Laboratory Exercise 3
Basic Chart Diagnosis and Interpretation
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. However, in this exercise you will be working with the following for 1200 UTC 3 Feb 2012: (i) abridged surface data plot; (ii) data plot and analysis for 850 mb, 700 mb and 300 mb; (iii) data plot for 500 mb; and (iv) a color version of the sea-level pressure field overlaid with 1000-500 mb thickness contours and a color version of the 500 mb analysis with absolute vorticity contours.
B. Obtaining the Weather Data--Radiosonde Observation (RAOB)
The data which is utilized to construct these charts are obtained by RADIOSONDE. The radiosonde at least monitors conditions at these levels. If unusual temperatur or moisture 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 201, 410, 420, and 430 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.
C. Mandatory Level Charts
In this lab 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 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:
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 ERTH 260) and plotted at upper right.
3 Hr Pressure Change (mb) is coded (discussed ERTH 260) and plotted at bottom right.
Wind Velocity is coded (discussed ERTH 260) 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
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"
(Note: on the 500 mb chart you will generate below, the originator decided to make the coding different, as explained in class.)
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 12200 meters 1220 dm 36000 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, except the speeds are in knots.
D. Exercises: Diagnostic Map Skills
I. Print the following for 1200 UTC 3 February 2012: (i) abridged surface data plot* ; m (ii) data plot and analysisor 850 mb, 700 mb, 300 mb and 200 mb; (iii) data plot for 500 mb ; and (iv) meteogram for Dodge City, KS for the 24 hour period centering on the time above.
Chart 1: Surface Weather Map
Chart 3: 500 mb Data Plot
Chart 4: Dodge City Meteogram
II. Surface Chart: (make sure you read through the resources section for this lab at the top level of the website) (Chart 1)
1. Give the following surface information for KDDC (Dodge City, KS).
(c) Wind information.
(d) Dew point temperature.
(e) Pressure 3 hours before current observation (think!)
(f) Visibility (if plotted)
2. Examine the meteogram for KDDC (Chart 4). (a) What was the present weather observed at 0500 UTC 3 February 2012? (b) How would you characterize the trend of temperature between 2100 UTC 2 February 2012 and 0100 UTC 3 February 2012.
3. Shade in (color pencil, NOT grease pencil) ALL the present weather symbols, as discussed in class on one copy of your surface chart. Careful, this one will be the one that you will be working on in Lab 4.
4. Sketch streamlines over the southern half of the central third of the United States on the other copy of the surface chart. What type of pressure system does your analysis suggest is located in that region and how could you tell?
III. Upper Air Charts
Where required, verbal responses should be on separate sheets, and should be made with complete sentences. To a certain extent, this is a review of material we covered in ERTH 260.