Isentropic Analysis

1. N–S Cross section along 78°W Isentropic Analysis Potential temp, θ(K) Pressure (hPa) 30 April 2012 Heather M. Archambault South North

2. Why use potential temperature θas a vertical coordinate?

3. Why use potential temperature θas a vertical coordinate? • Can visualize 3D atmospheric flow

4. Why use potential temperature θas a vertical coordinate? • Can visualize 3D atmospheric flow • Can track moisture moisture moves along θ surfaces, not along p or Z surfaces

5. Why use potential temperature θas a vertical coordinate? • Can visualize 3D atmospheric flow • Can track moisture moisture moves along θ surfaces, not along p or Z surfaces • Useful for “potential vorticity thinking”

6. Why use potential temperature θas a vertical coordinate? • Can visualize 3D atmospheric flow • Can track moisture moisture moves along θ surfaces, not along p or Z surfaces • Useful for “potential vorticity thinking” • Can use vertical profile of θ to diagnose static stability, frontal zones

7. Potential temperature definition The temperature that a fluid parcel would acquire if adiabatically brought to a standard reference pressure

8. Why are θ surfaces called “isentropes”? • Relationship between entropy (s) andθ • If entropy is constant, so is θ (a constant entropy surface is isentropic surface)

9. Need to understand vertical variation ofθ to use it as a vertical coordinate

10. Need to understand vertical variation ofθ to use it as a vertical coordinate • Atmosphere is stable: θ increases with height

11. Need to understand vertical variation ofθ to use it as a vertical coordinate • Atmosphere is stable: θ increases with height • Atmosphere is neutral: θ is constant with height

12. Need to understand vertical variation ofθ to use it as a vertical coordinate • Atmosphere is stable: θ increases with height • Atmosphere is neutral: θ is constant with height • Atmosphere is unstable: θdecreases with height

13. Static Stability • Assessed by vertical spacing of θ • The lower the static stability, the greater the spacing of θ in the vertical (and vice versa)

14. Identifying warm and cold air • Unlike for p surfaces, θ slopes down toward warm air and up toward cold air

15. Identifying warm and cold air • Unlike for p surfaces, θ slopes down toward warm air and up toward cold air Z θ1 θ2 Warm surface

16. Identifying warm and cold air • Unlike for p surfaces, θ slopes down toward warm air and up toward cold air Z Z θ1 θ1 θ2 θ2 Warm Cold surface surface

17. Frontal zones • Regions of sloped, highly compact θ N–S Cross section along 78°W Potential temp, θ(K) Pressure (hPa) South North

18. Frontal zones • Regions of sloped, highly compact θ