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<br />wind and rain mass flux after 2030 MDT,
<br />consistent with the hypothesis that local
<br />enhancements in the easterly wind re,
<br />suIted in a corresponding increase in
<br />heavy rainfall over FCL Caracena et al.
<br />(1979) noted a similar correlation be,
<br />tween changes in the easterly flow and
<br />corresponding changes in rainfall for the
<br />BT flood,
<br />The resultant quasi, stationary nature
<br />of the storm during the last 1.5 h of the
<br />event appeared to be the result of an in,
<br />crease in the magnitude of the system
<br />propagation vector, which, in the net, ex-
<br />tended toward the south,southwest. The
<br />appearance of new cells and heavy rain
<br />along the southern flank of the storm was
<br />likely the result of interaction between
<br />the enhanced, moist east-southeasterly
<br />flow, a weak low-level outflow bound-
<br />ary (Fig, 10), and local topography
<br />(Fig, 2), Hence, the sum of cell move-
<br />ment and system propagation vectors
<br />resulted in a quasi, stationary storm (e,g"
<br />Miller 1978; Chappell 1986),
<br />
<br />FIG. 11. Cloud-ta-ground lightning detected between 1800 and 2300 MDT on
<br />28 July 1997. Grid origin as in Fig. 9. Colors represents CO flash locations de-
<br />tecLed in I,h intervals, J 800-1900 (btue), t 900-2000 (purple), 2000-2tOO (green),
<br />2100-2200 (red), and 2200-2300 MDT (black), The locations of negative and
<br />positive polarity CO flashes are indicated by a (-) and (+), respectively. The CO
<br />flash 'counts by polarity are indicated in the upper-right comer of the figure. The
<br />small box embedded within the larger grid encloses the area plotted in Fig. 9.
<br />
<br />Radar reflectivity data (Figs, 9d-i) indicate that the
<br />last group of convective cells and heavy rainfall be,
<br />gan 10 stall over southwestern FCL after 2030 MDT,
<br />nearly coincident with the onset of increased low,level
<br />east,southeasterly flow. To further illustrate the rela,
<br />tionship between the increased easterly wind compo,
<br />nent and the onset of heavy convective rainfall after
<br />2030 MDT, a time series (2032-2215 MDT) of line-
<br />averaged easterly wind speeds and the relative devia-
<br />tion of those speeds from the value at 2032 MDT was
<br />computed utilizing the dual, Doppler analyses, The
<br />mean easterly wind components were computed along
<br />a 25 km long north-south,oriented line, located at x
<br />= 15 and extending from y = 10-15 in Fig, 12, The
<br />relative deviation and mean of the easterly wind com-
<br />ponent for each time was then overlaid on a time se,
<br />ries of rain mass flux for the FCL storm computed
<br />from CSU-CHILL polarimetric radar data (cf, section
<br />7d), The resultant time series (Fig, 14) exhibits a high
<br />positive correlation (r = 0,91) between the easterly
<br />
<br />204
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<br />c, Combined polarimetric and
<br />NEXRAD observations of stonn
<br />vertical structure
<br />Observations of deep convection us'
<br />ing dual-polarized radar have provided
<br />new insights into the microphysical and electrical de,
<br />velopment of convective clouds (e,g" Goodman et at
<br />1988; Bringi et aL 1997; Jameson et aL 1996;
<br />Ramachandran et aL 1996; Carey and Rutledge 1996,
<br />1998), Combinations of multiparameter variables such
<br />as differential reflectivity (ZOR)' specific differential
<br />phase (KDI')' and linear depolarization ratio (LDR) pro,
<br />vide information on hydrometeor size, shape, and ther-
<br />modynamic phase (the presence of oblate raindrops,
<br />hail, melting, etc,; cf, Jameson and Johnson 1990;
<br />Doviak and Zmic 1993), thereby reducing the ambi,
<br />guities involved with inferring cloud microphysical
<br />properties based solely on radar reflectivity (2),
<br />For example, ZOR is the ratio (expressed in dB) of
<br />reflectivity measured at horizontal polarization (ZH)
<br />to that of vertical polarization and provides informa,
<br />tion on the reflectivity' weighted mean hydrometeor
<br />axis ratio, The presence of large, oblate particles (e,g"
<br />large raindrops) tends to increase the ZnR from near
<br />zero to several decibels, The Kol' (expressed in degrees
<br />
<br />Vol, 80, No, 2, February 1999
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