ATM OCN (Meteorology) 100
Answers for Homework 4
Summer 1998
Date Due: Wednesday, 5 August 1998
The total maximum points were 200. Point distribution
for each question is noted below.
PART 1. PLANETARY ATMOSPHERIC CIRCULATION
(50 pts for the section)
1. Suppose that in 1849 you would have wanted to join the California
Gold Rush and you would have booked passage from New York City to San Francisco
on a 19th century Clipper sailing ship that went around Cape Horn. Briefly
describe in a paragraph the sequence of weather events that you would have
probably experienced leaving New York in October and traveling south to
Cape Horn across the North and South Atlantic Oceans. Relate these weather
features to the prevailing wind regimes and the semipermanent pressure
features of the general atmospheric circulation. (You may want to consult
an atlas!)
[20 pts.]
| A paragraph (with reasonably good grammar) should have included
the fact that leaving New York, one is in the prevailing westerlies with
some possible storms. Nearing the Florida coast, one would have reached
the horse latitudes, with fair skies and weak winds, under the subtropical
high pressure in the western Atlantic. Farther south, the northeast trades
can be found with a persistent northeast wind and some cumuliform clouds.
Near the equator, the doldrums also known as the Intertropical Convergence
Zone, would have been reached, with hot, humid and cloudy conditions, with
abundant rainshowers or thunderstorms. Sailing into the Southern Hemisphere,
the southeast trades would be encountered first, then the subtropical highs
of the South Atlantic (near southern Brazil and northern Argentina) then
as one approaches Cape Horn, the prevailing westerlies would have dominated.
The passage around the Cape would have probably been stormy in with migratory
cyclones moving eastward across the Southern Oceans. |
2. Using the January and July sea-level pressure charts found on
page 229 of your textbook, answer the following:
a. What is the approximate mean value of the sea-level pressure
along the equator in January? and in July? Do you detect any significant
seasonal change in intensity or location? If you do, please describe.
What is its average pressure in January? [1pt.]
Do you detect any seasonal change in intensity? [1pt.]
| Some small variations in pressure occur between January and July
immediately along the equator. More importantly, the Intertropical Convergence
Zone has shifted farther north away from the equator between January and
July, which would be responsible for a slight increase in pressure. |
b. Locate the high pressure feature situated over the North
Pacific Ocean near Hawaii.
What is its average central pressure in January? [1pt.]
How has its position changed between January and July? [2pts.]
The cell has moved to the Northwest
|
c. Inspect the corresponding subtropical high pressure cells
in the other ocean basins. What general statement can be made concerning
the seasonal variation in the location and intensity
of these features? [4pts.]
They tend to follow the sun, shifting away from the equator
and intensifying in the summer hemisphere, while moving toward the equator
and weakening in the winter hemisphere.
|
d. Locate the low pressure feature located over the North Atlantic
Ocean near Iceland.
What is its average central pressure in January? [1pt.]
How has its position changed between January and July? [2pts.]
The cell has moved to the west-northwest
|
e. Use the major pressure features over the North American and
the Eurasian continents to make a statement as to the seasonal (winter
versus summer) variations in the air pressure over the continents. [4pts.]
| During winter, the continents are dominated by large high pressure
cells, with relatively lower pressure over the oceans, while in summer
the continents have lower pressure than over the oceans. |
PART #4.1. SURFACE WEATHER MAP ANALYSIS
The total maximum points were 100 for Part #4.1 and 50 points for Part
#4.2.
The attached map contains the surface weather features observed over
the upper Midwest at 6:00 AM CST on Friday, 1 November 1991.
A. STATION MODEL
At each station the following abbreviated surface station model has been
utilized:
Wind arrows fly with the wind. Each full barb equals 10 knot wind speed.
[Sample station reports a 15 knot wind from the southeast
(SE)]. Missing values are indicated by an " M".
What were the conditions reported at Madison, WI at map time? (Include
units)
Temperature:
Dewpoint:
Pressure:
Sky cover:
Wind Speed:
Wind Direction:
Precipitation type or significant weather phenomenon:
|
52 degrees F
50 degrees F
1002 mb
Overcast (10/10 clouds)
15 knots
Southeast
Rain showers
|
B. ISOBAR ANALYSIS
Remember that isobars portray the sea-level corrected pressure field. Analyze
the sea-level pressure field by drawing isobars spaced every
4 mb, centered upon 1000 mb (i.e., ..., 992, 996, 1000, 1004, 1008,
..., etc.). The reported values may not necessarily equal the desired
values. Therefore, you would have to interpolate between those stations
with pressures within + 4 mb of the desired value. When performing
your analysis make a light sketch of the isobars with a pencil initially;
then smooth the isobars. After smoothing and checking your analysis, isobars
may then be inked with a black pen and each labeled neatly.
Points to consider during your isobar analysis:
1. Where is the low or high pressure center(s) located?
(Identify the pressure feature and state or province)
The surface low was in southeast Iowa, near Cedar Rapids.
The surface high was in the Prairie Provinces - off the
map
|
Hint: you should look for the regions with a distinct, closed
wind circulation. Locate the center of the low or high pressure system
and mark with a large red block "L" (for the low) or a large blue
"H" (for the high). Give an estimate of the value of the central
pressure.
The central pressure of the low over Iowa was approximately
995 mb.
|
2. Since the atmosphere is a continuous fluid, no discontinuities
nor sharp kinks should appear in your isobar analysis. The spacing between
isobars should suggest a smooth horizontal pressure gradient, if possible.
Some kinks in the isobars should be expected, especially near wind shift
lines and fronts.
3. The isobars are usually spaced closer in regions where the
wind speed is greatest.
4. Does your smoothed analysis conform to the above items ?
C. ISOTHERM ANALYSIS
Analyze the surface temperature field by drawing isotherms every
10° F (e.g., 10°
, 20° , 30°
,...). Some smoothing may be necessary. Attempt to follow the same rules
as for drawing isobars noted above; however, the temperature field is usually
more variable than the pressure field. The finished isotherm analysis should
be drawn in dashed red lines and each isotherm should be
labeled accordingly.
Points to consider during your isotherm analysis:
1. Where are the warmest and coldest regions
(by state) found?
a. Warmest Southern Indiana, Illinois or central Kentucky
and Tennessee.
b. Coldest Nonwestern North Dakota, extending into Manitoba
and Saskatchewan.
|
2. Where are the sharpest horizontal temperature contrasts
located?
Along the Mississippi River, extending from Eastern Iowa
(near the low) to Missouri.
|
D. INTERPRETATION OF THE SURFACE ANALYSIS
Answer the following questions using your analyzed Midwest map.
1. Where are the regions of a) highest and b) lowest
dewpoints?
a) Highest (humid) Kentucky, Tennessee and southern Illinois
b) Lowest (dry) Western South Dakota, into Wyoming and Montana
|
2. What type of precipitation is found in:
a) Illinois Steady Rain
b) Nebraska Steady Snow
c) North central Iowa - Southeast Minnesota Freezing rain
|
3. What is the prevailing wind direction that is observed in the
following locales?
a) North Dakota = Northwest winds
b) Central Illinois = Southeast winds
|
4. In what region do you find the strongest winds
on the map?
Kansas (also Nebraska is acceptable)
|
Are the isobars closer or farther apart in this area?
| Yes, they are closer in Kansas |
5. What region(s) is experiencing warm advection ,
where winds are transporting warm air toward colder regions?
Illinois and South-central Wisconsin
|
What region(s) is experiencing cold advection, where winds
are transporting cold air toward warmer regions?
| Missouri and essentially most of states in region
that are west of Mississippi River. |
E.. FRONTAL ANALYSIS
With the aid of your isotherm analysis, the distribution of dewpoint temperatures
and the wind field through the regions of distinct wind shifts, can you
identify a cold front? A warm front? Using the conventional frontal symbols
(see Figure 1.3 on page 4 of your text), draw the appropriate surface fronts
on your analysis.
PART #7.2 WEATHER SEQUENCES & FRONTAL PASSAGES
For each of the weather elements listed, describe the time sequence that
you would expect to observe at your "weather station" during:
1. A warm frontal passage in autumn resulting when
an idealized cyclone passes from south to north, while staying west of
you:
| |
PRE-FRONTAL
|
FRONTAL
|
POST-FRONTAL
|
CLOUDS (type) Cirrus/cirrostratus
to altostratus
to nimbostratus/stratus to fog to clear
|
PRECIPITATION Steady rain to drizzle 'till frontal passage then none
(type)
|
TEMPERATURE
(trend) Cool to Warm
|
WINDS (direction) Northeast --->East -----> Southeast to south
|
PRESSURE TENDENCY Falling Unsteady Steady to slow rise
|
2. A cold frontal passage in the spring of the year
when an idealized cyclone moving eastward passes from the northwest to
the northeast of you:
| |
PRE-FRONTAL
|
FRONTAL
|
POST-FRONTAL
|
CLOUDS (type) Clear to Cumulus to Cumulonimbus to clear and stratocumulus
|
PRECIPITATION Rain (snow) Showers Thunderstorms Snow (rain) showers
(type)
|
TEMPERATURE
(trend) Warm to Cold
|
WINDS (direction) South Southwest West Gusty Northwest
|
PRESSURE TENDENCY Falling Unsteady Rapid rise
|
3. The passage of an idealized snow producing cyclone (and attendant
trough line) in winter traveling from southwest to northeast, but remaining
just south of you:
| |
BEFORE TROUGH
|
TROUGH PASSAGE
|
AFTER TROUGH
|
CLOUDS (type) Cirrus/cirrostratus to altostratus to nimbostratus to stratocumulus
|
PRECIPITATION light snow to heavy snow ---> decreasing to snowshowers
(type)
|
TEMPERATURE
(trend) Cool -------> to ------> Cold
|
WINDS (direction) Northeast -----> North ---------> Northwest
|
PRESSURE TENDENCY Falling -----> steady -------> Rising
|
Last revision: 5 August 1998
Produced by Edward J. Hopkins, Ph.D.
Department of Atmospheric and Oceanic Sciences
University of Wisconsin-Madison, Madison, WI 53706
hopkins@meteor.wisc.edu
URL Address: aos100/homework/98hmk04a.htm