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<br />5
<br />
<br />.
<br />
<br />in, the above airflow models break down and the convective component
<br />
<br />of the orographic cloud system establishes itself. The degree of in-
<br />
<br />stability dictates whether the convective motions will remain embedded
<br />
<br />within the cloud system or if they will dominate the vertical velocity
<br />
<br />field.
<br />
<br />~
<br />
<br />A convective vertical velocity component, > 1 m/sec, can be in-
<br />
<br />duced and modified by thermodynamic stratification of the atmosphere,
<br />
<br />the orographic lift encountered, advective processes of thermodynamic
<br />
<br />variables, daily cycles of solar insolation, radiative losses, surface
<br />
<br />roughness, vegetation, snow cover, small to medium scale turbulent
<br />
<br />motions, wind shear, and microphysical processes (Orgill, 1971; Orgill,
<br />
<br />et a1., 1971; Derrickson, et a1., 1973; Nickerson, et a1., 1976; Reid,
<br />
<br />
<br />1976; Reid, et a1., 1976; Walsh, 1977). Convection may be superimposed
<br />
<br />
<br />upon the orographic cloud and this instability can be caused by a com-
<br />
<br />
<br />bination of destabilization by synoptic scale advective processes and
<br />
<br />airmass lifting by the topography (Scorer, 1953; Saucier, 1955; Hess,
<br />1959; Elliott and Schaffer, 1962; Myers, 1962; Elliott and Hovind, 1964;
<br />Furman, 1967; Rhea, et a1., 1969; Wong and Kao, 1970; Rhea, 1972;
<br />Fraser, et a1., 1973; Grant, ~ a1., 1974; l1arwitz, 1975; }1arwitz,
<br />et a1., 1976; Walsh, 1977; Reynolds and Morris, 1978). This component
<br />
<br />of the total cloud system is highly variable in time and space as
<br />
<br />temperature and moisture fields are advected into the cloud system and
<br />
<br />the air is lifted over the barrier (Elliott and Schaffer, 1962; Dept.
<br />
<br />of the Air Force, 1969; Fraser, et a1., 1973; Vardiman, et a1., 1976).
<br />
<br />
<br />The variability may occur with a diurnal cycle, a synoptic storm pro-
<br />
<br />gression, or from storm to storm as each storm track, moisture content,
<br />
<br />.
<br />
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