Isentropic Analysis

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

Why use potential temperature θas a vertical coordinate?

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

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

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”

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

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

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)

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

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

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

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

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)

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

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

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

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

Frontal zones • Regions of sloped, highly compact θ

Isentropic Analysis