initial (pre-split) storm move? (0.5%)

e) Locate both the left-moving or right-moving storm motions on the hodograph [hint: use the mean flow/shear rule-of-thumb discussed on the CD]. (1%)

f) 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. (1%)

g) Climatologically, what type of supercell is more likely in southeastern Australia, rightmovers or leftmovers? Explain. [hint: consider large-scale thermal advection and increase of CAPE] (1%)

4. Calculate the buoyancy (m s^-2) and vertical motion (m/s) of a parcel of air in a downburst (microburst), as a function of height below cloud base. The ground is at 1000 mb and the cloud base is at 2.5 km above the ground. Assume that at cloud base the parcel has the same virtual temperature as the environment, but it contains 2 g kg^-1 of hydrometeors. Assume also that the parcel will continue to be saturated until it has consumed (evaporated) all its hydrometeors. The environment below cloud-base is well-mixed, i.e. it has a constant mixing ratio and a constant potential temperature. The cloud base is at the convection condensation level and the ambient surface temperature equals the convection temperature, which is 30ºC.

a) (graphical) Use a skew T log p (there are some blank ones in the lab) to plot the variation of temperature and dewpoint, both for the parcel and the environment, between cloud base and the ground; highlight the ‘negative area’. (2%)

b) (numerical) Calculate the downward parcel acceleration. Ignore entrainment. Assume that the BPGA, which opposes the buoyancy, remains at 50% of the buoyancy itself, and that the initial vertical velocity at cloud base is zero. The only term in the buoyancy you can ignore is the perturbation pressure term. Plot, as a function of height between cloud base and ground, both the buoyancy and the vertical velocity (In reality the vertical velocity will rapidly decrease near the ground because of continuity) [hint: as a first order approximation, assume a parcel lapse rate of say 6 K/km. To do better, you need to calculate the MALR at every step in a finite difference approach](8%)