SubVORTEX-RFD Mission Log

Brief Summary of SubVORTEX-RFD Missions (May-July 1998)

980513 - This was a dry run mission to work out any problems. The emphasis was on "dry" as no convection formed. One vehicle transmission blew a seal and left an oil slick on I-40. A replacement vehicle will be ready for pickup on Monday morning.

980514 - A strong, negative-tilt wave and large convective instability were present in western Oklahoma and NW Texas. We feared the dynamics would favor a squall line and not supercells. We were right, and chose to abort the intercept shortly after convection was initiated. The combination of late day, low visibilities from smoke, and a squall line made intercepts difficult. TAMU launched a rawinsonde near Clinton, Oklahoma, early in the afternoon. TAMU and NSSL1 launched two rawinsondes from Erick, Oklahoma, for intercomparison of sondes. NSSL1 launched a rawinsonde south of Childress, Texas, shortly after gust front passage. PROBE vehicles crossed the dry line numerous times doing east-west traverses along I-40 and again while travelling south on US-83 towards Childress. Dewpoints dropped from 15-17C to 9C in 6-12 seconds (6 s sampling rate). Visibilities changed from ~3 miles on the moist side to ~7-10 miles on the dry side.

980517 - Our target was the dry line in the Texas panhandle. As in previous days, we were plagued by low visibility from the smoke making visual identification of targets difficult. Since no significant convection developed before dark, this was not a problem. Two mobile mesonet vehicles made multiple passes through the dry line collecting high resolution data on this feature. Not suprisingly, the structure was quite complex and showed large variations from one pass to the next. Sounding data from AMA indicated that there was little or no CIN left and convection should have initiated during the afternoon. This case should provide some interesting data for the analysis of dry line structure at the surface.

980519 - The VORTEX armada left Norman mid-morning for Nebraska. Our eventual target was a supercell near Sydney, Nebraska, late in the afternoon. The fortuitous location of I-80 and US-30 gave us ample opportunity to make repeated passes through the rear flank downdraft and inflow region of the storm until darkness ended operations. Excellent lighting conditions provided opportunities for video data collection to complement the mobile mesonet observations.

980520 - The targeted area was northeast Colorada and by late afternoon a good candidate supercell was intercepted in Yuma County. A lack of paved roads limited our options but numerous north-south passes were made ahead of and behind the supercell with excellent data collection in the RFD region of the storm. Good video data was collected from the south and southwest of the storm to complement the mobile mesonet observations.

980521 - Initial target was southwest Nebraska, but a long wait was required for initiation. A surge of cool air from day-long convection in northern Nebraska complicated the picture, but a strong supercell was intercepted near Cambridge, Nebraska. The RFD was very strong and downward motion of cloud tags was impressive. Multiple passes by all mobile mesonet vehicles was accomplished on the few paved roads and occassional dirt road (and we were reminded once again how tricky dirt roads can be when wet). Video data was collected as well as DOW radar data. Data collection was terminated shortly after dark.

9805122 - Target area was between Salina and Hutchinson, Kansas, along the warm front. A long waiting game ensued and operations were finally terminated during the late afternoon when it was clear convection would not form in time for operations. The DOW radars set up a dual-Doppler baseline to observe the wind shift boundary and the mobile mesonets made numerous transects across the wind shift line collecting high temporal and spatial resolution surface data. This should provide an interesting case for studying frontal boundaries and, in this case, the lack of convection along it.

980523 - Operations were terminated shorly after leaving Norman owing to deteriorating surface conditions in the target area.

980524 - The target area was northwest Oklahoma where upslope moisture advection was occurring and the shear profile would support supercells. By late in the afternoon it was clear that the atmosphere had other ideas as numerous supercells formed in south central Kansas and moved southeast into northern Oklahoma. VORTEX targeted a developing storm near Medicine Lodge and made numerous passes with all the mobile mesonet vehicles. Data collection was terminated when it became too dark to work. All vehicles carefully navigated their way home through the numerous supercells that continued to develop during the evening hours. The NOAA P-3, as part of MEaPRS, targeted these nocturnal storms and the developing mesoscale convective complex for a night of data collection.

980529 - VORTEX started the day in Salina, Kansas, after a travel day on 980528. The outlook was for increasing upslope flow and advection of moisture north of the front into western Kansas, western Nebraska, and eastern Colorado. By early afternoon, it became evident that advected moisture was mixing out and the likelihood of convection was small. Operations were terminated and the caravan travelled to Kearney, Nebraska, to set up for operations on the following day.

980530 - The forecast models and data all suggested that there would be supercells in northwest Iowa and that initiation would occur early in the day. A departure time of 8 a.m. was set and the caravan headed east into Iowa. By 1245 pm, the first tornado warning was issued for a fast moving cell that we hoped we could intercept in a few hours. We continued north and east toward Ames, Iowa. Continuous tornado warnings were issued for this storm for many hours as it travelled southeast at close to 40 mph. As we moved in for the intercept southeast of Ames, the storm went from 70 dBz to 55 dBz to 35 dBz in three successive scans and quickly dissipated. Post analysis showed the storm had moved south of the jet core into a region of warm temperatures aloft and the loss of CAPE and increase of CIN contributed to its quick demise. VORTEX turned back west but was unable to make an intercept in South Dakota.

980604 - A very strong cold front was nearly stationary across portions of Oklahoma and Texas. Cool, dry air north of the front was very stable, while hot, humid air south of the front was very unstable. The target area was north Texas where convection was expected to become supercellular. VORTEX targeted these storms but none of the convection was able to maintain supercellular characteristics for more than a brief period. Large dewpoint depressions and a relatively high LCL and LFC lead to storms with strong outflows. Within a few hours of initiation, convection had organized into a linear system and continued to grow upscale into a mesoscale convective system. The VORTEX mobile mesonets got a well deserved washing from the heavy rain encountered.

980607 - VORTEX crews intercepted a supercell south of Melrose, NM and worked the storm as it moved northeast into Texas. Operations were finally halted because of darkness as the storm moved through Dimmitt. Visibility was generally poor with light fog and dust obscuring the view on the inflow side. We consistently saw SE/25-35 winds on the inflow, and W/30-50 winds in the RFD. Cloud base was very low and made visual observations difficult. However, while the storm was still southwest of Melrose, visibilities were much greater, cloud base was higher, and structure was impressive.
Another great NULL case! Now, can we please have a non-null case?

980608 - Convection initiated quickly along the rapidly moving dry line in western Oklahoma. Operations began near Cordell, Oklahoma, and continued until the rapidly moving storm sped out of reach as it approached El Reno, Oklahoma. No long-lasting, organized, low level mesocyclone was noted. A second storm was intercepted later in the evening when new convection developed along the dry line and moved over the boundary left by the earlier convection. This storm was LP in character for much of its early life cycle, becoming more CL or even HP as it matured and moved into the OKC metro area shortly after sunset. Despite being in a metro area, PROBE vehicles were still able to gather data from this supercell as it crossed the city.

980609 - VORTEX targeted numerous supercells in north Texas during the afternoon. Most of these storms were outflow dominated and a large outflow boundary began to undercut all new convection. The data from these storms will be used to test the various hypothesis concerning supercells elevated above a cold dome.

980622 - VORTEX crews were mobilized to intercept storms that were forecast to form on an old outflow boundary (from the previous nights' MCS activity). It was hoped that these storms would move northeast and into both better shear and better moisture. As is often the case, the atmosphere had other ideas. Convection formed on the foothills and moved very slowly to the east. This air was considerably drier than was necessary for supercells. VORTEX played a waiting game until the storms moved farther east. By late afternoon, one cell had begun to acquire supercell characteristics and intercepts began. A lack of good westbound roads limited our options but numerous north-south transects were completed in the inflow region. The storm diminished in strength during the early evening hours and operations were terminated at sunset. Shortly thereafter, the cell moved into better moisture and became severe once again with large hail and significant rotation.

980623 - ANOTHER GREAT NULL CASE! A supercell moved to the southeast along and over US26 (how convenient!) near Broadwater and Lisco, Nebraska, during the evening and we sampled it extensively. Tornado-strength low-level rotation was noted on several occasions, but no tornado formed. VORTEX observed an interesting variety of precipitation in the RFD's. An early transect indicated tiny droplets with no real discernible divergence signature at the surface. On a later transect, there was moderate rain and 1" hail, with a very dramatic divergence zone (on the order of a few hundreds of meters scale) colocated with an intense surface anticyclone in the hook precipitation. It's fairly certain that we were in a anticyclonically flared hook immediately west of the surface cyclone position. The RFD was "scuddy", and the anticyclonic shear/rotation in the RFD area was very pronounced ... often tornado strength. In fact, we saw about 6-12 laminar anticyclonic funnels in the RFD periphery.

980701 - An excellent data set was collected today from a developing Denver convergence vorticity zone (DCVZ). An early eastward transect along US36/I70 found NW and NE winds almost all the way out to Strasburg. A repeat westbound transect indicated that strong SE winds were quickly pushing westward, tightening up the gradient along the convergence zone. Simultaneous transects were done along US36 and Road 88, approximately 7 miles north. Winds were generally weaker along the northern route.
Later, a weak outflow boundary from convection farther south moved north along the existing DCVZ and initiated new convection over DIA. This quickly became severe with 1"+ hail encountered.
Examination of radar data upon return indicated that the Denver cyclone was very well established and cyclonic turning of winds could be seen from the foothills eastward out to Byers - a distance of about 60 miles. Some scale contraction of the cyclone was noted during the course of the afternoon. By early evening, the cyclone had collapsed into a strong convergence zone that was located just east of DEN.

980709 - Another DCVZ mission was conducted today. Radar data from early afternoon indicated a reflectivity gradient associated with the boundary although velocities were generally less than 10 knots on either side. The two MM vehicles completed an eastbound transect along CO52 between I25 and I76 and found only weak convergence and a diffuse boundary. Winds were NW to NE on the west side of the boundary and E on the east side. A reverse transect showed little change. A slight enhancement of the NW flow began during the mid-afternoon and some convective growth was noted on the convergence line. The mobile mesonets completed two box patterns of the convergence line along CO52 and CO7 on the north and south, and I76 and I25 on the east and west. A sharp convergence zone was noted on both of these passes and convection quickly grew into cumulonimbus. Operations were terminated after the second box pattern since the area was quickly becoming dominated by multiple outflows from widespread convection.

980715 - A large and diffuse Denver Convergence Vorticity Zone (DCVZ) began to contract during the day and over the course of a few hours became a very tight convergence line that pushed to the west. There was plenty of vorticity present along the line with southeast winds of 10-20 knots on the east side and northwest winds of 5-10 knots on the west side. As the DCVZ contracted, the wind shift line and associated band of large vertical vorticity exhibited an abundunce of dust whirls, some of which reached a considerable height. CAPE was marginal so that stretching of the ambient vorticity into landspouts was generally unsuccessful.
Despite the low CAPE, some storms were able to form on the boundary and ultimately produced their own outflow that was added to the existing boundary.
The DCVZ also had a distinct circulation center located north of DIA in Adams County during the early part of the day. This circulation also underwent a scale contraction from ~15-20 km to about 5-10 km in diameter and moved north into western Weld County.
Mobile Mesonets made numerous east-west transects along I-70 during the early stages as we attempted to locate the boundaries. As it became clear that the boundary was strengthening and moving westward, we completed some box patterns that extended well east and west of the boundary.

980716 - The Denver convergence vorticity zone (DCVZ) was present again today. Unlike yesterday, the boundary remained relatively stationary during the afternoon. The mobile mesonets intercepted the boundary on US36 just east of Byers. Over two dozen transects were done during the course of a few hours. Most notable was the strong southerly wind on the east side, and the weak northwest wind on the west side. The transition between these two wind environments was on the order of 100 meters. Dewpoints were about 10-11C on the east side and 4-5C on the west side. The transition of the dewpoint from moist to dry also occurred on this short spatial scale.
Many shear eddies were encountered. In these eddies we found easterly winds on the north side and westerly winds on the south side. Moisture was advected from the east side of the DCVZ boundary onto the drier west side via the eddies. We also found eddies on the west side of the DCVZ that had become completely detached from the boundary and still contained high dewpoint air. At least a few of the eddies managed to converge enough angular momentum to produce strong dust whirls in their central region.
This case represents a potential leap in the understanding of the small-scale structure of shearing boundaries and the processes through which mixing occurs along the boundary and how moisture is lofted vertically through the vortices.

980723 - Severe weather often occurs a day or two after frontal passage in Colorado and today proved to be no exception as high dewpoints and deep moisture were present over eastern Colorado. By noontime it became evident that the Denver Convergence Vorticity Zone (DCVZ) would establish itself later in the day. Earlier missions had demonstrated that during periods of shallow moisture the DCVZ would manifest itself as a dry line. Our intent was to collect transect data of the DCVZ during a period of deep moisture. We also wanted to test a hypothesis that landspouts are more likely to form if both sides of the DCVZ are moist.
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.

980724 - Another Denver Convergence Vorticity Zone (DCVZ) was present today in a very moist, moderately unstable airmass. The first few transects indicated that the wind shift line was broad and diffuse and was displaced a fair distance from the convection. Within an hour, a band of convection began to rain hard. Because there has been heavy rain in this area for a few days in a row, many dirt roads are muddy and difficult to navigate. With the sudden deluge of precipitation, the mission was scrubbed early.

980729 - A DCVZ was in place early on this day in a moist and conditionally unstable atmosphere. We first intercepted the boundary eight miles east of Byers on US36, where we encountered a sharp 6-8 degree C dewpoint gradient and wind shift. Of the two mobile mesonets, one executed brief and very slow east-west transects along the moisture gradient, while the other performed longer transects covering both the windshift and moisture gradient.
The moisture gradient was found to be on a scale of 10s of meters across. The wind shift however, was more defuse and consisted of 8-16kt southerly winds with 5-10kt northerly winds over a distance of 1-1.5km. The sharpness of the boundary did not change or move during the first hour and a half. Deep convection did develop northeast of our location, producing a strong southwestward moving outflow boundary. We transected the outflow boundary as it passed over us on US36. Once passed we followed the outflow boundary westward on US36 and made over a dozen transects along it east of Bennett where it slowed and mixed out. At this point a three degree Centigrade dewpoint gradient and 90 degree wind shift denoted the boundarys position.

980731 - This was another Denver Convergence Vorticity Zone (DCVZ) intercept. Atmospheric conditions were moist with a moderate cap in a weak 0-6 km shear profile. The westward moving DCVZ was first intercepted two miles west of Strasburg on US36. Over the course of three hours over two dozen east-west transects were completed along US36. A 3 degree C moisture gradient (15-18C) over a distance of 0.3 km was initially observed and sustained throughout the sampling period. Strong southerly surface winds to 24kts were observed on the east side of the convergence zone, with 10kt northerly flow on the west. This wind shift varied in width from 0.3km-1km. Several shear eddies were encountered on the west side of the boundary, as well as two meso lows moving northward along the boundary. Most notable was that the convergence zone would move westward at 10-13m/s for several kilometers before becoming stationary. The boundary would remain stationary for 20-25 minutes before resuming its westward track. We observed this cycle several times.
Convective temperature was not realized on this day. A weak shortwave moving across northern Colorado helped to initiate deep convection over the foot hills. This convection sent out an eastward moving ouflow boundary and avil that would force convection along the DCVZ but, given a warm temperature regime aloft, the TCU quickly glaciated and dissipated. No severe activity was associated with this DCVZ.

980821 - A DCVZ mission to Bennett. East-west transects were conducted along US36 through a defuse DCVZ. The best deltaV across the boundary was 25 kts spanning 2km. The moisture gradient 3.5 C dewpoint extended over a 1.5 km distance bounded by the wind shift. Transects concluded when an outflow boundary, generated by deep convection south of the target region, moved up and intersected the DCVZ. Limited road options and construction prevented most attempts at transecting the outflow. Several well defined vortices 10-15 m in width developed along the DCVZ/outflow intersection