on November 09, 1997 at 17:19:59:
CIN can be removed by mid-tropospheric ageostrophic motions resulting
in oscillations of the isentropic surfaces. This process may be
more effective if the normalized CIN (NCIN) is small (i.e., the
CIN is "tall and thin" rather than "short and wide"). The genesis
and source of the oscillations and waves is less important than
is their presence in a region of potential instability.
We rely on a simple physical consideration: intense convection
will occur provided that large convective available potential energy
(CAPE) is present in the air column and provided that the typical
negative area (CIN, or convective inhibition) below the level of
free convection for surface air is somehow removed or reduced to
a small value that can be overcome by random cloud-scale pulses
at the top of the surface boundary layer.
A negative area can be eliminated simply by heating the boundary
layer until "convective temperature" (Tc) is reached, with no modification
above. If Tc is not reached, the removal of the lid over the moist
surface layer must be a consequence of an ageostrophic circulation.
The ascending limb may produce adiabatic cooling of the lid while
having little effect on the boundary layer. The effectiveness of
this removal mechanism may depend, in part, on the aspect ratio
of the CIN. The aspect ratio of CIN is termed normalized CIN (NCIN).
In the presence of a jet streak, Sanders and Blanchard (1993) found
that middle tropospheric waves may have represented an instability
at relatively low Richardson number of the type discussed by Kaylor
and Faller (1972), Gossard and Hooke (1975), and Einaudi et al.
(1978/79). The wave development was viewed as a type of shearing
instability of a highly nongeostrophic base state. Circumstantial
evidence linked the growth of ageostrophic shear on the synoptic
scale to the development of mesoscale oscillations of the isentropic
surfaces in the negative area overlying the surface boundary layer.
These oscillations, apparently extending well aloft into the midtroposphere,
were linked to the removal of the CIN and the subsequent strongly
circumscribed outbreak of deep and severe convection.
Gossard, E. E. and W. H. Hooke, 1975: Waves in the Atmosphere.
Elsevier, 456 pp.
Einaudi, F., D. P. Lalas, and G. E. Perona, 1978/79: The role of
gravity waves in tropospheric process. Pure Appl. Geophys., 117,
Kaylor, R. and A. J. Faller, 1972: Instability of the stratified
Ekman boundary layer and the generation of internal waves. J. Atmos.
Sci., 29, 497-509.
Sanders, F. and D. O. Blanchard, 1993: The origin of a severe thunderstorm
in Kansas on 10 May 1985. Mon. Wea. Rev., 121. 133-149.