Download department of geosciences - San Francisco State University

DEPARTMENT OF GEOSCIENCES
San Francisco State University
Spring 2009
NAME________________________
Synoptic Metr Quiz #2
100 pts.
(Test will be collected at 10:40 AM)
1. Definitions. Choose two of the terms below. Provide a one sentence definition (if the term is an
abbreviation, first “unabbreviate” it and then define it). (5 pts each for a total of 10 pts).
(a) CAPE –
Convective Available Potential Energy – a measure of the total potential buoyancy
acceleration experienced by an air parcel rising from the LFC to the EL.
Directly related to the “positive” area on a Skew T/Log P diagram bounded by the parcel’s
ascent curve on the right and the sounding (ELR)on the left, from the LFC (CCL) to the
EL.
(b) Equilibrium Level
The location at the top of the positive area on a sounding at which
the air parcel becomes the same temperature as its surroundings
and above which it becomes colder than it surroundings.
Alternatively, it is the elevation that separates the region of the atmosphere in which the
buoyancy acceleration is positive (beneath the level) from that in which the buoyancy
acceleration is negative (above the level).
(e) Lifted Index
The difference (in C or K) between the temperature of a lifted
parcel and the surrounding atmosphere at the 500 mb level
(negative for unstable conditions).
(c) Convective Temperature
The temperature to which the surface temperature must be warmed in the afternoon in
order to both eliminate the “loaded gun” sounding inversion, and to make the sounding
nearly absolutely unstable.
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2. Units. Provide the units (metric) used conventionally for the following (8 pts)
horizontal winds meters per sec or m s-1 (or km h-1)
pressure gradient
mb km-1
lifted index
o
CAPE
Joules kg-1 or m2s-2
CIN
K or oC
Joules kg-1 or m2s-2
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Map Questions (82 pts) (Use back of sheets if necessary. Answer in complete sentences. )
3.
The sounding for Fort Worth, TX (KFWD) for 0000 UTC 4 May 1999 is shown in Fig 1. (30
pts total)
(a)
Determine the elevation (mb) of the LCL, LFC, and EL and the value of LI for the
sounding. (Show the ascent curve, and make sure to plot the positions of the levels on
the ascent curve) (28 pts)
(b)
Shade in the portion of the modified sounding that has positive buoyancy (CAPE) in
red. (2 pts)
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4.
The sealevel pressure map for 2200 3 May 1999 is given as Fig. 2. Note the lines indicated as
Lines A, B and C. The area experiencing dew point temperatures exceeding 60 oF is shaded
green.(22 pts total)
(a) Identify (4 pts each for 12 pts here):
Line A cold front
Line B warm front
Line C dry line
(c)
The low pressure area shown on the chart is known as a “wave cyclone”. Why is the
low a “cyclone”? (5 pts here)
The low pressure area is a cyclone because it is an area of low pressure completely
encircled by at least one isobar.
(d)
The low pressure area shown on the chart is known as a “wave cyclone”. What does the
adjective “wave” refer to in this term? (5 pts here)
The low pressure area is a wave cyclone because it is an area of low pressure completely
encircled by at least one isobar within which is found a frontal system in a wave-like
configuration.
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5.
The CAPE/CIN field for 2200 UTC 3 May 1999 is given in Fig. 3. Compare the pattern
shown with the dew point field depicted in Fig. 2.
(a) Briefly explain why the CAPE field is consistent with the dew point field (15 pts);
The dew point field gives an estimate of the mixing ratio field at surface pressure. Mixing
ratios determine the amount of latent heat released in an ascending air parcel. For a
given temperature, the latent heat released accounts for why an air parcel lifted to, say,
500 mb is considerably warmer than it would be if lifted dry adiabatically. Thus, for a
given Environmental Lapse Rate, the higher the surface dew point temperature that
more warm a force lifted air parcel would be relative to the surrounding air at the same
elevation. Hence, the surface dew point field is directly related to the surface based
CAPE field.
By the way, the argument that many of you used that the higher the dew point the
greater the chances that saturation would occur (and, hence, a rising parcel would cool
wet adiabatically) can be used in the context of the above charts ONLY if the surface
temperatures were everywhere the same. That is an unrealistic assumption. That
argument only works when looking at one sounding for one place at one time (and
increasing the surface relative humidity). See this graphic.
(b) Describe why the fact that thunderstorms were erupting over a broad area of southern
Kansas, Oklahoma and north Texas would be expected given the information depicted in
both Figs. 1 and 3. (Careful, it’s not just that CAPE is present) (15 pts)
Both figures depict large values of CAPE, both in the Fort Worth sounding, and
geographically over the southern Great Plains. But this would be not enough to expect a
widespread outbreak of thunderstorms if large amounts of CIN were present in all the
soundings in the area.
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But, as can be seen from the Fort Worth sounding and the geographic distribution shown
in Fig. 3, it is clear that the CAPE is occurring in an environment in which there is
virtually no CIN. Hence, the soundings in the area are “uncapped” (nearly absolutely
unstable) and thunderstorms would be expected to occur over the area.
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Figure 1: KFWD Sounding, 0000 UTC 4 May 1999
Figure 2: Sealevel pressure map, 2200 UTC 3 May 1999.
Boundaries are indicated as Lines 1, 2 and 3, and area of
surface dew point exceeding 60oF is shaded in green.
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Figure 3: Distribution of CAPE (red) and CIN (blue),
2200 UTC 3 May 1999.
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