WMOD00400

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

I

13

I

A. Storm Structure and Sequence

1. Typical storm

Fig. 3.1 is a schematic of the typical height-time cross-section for a

storm as it moves across Holf Creek Pass. The schematic was derived by con-

I

I

structing a height-time cross-section for each storm studied over the San

I

Juans with instrumented aircraft during the 1973, 1974 and 1974-75 winter

seasons. In all there were 12 separate storms studied. All but one of the

I

storms contained most of the significant characteristics displayed in Fig.

I

3.1. The special storm ~vill be discussed separately.

Contained in Fig. 3.1 is the time sequence of cloud tops as determined

from the Durango rawinsonde. Cloud tops are assumed to be at saturation with

I

respect to ice. The clouds tops during the front half of the storm were

often not precisely determinable because the temperature and dewpoint would

I

slowly separate with height. \{hen the cloud tops began to lower they did so

because of subsidence from aloft, thus well defined cloud tops associated ~vith

I

I

subsidence inversions developed later in the storm.

The time sequence of cloud bases was easier to determine. Throughout

I

the storm the saturation level was generally well defined on the Durango

rawinsonde. The typical sequence of cloud bases was as shown in Fig. 3.1.

The cloud bases lowered rapidly at the front end of the storm because the

mid-level moist air was rapidly approaching from the southwest. ~~en pre-

I

I

cipitation began to fall over and upwind of the San Juans, the air mass was

I

soon saturated to the ground. Hith the formation of convection the convec-

tive cloud bases became distinct and typically occurred at 2.5 to 3.0 km.

The region of convective instability was determined by examining th~

I

I

vertical profile of B (the equivalent potential temperature) as a function
e
of time and by examining the individual wind profiles and soundings plotted

I
I