FLOOD01363

• County: Statewide
• Title: Colorado High Plains Thunderstorm Systems - A Descriptive Radar-Synoptic Climatology
• Date: 6/1/1974
• Prepared By: CSU
• Floodplain - Doc Type: Educational/Technical/Reference Information

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Figure 1 presents the Denver, Colorado 1200GMT sounding for August 18, 1993. Points
A and B on the sounding reference the observed (predicted) temperature and dew points
respectively. If this "surface parcel" is lifted to the level of condensation, Point C, we get
the lower reference point for the calculation of the depth of the updraft warmer than Oc.
The moist adiabatis then followed until itcross the OC isotherm at point D. The depth of
the updraft is then noted in kilometers between Points C (3.5km) and D (5.2km) to arrive
at a value of 1.7 km. In this case the depth of the updraft's warm layer warmer than OC
is 1.7 km. It is thought that this layer represents the region of the cloud that is favorable
for the development of coalescence type precipitation mechanisms to dominate and
produce warm rainfall efficiently. Obviously mixed phase processes are operative also
but it should be noted that the warm type processes should be able to perform in this
layer on their own. Note that the PWI in this atmosphere was calculated to ee almost
1.00 inches.

The warm layer calculated above is used the equation below to calculate the potential
peak point 60-minute rainfall for the precipitating portion of a thunderstorm's reflectivity
pattern that is equal for greater than a 5-level. The equation is presented below:

(1) PWI x UWL / RWL x 2 = Peak 6O-minute rainfall where

PWI = Surface - 500MB precipitable water index
UWL = the calculated updraft warm layer
RFL = the regional warm layer

In the case above (1) equates to: 1.00" x 1.7km /1.5km x 2lhr = 2.27 incheslhr .
In essence an updraft achieving a warm layer of atleast or than 1.5 km in depth on this
day is capable of producing over 2,00 inches of rain in one hour! Thus the depth of the
warm layer is a measure of the opportunity for warm coalescence droplet growth to
dominate the rain formation process in the cloud.

The regional warm layer (RWL) varies with elevation and to some degree with location
across the United States. In general the values below have been used effectively to
accurately estimate QCP2:

Regional Warm Layer Depth (RWL) Changes with Elevation:

or

Elevation above 3,500 feet:

1.5 km

Elevations below 3,500 feet:

2.5 km

Sub-tropics/tropics (Dew pt > 70F)

3.5 km

To some no doubt this variation in the RL W will prove discomforting but appears to be
directly related to the typical vertical temperature updraft structure found in these areas.
Typically in regions at elevations above 3.500 feet a moist sub-cloud layer is overlain by

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