BY: NWS LOUISVILLE
- An isentropic process is an adiabatic process (i.e., no parcel heat exchange with its environment). For synoptic scale weather systems, air parcels generally move along constant potential temperature/theta (i.e., isentropic) surfaces, NOT constant pressure (isobaric) surfaces (Figs. 5a and 5b). In other words, air moves in 3 dimensions, not on horizontal pressure surfaces.
| Fig. 5a: Example of an isentropic surface in 2-dimensions. Bold solid (dashed) lines are lines of constant pressure/isobars (mixing ratio/isohumes) while the bold arrow is wind direction on the surface. Flow is from higher-to-lower values of pressure and moisture. Thus, ascent/upward moisture transport is occurring.
||Fig. 5b: Same as Fig. 5a except the isentropic surface is shown in 3 dimensions to more clearly show ascent and upward moisture transport.
- Isentropic analysis allows the ability to attain quantitative estimates of vertical motion and coherently track air flow and the 3-dimensional transport of moisture in space and time (unlike pressure coordinates).
- Vertical motion on an isentropic surface is determined via pressure advection, which is analogous to temperature advection on a constant pressure surface (e.g., 850 mb). In other words, an area of warm air advection at 850 mb likely also is an area of isentropic lift.
- Isentropic ascent and upward moisture transport (Fig. 5) are present in areas where winds on the theta surface cross isobars and isohumes (mixing ratio lines) from higher-to-lower values of pressure (similar to warm advection on a pressure surface) and mixing ratio.
- For descent, wind flow is from lower-to-higher pressure, and often from lower-to-higher values of mixing ratio which produces drying.
- The stronger the winds, the tighter the pressure gradient (i.e., the steeper the slope of the isentropic surface), and the more perpendicular the winds are to the isotherms and isohumes, the stronger the upward motion and moisture transport will be. This can lead to significant precipitation.
- Vertical motion values associated with isentropic lift usually are "synoptic-scale" values, i.e., on the order of several (perhaps 5-10) cm/s.
- Divergence within entrance and exit regions of jet streaks can increase the flow along isentropic surfaces and isentropic lift.
- Significant diabatic effects, e.g., latent heat release or diurnal heating/cooling, and isentropic analysis near the ground are limitations and can make accurate isentropic analysis difficult.