From: "Don MacGorman" To: ljmill@mmm.mmm.ucar.EDU Date: Wed, 27 Oct 1999 16:06:28 -0500 Subject: Re: Evaluate STEPS options CC: dave.rust@nssl.noaa.gov, wbeasley@ou.edu, conrad.ziegler@nssl.noaa.gov, jstraka@pig.gcn.ou.edu, rasm@ncar.ucar.edu Jay, Dave Rust and I have been working on a response, with input also from Conrad Ziegler. I don't think we say anything that others have not said in one form or another, but we still wanted to offer our input to the arguments against moving STEPS. First, the wording of the original question sounds as if the committee is dismissing 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. The motivating thrust of STEPS is the study of precipitation formation, lightning production, and electrification in severe storms. Although such data sets from nonsevere 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 answer the question as asked and dismiss the severe storms element (we will come back to it), there are problems acquiring the coincident data sets mentioned above if the program is moved farther west. The committee, by asking us to consider whether either the Denver WSR-88D or the Cheyenne WSR-88D could be used with S-POL and with CHILL at Greeley, appears to recognize the need for three good non-colinear dual-Doppler baselines (to avoid coverage problems along a single baseline) and for some triple Doppler capability. Cheyenne is too far from Greeley (roughly 80 km, instead of the 60 km planned by STEPS) to provide good dual-Doppler data from three baselines in a roughly equilateral triangle, much less to provide triple Doppler data. Denver may be a bit closer, but it still is farther than the desired 60 km. Even worse, Denver's traffic would impede or prevent STEPS surface operations in much of the region of the WSR-88D's coverage, getting airspace might be a problem over at least some of the region, and moving that far south moves the program even farther from the region in which supercell storms tend to occur. If the committee will grant that the main reason most investigators are planning to participate in STEPS is to study severe storms and that addressing STEPS objectives concerning severe storms is valuable, then the case against moving to Greeley becomes even more overwhelming. All the climatologies we have seen indicate that severe storms are much more likely east of Greeley, near the Colorado-Kansas border. This is also true of storms whose CG activity is dominated by +CG flashes. Obviously, severe 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, often being completely west of it. 2. Although orogenically initiated convection may account for a substantial fraction of severe storms in the 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 completely eliminate the possibility of an important ancillary experiment, study of sprite production, because sprite observing systems would be too close to see above storms 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, I 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 non-essentials 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 severe storms. 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. Regards, Don MacGorman and Dave Rust