Department of Earth & Climate Sciences
San Francisco State University
Name______________________
Spring 2015
Quiz #7 Key
150 pts.
1. Definitions. (5 points each for a total of 20 points in this section).
(a) Downburst -- a thunderstorm downdraft that strikes the ground and exceeds
the speed criterion for severe thunderstorm winds (>56 mph).
(b) Geostrophic Wind -- the theoretical wind that would result from an exact
balance between the Coriolis acceleration and the pressure gradient
acceleration so that, in the Northern Hemisphere, air would flow parallel to the
isobars (height contours) clockwise around highs and counterclockwise around
lows, and faster in regions in which the pressure (height) gradients are the
largest.
(c) mesocyclone – a persistent counterclockwise rotating
thunderstorm updraft.
(d) vertical wind shear – the change in wind direction and
speed with height.
2.
Units. Provide the convetional metric unit for each (3 pts
each for a total of 12 pts)
Example
Acceleration of gravity m/s2 or m s-2
Coriolis acceleration
__ m s-2_____
CAPE
__Joules kg-1_________
CIN
___ Joules kg-1
horizontal pressure gradient acceleration
m s-2_____
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3. Short answer Questions (105 points). Answer neatly in complete sentences in the
spaces provided or on the back of the exam sheets. Show all steps for calculations.
(a)
You are provided with a diagram that is used by meteorologists to evaluate
vertical wind shear. The boxes indicate the information at 1 km inteverals
from the ground (0 km) to approximately the 500 mb level (6km). (30 pts)
i. This diagram is called a hodograph (5 pts)
ii. What does each labeled point on the diagram indicate? (5 pts)
The labeled points indicate the tip of a wind vector (arrow indicating conceptual
wind direction) for each level shown.
iii. Explain how the information for shown for 0 km and 6 km is consistent with the
calculated value of storm relative helicity of 474 (not shown). (20 pts)
The wind at the surface is southerly at 15 knots, and the wind at 6 km is
west-southwesterly at around 55 knots. The important thing to note is
that the surface wind is nearly (but not exactly) at right angles to the
winds aloft. This means that the surface wind streamlines will develop a
rotation or twirl that is measured by the storm relative helicity. Since the
wind streams at the surface and aloft are nearly at right angles to one
another, one would expect high values of the helicity. 474 m2/s2 is large
value that indicates the updraft would be rotating strongly.
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(b)
You are provided with the paired (a) CAPE/CIN charts; and (b) Visible
Satellite Images for 1700 UTC and 2200 UTC on 29 April 2010. Note the
locations labeled A and B. (Note: no radar images given...so even though
the question was misworded, you should recognize that you are looking at
a satellite image and not a radar image).
Explain the differences in the satellite presentation at locations A and B given
the evolution of the CAPE/CIN field you see on the top chart. [Note: this is an
essay question. It does not need to be long. However, it needs to be written in
proper English, NEATLY, with sentences that have a subject, verb and object]
(30 points)
The morning CAPE and CIN field suggests that all of the soundings in the area are
potentially unstable, but that large values of CIN need to disappear from these
soundings before the soundings become nearly absolutely unstable. The values of
CAPE at the two locations are roughly similar, but it is clear that by the afternoon
while the CIN disappeared at location A, there was still significant values of it at B.
Thus, the convective temperature was probably attained at A, and thunderstorm
initiation should be observed at location A, but not at B. The radar reflectivity and
visible satellite plots substantiate this, with both showing heavy precipitation
associated with a cumulonimbus cloud at A, but clear skies and no precipitation at B.
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(c) The chart on the next page is the 500 mb chart for the eastern Atlantic for 12 UTC
10 May 2013. Note the station reporting a 40 knot (20.7 m/s) wind highlighted in
red. The orange lines show the natural coordinate system centered on the observation
in question. Locations 2 and 1 are 500 km apart.
i. Label the two axes for the coordinate system. (10 pts)
This is a natural coordinate system. The s-axis is tangent to the wind with the
positive end on the downstream side. The n-axis is at right angles to the wind,
with the positive direction to the left. See diagram.
ii. The geostrophic wind relation is
g Dz
Vg = f Dn
Compute the value of
Dz
. Show all steps. (35 pts)
Dn
Dz z2 - z1
=
Dn
Dn
Dn = 500 km = 500000 m
The 500 mb heights at the two locations can be read right off of the chart.
Z2 = 576 dm = 5760 m
Z1 = 585 dm = 5850 m
Substituting these values into the expression gives:
Dz ( 5760 - 5850 ) m
0.18 m
=
=Dn
500 km
km
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3. Sounding Interpretation. You are provided with the sounding for Brownsville TX
(KBRO) for 12 UTC 9 May 2013. (25 points)
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Your task is to transform the sounding to one that estimates conditions for afternoon
heating. On this skew-T log p diagram, the saturation mixing ratio lines are dark grey
and slope from left (at the bottom) to right (at the top) and are labeled at the bottom in
g/kg.
a. Find the Convective Condensation Level, and the Convective Temperature (indicate
on the diagram) (10 pts) 772 mb, 41C
b. Draw the parcel ascent curve that would occur if the afternoon temperature achieves
the Convective Temperature. (5 pts) On diagram.
c. Shade in the CAPE for the ascent curve that results. (5 pts) On diagram.
d. This sounding is an example of the type of sounding that has great potential for
explosive convection in the afternoon, even though the morning sounding has a
strong inversion. What is the “popular” name that severe weather forecasters give
this type of sounding? Loaded Gun Sounding__________(5 pts)
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