GEO165 LAB EXERCISE #3

Transcription

1 GEO165 LAB EXERCISE #3 Tools of the Meteorologist Part 2: Fronts Introduction 1. A front is the leading edge of a new air mass invading a region. Because Vilhelm Bjerknes was reminded of the bloody battle fronts of WWI he named these air mass boundaries, which is where cold and war air battle for dominance after the front in war. 2. All fronts are found in troughs of low pressure because surface convergence in a low pressure system causes the differences in the two air masses to be concentrated along the front, as a front approaches the pressure drops. Pressure bottoms out as the front passes and then rises. As is the case with most atmospheric variables pressure change is usually more distinct with a cold front. 3. A trof (meteorological shorthand for trough) is an elongated area of low pressure. All fronts are found in trofs but not all trofs are fronts. 4. A cold front is the leading edge of a cold air mass (ca, cpk, mpk) and it moves in the direction the triangles point. 5. A warm front is the leading edge of a warm air mass (ct, mt, cpw, mpw) and it moves in the direction the semi-circles are pointing. 6. Fronts are not paper-thin boundaries, in fact weak fronts may be 100 miles or more across and for that reason a front is often called a frontal zone. 7. Cold fronts are easier to find than warm fronts, they are more distinct. Sometimes satellite images, precipitation data or fog occurrence is necessary to locate a warm front, this is rarely the case with a cold front. 8. Review the information on the station model, map symbols and air masses found at: 9. A stationary front is a front that is almost stationary, more accurately it is called a quasi-stationary front. 10. An occluded front, which we will cover in the next lab, is a special case found late in the life cycle of the mid-latitude cyclone.

2 Maps for Lab #3 COLD FRONT MAPS 23z 30 DEC 2002, 12Z 24 JAN 2002 WARM FRONT MAPS 12Z 16 MAY 1996, 21Z 17 JUL 1996 Assignment 1. Analyze all maps following the procedures found at: and especially the detailed information at: 2. Create 3d surface graphs of pressure pattern using the MS Excel data sheet for the 24 JAN 2002 cold front and for the 17 JUL 1996 warm front. DPlot instructions: 3D SURFACE GRAPHS You are to create and save 2-3D surface graphs, one for the Cold front and another for the warm front. COLD FRONT 3D GRAPH 24JAN Z Select all values in the 3 columns for the anticyclone data. Click on DPlot on the menu bar. Click on XYZSurface Save the graph as a jpeg file. Select Options and click on Contour Options Click on View in 3D Set Azimuth to 76 Set Elevation to 41 Click on Draw Sides Set Intervals to 18 Set Lower to 1006 Set Higher to 1024 Click OK Double Click on the title data from lab2.xls and create an appropriate title that tells the viewer of the 3D Surface graph What he/she is viewing The date Axis Values When finished save the 3D surface graph.

3 WARM FRONT 3D GRAPH 17JUL Z For the middle warm front follow the same instructions but substitute the following values Set Azimuth to 76 Set Elevation to 41 Click on Draw Sides Set Intervals to 19 Set Lower to 1004 Set Higher to 1023 If the scale on the right has decimal numbers (it should not), go back to contour options and be sure the lower, higher, and interval values are correct. 3. Use the data in the other columns to create line graphs that illustrate the change of weather variables across a front. a. temperature and dew point temperature for 16MAY1996 b. temperature and dew point temperature for 30DEC2002 COLD FRONT LINE GRAPHS 30DEC Z Select the upper left temperature cell. Hold shift and click to lower right temperature cell. RELEASE SHIFT HOLD CTRL and select the upper left dew point cell RELEASE CTRL HOLD SHIFT Select lower right dew point cell. Select DPlot Select XYXY Label the graph as explained above. WARM FRONT LINE GRAPHS 16MAY Z REPEAT INSTRUCTIONS IMMEDIATELY ABOVE 4. Answer the questions.

4 NAME Lab #2 Questions 1. Compare the trofs of the cold front and warm front shown on the 3d surface graphs. a. Which trof is more distinct the cold front or warm front? b. Would you be able to locate the warm front based on the pressure pattern alone? c. Would you be able to locate the cold front on the pressure pattern alone? d. When analyzing a weather map which type of front should be easier to find? Why? 2. In what general directions are the winds from on either side of the cold front? WEST SIDE EAST SIDE

5 3. In what general directions are the winds from on either side of the warm front? NORTH SIDE SOUTH SIDE 4. On the 16May1996 warm front map what do you find that helps locate the warm front? Though pressure and dew point change are often weak this occurrence often helps the meteorologist to locate the warm front. Look in your text book and explain why this happens.

6 5. Describe what happens on the line graphs as you proceed from west to east across the cold front and from north to south across the warm front. Explain why this happens by referencing the general wind directions on either side of both fronts. 6. The last map (page 11) shows wind streamlines and indicates convergence and divergence at the surface at 23z 30DEC What is convergence, and explain why it is found where it is using the wind streamlines and pressure pattern.

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