First, the wording of the original question sounds as if the OFAP/FAC may not be considering much that was a major motivation for STEPS. We are not seeking coincident collection of radar data, lightning mapping data, and electric field data in just any type of storm, but in severe storms, particularly LP-MP-HP supercell storms and severe storms having frequent positive cloud-to-ground lightning. Although such data sets from other types of storms could be used to address some of the STEPS objectives concerning polarimetric radar and a few aspects of objectives concerning lightning phenomenology and electrification, a program addressing these elements alone would be a fundamentally different program than was proposed for STEPS.

Even if we do not consider the supercell storm element (we will come back to it), there are problems acquiring the coincident data sets mentioned above if the program is moved farther west. OFAP/FAC, by asking us to consider whether either the Denver or the Cheyenne WSR-88D radar or the CSU Pawnee research Doppler radar could be used with S-Pol and with CHILL at Greeley, appears to recognize the need for three good non-collinear dual-Doppler baselines (to avoid problems with deriving Doppler winds along any single baseline) and for some triple-Doppler capability. Cheyenne (KCYS) is farther from Greeley than the baseline in the network designed by STEPS (roughly 80 km, instead of the nominal 60 km planned by STEPS), so resolution would be poorer than desired, though coverage would be comparable in size to that proposed by STEPS. Denver (KFTG) is close to the desired distance, but air traffic into and out of DIA would make getting airspace difficult over much of the region of coverage, and highway traffic would impede or prevent surface vehicles from operating in the Denver metropolitan area during late afternoon, when the probability of storms is largest. The baseline between Greeley and the Pawnee is only 48 km, which provides a much smaller region of coverage than desired. However, the total coverage for every one of these options is much smaller than the region designed by STEPS, because roughly half of the region of coverage is over or near the front-range of the Rocky Mountains. STEPS cannot operate in mountainous regions, and severe storms are unlikely to occur there. We think the resulting coverages are too small.

If the S-Pol radar were placed nearly east of CHILL, this would compromise the multi-Doppler measurements but would move part of the coverage from over the mountains to over the high plains of the front range. However, coverage would not extend eastward beyond about Akron Colorado which will typically be well into the dry-air side of the usual location of the dry line nearer the Colorado-Kansas border. Furthermore, since the dominant track of most storms is west-to-east, the north-south orientation of the dual-Doppler lobes would mean that convective storms could be studied only while they are crossing one or the other of lobes. On the other hand, the flexibility of siting both CHILL and S-Pol near Goodland allows us to orient the dual-Doppler, dual-polarimetric lobes in such a way that coverage by the research radars extends more along the general direction of travel of storms. This then allows us to study some storms as they track eastward from their inception or invigoration near the dry line on into Kansas as they continue to intensify as moister air is encountered.

If OFAP/FAC will grant that the main reason most investigators are planning to participate in STEPS is to study supercell storms or severe storms having frequent positive ground flashes and that addressing STEPS objectives concerning these types of severe storms is valuable, then the case against moving to Greeley becomes even more overwhelming.

All the climatologies we have seen indicate that both types of severe storms are much more likely east of Greeley, near the Colorado-Kansas border. (The relative maximum in severe weather reports in north-central Colorado is due partly to population bias; supercell storm and positive CG climatologies show much larger frequencies of occurrence near the Colorado-Kansas border and eastward than near Greeley.)

Obviously, supercell storms can occur further west, since STERAO was able to observe at least one severe storm that appears to have gone through a supercell phase. However, our chances of observing the main targets of the program (supercell storms and +CG dominated storms, though other types of storms will also provide useful data) are diminished considerably by moving as far west as Greeley. Because we expect to observe only a few supercell storms even in the most favorable region we could find, it seems to us that we would seriously jeopardize achieving our objectives by moving the field program to avoid moving CHILL.

This argument isn't based only on climatological statistics. There are physical reasons the region farther east is more favored for the storms STEPS is targeting:

1. The dryline is an important ingredient in the formation and growth of many severe and supercell storms. Some of the planned scenarios for observing precipitation formation and electrification early in a supercell storm's lifetime rely on making observations near the dryline and then following storms as they move farther east and intensify. Thus, the dryline and the region east of the dryline are important for STEPS observations. Greeley is far enough west that it is a marginal location for observing this region, typically being completely west of it.
2. Although orogenically initiated convection may account for a substantial fraction of severe storms in the proposed STEPS target area, these storms typically do not become the supercell storms we are targeting until they approach the STEPS region. Atmospheric conditions in regions as close to the front range as Greeley tend to be unfavorable for producing strong supercell storms. Deeper moisture, greater convective instability, and deeper hodograph curvature (all key airmass requirements for supercell severe storms) typically occur over the planned STEPS domain much more frequently than over Greeley.

   Note also that moving as far west as Greeley would seriously harm an important ancillary experiment, study of sprite production, because sprite observing systems would be too close to see above most storms in the operations region and because mesoscale convective systems with stratiform regions large enough to cause sprites almost never occur that far west in Colorado.

   If adequate funds simply cannot be found to support all the major facilities requested for STEPS, then we STEPS investigators will have to decide if the program can be viable with fewer resources. As a matter of courtesy, we think the investigators as a group should be allowed to decide where to cut, instead of being asked to consider only a single option. However, STEPS has already cut what investigators could readily perceive as nonessentials and has cut back the duration of the field program more than many investigators thought was wise. We may have reached the point where there is not much more room for cutting without destroying the essence of STEPS.

   In summary, moving the field program to accommodate not moving CHILL would seriously compromise our ability as a community to achieve many of the primary objectives proposed for STEPS, including all those concerning supercell storms and storms with frequent positive ground flashes. We question whether having STEPS near Greeley would be a wise use of the considerable resources that would be required besides CHILL. We think the impact would be so great that a field program making good use of the proposed resources in that region should have a different set of objectives, few of which would overlap the original objectives of STEPS, except as low-probability targets of opportunity.

   Respectfully,
   STEPS Participants