WMOD00548

• Contract/Permit #: 6-07-DR-20020
• Project Name: Sierra Cooperative Pilot Project
• Title: Satelite Studies During the 1976-77, 1977-78 Sierra Cooperative Pilot Project
• Date: 2/1/1979
• Weather Modification - Doc Type: Report

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4.0 INTENSIVE CASE STUDIES

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4.1 9 March 1977 CASE STUDY

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The 9 March storm was one of the few significant storms that

occurred in the drought-stricken winter of 1976-77 in California. A

deepening middle-level trough formed along the West Coast in response

to strong cold advection to its rear. Pre-frontal precipitation was

falling over the mountains from a fairly warm (-10 to -15oC) orographic

cloud at 0945 GMT. The main part of the storm swept through southern

Oregon. As a result, no extensive high cloud covered the frontal band

in the target area.

Decreasing CTT preceded the surface cold front as the frontal cloud

band entered the target (Figure 2). Coldest cloud tops over the target

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were at 1445 just before the front, where mean CTT in the target was

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-33 C and the coldest cloud element seen in the target was -38 C. Radar

observed cloud tops agreed fairly well with the satellite cloud tops.l

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The radar top of -41 C at 1530 GMT was not verified by the satellite

(Figure 2), due to the lower resolution of the satellite IR sensor

averaging out this small perturbation.

Precipitation began increasing sharply around 1345 at Plavapa to

a maximum of 9 rom per hour with the frontal passage (Figure 3). No

rain fell at Auburn in the Central Valley until the frontal band passed

over. Precipitation rates at all stations shown in Figs. 3 and 4 are

closely related to the decreasing target area CTT. In a storm such as

this one, the above result is to be expected. The decreasing CTT is

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1 Using the Sheridan sounding the satellite cloud-top temperatures
could be converted to km above mean-sea level and the radar heights
could be converted to temperature for comparisons.