The redistribution of a low-level, passive constituent of the atmosphere under the influence of a growing baroclinic wave is examined by a series of analytical calculations based on a two-level, highly truncated model. It is shown that a constituent confined to the lower half of the atmosphere, and initially homogeneous in the horizontal, will tend to achieve maximum concentration in the low pressure/warm sector portion of the wave and minimum concentration in the high pressure/cold outbreak region with sharpest gradient between the maxima and minima roughly coinciding with the cold front. This distribution is further accentuated if an initial meridional gradient of the constituent exists. If we assume, as a rough first approximation, that water vapor can be considered to be such a passive constituent, it is shown that the implied relative humidity field and cloud distribution will tend to evolve into the comma-type form commonly observed on satellite images of mid-latitude cyclone waves. Moreover, the solution is shown to replicate the complex flow regime associated with the comma formation, elucidating the dynamical rôles of vertical motion and advection in the cloud evolution.DOI: 10.1111/j.1600-0870.1985.tb00269.x
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