Meteorology 201

 

 

Laboratory Exercise 4

 

 

 

Synoptic-scale Chart Analysis

 

 

 

 

 

 

 

 

 

 

Insert in ringed-three hole binder.  Work not

turned in in binder will not be accepted.

 

Point deductions for sloppy or late work.

 

 

 

 

 

I.  Introduction

 

A careful analysis of surface data is one of the most important techniques the operational meteorologist can master.  Since most users of weather information or forecast services live at sealevel, it is obvious that a an accurate portrayal of present conditions at that elevation is fundamental.

 

More importantly, surface boundaries (cold/warm air, high humidity/low humidity, wind shift or trough lines) are often foci for important weather developments.  Only an accurate analysis of the surface data can disclose these important features.

 

It is important that the beginning meteorologist accept the following:  although objective (computer) analysis of weather data has provided an important service to operational meteorology, it is not a substitute, by any means, for hand analysis.  Even at the synoptic scale, computer analyses tend to oversmooth ALL (even meteorologically-significant) smaller-scale irregularities in the pressure field, often obscuring the important boundaries alluded to in the last paragraph.  In addition, the smoothing-procedure (discussed in class) often results in isobars being drawn incorrectly with respect to station data.  Meteorologists who have experience in making field operational decisions WILL NEVER exclusively depend upon machine-generated analyses and will always complete an analysis of the data (even if it is a "quick and dirty" analysis) themselves.

 

In reality, there are many facets to the manual analysis of surface data.  Some of the more difficult tasks, including the more detailed technical aspects of frontal analysis, will be left to future classes (Metr 403).  The techniques we will center upon include:

 

II.  Synoptic-scale Analysis

 

A.  Establishing a Synoptic History

 

In order to provide onself with guidance about what the present analysis should basically look like, the analyst MUST examine the series of analyzed surface maps preceding the synoptic time of the data to be analyzed.  Pressure systems should maintain some continuity, that is to say, should not disappear from the map, should continue to

intensify or weaken (depending upon the trends determined from the recent "history" as depicted on the last few maps) and should shift position in smooth arcs.  Normally an analyst will have also drawn the previous maps and will have some knowledge of these prior conditions.

 

B.  Depiction of Present Conditions. 

 

         i.        Present Weather Symbol Depiction Routine (colored pencil shading)

 

Procedure described in class.

 

         ii.       Highs and Lows

 

Should generally have shifted slightly from their positions on previous chart.  However, previous positions may have been incorrect.  Guidelines given in class.  Red L's and blue H's.

 

 

         iii.      Construction of Isobars (Surface) (acetate)

 

Should generally have shifted slightly from their positions on previous chart. (Standard interval is 4 mb starting at 1000 mb--discussed in class).   However, previous positions may have been incorrect.  Guidelines given in class and in the first reading in the READER.

        

         iv.      Construction of Height Contours (Upper Air) (acetate)

 

Follow same procedure as for surface isobars.  However, use contour intervals as discussed in previous labs and in class.  Standard analysis includes:  a)  for 500 mb, height contours (solid black) and isotherms (dashed red, Centigrade, 5 degree intervals); for 300 mb, contours (solid black), isotherms (dashed red, Centrigrade, 5 degree intervals), and isotachs (purple, 20 knot intervals, starting at 50 knots).  Remember, winds tend to blow parallel to upper level height contours.

 

         v.       Fronts and troughlines. (As discussed in class)

Fronts

 

1.    Surface fronts are drawn on the warm-air side of packing in the surface isotherms.

2.    Fronts are found in regions of locally reduced pressure (sharp troughs evident as kinks on isobars)

3.    Usually (but not always) sharp wind shifts (veer of wind from warm air side to cold air side of fronts) occur

4.    Usually, pressures fall markedly ahead of fronts and rise behind.

 

Please use proper color conventions. 

 

Procedure: 

á      Sketch frontal positions in lightly on acetate on the basis of the steps above. 

á      Now draw isobars to fit the frontal position.

á      Once frontal positions are finalized, make sure isobars KINK AWAY from low pressure, as shown in class.

 

Troughlines

 

Regions of surface low pressure not associated with fronts.  Can often be associated with significant bad weather.  Indicate persistant troughlines with dashed black or brown line. 

Specialized Troughs Ðhave their own symbol set as discussed in class.  (e.g., Dry Line; outflow boundaries).

 

C.  Exercises

 

Surface Analyses

 

1.  (a) First pass frontal analysis on acetate based upon rules of thumb discussed in Metr 201, using the fax ua_300 etc. for 05111312.  As discussed in class.

 

2.  Perform an advection/frontal analysis on the 12 UTC and 00 UTC charts (eta_thick)  showing 1000-500 thickness and surface isobars

 

3.  Analyze the 13 November 2005 12 UTC and the 14 November 2005 00 UTC  surface charts included in the Map Set synoptically. Here's the chart from 24 hours previous and the 9 UTC chart to use as history.

 

 

 

Steps to a good surface analysis

 

1.    Assemble all your analysis tools:  (a) clean copy of plotted data; (b) for rough draft Ð grease pencils, clean acetate, alcohol and rags; (c) for final draft Ð colored pencils and pens.

 

2.    Examine previous chart for positions of highs, lows, fronts and boundaries.  ÒPreviousÓ means 3 hours to 6 hours in advance.  Even a chart for 12 hours previous will help. 

 

3.    Examine the plotted data to ÒcharacterizeÓ or ÒdiagnoseÓ the patterns on it. Mentally note any circulations apparent, large wind shifts etc.

 

4.    In colored pencil, lightly shade a circle (about 1/4Ó outwardÓ from each station that reports present weather using the color coding posted on the wall).

 

5.    On acetate, find the lowest or highest pressure and start drawing IN BLACK isobars as follows:  (a) 4 mb interval; (b) isobars on surface charts are drawn STARTING with 1000 mb (e.g., 1000, 1004, 1008, 1012 etc).  Make sure to label Highs (blue H) and Lows (red L) at the center of the patterns, and to label isobars clearly so you donÕt get confused when you transfer.

 

6.    Examine the previous analyzed chart (if you have one) (such charts are called the synoptic HISTORY) for locations of fronts, boundaries, highs and lows.  It is highly unlikely that such features either disappear or move erratically.  In other words, a cold front on the HISTORY should appear on your chart.  It should not disappear or back up.  If you analyze a series of charts, there should be a logical progression (based upon sound meteorology) of features.  Highs and Lows should NOT appear and disappear, or jump around the map.

 

7.    Draw a first guess for frontal positions based upon the techniques discussed in class.

 

8.    Adjust the position of fronts to match wind shifts, etc. and then kink the isobars at the front AWAY from low pressure.

 

9.    Show rough draft to instructor for OK.

 

10. Transfer to hard copy using the light tables. Final analysis MUST be neat. No erasures. No grease pencil.  Isobars Ð black (or pencil); cold fronts blue; warm fronts red; occluded fronts purple; stationary fronts alternating blue and red; dry line, brown broken open warm front symbols.  If isotherms are drawn, they should be dashed red; isodrosotherms, dashed green.