WMOD00299

• Title: Physical Structure of Northern Colorado River Basin Cloud Systems
• Date: 9/1/1985
• State: CO
• Weather Modification - Doc Type: Report

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15

frequently observed in replica samples, although ice nuclei

identification indicated that some material had reached the dendritic

growth region.

Regarding natural cloud system structure, Rheaet al. made three
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important observations regarding mesoscale cloud system structure which

have been confirmed in the present study (Chapter 3). These are:

a. "The highest precipitation rates (frequently exceeding 0.1 in hr-1)
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usually occur in the first 3 to 6 hours follOWing the surface passage of
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maritime pOlar fronts. This time period frequently coincides with the
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existence of 2400 to 2700 flow above 12000 ft M.S.L. but 2800 to 3100

flow at lower levels."

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b. "Longer periods of !.-i~!'1ter (0.003 to 0.050 in hr ), mainly
orographic precipitation usually continue after passage of the frontal
convergence area with cloud tops lowering to between 15000 ft M.S.L.
-==-"""'-~---"--' .-

and 11500 ft M.S.L."

c. "There is usually a convective precipitation component, which is

reflected in the unsteady precipitation rates observed. Convective band

passages at 2.5 to 4 hour intervals are quite commonly observed."

Rhea (1967) concluded that the first two components mentioned above

account for about 8~ of the Park Range winter precipitation.
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The PRP also made extensive measurEll1ents of ice crystal habit.

'l'h"~l_!~_und that dendritic crystals .El.ccounte.d._Jor SOft of the snowfall.

Less than 6" were of columnar or needle habit. This is one very

significant difference between the Park Range region and coastal regions

discussed in the previous section.