NOAA/National Severe Storms Laboratory

Koch - 4

Hypothesis concerning initiation of Narrow Cold Frontal Rainbands

Steve Koch on November 13, 1997 at 17:49:08:

A narrow band of intense but shallow convection (NCFR) is initiated along anafronts (characterized by rearward relative flow) when sufficient moisture and a strong low-level (LLJ) are present within 100 km ahead of the cold front, even in the absence of any CAPE. The strength of the lifting, which is associated with a circulation transverse to the LLJ, is dependent upon (1) the strength of the LLJ, (2) the magnitude of the deformation forcing in the Sawyer-Eliassen equation, and (3) the cross-jet balance between the vorticity associated with the vertical wind shear in the LLJ and the solenoidal vorticity associated with any diabatic cooling source that may exist behind the front.


Koch, S. E., and W. Clark, 1997: The dynamics of severe storm initiation along a nonclassical cold front revealed by remote sensing observations during COPS-91. J. Atmos. Sci. (submitted).

Adams, M. E., S. Businger, M. L. Kaplan, and S. E. Koch, 1997: Extraction of geopotential height and temperature structure from observed profiler winds over the Central Plains during STORM-FEST. J. Appl. Meteor. (cond. accept.).

Karyampudi, V. M., S. E. Koch, J. W. Rottman, and M. L. Kaplan, 1995: The influence of the Rocky Mountains in the 13-14 April 1986 severe weather outbreak. Part II: Evolution of an internal bore and its role in triggering a squall line. Mon. Wea. Rev 123, 1423-1446.

Koch, S. E., and P. J. Kocin, 1991: Frontal contraction processes leading to the formation of an intense narrow rainband. Meteor. Atmosph. Phys, 46, 123-154.

Determine existence of the LLJ from WSR-88D and Profilers, and estimate the thermal fields from thermodynamic retrieval applied to the Profiler + WSR-88D combined network using the Adams et al. (1997) & Karyampudi et al. (1995) divergence equation methodology. Low-level aircraft traverses can be used to obtain greater spatial resolution of these fields near to the front. Moisture distribution are obtainable from ground and Mobile Mesonets, M-CLASS, and GOES8 cloud distributions. Vorticity balance will be evaluated from cross-front wind shear using radars and profilers, but access to one or more 915 Mhz profilers is necessary to obtain information with sufficient detail in lowest 2 km, based upon our experiences in COPS-91 (Koch and Clark 1997). I argue for this additional invaluable instrumentation, since the emphasis in TIMEX is on low-level features.
If no transverse circulation exists surrounding observed LLJs, or no relation can be found between the strength of such circulations and the three governing parameters, or convection fails to initiate in the manner predicted, in a sufficient number of cases.

The following appear in order; discussion points may directly refer to one or more comments preceeding it.


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