WMOD00303

• Title: Observational and Numerical Studies of Cloud and Precipitation Development with a View to Rainfall Enhancement
• Date: 4/1/1992
• Weather Modification - Doc Type: Report

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regions and the distribution of precipitation resulting from large scale synoptic and local
terrain efFects.

iv) The physical chain of events that ultimately lead to precipitation formation and
its response to seeding.

Orographic :flows encompass all scales of motion, and disturbances forced by the
orography may extend above tropospheric altitudes. The two most important mechanisms
that force orographic :flows are thermal and mechanical in nature. While thermal
circulations are related to differential heating and cooling associated with diurnal
insolation, a wave disturbance is created when stable stratified air is forced to rise over
a topographic barrier. The energy associated with the disturbance is usually transported
away !rom the mountain by gravity waves. Gravity waves forced by mountains are also
referred to as mountain waves. The more complex the three-dimensional nature of the
topography the more complex the gravity wave structures that will be produced. The wave
structures will also change dramatically as the direction and/or speed of air:flow changes
in the lower layers. Air:flow patterns over complex terrain are, consequently, closely linked
to the orography and highly space and time dependent.

A review of the relevant literature immediately highlights the correlation between
the temporal and spatial evolution of CLW and the complexity of the terrain (Rauber
et al.,1986; Rauber and Grant, 1986; Marwitz, 1986; Deshler et al., 1990, Huggins and
Sassen,1990). This was particularly evident in comparisons between studies in the Sierra
Nevada and Colorado Rocky mountains which found that the Sierra barrier can be viewed
as an approximate two-dimensional inclined plane, while the San .Juans in Colorado are
a complicated three dimensional structure (Marwitz, 1986). Measurements of supercooled
water over the Mogollon Rim. in Arizona and in Utah have also revealed considerable
temporal variability in cloud liquid water as monitored by a microwave radiometer (Super
and Holroyd, 1989; Super et al.,1989; Sauen et al., 1990). The bulk of the CLW was
produced by synoptic scale storms (Super and Boe, 1988). The orography around the
Mogollon Rim in Arizona and especially upstream of the Mogollon Rim displays a complex

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