FLOOD10114

• County: Saguache
• Stream Name: Saguache Creek
• Basin: Rio Grande
• Title: Radar Rainfall Reconstruction of Saguache Creek Flash Flood
• Date: 7/25/1999
• Prepared For: CWCB
• Prepared By: HDR Engineering, Inc.
• Floodplain - Doc Type: Flood Documentation Report

Since late 1981, HMS has used a combination of surface weather station
data and a 2-D cloud methodology to predict the peak rainfall rate associated
with convective rainfall. HMS has found that the depth of a thunderstorm's
updraft that is warmer than O. Celsius is directly related to the rain-production
potential of the cloud, When the warm depth of the updraft exceeds 1.5 km in
Colorado, for instance, the rain-production potential of the cloud doubles,

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radar reflectivity and the intensity of the rainfall rate. This relationship is
described by the equation (1) below:
(1) Z = A Rb

Where, Z is the radar reflectivity in dBZ, A is an empirically derived co-efficient
related to the cloud phYSiCS of the storm cloud water droplets and b is another
empirical co-efficient related to the type of storm cloud present. This
relationship has proven to produce highly variable results. Since the values of
both A and b are variables that must be assumed, opportunities for errors in the
calculation are possible.

The algorithms used to estimate the rainfall are standard for use around
the country and have not proven to be responsive to local cloud variations, The
r-squared or "goodness" of the rain to radar reflectivity statistical relationship has
varied from 0,15 to 0.90 on a daily basis and for most storm seasons has been
about 0.60.

The good r's (values >0.75) have been for the low volume and low
intensity rain events (stratiform rainfall), generally those of less than 0.25"/hr
accumulation rates. The high intensity, high volume, thunderstorms (convective
rainfall) have shown r-values of 0,15 to 0.45. Thus the standard prOducts
appear to be unreliable at this point. The storm rainfall has been both
overestimated and underestimated for periods of less than three hours for
storms within 25 miles of each other,

Finally, hail "contamination" of the equation has proven to be a
troublesome problem to deal with as well. Since the strength of the radar signal
is related objectively by the algorithm to the estimated rainfall, the strong radar
return value of hailstones will usually cause an over-estimation of the rainfall
rate.

HMS uses its own method to solve these problems related to rainfall over
and under estimation. The HMS method uses the radar reflectivity to locate the
portion of the precipitating cloud where the heaviest rainfall is located rather
than to calculate a rainfall rate. In over 90 percent of the operational heavy rain
days in the Urban Drainage & Flood Control District since 1985, HMS has
observed that the heaviest rainfall has occurred when the strongest radar
reflectivity field passes over the rain gauges. Given the validity of this
assumption, the next step is to calculate the peak rainfall rate associated with
the storm, which can in turn be related to the strongest radar reflectivity values.