FLOOD01375

• County: Larimer
• Title: Mesoscale and Radar Observations of the Fort Collins Flash Flood of 28 July 1997
• Date: 2/1/1999
• Prepared By: American Meteorological Society
• Floodplain - Doc Type: Flood Documentation Report

per kilometer) is proportional to the
product of the mass, weighted hydrom,
eteor axis ratio and the precipitation
liquid water content (Jameson 1985),
and is derived from the range derivative
of the differential propagation phase
between vertically and horizontally po-
larized radiation, Also, KDP tends to in,
crease from 00 km-1 to several degrees
per kilometer in regions of substantial
precipitation liquid water content.
Qualitatively, LDR provides informa-
tion on the hydrometeor phase, shape,
and orientation by measuring the relative
amount of depolarized power (in dB) re-
turned from scatterers in a given radar
pulse volume, Pulse volumes with mix,
tures ofliquid water and ice (e,g" mixed
phase) or canted, irregularly shaped par,
ticles all act to increase the LDR. In the
case of the FCL flood, ZDR' KDP' and LDR
were collected by the CSU-CHILL ra-
dar and when combined with reflectiv-
ity and CG lightning data, provide
insight into the microphysical character,
istics of the flood convection,
To set the framework for discussion
of the polarimetric observations, first
consider the time-height series of KCYS
mean radar reflectivity computed for a
stationary cylindrical volume (lO,km
radius, l4-km height) centered on Taft
Hill and Drake Roads in FCL, shown in
Fig, 15 (28 July 19971725-2225 MDT),
Note that Fig. 15 also includes a time
series of rain mass flux (CSU-CHILL
radar estimate; see section 7d) and marks
indicating the time of occurrence of in-
dividual CG lightning flashes, Three pri-
mary pulses in convective rainfall
separated in time by approximately 1-
1.5 h are apparent in Fig, 15 (peaks at
minutes 85, 155, 260), The life cycle of

FIG. 12. Dual~Doppler-derived horizontal
wind vectors (m 5-1) and radar reflectivity (shaded)
for the l,km height level at (a) 2110 MDT 28 July
1997 and (b) 2130 MDT 28 July 1997. Grid ori-
gin as in Fig. 9. Vector scale (m 5-1) is indicated
in the upper-right comer.

Bulletin of the American Meteorological Society

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