Universal fluctuations in tropospheric radar measurements

Radar data collected at an experimental facility arranged on purpose suggest that the footprint of atmospheric turbulence might be encoded in the radar signal statistics. Radar data probability distributions are calculated and nicely fitted by a one-parameter family of generalized Gumbel (GG) distributions, Ga. A relation between the wind strength and the measured shape parameter a is obtained. Strong wind fluctuations return pronounced asymmetric leptokurtic profiles, while Gaussian profiles are eventually recovered as the wind fluctuations decrease. Besides stressing the crucial impact of air turbulence for radar applications, we also confirm the adequacy of Ga statistics for highly correlated complex systems. Copyright c © EPLA, 2010 Radar sensors are currently employed in a large variety of applications, e.g. surveillance and target identification [1], weather observations and forecasting [2], and geophysical investigations [3]. It is therefore of paramount importance to have a clear control on all possible sources of external disturbances, which could significantly alter the measurements response. In 2006, within the activities of the GALAHAD project [4], a large collection of ground-based radar data was recorded, and soon realized to constitute a unique opportunity for investigating the impact of external perturbations on the sought response. It was in particular observed that occasionally, but in coincidence with hard windy conditions, the radar interferograms were affected by an overall decorrelation. Motivated by this unexpected finding, it was hypothesized that the atmospheric turbulence could drastically influence the radar propagation even over short ranges, an observation which however rested on purely qualitative ground. No further experimental realizations were in fact subsequently developed (a)E-mail: [email protected] to scrutinize the process and reach an unambiguous proof of concept. At variance, on the theoretical side, pioneering studies on electromagnetic signal propagation through a turbulent atmosphere [5,6] date back to the 1970s. Punctual fluctuations of air refractive index materialize in a recorded imprint, which could therefore encode an indirect signature of turbulence. Indeed, the embedding medium can be ideally segmented in neighboring volume cells, each contributing to the signal according to an overall trend and its own statistics. The signal impinging on the junction between two adjacent cells is partly reflected and partly transmitted and the data collected at the receiver is sensitive to the sequence of scattering events occurred along the propagation path. In this sense, the radar echo is a global quantity indirectly representing the specific state of the crossed medium. A comprehensive interpretative framework for such phenomena is however still lacking, following the inherent difficulties to accommodate for the intimate, highly complex nature of radar/air interactions. In particular, the characteristic of air-induced fluctuations remain to be fully elucidated.