FLOOD09350

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Generalized Estimates of Probable Maximum Precipitation for the United States West of the 105th Meridian
• Date: 1/1/1960
• Prepared For: USDA
• Prepared By: US Weather Bureau
• Floodplain - Doc Type: Educational/Technical/Reference Information

from air containing a given amount of moisture
may be related directly t.o convergence and/or
orography.
2,3.2 Convergence is really a measure of in-
flow. It may also be visualized as the horizontal
shrinking of a mass or column of air. Converg-
ence is expressed in u.rms of shrinkage per unit
time. Thus, for example, a convergence of
2 X 10-' sec.-' would indicau. that the hori-
zontal cross-section area of a column or air was
being reduced by 0.00002 per sec.
2.3.3 The convergence required to produce
various precipitation rates from saturated air
layers of various temperatures and thicknesses
has been computed. The rate at which the amount
of water vapor required for saturation decreases
with lowering temperatures may be called the rate
of production of moist.ure excess over sat.urat.ion.
Assuming that. this moisture excess would all fall
out. as precipitation and convergence would de-
crease with height to zero at. 4.5 km. (rougWy
15,000 ft.) Peterson [12] constructed a graph
(fig. 2--1) relating the 6-hr. precipitation to the
temperature and convergence at the surface in a
pseudoa.diabatic saturau.d atmosphere. This
graph demonstrates that if t.he assumptions are
valid, considerable horizont.al convergence must be
associated with heavy rainfall rates. This appears
to be true even wit.h some allowance for horizontal
convergence of the falling raindrops, which would
cause the precipitation rate to be greater than the
rate of production of moisture excess, and for some
additional lift provided by orographic barriers.
2.3.4 Gilman and others [13] prepared sche-
matic illustrations (fig. 2-2) of the change in
shape of an initially cubic mass of saturated air
wit.h a surface temperature of 700 F. and a pseu-
doadiabatic lapse rau. when sufficient horizontal
convergence occurs to effect upward motion ade-
quate to produce 1, 2, and 5 in. of rain. Diagrams
B, C, and D are based on four assumptions: (1)
convergence decreases linearly with pressure to
zero at 600 mb., or roughly 14,000 ft.., (2) winds
at any given level are of uniform speed and radi-
ally directed, (3) rainfall intensity is uniform
over area., and (4) the air is lifted pseudoadiabati-
cally. Figure 2-2E is based on the same assump-
tions and in addit.ion assumes that another wind
component, constant in direction but with speed
increasing from zero at 1,000 mb. to 50 knots at
200 mb., is superimposed on the radially-directed
wind, or convergence, field of figure 2--2C. Figure

2--2 provides an indication of the degree of hori-
zontal convergence required to produce large
amounts of precipitation.
2,3,5 The effect of orographic lifting on pre-
cipitation intensity is a perplexing problem. It
is difficult to deu.rmine within a particular storm
how much of the variation in precipitation is
related to changes in the storm mechanism and
how much is related to orography. Also, the same
orographic barrier that is a precipitation-produc-
ing factor on t.he windward slope acts as a precipi-
tation-inhibiting agent on the lee slope. In
rugged, irregular topography such as in western
United States, most slopes will exhibit windward
and lee characteristics at different times depending
on the storm path and circulation. The amount of
lift produced by a given flow with specific thermal
and humidity characu.ristics across an orographic
barrier is dependent, however, only on the height,
slope, and other topographic characteristics of
t.he barrier.
2.3.6 Lack of proper instrumentation pre-
cludes an accurate analysis of orographic effects
on precipitation intensities in storms. However,
computations based on reasonable assumptions of
wind field, drop-size distribution, and precipita.
tion-element. trajectories over a generalized barrier
indicau. that storm precipitation may be distrib-
uted an appreciable distance downwind from the
ridge. Moreover, precipitation profiles across an
orographic barrier may vary widely from storm to
storm. Figure 2-3 is a simplified schematic dia-
gram illustrating some of the physical processes
effecting these variations. It presents an idealized
cross sect.ion of a barrier such as the Sierra Ne-
vada., with a high plateau on the lee side.
2.3.7 The heavy lines (fig. 2-3) represent the
streamlines of air flow across the barrier. On the
left, or windward side of the ridge, points L and
H represent the bottom and top, respectively, of
the condensation or cloud layer. Precipitation-
formation rates throughout the layer are indicated
by the profile A. Dashed curves B, through B,
represent trajectories of falling raindrops or snow
crystals. Those formed at the higher elevations
are carried farthest downwind and fall on the lee
side. Those formed at lower altitudes fall on the
windward slope. Curve C presents a rongh indi-
cation of the precipitation distribution. Precipi-
tation which is produced on the windward side of
t.he barrier and falls on the lee side is called
spill-over.

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