On Moist Instability

1. On Moist Instability Steven C. Sherwood (2000)

2. Abstract • An argument is made that the concepts of conditional instability and conditional symmetric instability need to be revisited. Confusion in the profession has led to two extant definitions of conditional instability that are superficially similar but fundamentally inconsistent. Only one definition corresponds to a real instability. Further, spillover of this confusion into the analysis of slantwise instability mechanisms appears to be producing inappropriate diagnosis of conditional symmetric instability. Concepts from hydrodynamic stability theory are helpful in discussing the situation.”

3. Traditional definitions of conditional instability (CI) • “…the occurrence of an atmospheric lapse rate that is steeper than a moist adiabat but less steep than a dry one.” • A rising parcel can become positively buoyant. • Sherwood writes that these definitions are inconsistent and need attention. Temperature profiles showing the moist adiabatic lapse rate (pink), a conditionally unstable lapse rate (black), and the dry adiabatic lapse rate (green). Image source: Lyndon State College

4. Problems with this definition • Instability requires some form of stored energy (e.g. CAPE), but stored energy does not guarantee that an instability will occur. • Sherwood argues that CI does not constitute a true instability, but rather that a parcel could become unstable and that the parcel is “preconditioned”. • “…lapse-rate CI is not an instability in any meaningful sense.”

5. “Marginal Stability Curve” • A curve in state space that separates stable basic states from unstable states. This is an ideal curve that does not realistically exist. • Though we can draw regions in state space of stability, instability, and unknown.

6. “The problem is that, too often, CI is not thought of as a statement of uncertainty but as a real instability, even by those who stick to the lapse-rate definition. The confusion is propagated in many popular textbooks, which give this definition and immediately afterward begin to discuss conditions for the “release” of CI. The common interpretation of lapse-rate CI in terms of a quantitative “condition” for instability is fundamentally specious, since the required condition is a fictitious, saturated parcel that is different from any air actually measured. One may just as well posit “conditional” instability in a sounding if a hypothetical parcel 10 K warmer than any observed could have been buoyant in that sounding! In fact neither of the words conditional instability properly describes what is really meant by the lapse-rate definition (regrettable, but these things do happen). Lapse-rate CI is ignorant of any specific mechanism for initiating a disturbance.”

7. Alternatives for Quantifying Instability • Convective available potential energy (CAPE): integrated area between parcel temperature sounding and environmental temperature sounding. Larger numbers (especially over 1,000 J kg-1) indicate larger instability. • Lifted index (LI): difference between parcel and environmental temperatures at a fixed level, typically 500 mb. Negative numbers indicate instability. • Lapse-rate conditional instability is weak when it comes to diagnosing convective potential.

8. The Intricacies of Instability David Schultz, Philip Schumacher, and Charles Doswell III

9. Abstract • In response to Sherwood’s comments and in an attempt to restore proper usage of terminology associated with moist instability, the early history of moist instability is reviewed. This review shows that many of Sherwood’s concerns about the terminology were understood at the time of their origination. Definitions of conditional instability include both the lapse-rate definition (i.e., the environmental lapse rate lies between the dry- and the moist-adiabatic lapse rates) and the available-energy definition (i.e., a parcel possesses positive buoyant energy; also called latent instability), neither of which can be considered an instability in the classic sense. Furthermore, the lapse-rate definition is really a statement of uncertainty about instability. The uncertainty can be resolved by including the effects of moisture through a consideration of the available-energy definition (i.e., convective available potential energy) or potential instability. It is shown that such misunderstandings about conditional instability were likely due to the simplifications resulting from the substitution of lapse rates for buoyancy in the vertical acceleration equation. Despite these valid concerns about the value of the lapse-rate definition of conditional instability, consideration of the lapse rate and moisture separately can be useful in some contexts (e.g., the ingredients-based methodology for forecasting deep, moist convection). It is argued that the release of potential (or convective) instability through layer lifting may occur in association with fronts, rather than with isolated convection, the terminology ‘‘convective’’ being an unfortunate modifier. The merits and demerits of slantwise convective available potential energy are discussed, with the hope of improving diagnostic methodologies for assessing slantwise convection. Finally, it is argued that, when assessing precipitation events, undue emphasis may appear to be placed on instability, rather than the forcing for ascent, which should be of primary importance.

10. History of Conditional Instability • Originated in the 1860s when the ideas of absolute stability, absolute instability, and some conditional instability • Normand in 1938 recognized the same Problems as Sherwood. He subdivided instability into divisions similar to CAPE and CIN • Normand’s classification scheme is illustrated in Table 1

11. Lapse Rate Instability Taylor expanding (1) Reinserting linear terms from (2) into (1). Where Γp is the parcel lapse rate and γ is the environmental lapse rate Solution shows Γp – γ < 0 would grow exponentially with time given Γp constant (the parcel remains saturated/unsaturated) and γ remains constant over displaced z Since those two conditions are rarely met, therefore this solution does not apply for conditional instability

12. Potential Instability • Introduced by Rossby (1932) • 1) the lapse rate of wet-bulb temperature exceeds the moist-adiabatic lapse rate • 2) the equivalent potential temperature Θe decreases with height • 3) the wet-bulb potential temperature Θw decreases with height • Involved in layer lifting not typically associated with deep convection

13. SCAPE • Slantwise CAPE • Shallower vertical instability • Instability must be near lifting mechanism • Computation problematic since following a slantwise path • SCAPE tends to be very smaller than values found in numerical models • Potential for advancement in forecasting

14. Conclusions • Agree with Sherwood’s assessment on terminology • Slantwise convection initiated by finite disturbances rather than infinitesimal perturbations • Lifting mechanism more important to the cause of precipitation • Instability a response to forcing

15. Citation Map