Open book – take home
Use any resource you like,
including the textbook, the peer-reviewed literature, COMET/METED material such
as “Anticipating Convective Storm Structure” (on the web or CD). But your
answers must represent an individual effort.
1.
(11%) We have not studied microburst dynamics, but we use the concept to test
your understanding of convective dynamics in general. A microburst is a small
downburst that develops at or above the base of a cumulonimbus cloud. The
downward acceleration results from the negative buoyancy that the microburst
develops on its way towards the ground. I saw two spectacular ones from the air
in IHOP. The air was at about 100F, the convective BL very deep, and the air
dry. One produced a deep dust bowl spreading out radially [our ex-navy aircraft
carrier pilot Wayne Sand banked about 90 degrees to avoid it], the other
produced a beautiful gustnado. Anyway, the objective of this exercise is to
calculate the buoyancy and vertical motion of a parcel of air in a microburst,
as a function of height below cloud base. Assume the following:
a) the ground is at 1000 mb,
where the ambient temperature is 35ºC
b) the cloud base is at 3 km
above the ground
c) at cloud base the parcel has
zero vertical velocity, the same virtual temperature as the environment, but
its negative buoyancy is due to 2 g kg-1 of hydrometeors
d) the descending parcel will
continue to be saturated until it has consumed (evaporated) all its
hydrometeors
e) the environment below
cloud-base is well-mixed, i.e. it has a constant mixing ratio and a constant
potential temperature
Now
do the following:
a) (graphical) Use the parcel
technique to plot the variation of temperature and dewpoint, both for the
parcel and the environment, between cloud base and the ground, on a skew T log
p (there are some blank ones in the lab). Also highlight the parcel’s ‘negative
area’. (2%)
b) (write-up) Discuss the
dynamics of this microburst: what accelerates it downward, what retards this acceleration?
When the microburst is about to hit the ground, its vertical velocity will
rapidly decrease, because of continuity. What is the real force causing this
deceleration? (4%)
c) (numerical) Plot, as a
function of height between cloud base and ground, both the buoyancy (m s-2)
and the vertical velocity (m s-1). To do this, you need to use the
vertical equation of motion. Ignore all terms on the right hand side of this
equation, except the buoyancy. In the buoyancy term itself you can ignore one
term: the perturbation pressure term. [hint: as a first order approximation,
you can assume a parcel lapse rate of say 6 K/km] (5%)
d) (extra credit) repeat the
above, but do it correctly, i.e. calculate the moist adiabatic lapse rate, and
keep track of the water vapor and the remaining liquid water, at every step in
a finite difference approach (2%)
2.
Courtesy of Cory, I showed an example in class of a spreading arc of clouds in
northern
3. Choose one of the following two. (7%)
Either
Question 3.1 on p. 577 in the textbook, Bluestein 1993 (you must limit
your answer to 2 pages typed, font size 12, or else 3 p handwritten)
Or
else this question: A tornadic storm occurred near
a) Go to Dr. Oolman’s website
(weather.uwyo.edu) to retrieve the sounding, and the hodograph, at YMML
(Melbourne) at 12 Z on 3 Oct 200.1 (1%)
b) On the hodograph, roughly
draw the mean shear vector between 1000-400 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. (1%)
c) Does the 0-6 km hodograph
turn clockwise, counterclockwise, or is the shear straight? (1%)
d) Locate the 0-6 km mean wind
on the hodograph (you can eyeball this) (1%)
e) Locate both the left-moving
and right-moving storm motions on the hodograph [hint: use the ID method
discussed in class]. (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