|
<br />a)
<br />
<br />
<br />
<br />monsoonal flow likely promoted high
<br />precipitation efficiencies due to mini-
<br />mal entrainment of dry air with its
<br />associated evaporation in developing
<br />convection, Further, the depth of the
<br />"warm" layer between the cloud base
<br />(-700 m AGL) and the freezing level
<br />(-3,6 km AGL) was nearly 3 km, fa,
<br />cilitating the production of rainfall via
<br />warm-rain collision-coalescence,
<br />Once formed, high boundary layer hu-
<br />midities coupled with low cloud bases
<br />likely prevented substantial evapora,
<br />tion of precipitation below cloud base,
<br />Additionally, the Fort Collins
<br />storm formed in a region characterized
<br />by I) focusing mechanisms for con,
<br />vection such as orography and outflow
<br />interactions; 2) a veering wind profile
<br />between the surface and 3 km; 3) mod,
<br />erate southwesterly flow aloft (Fig, 6);
<br />and 4) low-level advection of moist,
<br />high,8, air by an increasing upslope
<br />flow (Figs, 8a,b), From a convective
<br />organization viewpoint, these factors
<br />led to an environment that was higWy
<br />conducive to the development of "train-
<br />ing" convection (cf. section 7a) and
<br />the development of a quasi-stationary
<br />rainfall system (e,g" Miller 1978;
<br />Chappell 1986; Doswell et a!. 1996),
<br />
<br />7. Radar observations
<br />
<br />Data collected by the KCYS-
<br />NEXRAD (closest NEXRAD to FCL;
<br />5-min volume scans; VCP, I I mode)
<br />and the CSU-CHILL (dual-polarized
<br />volume scans, 5-15-min intervals)
<br />radars were used to document 3D precipitation struc-
<br />ture, evolution of the low-level mesoscale wind field
<br />(l,km height level), and to create storm total rainfall
<br />maps using a variety of radar rainfall estimators for
<br />comparison to the rain gauge data (e,g" Fig, 3b),
<br />
<br />FIG. K Cloud drift winds derived from GOES,9, 28 July 1997: (a) 2016 (1416)
<br />and (b) 2315 UTC (1715 MDT), Winds barbs are ploued in knots; half barb = 5 kt
<br />(2.5 m S-l), full barb = 10 kt (5 ill S-I). The location FCL is indicated.
<br />
<br />observed in the BT case; e,g" Caracena et a!. 1979)
<br />was nearly coincident with an increase in convection
<br />along the northern and central sections of the Front
<br />Range (Fig, 8a,b) and just prior to the first pulse of
<br />rainfall over the FCL area (e,g" Fig, I),
<br />Numerous precursors in the storm environment
<br />shaped the character and motion of the convection,
<br />including a warm, moist soulh,southweslerly flow
<br />above the boundary layer, a 10w,Ievel east,southeast-
<br />erly wind, a relatively low CAPE and LFC, and high
<br />humidity through the depth of the sounding, The moist
<br />
<br />a, Overview of storm evolution on the evening of 28
<br />July 1997
<br />The first two episodes of heavy rainfall on the
<br />evening of 28 July 1997 occurred between 1800 and
<br />2000 MDT (e,g" Fig, I) in association with two small
<br />
<br />200
<br />
<br />..
<br />
<br />Vol. 80, No, 2, February 1999
<br />
|