ENSC 408: Lab 3Tephigrams

September 23, 2019

Weather Forecasting Results

Average Score: 6 / 10

Low Temp. (C) High Temp. (C) Precipitation (no/yes/type)

Observed 7 17 0Forecasted 6 16 0

Class Average 7 17 1Class 6 17 0

7 16 17 16 18 17 16 18 15 16 14 19 1

Objective:

• To learn techniques to study vertical atmospheric structure using a tephigram

Materials:

1. A blank tephigram

2. Coloured pens/pencils

Mixing Ratio (rs)

Isotherms (T)Dry Adiabat (θ)Moist Adiabat (θw)

Isobars (kPa)

Tephigrams

Plotting on a Tephigram

For the first part of the lab you will be plotting radiosonde data on your tephigram copy

1. Add points for Temperatureand Dewpoint Temperature at each pressure level

2. Connect points with a straight line for each field

3. Identify each line as ‘T’ and ‘Td’

4. You will also add a wet-bulb temperature (Tw) line in a later question (#7) in a different colour

Then there are 14 questions to answer (Questions 2 – 15 in your lab manual)

Td T

Pressure (kPa) T (C) Td (C)

100 25 10

90 25 15

70 6 4

Stability of Vertical Levels

Levels can be:• Stable • Unstable• Conditionally unstable• NeutralDetermined by comparing the slope of the ELR with that for the dry and moist adiabats

Identification of Tropopause, Isothermal

Layers, and Temp. Inversions

Tropopause• The boundary between the

troposphere and stratosphere• Identified as an inflection point in

the sounding where the ELR decreases to 2 C/km or less –usually near 20 kPa in mid-latitudes

Isothermal Layers• Layers of constant temperature• Slope of ELR = Slope of IsothermsTemp. Inversions• Layers which warm with height• Slope of ELR > Slope of Isotherms

Source: Stull, R (2017)

Lifting Condensation Level (LCL)

Lifting Condensation Level (LCL)• The pressure level at which a

parcel, when lifted adiabatically, becomes saturated (typically the cloud base)

• Determined by joining a line from the surface temperature, parallel to the dry adiabat, with a line from the surface dewpoint temperature, parallel to the isohumes. The LCL is located at the confluence of the two lines.

Source: https://weathertogether.net/weather-101/how-to-read-skew-t-charts/

Level of Free Convection (LFC)

Level of Free Convection (LFC)• The pressure level at which a

parcel of air, lifted adiabatically, first becomes warmer than its surroundings in a conditionally unstable atmosphere

• Determined by drawing a line from the LCL, parallel to the moist adiabat. The LFC is located at the intersection of this line and the ELR

Source: https://weathertogether.net/weather-101/how-to-read-skew-t-charts/

Amount of Water Condensed in a

Parcel Lift

Amount of Water Condensed in a Parcel Lift • Determined by subtracting the

mixing ration at the top of the parcel lift from that for the bottom

In this example:• rT – rnew = 2 g/kg – 0.18 g/kg

= 1.82 g/kg

Source: Stull, R 2017

Latent Instability

Latent Instability• if a parcel somewhere along

the profile can be lifted so that it eventually reaches a Level of Free Convection (LFC) where it is warmer than its environment and therefore buoyant.

• A vertical zone of latent instability can be identified on a tephigram by tracing the coldest moist adiabat line that cuts the temperature profile, to the ground.

• Zones where this line is cooler than the wet bulb profile, are zones of latent instability.

Source: https://slideplayer.com/slide/4348000/

Potential Instability

Potential Instability• If a portion of the profile has

a wet bulb potential temperature which decreases with height, then lifting this vertical zone will eventually lead to an unstable profile.

• Zones of potential instability will have a wet bulb potential temperature profile that decreases with height

• Slope of wet bulb temperature < Slope of moist adiabats Source: http://tornado.sfsu.edu/geosciences/classes/m201/buoyancy/SkewTMastery/mesoprim/skewt/stability_potential5.htm

Convective Condensation Level

(CCL)

Convective Condensation Level (CCL)• The level at which a parcel of air

becomes saturated when lifted adiabatically

• Identified as the intersection of the temperature curve with a line drawn from the average mixing ratio of the surface layer (bottom ~5kPa, parallel to the isohumes

Source: http://kejian1.cmatc.cn/vod/comet/mesoprim/tephigram/media_gallery.php.htm

Convective Temperature (Tc)

Convective Temperature (Tc)• lowest temperature to which the

surface air temperature must be heated before a parcel can rise dry-adiabatically to its LCL without ever being colder than its environment

• Determined by drawing a line from the CCL to the surface, parallel to the dry adiabat

Source: http://kejian1.cmatc.cn/vod/comet/mesoprim/tephigram/print_2.php.htm

Discussion Questions

In addition to the annotated tephigram, there are several questions to answer• Questions 2 – 5, 7 (at the end!), and 12 – 15 will require additional write-up

• Some may be able to be included on the tephigram instead if desired!

• The remaining questions can be answered on the tephigram plot• Please indicate in your write up that the answer is located on the tephigram plot if so

Assignment

This lab assignment is due at the start of next week’s lab (October 1st at 8:30 am)It is worth 4 % of the final course grade

Includes the annotated Tephigram with: 1. Temperature (#1), dewpoint temperature (#1), and wet bulb temperature (#7)2. Layer stabilities (stable, unstable, etc) (#6)

3. Additional annotations (can also be submitted with discussion questions)• Tropopause, CCL, etc

As well as a write-up of the remaining discussion questions