WMOD00400

• Contract/Permit #: 14-06-D-6801
• Title: Structure and Seedability of San Juan Storms
• Date: 11/1/1976
• Weather Modification - Doc Type: Report

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on standard thermodynamic diagrams. The base of the convective instability

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was located where observed upslope winds would first begin to release the

convective instability. There was one particular case (30 January - first

flight) where there was significant conditional instability but the winds

were do\vuslope and unable to release the thermodynamic instability. The

top of the convective instability has the same 8 as at the convective con-
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densation level. The convective instability forms initially due to cold

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air advection aloft and sometimes warm, moist air advection below. More

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often the \Varm, moist advection is delayed until the blocked flow is removed,

thus allowing the warm, moist air to advect into the area from the south or

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sou th\ves t.

The blocked flow is a serious problem on many storms. The blocked flow

level is defined as that level below which the wind directions are less than

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1600. The blocked flow seems to form from two or three processes. ~~en a

\vell organized storm passes over the San Juans the end of the storm occurs

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when significant cold air advection begins to occur in the lowest levels.

Thus, a mass of cold, stable air is left behind to affect the next storm to

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pass over the area. The other processes are more speculative but there seems

to be some supporting evidence in the data. It is speculated that the high

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mountain massives in southern Colorado cool by snow sublimation and to a

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lesser degree by radiative cooling during clear winter periods. That these

cooling effects are possible seems likely because the mountains are above most

of the moisture and the \\finds during non-storm situations are typically light

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because the jet stream is well north of the region.

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The evidence supporting the above hypothesis concerning blocking flow

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is that the stability after the passage of a major trough is as descrj~ed

(see post-storm stability on 29 December 7Q, 28 January 75, 1.5 l'larch 7.5

and 17 Harch 75). On the other hand, thermodynamically stable blocking fTow

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