High resolution airborne radar dual-Doppler technique

An airborne-based meteorological radar adds new dimensions to remote sensing of clouds, extending the coverage of ground-based installations and allowing for more direct comparison with in situ data. The Wyoming Cloud Radar (WCR) on-board the University of Wyoming KingAir research aircraft is a 95 GHz Doppler radar capable of quasi simultaneous multi-beam scanning. A technique is presented to analyze and merge the data collected by pairs of WCR beams (directed 30 deg apart) leading to two-dimensional wind field syntheses in the horizontal or vertical planes. The gridding methodology can be customized depending on the type of flight pattern. The grid can be made to strictly follow the actual scanned surface, or it can be simplified to plane surfaces, with a resolution between 30 and 45 m. A weighted least-squares method is employed to solve the velocity inverse decomposition problem for each cell. The results of the technique are illustrated in two case studies of cumulus cloud kinematics. The robustness of the technique is demonstrated by the retrieved fine-scale velocity fields: although revealing highly complex flow structures, they are characterized by a high degree of smoothness despite each grid data point being the result of a single independent calculation, and no assumption of mass continuity.

A detailed analysis of the error sources affecting an airborne dual-Doppler synthesis is offered focusing in particular on the accuracy achievable with the proposed technique. The upper-bound values together with a methodology to combine the uncertainties in the measured radial velocities into the error in the dual-Doppler retrieved velocity are estimated. However, the characteristics of the target and its environment (e.g., cloud microphysical structure, mean winds, shear) directly affect elements such as the width of the Doppler power spectrum, or the temporal uncertainties deriving from target advection and evolution in the time lapse between beam illuminations. Idiosyncrasies of the radar system design, the data collection process, and of the aircraft navigation devices also influence the final accuracy. The quantification of the uncertainties and their distribution is realized for a case study illustrating the kinematic field in a vertical transect of a cumulus cloud.