Isentropic Analysis Techniques: Basic Concepts

March 5, 2004Adapted from Professor Jim Moore of St. Louis University

The troposphere, except in shallow, narrow, rare locations, is stable to dry processes. For the purpose of synoptic analysis, these areas can be ignored

and potential temperature used as a vertical coordinate.

Potential temperature increases with height

International Falls, MN Miami, FL

Potential temperature as a vertical coordinate

Isentropic Analysis: Advantages

For synoptic scale motions, in the absence of diabatic processes, isentropic surfaces offer greater continuity with the depiction of 3D motions

Horizontal flow along an isentropic surface contains the vertical motion often neglected in a Z or P reference system

Moisture transport on an isentropic surface is three-dimensional

Advection of Moisture on an Isentropic Surface

Advection of Moisture on an Isentropic Surface

Moist air from low levels on the left (south) is transported upward and tothe right (north) along the isentropic surface. However, in pressure coordinates water vapor appears on the constant pressure surface labeled p in the absence of advection along the pressure surface --it appears to come from nowhere as it emerges from another pressure surface. (adapted fromBluestein, vol. I, 1992, p. 23)

Isentropes near Frontal Zones

300 K Surface Pressure in mb

Wind blowing from lowpressure to high pressure-air descending

Wind blowing from highpressure to low pressure-air ascending

Isentropic Analysis: Advantages

Atmospheric variables tend to be better correlated along an isentropic surface than on a constant pressure surface

The vertical spacing between isentropic surfaces is a measure of the static stability. Convergence (divergence) between two isentropic surfaces decreases (increases) the static stability in the layer

The slope of an isentropic surface (or pressure gradient along it) is directly related to the thermal wind.

Isentropic Analysis: Disadvantages

In areas of neutral or superadiabatic lapse rates isentropic surfaces are multi-valued with respect to pressure

In areas of near-neutral lapse rates there is poor vertical resolution of atmospheric features. In stable frontal zones, however there is excellent vertical resolution

Isentropic surfaces tend to intersect the ground at steep angles

Neutral-Superadiabatic Lapse Rates

Vertical Resolution is a Function of Static Stability

Isentropic Analysis: Disadvantages

The “proper” isentropic surface to analyze on a given day varies with season, latitude, and time of day. There are no fixed level to analyze (e.g., 500 mb) as with constant pressure analysis.

If we practice “meteorological analysis” the above disadvantage turns into an advantage since we must think through what we are looking for and why!

Choosing the “Right” Isentropic Surface(s)

The “best” isentropic surface to diagnose low-level moisture and vertical motion varies with latitude, season, and the synoptic situation. There are various approaches to choosing the “best” surface:

Use the ranges suggested by Namias (1940) :

Season Low-Level Isentropic Surface

Winter 290-295 K

Spring 295-300 K

Summer 310-315 K

Fall 300-305 K

Choosing the “Right”

Isentropic Surface