The Bow Echo and MCV EXperiment (BAMEX): air and ground-based strategies

Figure 1

Figure 2

Bow echoes are one of the best-known modes of high surface winds associated with severe weather events. Bow echoes can be over 300km long, last for several hours, and generate mid-level cyclonic vortices that often consolidate and grow in scale to become a mesoscale convective vortex (MCV). MCV's can have diameters of 100-200km and often persist long after the parent convective system has dissipated. Some MCV's re-initiate convective storms for several days as they travel distances of 1000km or more. BAMEX, a field experiment involving scientists from NSSL, NCAR, the NWS, and OU, seeks to understand both of these related mesoscale phenomena.

BAMEX proposes to use three aircraft: two Doppler-radar-equipped turboprops, and a high altitude jet for dropsonde deployment (Figure 1). Once inside the mesoscale convective system (MCS), the turboprop aircraft will use Doppler radars and in-situ data to map the three-dimensional circulation of these systems, particularly the "rear-inflow" jet and the relationship between convective line structure and surface wind. Deployed dropsondes, according to Project Director Chris Davis of NCAR, will provide unique observations of the vertical thermodynamic structure, which will be crucial in determining why some systems produce damaging surface winds while other similar systems keep their winds aloft. Dropsondes will also help map the thermodynamic structure of the developing and mature MCV.

In addition to the aircraft, a mobile array of ground-based instruments will be used to probe the structure of bow echoes, document the thermodynamic structure of the primary boundary layer, and any existing convergence boundaries, probe the surface cold pool, and measure surface horizontal pressure and wind variations behind the leading convective line (Figure 2). Some ground-based observing systems being proposed for BAMEX include two SMART-Radar truck-mounted radars, a mobile profiling system (MIPS), two mobile GPS sounding systems, and several mobile mesonet vehicles. "The combination of aircraft and ground-based measurements is important for understanding the coupling between boundary-layer and free-tropospheric circulations within MCS's, and, in particular, how the rear-inflow penetrates to the surface in nocturnal severe wind cases," said co-PI Mike Biggerstaff of OU.

The spatial distribution of these events is clearly centered in the midwest United States with some extension into the Ohio Valley (especially bow echoes). Hence, BAMEX operations will be based at the St. Louis, MO NWS office, and aircraft will be based at Mid-America airport east of St. Louis in Illinois. BAMEX forecasts will be provided by a team of experts drawn from St. Louis and surrounding NWS offices and the SPC in Norman, OK. The domain for operations will be approximately 650km radius of St. Louis, or a 1.5 hour ferry for the turboprop aircraft. Once airborne, aircraft scientists will be in constant contact with Operations Center personnel through an aircraft-satellite 9600-baud data link. This link will keep scientists updated on the rapidly-evolving weather through voice, e-mail, and image transfers. Communications with ground teams will be via conventional cell phone. BAMEX is scheduled from May 20, 2003 until June 6, 2003.

By Dave Jorgensen and Jeff Trapp

More information about BAMEX can be found on the Web at: http://www.mmm.ucar.edu/bamex/science.html, and in NSSL Briefings Volume 4 Number 4.