WMOD00416

• Title: Wintertime Cloud Systems Over the Rockies: Three Case Studies
• Date: 12/1/1980
• Weather Modification - Doc Type: Report

15

CHAPTER II ANALYSIS METHODS

2.1 Stability Analysis

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The convective component of the orographic cloud system takes

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several different forms as the degree of instability increases; an

embedded Bernard Cell type of convection, stationary convective lines,

and vigorous, moving convective bands (Marwitz, 1976). Many different

stability analyses and indices have been used by various investigators

(Saucier, 1955; Schuetz, 1957; Hess, 1959; Gates, 1960; Elliott and

Schaffer, 1962; Elliott and Hovind, 1964; Rhea, 1967; Dept. of the Air

Force, 1969; Chappell, 1970; Fraser, 1970; Orgill, 1971; Grant, et a1.,

1974; P100ster and Fukuta, 1974; Seinfie1d, 1975; Marwitz, et a1.,

1976; Vardiman, ~ a1., 1976).

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Parcel methods for analyzing stability can be used to detect large

scale instability (Elliott, 1969). If a parcel of air is lifted to its

LCL (Lifted Condensation Level), by an orographic lift for example, and

the environmental lapse rate in the vicinity of the LCL is such that

r < r. < r d' b . (conditional instability),
saturated env~ronment dry a ~a at~c
the parcel will continue to rise due to buoyancy (r = -dT/dZ = lapse

rate). As the parcel rises, the temperature difference between the

parcel and the environment will accelerate the parcel upwards while

entrainment of environmental air will tend to decelerate it. \Vhen the

temperature of the parcel again equals the temperature of the environ-

ment, the parcel will no longer be accelerated by buoyancy.

There are two types of layer methods which can be used to detect

smaller scales of instability: layer conditional instability and po-

tential, convective instability. The first method applies the concepts

of conditional instability to an atmospheric layer. The layer is