Meteorology 301

A. Overview

This exercise will get you used to the so-called "mandatory-level" charts used often by operational meteorologists. The mandatory levels are SURFACE, 1000 MB, 925 MB, 850 MB, 700 MB, 500 MB, 300 MB, 250MB, 200 MB, 150 MB and 100 MB. Charts are produced for each of these levels except for 1000 and 925 mb.

The data which is utilized to construct these charts are obtained by RADIOSONDE. The radiosonde at least monitors conditions at these levels. If unusual temperature, moisture or wind conditions exist at other levels besides the mandatory levels, then the radiosonde will also relay the information from these levels (known as the SIGNIFICANT LEVELS). However, "map view" weather charts are only routinely produced for the mandatory levels listed above. Note that a RADIOSONDE OBSERVATION is also known by the acronym RAOB. Another synonym is RAWINSONDE, which refers to an upper air sounding that also contains a RADIO WIND OBSERVATION. In operational use, the term "radiosonde" and "rawinsonde" are used synonymously....all raobs have wind information now.

The radiosonde monitors two of the three STATE VARIABLES (pressure and temperature), the dew point depression (from which SPECIFIC HUMIDITY may be obtained) and wind speed and direction (together termed the WIND VELOCITY). The process by which the radiosonde monitors this information is termed TAKING A SOUNDING and the information obtained is often termed THE SOUNDING. By international agreement, soundings are taken at relatively evenly spaced (300 km) apart radiosonde stations worldwide at two times 0000 UTC and 1200 UTC. The local sounding site is at Oakland International Airport (OAK).

The information contained in the soundings observed worldwide may be portrayed graphically in a number of ways. Discrete information from each radiosonde, say, 500 mb height and 500 mb temperature, may be plotted for North America and contoured, thus producing the North American 500 mb Height Map. Alternatively, the variation of temperature with height at a given radiosonde station may be plotted, thus producing a sounding. The methodology for each of these is an important skill for the beginning meteorologist. It involves not only the learning of the techniques for contouring and plotting but also the mastering of the manner in which the information is coded and decoded.

Besides being used for portraying the present (or, initial) conditions in the atmosphere, the data obtained by the radiosonde is also used in establishing the "starting point" to forecast future conditions by NUMERICAL MODELS. In Metr 401, 402 and 403 you will learn that there are five basic equations (called the PRIMITIVE EQUATIONS) which individuals in each of the branches of meteorology use to understand the present state or to characterize the evolution of the future state of the atmosphere. These equations require certain "startup" conditions to be INITIALIZED. The initialization of the models is dependent upon the information acquired by the radiosondes.

B. Mandatory Level Charts

Today you will be working with some of the MANDATORY LEVEL CHARTS, in particular the 200, 300, 500, 700, 850 mb Charts and the Surface Chart (to proxy for the 1000 mb chart). You are to obtain these charts from the WGSL for the date and time specified by the instructor.

You will note that the charts contain both plotted information and contours. The plotted information is arranged around a circle which denotes one of the radiosonde sites. There are some relatively simple conventions governing the plotting of radiosonde information around the station circles. These are described below:

1. Surface

Temperature (Fahrenheit in US publically-distributed charts, Centigrade everywhere else and on charts produced for scientific purposes) is plotted at upper left. Units or degree symbols are NEVER plotted.

Dewpoint Temperatures are plotted at bottom left, with the same convention. (Note: on the Pacific and Atlantic Analyses the sea-surface temperature is plotted beneath the dewpoint temperature in ship data.)

Pressure (millibars) is coded (discussed in class) and plotted at upper right.

3 Hr Pressure Change (mb) is coded (discussed in class) and plotted at bottom right.

Wind Velocity is coded (discussed in class) and plotted on station circle.

2. Upper Air Charts

Mandatory Level Charts for levels above the surface are all CONSTANT PRESSURE charts (as opposed to the surface chart, which is a CONSTANT LEVEL or ELEVATION chart). All the upper air charts show temperature in Centigrade, plotted at upper left in whole degrees.

a. NWS (used to be referred to as DIFAX) Charts (If not available skip to b)

Dew Point Depression (Centigrade) (discussed in class) is plotted at bottom left. The height of the constant pressure surface is plotted at upper right but is coded differently at each different mandatory level in the following manner:

850 mb In meters, with the leading "1" digit dropped. "1391 meters" = "391"

700 mb In meters, with the leading "3" or "2" digit dropped. "3011 meters" = "011"

500 mb In decameters, rounded to the nearest decameter. "5666 meters" = "567"

300 mb Same as 500 mb. "30

200 mb In meters, with the leading "20" or "19" dropped. "19940 meters" = "199"

Note: contour labeling differs from this scheme (discussed in class).

Typical Heights (Rough Approx) for Constant Pressure Surfaces in the Middle Latitudes

850 mb 1400 meters 140 dm 5200 feet

700 mb 3000 meters 300 dm 10000 feet

500 mb 5700 meters 570 dm 18000 feet

300 mb 9300 meters 930 dm 30000 feet

200 mb 19900 meters 2000 dm 60000 feet

The Height Tendency (meters) (discussed in class) is plotted at bottom right.

Wind Velocity information is plotted as it is on the Surface Analysis.

b. WXP-generated charts

Dew Point (Centigrade) (discussed in class) is plotted at bottom left. The height of the constant pressure surface is plotted at upper right in meters.

The Height Tendency (meters) (discussed in class) is plotted at bottom right.

Wind Velocity information is plotted as it is on the Surface Analysis.

3. Exercises: Map Skills

Surface Chart: Obtain a plot of the surface data over the continental United States for 06 UTC 24 October 2005.

The command line syntax will be:

sfcwx 05102406 -re=us -var=full -stat_prior=1 -cod=green:hi=.8 -dev=d

1. Give the following surface information for KTPA (Tampa, Florida).

(a) Pressure

(b) Temperature.

(d) Dew point temperature.

(e) Pressure 3 hours before current observation (think!)

(f) Visibility (if plotted)

2. Shade in (color pencil, NOT grease pencil) ALL the present weather symbols, as discussed in class.

3. Sketch (on acetate) a few streamlines over the southeastern portion of the United States. What type of pressure system does your analysis suggest is located in that region and how could you tell?

4. Contour the surface pressure field (without regard to fronts...do not attempt to find fronts). Isobars at 4 mb intervals (1000, 1004, 1008 etc.), Highs (blue), Lows (red).

Upper Air Charts Obtain a plot of the following for 12 UTC 24 October 2005: (a) Using wxp "training wheels"- 500 mb; (b) Using the script "fax" the 850, 700 and 300 mb height maps with data (fax ua_700 -p; etc)

1. Contour the 500 mb chart. Height contours at 6 dm intervals starting at 546, (e.g. , 564, 570, 576; or 564, 558, 552, 546 etc.) solid, black, isotherms at 5C intervals (0, -5, +5 etc) in dashed red.

2. The solid lines on each of the upper air charts connect locations experiencing the same value of height. How would you characterize the WIND FLOW's relationship to the contours on the 500 mb chart? (Think!)

3. Does this relationship generally hold true for all the MANDATORY LEVEL upper air charts? Explain. Also, if not for which one(s) or for what areas (geographic or contour) does the relationship not seem to hold?

4. The 500 mb chart is often used by meteorologists to portray the wind and height patterns in the upper troposphere. In reality, the 500 mb level is in the mid troposphere. To what extent does the 500 mb pattern depicted on the charts attached compare to the 300 mb pattern. (Hint: comment on comparative geometry [does the pattern "look" basically similar at both levels], wind directions and speeds, locations of troughs and ridges etc.).