Tutorial A - Understanding and Using Soundings

Activities (2 parts)

Activities A. Instructions/Exercises (with the instructor demonstrating and the student following):

The activity numbers below correspond to the section numbers in the online Tutorial Readings. You should already have with you hard copies of printouts S1 , S2 , and S3 .

1. The Environment - Supportive or Oppressive

No activities other than the online tutorial readings.

2. Soundings - To Measure the Environment

a. Thermo Diagram Basics: Start by identifying the temperature (T) and pressure (P) lines (isotherms and isobars) that form the background of these graphs (Fig 1, in the on-line Lab Readings, which is also included in the S1 Printout that the student should bring to the lab). Pressure is used as a surrogate measure of altitude, with lower pressures at higher altitudes. This graph is a "semi-log" graph, with P decreasing logarithmically along the vertical axis, and T increasing linearly along the horizontal axis.

b. Plotting Environmental Soundings:

1) Plot the following environmental sounding of temperatures (T) and dew-point (Td) on this background chart, to show thermal layering of the atmosphere. Use large dots for T, and use X's for Td (but students and TAs might prefer to use red colour for T and blue for Td). Connect all the T dots sequentially with lines, and connect all the Td points with separate lines. The answer is shown in Fig 2 of the online lab readings.

P(kPa)
T (°C)
Td (°C)
20
-30
-60
30
-30
-45
50
-10
-35
65
10
-20
80
25
-5
90
25
20
100
40
25

Table 1. An atmosphere sounding of the environment.

2) Calculate the height of the cloud base (called the Lifting Condensation Level, LCL), from ZLCL = a • (T - Td) where a = 0.125 km/°C, and where T and Td are temperature and dew point of the air at the bottom sounding point.

3) Locate and label the tropopause, as the height (or pressure) at the bottom of the isothermal layer in the top half of the graph.

4) Identify layers of stratiform (layer) clouds. They are regions where Td nearly equals T.

Beware of some common errors students make when plotting a sounding:

3. Air Parcels - Tracking Them on Thermo Diagrams

a. Humidity of the Air: Identify the humidity mixing ratio (r) lines (isohumes, Fig 3).

1) Plot the near-surface air parcel from the above sounding (i.e., the air initially at P = 100 kPa with T = 40°C and Td = 25°C) on Fig 3 of the S1 Printout (the result is shown on Fig 4 of the Lab Readings). Then, use those isohumes to determine the following:
2) actual mixing ratio (r) __________ g/kg [Hint: based on Td]
3) saturation mixing ratio (rs) __________ g/kg [Hint: based on T]
4) relative humidity (RH%). __________ % [Hint: RH% = 100 • r / rs]

Common errors in this part include:

b. Rise of Unsaturated Air Parcels: Identify the dry adiabat lines (, isentropes, Fig 5). Plot the surface air parcel from before (i.e., plot the T and Td points at 100 kPa on Fig 5), and then lift it dry adiabatically. Namely, draw a line for air-parcel temperature (T) that follows or goes parallel to the adiabats, and draw a separate line for Td that follows or goes parallel to isohumes. Stop at the height where those two lines cross; namely, where T = Td. This is how you find the lifting condensation level (LCL), which is cloud base for the thunderstorm (answered in Fig 7).

1) LCL is at what pressure altitude? _______________ kPa
2) Find T, Td, r, rs, and RH% at the LCL.

A common error in this part is:

c. Rise of Saturated Air Parcels: Identify the saturated (moist) adiabat lines (L) in Fig 8. Plot a point in Fig 8 at the exact T and P of the LCL from the previous exercise. Then continue lifting the air parcel up to an altitude where P = 40 kPa, still drawing separate lines for T and for total water content (now indicated by rT instead of by Td). The temperature follows (or goes parallel to) the saturated adiabat on the way up, and the humidity (now itentified as the total water mixing ratio rT, which equals the initial r value before the parcel was lifted) follows the isohume (answered in Fig 9). At this new location, determine the following for the parcel:

1) T = ___________ °C
2) rs = ___________ g/kg
3) Td = ___________ °C [Hint: Td = T for saturated air.]
4) r = ___________ g/kg [Hint: r = rs for saturated air.]
5) rT = ___________ g/kg
6) liquid-water mixing ratio rL = rT - rs = ___________ g/kg

where this last number shows how much liquid water (cloud and rain droplets) are being carried by the air parcel.

A common error in this section is:

d. Full Thermo Diagram: Examine Fig 10 (included in the S2 printout), which shows the complete thermo diagram with all the lines (isobars, isotherms, isohumes, dry and moist adiabats). This is just a superposition of all the previous graphs.

1) To get more accuracy from the thermo diagram, larger diagrams are usually used in real life, with more lines pre-drawn on them (S3 Printout). Using the S3 Printout, identify the isobars, isotherms, isohumes, dry and moist adiabats. [Hint: by tilting the S3 Printout in your hands to view it from an edge, it is easier to focus on any one set of lines. For example, view from the lower right corner.]

2) Although Fig 10 (or equivalently the S3 Printout) are used here because of their simplicity (they are basically semi-log graphs), there are other thermo diagrams that are used by various agencies, and which are used to plot some of the soundings that you can view on the web. Three types of diagrams are very similar to the one we used here (Fig 10): Emagram, Pseudoadiabatic diagram, and Stuve diagram. However, there are two other diagrams which look similar to each other, but a bit different from what we've learned so far. They are the Tephigram, and Skew-T Log-P diagram (called a Skew-T diagram for short)(see Fig 11 in printout S2).

a) Examine your copy of the Skew-T diagram in the S2 Printout, and identify the isobars, isotherms, isohumes, dry and moist adiabats.
b) Plot the same sounding from lab exercise (2) on the Skew-T of Fig 11 and the emagram of Fig 10, and compare the results.

A common error in this part is:

4. Static Stability

a. No activities, other than the online tutorial readings.

b. No activities, other than the online tutorial readings.

c. Following the instructions in the on-line Lab Readings, and using the sounding from exercise 2 (which is replotted here as Fig 12 and on printout S2), determine which regions of the environment are statically:

1) unstable (always do this first)
2) neutral
3) stable
4) Identify and label in Fig 12 strongly-statically stable regions from the sounding.

Indicate your results along the right edge of Fig 12. The answers are shown in Figs 13-15.

A common error in this section is:

d. Interpretation of the Resulting Soundings: Strong static stability are layers where T is constant with height (i.e., is isothermal), or where T increases with height (a temperature inversion). The stratosphere is strongly stable, and the base of the stratosphere (often somewhere in the 40 to 20 kPa pressure height range) is called the tropopause. Between the tropopause and the ground is the troposphere. The tops of most large thunderstorms are near the tropopause. The boundary layer is a relatively thin layer of unstable or neutral stability near the ground, capped by a statically stable lid or temperature inversion. The boundary layer holds the warm-humid air that is the fuel for storms. Sounding characteristics that are most conducive to severe thunderstorms are sketched in Fig 17 of the online Lab Readings.

1) Identify and label the boundary layer and its capping stable layer.
2) Identify and label the stratosphere, troposphere, and tropopause.
3) Would you anticipate the sounding of Fig 12 to be conducive to severe thunderstorms? (yes / no) Why?

The answers are shown in Fig 16.

5. Predicting Storm Intensity

a. How to Find Parcel Stability: To determine whether thunderstorms could be triggered to start at all, we need to compare the rising air parcel from the surface with the environmental sounding (Fig 18 of Printout S2). List the four steps involved in determining storm intensity.

b. Tracking Parcel Rise: Starting with Fig 18, draw a line showing how the temperature and total mixing ratio change in a rising air parcel starting from the surface with the same initial conditions as the environmental sounding. For our example, this is in Fig 19 with all the background thermo diagram lines removed for clarity.

c. Convective Inhibition (CIN) and Triggering: For a parcel rising from the surface to become a thunderstorm, it must first penetrate the cap on the boundary layer. However, the air parcel is colder than the environmental temperature in this cap, which would tend to prevent continued upward motion of the parcel (Fig 20) and would not allow a thunderstorm to from.

Trigger mechanisms can overcome this obstacle two ways: (1) by forcing the surface air parcel above the top of this cap (Fig 21); and (2) by heating the surface air such that rising parcels are buoyany enough to penetrate the cap on their own (Fig 22). Sometimes condition (2) is satisfied in later afternoon, after the sun has caused more heating of the earth's surface.

1) Using Fig 18, to what pressure altitude must the air parcel be lifted from the surface to trigger thunderstorms? This is called the level of free convection (LFC). LFC = ________ kPa
2) Or, to what temperature must the parcel be heated near the surface to give it enough buoyancy to penetrate the cap? _____ °C

A common error in this part is:

d. Convective Available Potential Energy (CAPE): Once the air parcel from the surface is above the cap, it is warmer than the environment and can continue to rise due to its own buoyancy (following the moist adiabat) until it hits the stratosphere (Fig 23). This max height is called the limit of convection (LOC), and gives the top of the thunderstorm anvil.

1) Starting from exercise 5.a.1, plot the additional parcel rise in Fig 18, and find the pressure altitude of the top of the thunderstorm. LOC = ____________ kPa

2) The area between the moist adiabat of the rising parcel and the environmental sounding (between the LFC and LOC heights) is proportional to the Convective Available Potential Energy (CAPE). Larger values of CAPE means that the thunderstorm can tap more of the buoyant energy associated with latent heat release, to become a more intense storm.

a) On Fig 25 or on the full thermo diagram (S3 Printout), plot the environmental sounding (if not already plotted), and plot the path that a surface air parcel would take if lifted to the top of the graph.

b) Using the technique discussed in the on-line Tutorial Readings, calculate the CAPE area: ______ °C km.

c) Determine the qualitative strength of the storm (no storm, weak, moderate, strong, severe) using table BB from the online Tutorial Readings.

6. Final Conclusions and Review:

You now know how to determine thunderstorm base, top, and trigger needed to get it started. You can identify the boundary layer, cap, troposphere, tropopause, and stratosphere. You can use the thermo diagram to help calculate various humidities, to determine temperature change of vertically moving air parcels, and to determine how much liquid water could be produced within the one rising air parcel. You can identify layers of stratiform and cumulus clouds. (Also, from the extra material of section 4, you can find the static stability of the environment. From the advanced materials, you can determine CAPE area and thunderstorm intensity, and you can interpret Skew-T diagrams.)

Activities B. Homework / Independent Practice / Test Preparation

To help solidify your new skills as summarized in section 5e above, you might want to try exercise 1 below. Exercise 2 below will not be covered on any tests, but provides real, current soundings to study.

  1. Practice this on your own as homework, with the following sounding on a blank thermo diagram (S3 Printout):

    P(kPa)
    T (°C)
    Td (°C)
    20
    -45
    -60
    25
    -45
    -45
    30
    -40
    -40
    40
    -25
    -60
    60
    0
    -35
    75
    15
    -15
    80
    7
    5
    98
    25
    15
    100
    30
    20

    Table 2. A new atmospheric sounding of the environment.

    a) Plot this new sounding on the full thermo diagram (S3 Printout).
    b) Determine the static stability layering.
    c) Identify the boundary layer, cap, troposphere, tropopause, and stratosphere.
    d) For an air parcel at the surface with the same T and Td as listed in the table above at P=100 kPa, use the thermo diagram to estimate its mixing ratio, saturation mixing ratio, and relative humidity initially.
    e) For this air parcel starting from the surface, draw lines on the thermo diagram showing how the temperature and dew-point of this air parcel change as it rises.
    f) Find the pressure heights of the LCL, LFC and LOC.
    g) Calculate the CAPE, and determine thunderstorm intensity.
    h) Indicate what is needed to trigger this thunderstorm.
    i) Identify any layers of stratiform or cumulus clouds, if any.
  2. Use the following web link (or other links as suggested by the TA or prof), to examine plots of real soundings near thunderstorms.
    a)First look at national radar maps to find location of strong thunderstorms, often indicated with the yellow and red shading. Any of the following web sites can be used to view radar displays:

    US National Radar (R) web sites. The first site (Ra) is often the quickest and simplest. The other two sites provide more detail, and allow you to click on the map to zoom into a region. Also, the last site (Rc) gives storm-top altitutde, storm movement, and indications of hail or tornadoes.
    (Ra) American Meteorlogical Society DataStreme
    (Rb) US National Weather Service
    (Rc) Microsoft Intellicast

    b) Next look at a map of the "upper-air" rawinsonde sounding stations, and pick a station this is at, or just southeast, of the thunderstorm location.

    Atmospheric Sounding (S) web sites Pick a site that shows either Stuve diagrams or emagrams, to view the soundings on semi-log graphs similar to what was used in this course (i.e., similar to Fig 10 in S2 Printout). Other sites display the soundings on Skew-T diagrams (i.e., similar to Fig 11 in S2 Printout). Some sites allow you to display a sounding in either form.
    (Sa) U Wyoming: either Stuve and Skew-T (international, friendly, with CAPE)
    (Sb) American Meteorlogical Society DataStreme: Stuves for US (simplified)
    (Sc) Ohio State Univ: Skew-T for US and Canada
    (Sd) UCAR RAP: Skew-T (US and Canada) with CAPE
    (Se) Northern Illinois Univ. Weather Machine (draws graphs on demand): Emagram, Skew-T, or Stuve (international, with CAPE, but no station list)

    c) Retrieve the sounding for that station from the web. If your sounding doesn't have large CAPE, try a diferent location near other thunderstorms.
    d) For each such plot, first identify which type of thermo diagram it is (i.e., the type used here, or a Skew-T/Tephigram type).
    e) Then identify all of the isopleths (isobars, isotherms, isohumes, dry and moist adiabats, height contours if any).
    f) Look at the sounding and identify the boundary layer, cap, troposphere, and stratosphere.
    g) For an air parcel starting from the surface, find the LCL, LFC, LOC, CAPE, and thunderstorm intensity (often, these numbers are computed automatically by the computer program that plotted the graph, and are displayed near the graph).

    Copyright © 2002 by Roland Stull
    UBC ATSC
    .