Introduction to Thermodynamic Diagrams How thermodynamic diagrams are used to forecast thunderstorms? Thermodynamics M. D. Eastin

Introduction to Thermodynamic Diagrams Outline: Basic Idea of Thermodynamic Diagrams Possible Diagrams Skew-T Log-P Diagram Rawinsondes Dropsondes Skew-T Applications Thermodynamics M. D. Eastin

Basic Idea of Thermodynamic Diagrams Advantages: A visualization tool We can always use the mathematical formulas… Many of us learn better through visualization… Eliminates or simplifies the equations Can determine many quantities in a graphical format Desired Qualities in a Thermodynamic Diagram: 1. For cyclic processes, the area should be proportional to the work done or the heat exchanged 2. The lines should be straight (easy to use) 3. The angle between adiabats and isotherms should be as large as possible (easy to distinguish variations in atmospheric stability → will air rise or sink) (more on stability later…) Thermodynamics M. D. Eastin

Possible Thermodynamic Diagrams P-V Diagrams? Pros: Satisfies Requirement #1 Good for illustrating basic concepts Cons: Angle between isotherms and adiabats is very small Isotherms and adiabats are not straight lines We don’t observe volume We need to use a different diagram that satisfies all three requirements and uses a coordinate system for observable variables p Isobar i Isochor Adiabat f Isotherm V Thermodynamics M. D. Eastin

Basic Idea of Thermodynamic Diagrams Area-Equivalent Transformations: P-V diagrams only satisfy Requirement #1: Enclosed area proportional to energy Thus, we need to consider other variables for the coordinate systems Create a generic transformation from P, V → A, B P A B V W Thermodynamics M. D. Eastin

Basic Idea of Thermodynamic Diagrams Area-Equivalent Transformations: P A B V WpV WAB Thermodynamics M. D. Eastin

Possible Thermodynamic Diagrams Tephigram: Area proportional to energy 3 sets of nearly straight lines Isotherms (T) “Dry” Adiabats (θ) Saturation Mixing Ratio (w) Isobars (p) are curved Moist adiabats (θe) are curved 90º angle between dry adiabats and isotherms 1000 mb 800 mb 600 mb 400 mb Pressure -40oC Temperature -20oC 0oC q = 0oC 20oC 40oC 60oC qe w Note: We will talk about the moist adiabats (θe) and saturation mixing ratio (w) lines later in the course Thermodynamics M. D. Eastin

Possible Thermodynamic Diagrams Emagram: Area proportional to energy 4 sets of nearly straight lines Isobars (p) Isotherms (T) “Dry” Adiabats (θ) Saturation Mixing Ratio (w) Moist adiabats (θe) are curved 45º angle between dry adiabats and isotherms 1000 mb 800 mb 600 mb 400 mb -20oC 0oC 20oC 40oC -40oC Pressure Temperature q = 0oC 60oC 80oC 100oC qe w Note: We will talk about the moist adiabats (θe) and saturation mixing ratio (w) lines later in the course Thermodynamics M. D. Eastin

See Example on Next Slide Possible Thermodynamic Diagrams Skew-T Log-P Diagram: Area proportional to energy 3 sets of nearly straight lines Isobars (p) Isotherms (T) Saturation Mixing Ratio (w) Dry adiabats (θ) are slightly curved Moist adiabats (θe) are curved ~90º angle between adiabats and isotherms See Example on Next Slide Note: We will talk about the moist adiabats (θe) and saturation mixing ratio (w) lines later in the course Thermodynamics M. D. Eastin

Possible Thermodynamic Diagrams Skew-T Log-P Diagram: Pressure (mb) 1000 900 800 700 600 500 300 200 400 Temperature (oC) 30 40 20 10 -10 Isobars (p) Isotherms (T) Saturation Mixing Ratio (w) Dry Adiabats (θ) Moist adiabats (θe) Thermodynamics M. D. Eastin

The Skew-T Log-P Diagram Most commonly used diagram (we will use it too…) Come in a variety of shapes, sizes, and colors. All Skew-T Log-P diagrams provide the exact same information! Thermodynamics M. D. Eastin

The Skew-T Log-P Diagram Note how the lines of constant temperature slope (or are skewed) toward the upper left Hence, “Skew-T” These lines are always solid and straight but vary in color Our Version: Red solid lines Thermodynamics M. D. Eastin

The Skew-T Log-P Diagram Note how the change in pressure along the Y-axis in non-uniform Rather, it changes logarithmically Hence, “Log-P” These lines are always solid and straight but may vary in color Our Version: Blue solid lines Thermodynamics M. D. Eastin

The Skew-T Log-P Diagram The dry adiabats, or lines of constant potential temperature slope at almost right angles to the isotherms These lines are always solid and slightly curved, but may vary in color Our Version: Light Blue Solid Lines Thermodynamics M. D. Eastin

The Skew-T Log-P Diagram The lines of constant saturation mixing ratio are also skewed toward the upper left More on these in a future lecture… These lines are always dashed and straight, but may vary in color Our Version: Pink dashed Lines Thermodynamics M. D. Eastin

Skew-T Applications The moist adiabats are lines of constant equivalent potential temperature and they represent pseudo- adiabatic processes More on these in a future lecture… These lines are always dashed and curved, but may vary in color Our Version: Dashed bluish-green Thermodynamics M. D. Eastin

Saturation Mixing Ratio (10 g/kg) Skew-T Log-P Diagram Pressure (200 mb) Moist Adiabat (283K) Dry Adiabat (283K) Isotherm (T= -10ºC) Saturation Mixing Ratio (10 g/kg) 10ºC = 283K Thermodynamics M. D. Eastin

Skew-T Log-P Diagram Plot Rawinsonde or Dropsonde Observations: Temperature Dewpoint Temperature Thermodynamics M. D. Eastin

The Rawinsonde Instrument Package attached to a Balloon: Launched twice daily (00 and 12 UTC) Regular launch locations Rise from the surface into the stratosphere before the balloon bursts Observe pressure (p), temperature (T), dewpoint temperature (Td), altitude (z), and horizontal winds (speed, direction) at numerous regular levels through the atmosphere. Temperature and Humidity Sensor Thermodynamics M. D. Eastin

Standard 1200 UTC Rawinsonde Sites The Global Rawinsonde Network Standard 1200 UTC Rawinsonde Sites Thermodynamics M. D. Eastin

The Dropsonde Instrument Package attached to a Parachute: Launched from aircraft or hot air balloons over data sparse regions (e.g. the oceans) Used to improve “high-impact” forecasts Hurricane forecasts Winter storm forecasts Irregular launch times and locations Fall the from launching platform down to the surface using a parachute that controls the rate of descent Observe pressure (p), temperature (T), dewpoint (Td), altitude (z), and horizontal winds (speed, direction) at numerous regular levels through the atmosphere. Thermodynamics M. D. Eastin

Skew-T Applications Identify Temperature Inversions Inversions are layers where temperature increases with height Thermodynamics M. D. Eastin

Skew-T Applications Identify Dry Adiabatic Layers Dry adiabatic layers have the temperature profile parallel to a dry adiabat Thermodynamics M. D. Eastin

Skew-T Applications Determine the Potential Temperature (θ) of any Air Parcel Bring air parcel down a dry adiabat to 1000 mb Add 273 K to the T-value Begin with parcel at 400 mb T = 40ºC θ = 313 K Thermodynamics M. D. Eastin

Introduction to Thermodynamic Diagrams Summary: Basic Idea of Thermodynamic Diagrams Possible Diagrams Skew-T Log-P Diagram Rawinsondes Dropsondes Skew-T Applications Thermodynamics M. D. Eastin

References Thermodynamics M. D. Eastin Petty, G. W., 2008: A First Course in Atmospheric Thermodynamics, Sundog Publishing, 336 pp. Tsonis, A. A., 2007: An Introduction to Atmospheric Thermodynamics, Cambridge Press, 197 pp.   Wallace, J. M., and P. V. Hobbs, 1977: Atmospheric Science: An Introductory Survey, Academic Press, New York, 467 pp. Also (from course website): NWSTC Skew-T Log-P Diagram and Sounding Analysis, National Weather Service, 2000 The Use of the Skew-T Log-P Diagram in Analysis and Forecasting, Air Weather Service, 1990 Thermodynamics M. D. Eastin