FLOOD02086

• County: Gilpin
• Community: Black Hawk/ Central City
• Title: Black Hawk/Central City Colorado Radar Rainfall Reconstruction
• Date: 7/30/1998
• Prepared For: Gilpin County
• Prepared By: HDR Engineering, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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2.0 General radar-rainfall estimate methodology

The utilization of radar to estimate rainfall has been in use for over 30 years by meteorologists in
both the government and the private sector. In general, most current radar-derived rainfall
techniques rely an assumed relationship between the strength of the 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, thunderstorm rainfall events (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 estimated rainfall.

HDR meteorologists use their own method to solve these problems related to rainfall over and
under estimation, The HDR 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, HDR meteorologists have observed that the heaviest rainfall occurs when the
strongest radar reflectivity field passes over the rain gauges. Given the validity of this
assumption, the next step entails the calculation of the peak rainfall rate associated with the
precipitating cloud, which in turn can be related to the strongest radar reflectivity values,

Since late 1981, HDR meteorologists have used a combination of surface weather station data
and a 2-D cloud methodology to predict the peak rainfall rate associated with convective rainfall.
HDR has found that the depth of a thunderstorm's updraft that is warmer than 00 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.
Equation (2-4) shows a simplified form of this relationship:

(2) Peak 50-minute rainfall = PWI X (DeDth of uDdraft warm laver) X 2
1.5km

2

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