WMOD00416

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

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At some point, e must begin to increase with height in order to
e

determine the top of the layer as defined by B. This seems to imply

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some degree of potential stability within the layer of potential in-

stability. As long as the base of the layer is determined as defined

by A, 8 will decrease with height from layer base to layer top and an
e

average conditional instability will exist throughout the layer.

2.2 Blocking Analysis

A serious difficulty encountered in a two-dimensional analysis of

airflow past a ridge is the inability to describe the motion of air

which fails to pass over the barrier. This may be caused by any of the

,
following: downslope valley winds in lower layers, thermodynamic

stagnation, cold air pooling, light winds which pile air up against the

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ridge, or winds which encounter the barrier and turn to parallel it

without ascension.

These conditions are termed blocking flows (Kao,

-;;;

1965; Wong and Kao, 1970; Orgill, 1971; Fraser, et a1., 1973; }1arwitz,

et al., 1976; Reid, 1976; Rhea, 1978).

Blocking conditions may vary in time and have a large effect upon

the orographic lift which, in turn, determines the orographic as well as

the convective cloud components which form over the ridge. The b1ock-

ing flow has been defined with respect to wind direction, a transbarrier

wind component, and even a combination of transbarrier wind component

and temperature profile (Myers, 1962; Elliott, 1969; Fraser, ~ a1.,

1973; Harwitz, et a!., 1976; Rhea, 1978; Marwitz, 1980).

The criteria for blocking chosen for this study is similar to that

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of Rhea (1978). A layer will be termed blocked if the transbarrier wind

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component is less than 2 m/sec or if dT/dP < 0.4 K/5 k Pa. This