WMOD00279

• Title: Applied Weather Associates Responses to Corps of Engineers Questions on the AWS Study of Cherry Creek PMP, Sept 2006
• Prepared For: USACE
• Prepared By: Applied Weather Associates
• Date: 1/29/2007
• County: Douglas
• Weather Modification - Doc Type: General Information

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Monument, Co 80132
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January 29,2007

Applied Weather Associates Responses to Corps of Engineers Questions on
the A WS study of Cherry Creek PMP, September 2006

1. Page xix. The definition of Precipitable water indicates the 300mb level is considered the top
of the atmosphere in this study. Was the precipitable water above 300mb truncated and not
used in the maximization and transposition adjustments?

A W A response to Ouestion 1,

The World Meteorological Organization (WMO) Manual for Estimation of Probable Maximum
Precipitation (1986) discusses precipitable water in Section 2.2.6,
page 14.
This (precipitable water) is a term, used mostly by hydrometeorologists, to express the
total mass of water vapour in a vertical column of the atmosphere. A statement that the air
contains 3 cm of precipitable water signifies that each vertical column of 1 cm2 cross-section
from the surface to the "top" of the atmosphere contains 3 g of water in vapour form. If the
water vapour were all condensed into liquid and deposited at the base of the column, the
accumulated liquid would be 3 cm deep, since the density of water is 1 g cm-3. Precipitable
water is, in fact, a misnomer, because no natural process will precipitate all the water vapour in
the atmosphere. For this reason, the substitute terms, "liauid equivalent of water vapour" or,
simply "liauid water equivalent. " are sometimes used.
Tables of precipitable water for saturated air with pseudo-adiabatic lapse rate as a
function of the 1 OOO-hPa dew point are presented in Annex 1. These tables are used for moisture
adjustments.

The WMO Manual has further discussions related to precipitable water in Section 2.3.2 Depth of
precipitable water, p 14,
.... In maximizing storm rainfall, only the depth of precipitable water from the ground to
some arbitrarily selected level from 400 to 200 hPa is used. The 300-hPa level is accepted
generally as the top of the storm, but it makes little difference which level from 400 hPa on up is
selected as there is very little moisture at those heights, and the effect on the moisture adjustment
is negligible. ....

A W A has historically used the 300mb level (the same as 300 hPa, approximately 30,000 feet) to
determine the precipitable water associated with the storm representative 1000mb dewpoint and
the maximum 1000mb dewpoint, consistent with the discussions in the WMO Manual. As