Listed below are 8
questions. Please attempt all questions. The last question is to be partly
completed on a tephigram, to be handed in with your
answers.
1.
Define the frontogenesis function mathematically and
in words (1%). (No need to expand the definition in the various terms.)
2.
Schematically, draw the various rainbands that can be
found near a frontal system. Please label them. (1%)
3.
Draw a cross section across a cold front aloft, trailed by a surface cold
front. Draw the following (2%):
4.
Define the helicity, say between 0-3 km,
mathematically; also show a hodograph, illustrating the various terms that are
needed to compute helicity (2%).
5.
Discuss how, in a high-shear, sufficient-CAPE environment, the dynamically-induced
perturbation pressure gradient force can explain (2%):
a. storm splitting
b. the survival of the
right-moving storm, in the case of clockwise-turning wind shear
[hint: there is no need to show the equations; it is
sufficient to show a schematic for both. Be sure to plot the wind profile, the
buoyant updraft, the distorted vortex tube, the direction of the wind shear,
the perturbation low(s) (and highs), and the direction of the
dynamically-induced perturbation PGF]
6.
Cold pool-shear interactions are believed to be important in the maintenance of
multicell storm clusters. Schematically explain how
optimal cold pool-shear interaction can sustain the storm system. Assume that
the LFC of the ambient air is, say, twice as high as
the cold pool depth (2%).
7.
Show schematically the location of the buoyancy-induced perturbation high and
low both in and around a buoyant bubble in an unsheared
environment. Also draw some perturbation isobars, and the direction of the
pressure gradient force (the BPPGA), as field lines both in and around this
bubble. In 1-2 sentences, explain the purpose of this force (2%).
8.
Visualizing an entrainment event and the ensuing downdraft on a tephigram. (6%)
You
are to perform a highly simplified simulation experiment to demonstrate that
mid-level entrainment can trigger convective downdrafts in a thunderstorm. Assume
the following:
a) the environment has these
characteristics:
a. the ground is at 1000 mb, where the ambient temperature is 35 ºC, and the dewpoint is 20.3 ºC
b. the convective
boundary-layer is well-mixed up to 850 mb, i.e. it
has a constant mixing ratio qv and a
constant potential temperature q
c. an isothermal layer is
present from 850 to 830 mb
d. a temperature lapse rate of
20 K/230 mb exists between 830 and 600 mb
e. the ambient dewpoint at 600 mb is -15 ºC
f.
the ascending parcel somehow penetrates through the CIN layer and, once
past the LCL, it conserves all its condensate qh
(i.e. no rainfall)
b) the entrainment event
a. occurs in a single burst at
600 mb
b. the environment mixes with
the undiluted ascending parcel at 50-50% mass ratio
c. the buoyancy of the mixture
(called “mixed parcel”) is due to the T perturbation only (ignore water loading
and water vapor perturbations)
d. the descending mixed parcel
does not entrain, and it will continue to be saturated until it has consumed
(evaporated) all its liquid water
Now
do the following:
a) (graphical, using a tephi, please hand in the analyzed tephigram)
Use the parcel technique to plot the variation of temperature and dewpoint, both for the undiluted parcel and the
environment, between the ground and 600 mb, on an aerological diagram (hand-out). Then infer the temperature
and mixing ratio of the mixed parcel at 600 mb.
Decide whether the mixed parcel is negatively buoyant (compared to the
environment), and if so, track it downward. Also highlight the ‘negative area’
of the developing downdraft. (2%)
b) (table) List the following:
(4%)
a. T, qv,
qh of the undiluted parcel at 600 mb
b. T, qv
of the environment at 600 mb
c. T, qv,
qh of the mixed parcel at 600 mb
d. qv, and pressure p of the
mixed parcel at the level where it just dries out (ql
=0, but qv =qv,sat)
e. qv, and p of the mixed parcel
at the level where the downdraft stalls, because it becomes neutrally buoyant
(same T as the environment) – this of course may never happen, i.e. the mixed
parcel may remain negatively buoyant down to the ground.