Moist absolute instability: The sixth static stability state

It is argued that a sixth static stability state, moist absolute instabilit y, can be created and maintained over mesoscale areas of the atmosphere. Ex amination of over 130 000 soundings and a numerical simulation of an observ ed 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 parti cular interest in the present study are the deep (greater than or equal to 100 mb) Layers that occur in conjunction with mesoscale convective systems (MCSs). A conceptual model is proposed to explain how moist absolute instab ility is created and maintained as MCSs develop. The conceptual model state s that strong, mesoscale, nonbuovancy-driven ascent brings a conditionally unstable environmental layer to saturation faster than small-scale, buoyanc y-driven convective elements are able to overturn and remove the unstable s tate. Moreover, since lifting of a moist absolutely unstable layer warms th e environment, the temperature difference between the environment and verti cally displaced parcels is reduced, thereby decreasing the buoyancy of conv ective parcels and helping to maintain the moist absolutely unstable layer. Output from a high-resolution numerical simulation of an event exhibiting t his unstable structure supports the conceptual model. In particular, the mo del indicates that MAULs can exist for periods greater than 30 min over hor izontal scales up to hundreds of kilometers along the axis of the convectiv e region of MCSs, and tens of kilometers across the convective region. The existence of moist absolute instability suggests that some MCSs are bes t characterized as slabs of saturated, turbulent flow rather than a collect ion of discrete cumulonimbus clouds separated by subsaturated areas. The pr ocesses 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 regi on of mesoscale convective systems.