Download Exam 3 ACTUAL Key - Kelvin K. Droegemeier

Dr. Droegemeier
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Meteorology 1004
Introduction to Meteorology
Fall, 2001
Examination #3
Wednesday, 28 November 2001
BEFORE YOU BEGIN!! Please be sure to read each question CAREFULLY and, if time
permits, check your answers. Answer all questions on the pages provided - do not use
additional sheets. Be sure to submit your computer form as well as the exam before leaving
the room. This exam is worth 100 points.
Part I: Multiple Choice (3 points each) - Indicate the correct answer on the computer form
using a #2 pencil.
1. The thermal wind vector is
a.
b.
c.
d.
e.
The wind associated with temperature gradients
Something that cannot be "felt" physically
A fictitious force
The vector difference between the geostrophic wind at two levels in the atmosphere
Both b and d.
2. A veering wind with height indicates
a.
b.
c.
d.
e.
Cold-air advection
Sinking motion
Warm-air advection
Increasing pressure
Increasing humidity
3. Warm air advection tends to
a.
b.
c.
d.
e.
Decrease the thickness
Increase the surface pressure
Decrease the surface pressure
Increase the thickness
Both c and d
4. Baroclinicity refers to
a.
b.
c.
d.
e.
Wind shear
Pressure gradients
Temperature gradients
Moisture gradients
None of the above
5. A continental tropical air mass typically originates over
a.
b.
c.
d.
e.
Washington state
Florida
Mexico
California
None of the above
6. The optimal location of a surface low with regard to intensification is
a.
b.
c.
d.
e.
Along the axis of the upper-level trough
West of the axis of the upper-level trough
East of the axis of the upper-level trough
In vertical alignment with the upper-level low
Insufficient information given to answer the question
7. The favored location for the intensification of a surface cyclone is in which quadrant of a jet
streak?
a.
b.
c.
d.
Right front
Left front
Right rear
Left rear
8. Net convergence acting on a vertically-oriented vortex will
a.
b.
c.
d.
e.
Spin the vortex down
Spin the vortex up
Stretch the vortex
Compress the vortex
Both b and c
9. Which of the following is not a condition associated with air mass (single-cell)
thunderstorms:
a.
b.
c.
d.
e.
Conditional instability
Localized lifting mechanism
Strong vertical wind shear
Abundant low-level moisture
None of the above
10. Microbursts are a hazard to low-flying aircraft because they
a.
b.
c.
d.
e.
Reduce visibility abruptly
Create strong temperature gradients
Create strong wind shear
Are short-lived and very small scale
Both c and d
Part II: True/False (3 points each) - Indicate the correct answer on the computer form using
a #2 pencil.
11. The greatest frequency of thunderstorms is coincident with the greatest frequency of hail
a. True
b. False
12. Absolute vorticity of an air parcel is the sum of the relative vorticity and the planetary
vorticity.
a. True
b. False
13. A decrease in thickness implies winds backing with height
a. True
b. False
14. If the upper-level pressure surfaces are purely horizontal, the thermal wind will be zero
a. True
b. False
15. Rising motion typically is found east of the axis of an upper-level trough
a. True
b. False
Part III: Definitions (5 points each) - Provide a short definition for each term in the space
provided.
1. Entrainment
The process by which environmental air is mixed into a cloud through turbulent processes. It is a
key aspect of downdraft formation in thunderstorms.
2. Relative vorticity
The microscopic (at-a-point, or local) spin at a point within a fluid measured relative to the
rotating Earth. It is a vector and has three components.
3. Gust front
The leading edge of the surface cold pool that spreads out beneath a precipitating thunderstorm
or mesoscale precipitation system.
Part IV: Short Answer (points as shown) - Provide a concise answer to each of the questions
listed below.
1. (10 points) In the context of polar front theory, sketch and briefly describe the stages of a
developing mid-latitude cyclone, showing the low, fronts, air flow, pressure patterns, and
weather.
2. (10 points) Sketch and briefly describe the three stages of an airmass (single-cell)
thunderstorm. Be sure to describe why such storms never last longer than approximately 1 hour.
Cumulus Stage -- Cloud consists entirely of updraft and is growing rapidly; significant
entrainment occurs.
Mature Stage -- Cloud consists of both updraft and downdraft, marking the most intense phase of
the storm. The downdraft is driven by entrainment, precipitation drag, and evaporative cooling
below clouds base. The resulting cold pool spreads laterally on the ground.
Dissipating Stage -- The cloud consists entirely of downdraft, with the gust front spreading in all
directions, thereby cutting off the supply of moist, warm air to the updraft.
Such storms never last longer than about an hour because, owing to the lack of significant
vertical environmental shear, the cloud grows vertically upright. No mechanism exists to
separate the updraft and downdraft, and thus precipitation falls back through the updraft, killing
it.
3. (20 points) Suppose a strong east-west temperature gradient exists across the southern Great
Plains, with temperatures increasing toward the west at 10 C/300 km. Westerly winds of 20 m/s
are howling from Amarillo (located 400 km due west of Oklahoma City) to the eastern
Oklahoma border.
a. At what rate (in degrees per hour) will the temperature change at Oklahoma City? Note that
the advection equation is given by
DT
DT
DT
DT
=- u
-v
- w
Dt
Dx
Dy
Dz
Given: dT/dx = -10C/300,000 m
u = 20 m/s
Distance between OKC and Amarillo: 400 km
Find the time rate of change of temperature at OKC (that is, find the temperature advection).
Using the equation given on the previous page, and noting that only the east-west term is
involved, we have
DT
DT
=- u
Dt
Dx
Working this problem thus is simply a matter of plugging in the numbers on the RHS, which
yields
DT
æ - 10C ö
= - (20 m / s) ç
= 6.67 x10- 4 C / sec = 2.4 C / hr
è300, 000m ÷
ø
Dt
Does the answer make physical sense? Yes, because warm air is to the west in the presence of a
westerly wind -- thus, the temperature should rise at OKC. Note that you didn't need to use the
distance between Amarillo and Oklahoma City.
b. If the temperature at Oklahoma City is 30C at 11 am, and if the convective temperature is
40C, at approximately what time would you forecast thunderstorms to begin at Oklahoma City?
From part a, we have the rate of change of temperature. In this part, we're given the initial and
final temperatures and asked to find the time, which will give us the time interval because we're
also provided with the starting time. Thus,
T final - Tinitial 40C - 30C
DT
=
=
= 2.4C / hr .
Dt From Part a t final - tinitial
Dt
Solving for the t gives 4.17 hours, which is roughly 4 hours and 10 minutes. Thus, convection
will begin at 11 am plus 4 hours and 10 minutes, or at approximately 3:10 pm.