ATM OCN (Meteorology) 100

Answers for Homework # 7

Fall 1997

The total maximum points were 100 for Part #7.1 and 50 points for Part #7.2.

PART #7.1. SURFACE WEATHER MAP ANALYSIS

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)

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)

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.

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?

2. Where are the sharpest horizontal temperature contrasts located?

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?

2. What type of precipitation is found in:

3. What is the prevailing wind direction that is observed in the following locales?

4. In what region do you find the strongest winds on the map?

Are the isobars closer or farther apart in this area?

5. What region(s) is experiencing warm advection , where winds are transporting warm air toward colder regions?

What region(s) is experiencing cold advection, where winds are transporting cold air toward warmer regions?

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 

        (type)                              'till frontal passage     
then  none 

TEMPERATURE                                                        
 (trend)           Cool                            to                     
   Warm

 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

 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

 PRESSURE TENDENCY   Falling   ----->                steady  
------->       Rising

Last revision 11 December 1997