The LaGrange Tornado during VORTEX2. Part I: Photogrammetry analysis of the tornado combined with single-Doppler radar data

This study presents a single-Doppler radar analysis combined with cloud photography of the LaGrange, Wyoming tornado on 5 June 2009 during VORTEX2 in an attempt to relate the hook echo, weak-echo hole (WEH), and rotational couplet to the visual characteristics of the tornado. The tornado damage was not extensive based on a post-storm survey and led to EF-2 rating. The circulation at low levels went through two intensification periods based on azimuthal shear measurements. The first intensification was followed by the appearance of a brief funnel cloud. The second intensification was coincident with the appearance of second funnel cloud that remained in contact with the ground until the tornado dissipated. A WEH within the hook echo formed before the appearance of a funnel cloud. The echo pattern through the hook echo on 5 June undergoes a dramatic evolution. Initially, the minimum radar reflectivities are near the surface (<15 dBZ) and the WEH does not suggest a tapered structure near the ground. Subsequently, higher reflectivities appear near the surface when the funnel cloud makes “contact” with the ground. During one analysis time, the increase of the radar reflectivity within the WEH at low levels results in a couplet of high/low radar reflectivity in the vertical. This increase in echo at low levels is believed to be associated with lofted debris although none was visibly apparent until the last analysis time. The WEH was nominally wider that the visible funnel cloud. The data set provided the first detailed analysis of the double ring structure within a hook echo that has been reported in several studies. The inner high reflectivity region is believed to be a result of lofted debris. At higher elevation angles, a small WEH appeared within the high reflectivity region owing to centrifuging of debris. A feature noted in past studies showing high-resolution vertical cross sections of single-Doppler velocity is an intense couplet of negative and positive values in the lowest few hundred meters. This couplet was also evident in the analysis of the LaGrange tornado. The couplet was asymmetric with stronger negative velocities owing to the motion of the tornado toward the radar. However, another couplet indicating strong rotation was also noted aloft in a number of volume scans. The decrease in rotational velocities between the low- and upper-level couplets may be related to air being forced radially outward from the tornado center at a location above the intense inflow.