The Intricacies of Instabilities

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.

Observation and numerical prediction of torrential rainfall over Korea caused by Typhoon Rusa (2002)

“…At 1800 UTC 30 August (Figure 12a), the southern warm (northern cold) air mass is identified by a region of high (low) e and . In Figure 12a, the boundary between the warm and cold air masses is well characterized by the strong moisture gradient and the demarcation between the upward and downward motions, so that a potentially unstable distribution of low-e air above high-e air develops over the location of maximum rainfall [Schultz et al, 2000]. The demarcation between the upward and downward motions begins in the midtroposphere, and results from a confrontation between the continental flows and the typhoon flow.…”

Formation and release of symmetric instability following Delta‐M adjustment

“…These sketches represent a region around 5 km deep and a few hundreds of kilometres wide. The shaded regions show where the saturated equivalent PV (EPV) is negative and hence where moist gravitational, inertial or moist symmetric instabilities are present (Schultz and Schumacher 1999;Schultz et al 2000). The EPV is defined as = (1/ρ) ζ · ∇θ * e , where ρ is the density, ζ is the absolute vorticity and θ * e is the saturated equivalent potential temperature.…”

Moist convective instability over the Arabian Sea during the Asian summer monsoon, 2002

“…CAPE over the Arabian Sea may play a role in causing intense rainfall on the west coast of India during the monsoon season. Although moderate CAPE (stored energy) in the atmosphere is a prerequisite for precipitation, the triggering mechanism plays a prime role in the ascent of surface air (Schultz et al 2000). Hence, variability in the monsoon rainfall must be related to both CAPE and the large-scale forcing for the ascent of surface air.…”