Moist Absolute Instability: The Sixth Static Stability State

It is argued that a sixth static stability state, moist absolute instability, can be created and maintained over mesoscale areas of the atmosphere. Examination of over 130 000 soundings and a numerical simulation of an observed event are employed to support the arguments in favor of the existence of moist absolutely unstable layers (MAULs). Although MAULs were found in many different synoptic environments, of particular interest in the present study are the deep (≥ 100 mb) layers that occur in conjunction with mesoscale convective systems (MCSs). A conceptual model is proposed to explain how moist absolute instability is created and maintained as MCSs develop. The conceptual model states that strong, mesoscale, nonbuoyancy-driven ascent brings a conditionally unstable environmental layer to saturation faster than small-scale, buoyancy-driven convective elements are able to overturn and remove the unstable state. Moreover, since lifting of a moist absolutely unstable layer warms the environment, the temperature difference between the environment and vertically displaced parcels is reduced, thereby decreasing the buoyancy of convective parcels and helping to maintain the moist absolutely unstable layer. Output from a high-resolution numerical simulation of an event exhibiting this unstable structure supports the conceptual model. In particular, the model indicates that MAULs can exist for periods greater than 30 min over horizontal scales up to hundreds of kilometers along the axis of the convective region of MCSs, and tens of kilometers across the convective region. The existence of moist absolute instability suggests that some MCSs are best characterized as slabs of saturated, turbulent flow rather than a collection of discrete cumulonimbus clouds separated by subsaturated areas. The processes in MAULs also help to explain how an initially unsaturated, stably stratified, midlevel environment is transformed into the mesoscale area of saturated moist-neutral conditions commonly observed in the stratiform region of mesoscale convective systems.