The following mobile mesonet log was written by Dr. David Blanchard.
Our target area was just east of Byers, Colorado, where the DCVZ crossed US36 - the same area we operated in last week. A rapidly growing Cb was located at the intersection of the DCVZ and US36. How convenient! A circulation at cloud base was present as we approached the cloud and soon thereafter a nearly transparent dust column became visible. The two mobile mesonets did multiple transects along a north-south road ahead of the approaching landspout. As the weak tornadic circulation approached the road, both mobile mesonets carefully positioned themselves to collect proximity data. After the vortex crossed the road, both mesonets resumed their north-south transects collecting data behind the circulation. We intercepted this circulation again on another north-south road one mile to the east. This weak tornado lasted for 13 minutes.
A second cloud base swirl was quickly forming to the southwest and moments later another landspout formed. This circulation was multi-vortex in nature and the strength of the ground circulation was much stronger than the first, so a greater distance was maintained as the circulation crossed the road. As before, both mesonets collected data ahead and behind the tornadic circulation which lasted 7 minutes. Neither tornado produced a prominent dust cloud or a funnel cloud.
First tornado - At 2130 UTC, ~25 minutes before the first tornado, the parent vortex associated with the first tornado was resolved in the velocity data at the 0.5° and 1.5° elevation scans. The vortex was embedded within the Denver convergence vorticity zone. The circulation had a height of 1.4 km and a diameter of no more than .75 km. A developing updraft existed ~ 6 km east of the vortex with a reflective core maximum of 20 dBZ located at a height of 6.1 km arl.
Over the next 30 minutes the velocity couplet associated with the vortex moved east away from the boundary while it become entrained in the storm’s updraft. The updraft rapidly increased in height with its echo top by 2159 UTC at a height of 15.2 km. The storm top divergence velocity signature was now resolved in the data, at ~61 kt. Additionally, by 2159 UTC the vortex associated with the tornado had ascended within the updraft to a height of 3048 m arl. Note, cross sectional analysis of the velocity data revealed the velocity couplet titled with height toward the east. The tilt in the radar data is consistent with the mobile mesonet crew’s visual observations.
At 2205 UTC the largest shear was sampled associated with the velocity couplet; 0.5° 44 kt gate-to gate shear, 1.5° 54 kt shear across 1.6 km. However, it was near this time when the tornado was observed to dissipate. At 2211 UTC the velocity couplet associated with the first tornado had nearly dissipated in the 0.5° and 1.5° elevation slices concurrent at a time when a westward surging outflow boundary produced by the storm was undercutting the updraft (although not shown, the same was true at the higher elevation tilts ).
Second tornado - The velocity couplet associated with the parent circulation of the second tornado was resolved beginning at 2211 UTC (Note, unlike the base velocity shown above, storm relative velocity was needed to best resolve the second vortex). The circulation was ~4.8 km southwest of the first vortex.. The circulation extended to a height of 2242 m arl with the largest shear at resolved in the 1.5° radar elevation scan; 32 kt gate-to-gate shear. The velocity signature dissipated by 2217 UTC. The brief duration of the circulation (>10 minutes) was consistent with the visual observations of the mobile mesonet crew; the second tornado persisted for seven minutes. Unlike the first vortex, the second vortex associated with the second tornado could not be traced to originally reside within the DCVZ. Furthermore, it could not be ascertained whether the velocity couplet ascend with time within its parent updraft.
Summary - On 23 July 1998 no Denver cyclone was identifiable in the radar or surface data. The first of two vortices was resolved in the radar data ~25 minutes prior to the first tornado. When first resolved, the vortex was found to reside within the DCVZ. Over the 25 minute time period preceding the tornado, the velocity couplet associated with the vortex became entrained in and ascended within a developing updraft. The vertical extension of the vortex was coincident when the updraft exhibited rapid vertical development. Unlike the first vortex, the second vortex associated with the second tornado could not be traced to originally reside within the DCVZ. Furthermore, it could not be ascertained whether the velocity couplet ascend with time within its parent updraft.
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