Blanchard
Hypothesis (Blanchard) concerning removal of CIN by mid-tropospheric ageostropic motions |
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David Blanchard 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. Background: 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. References: 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, 627-663. 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. |
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This hypothesis can be tested by using serial launches of two or more M-CLASS rawinsondes located near or on either side of a potential initiation boundary in which a lid, or CIN, is present. The passage of midtropospheric waves, if present, can be detected in the temperature profiles by noting the rise and fall of isentropic surfaces with respect to pressure and/or height. Surface mesonet data using instrumented vehicles (Mobile Mesonet) can be used to detect wave passage by noting high frequency pressure variations not associated with diurnal or synoptic features. Cases with both large and small NCIN can be compared to determine the effectiveness of the oscillations in removing the CIN. |
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(a) midtropospheric oscillations have no effect in the removal of CIN; (b) large and small NCIN show similar results in the removal of the CIN; (c) midtropospheric waves are not observed in a large sample size. |
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The following appear in order; discussion points may directly refer to one or more comments preceeding it. Click here to comment on this hypothesis. Please reference: BLANCHARD. |