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3242414 
Journal Article 
Moist absolute instability: The sixth static stability state 
Bryan, GH; Fritsch, JM 
2000 
Yes 
Bulletin of the American Meteorological Society
ISSN: 0003-0007
EISSN: 1520-0477 
81 
6 
1207-1230 
WOS:000087330300002 
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 (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 instability is created and maintained as MCSs develop. The conceptual model
states that strong, mesoscale, nonbuovancy-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.