Photogrammetric Analysis of Rotor Clouds Observed during T-REX

One of the key objectives of the T-REX field campaign, which took place in 2006 in the lee of the Sierra Nevada in eastern California, was to explore the origin, dynamical structure and temporal evolution of atmospheric rotors. On several days during T-REX, a largely stationary rotor cloud was observed over Owens Valley, in the lee of the Sierra Nevada. Small cloud fragments often formed just upstream of this stationary cloud, moving towards the main cloud and eventually being subsumed by it. Studying the evolution of these upstream cloud fragments could provide significant new insights into the structure and dynamics of the airflow upstream of and within the rotor. However, because of the clouds' small spatial scale and high temporal variability they are difficult to investigate with traditional tools such as Radar and Lidar.

Stereo photogrammetric analysis is a little utilized but highly valuable tool for studying smaller, highly ephemeral clouds. In this study, we make use of stereo digital photographs that were collected during T-REX but little used thus far. The data set consists of matched stereo pairs of photographic images obtained at high temporal (on the order of seconds) and spatial resolution (limited by the pixel size of the cameras). Using computer vision techniques we have been able to develop algorithms for camera calibration, automatic feature matching, and ultimately reconstruction of 3D cloud scenes. Applying these techniques to the small scale cloud fragments at the upstream edge of the rotor cloud we can follow their evolution in 3D space and time until their merger with the main cloud in great detail. To quantify their physical properties and dynamics such as growth, horizontal and vertical movement, allows new insights into the composition and parameters (e.g. the 3D structure of the lifting condensation level) of the highly turbulent environment at the upstream edge of rotor clouds.