Laboratory Exercise 3: Part II
II. Surface Weather Observations (METARS) and Synoptic-scale Frontal Analysis
Insert in ringed-three hole binder. Work not
turned in in binder will not be accepted.
Point deductions for sloppy or late work.
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.
II. METAR decoding
The surface data is coded on the basis of international convention. Observations are made hourly, and each "observer" is given a ten minute time window, between 10 minutes before the hour to the hour to make the mandatory surface observation. Any unusual conditions or events that occur out of the time window mandate that a "special" observation be taken. These reports are called "Metars".
The word METAR is from the French, "message d’observation météorologique régulière pour l’aviation," and is thought to have originated as a contraction from MÉTéorologique ("Weather") Aviation Régulière ("Routine"). The Federal Aviation Administration (FAA) may consider it to be erroneous to abbreviate METAR as METeorological Aerodrome Report. The FAA and National Oceanic and Atmospheric Administration (NOAA) specifically define a METAR as an "aviation routine weather report," an approximate translation of the French.
Once the observations are plotted on a map, they can be contoured carefully, as you did in the first part of the lab. It is important to keep in mind that a careful analysis of these observations is critical. Not only does such an analysis yield positions of low and high pressure areas, and troughs and ridges, but because a diagnosis of the patterns on such analyses can yield clues about weather development. For example, surface boundaries (cold/warm air, high humidity/low humidity, wind shift or trough lines) are often foci for the formation of thunderstorms, frontal waves etc.. Only an accurate analysis of the surface data can disclose these important features.
III. Synoptic-scale Analysis
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:
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)
1. Surface warm, cold and stationary fronts are drawn on the warm-air side of packing in the surface isotherms. Classic surface
occluded fronts are found under the thickness tongue (as shown in class) and join the cold and warm fronts at the triple point,
equatorward of which there is considerable packing of the thickness arrows.
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.
1. Sketch frontal positions in lightly on acetate on the basis of the steps above.
2. Now draw isobars to fit the frontal position.
3. Once frontal positions are finalized, make sure isobars KINK AWAY from low pressure, as shown in class.
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 -- have their own symbol set as discussed in class. (e.g., Dry Line; outflow boundaries).
1. METAR Decoding
You are provided with portions of the Federal Meteorological Handbook 1 (the complete Handbook is available as a pdf).
Decode the following METARs for Portland, Oregon. Indicate which of these are routine hourly observations and which are "specials."
KPDX 191553Z 21007KT 10SM FEW020 SCT055 BKN180 12/09 A2975 RAE03 SLPNO P0000 T01170094
KPDX 191453Z 22008KT 7SM -RA FEW010 BKN030 BKN055 12/11 A2970 RMK RAB17 SLPNO P0011 60014 T01220106 53045
KPDX 191353Z 21006KT 10SM FEW009 SCT032 BKN065 13/11 A2964 RMK RAE05 SLPNO P0000 T01330111
KPDX 191320Z 23005KT 7SM FEW009 SCT032 BKN065 13/12 A2962 P0000
KPDX 191253Z AUTO 22008KT 7SM -RA SCT007 BKN022 OVC045 14/13 A2960 RMK RAB07 SLPNO P004 T01370126
KPDX 191210Z COR 18013KT 7SM -RA SCT007 BKN020 OVC045 14/12 A2958 RMK RAB07 P0000
KPDX 191153Z 13003KT 7SM BKN008 BKN015 OVC040 13/12 A2956 RMK RAE10 SLPNO P0001 60026 T01270123 10134 20123 70058 52020
2. Surface Frontal Analyses
In this exercise, you will be contouring surface charts for 0000 and 1200 UTC 29 October 2009 and 0000 UTC 30 October 2009. You are also provided with the 0600 UTC and 1800 UTC surface plots, for your information. You will not have to contour the latter two charts. You are also provided with the 0000 UTC 29 October 500 mb and 1000-500 mb Thickness Fields and the 0000 UTC 30 October 500 mb and 1000-500 mb Thickness Fields
1. Perform an advection/frontal analysis on the 00 UTC charts (nam_thick) showing 1000-500 thickness and surface isobars for 29 and 30 October 2009. These will help you in visualizing the position of actual fronts.
2. Sequentially analyze the 0000 and 1200 UTC 29 October 2009 and 0000 UTC 30 October 2009 surface charts included in the Map Set synoptically. Remember to use the analysis from 6 hours previous (provided by instructor) as history.
Analyze each chart (procedure summarized above and here) using the steps below, in order:
The firstpass frontal analysis should be completed on a separate acetate (based upon rules discussed in Metr 400). Use your analyses in (1 to make a first guess. Then look at the temperatures (sharp temperature gradient into cold air begins at front), winds (winds usually veer across fronts) and isobars (isobars normally have a sharp kink outward from low pressure at fronts) to adjust the frontal positions. Once you are sure (consult instructor) about the frontal position, make sure that isobars kink across it (note: usually, but not always, there will be a wind shift and isobar kink along dry lines as well). Then transfer it NEATLY to the hard copy.
Once you have done the first chart, do the same for the second chart. You can use the surface plots at intermediate times (06 and 18 UTC) to check progression, but you do not need to analyze those charts for this exercise (though you would in an actual case study analysis). Remember