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<br />28 JULY 97 2115 MDT dBZ Z=1.2 km; VR Z=.3 kIn
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<br />due to a new cell growth (e,g"
<br />Chappell 1986), how might
<br />changes in the low-level flow
<br />have affected the sum of these
<br />two vectors in thc vicinity of
<br />FCL to produce zero net storm
<br />movement?
<br />A vailability of Doppler ra,
<br />dar data from the CSU-CHILL
<br />and KCYS-NEXRAD radars
<br />facilitated an examination of the
<br />coevolving horizontal wind
<br />field and precipitation structure
<br />using combined radial velocity
<br />estimates in a dual-Doppler syn,
<br />thesis (Figs, 12a,b), Due to the
<br />80-km baseline between CSU-
<br />CHILL and KCYS, the 70-km
<br />distance between KCYS and
<br />FCL, and beam curvature/re-
<br />fractivity considerations, the
<br />lowest level for which horizon,
<br />tal winds could be synthesized
<br />was I km, Reflectivity and ra,
<br />dial velocity data for both radars
<br />were interpolated to a Cartesian
<br />grid using a horizontal (vertical)
<br />spacing of 2 (I) km; then the
<br />two-dimensional wind field was
<br />computed for the l,km height
<br />level using CEDRIC software
<br />(Mohr and Miller 1983), Note
<br />that Dilly the horizontal winds
<br />were computed in the dual-Dop'
<br />pIer synthesis, and Dilly the I,km level will be dis-
<br />cussed in this study, No attempt was made to compute
<br />vertical velocity due to the long baseline and ineom,
<br />plete sampling of cloud tops by CS U-CHILL (maxi-
<br />mum elevation angle 80),
<br />Perhaps the most intriguing result of the dual,
<br />Doppler analysis is the apparent influence of a bow
<br />echo system (located 60-80 km southeast of FCL;
<br />Figs, 12a,b) on the mesoscale flow field affecting
<br />convection located over western FCL' Prior to
<br />2000 MDT, radar-measured winds in the vicinity of
<br />FCL were southeasterly at the I,km level (consistent
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<br />E- W of Taft and Drake (KM)
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<br />~ln the storm-relative framework of the bow echo (not shown), a
<br />cyclonic vortex existed on the northern end of the bow echo, with
<br />a weaker, but discernible anticyclonic vortex located on the south-
<br />ern end.
<br />
<br />Fro. 10. CSU-CHILL 2] 15 MDT28 July 1997: radial wind velocity (shaded in In S-I) at
<br />30G-m AGL and radar reflectivity (1.2 km AGL) contoured every 5 dBZ, beginning at
<br />20 dBZ The CSU-CHILL radar is located at an azimuth of ] 160 and -40-45 km from the
<br />origin (as in Fig. 9). Warm colors indicate outbound radial velocities (-easterly, southeast-
<br />erly), cool colors indicate inbound (-westerly) radial velocities. A portion of Spring Creek
<br />is also indicated by the bold blue line running through the origin. Note that wind data are
<br />missing on the west side of the figure where elevated terrain is located.
<br />
<br />Bulletin of the American Meteorological Society
<br />
<br />with winds near the 750-hPa level in Fig, 6), However,
<br />after 2000 MDT scattered convection located along the
<br />foothills of the Front Range to the south of FCL
<br />(Figs, 7a-d) began to organize and move slowly north-
<br />eastward, taking the form of a bow echo (e,g" Fujita
<br />1978; Weissman 1993; Weissman and Davis 1998),
<br />Dual,Doppler analyses (e,g" Figs, I2a,b) suggest that
<br />the low-level winds backed slightly in an area located
<br />along and to the west of the northern edge of the bow
<br />echo system (e,g" Figs, 12a,b, X = 40-60, Y = -20-5
<br />and X = 0-15, Y = 5-10) as it passed to the southeast
<br />of the CSU-CHILL radar. The change in low-level
<br />flow forced by the bow echo, combined with inflow
<br />associated with the convection over FCL, resulted in
<br />a narrow ribbon of flow dominated by an enhanced
<br />easterly wind component that terminated in convec,
<br />tion located over Spring Creek (Fig, 13),
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<br />203
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