New ABL measurements of Lagrangian relative dispersion by means of radiosonde clusters

Turbulent relative dispersion is a phenomenon of fundamental interest both for its theoretical implications and for its immediate applications, which in geophysical sciences range from pollutant spreading in the atmosphere to nutrient transport in the oceans. We present new results in the measurement of turbulent relative dispersion in the atmospheric boundary layer, which enrich the picture with respect to the current framework. The measurements were carried out using clusters of miniaturized radiosondes, carried by small (~40 cm in diameter) helium balloons [1]. These clusters enable the effective investigation of relative dispersion by computing inter-particle distances among radiosondes. This methodology represents a concrete attempt at realising the type of analysis originally conceived by L. F. Richardson in his 1926 paper [2], often regarded as the one that initiated the field of study of relative dispersion.The current accepted framework for the discussion of relative dispersion is the Kolmogorov-Obukhov scaling theory, which on dimensional grounds allowed for the derivation of the result (called the Richardson-Obukhov law) according to which the mean square distance in between particles advected by a turbulent flow field scales like the cube of time, , where ε is the energy dissipation rate and g is called the Richardson constant. However, this result is only valid for the case of homogeneous, isotropic turbulence, specifically in the inertial range of scales [2, 3]. Atmospheric turbulence, instead, is far from homogeneity and isotropy, and is characterized by local intense intermittency and entrainment [4, 5].We conducted six cluster launches across three distinct topographical environments: the near-maritime plains at Chilbolton Observatory, the western Alps near the Astronomical Observatory of Aosta Valley, and the hilly region surrounding Udine. The results reveal not only deviations from the RO law but also significant variations between launches and distinct dispersion regimes within each launch (Fig. 1). The implication is, as expected, that the dispersion law for the atmosphere does not have a universal character, and depends on specific details of the boundary layer flow. The next step in the analysis will be the identification of the relevant flow features which impact the dispersion law, which is especially challenging due to the wide range of possibly participating phenomena.Fig. 1. Mean square separation distance between mini-radiosondes within the cluster during six field experiment flights in different environmental topologies. Cross symbols show results from the MET OFFICE NAME dispersion model [6].1. Abdunabiev S. et al., Measurement 224, 113879 (2024)2. Richardson LF, Proc. R. Soc. Lond. A 110, 709 (1926)3. Devenish, BJ, Thomson DJ. JFM 867, 877–905 (2019)4. Van Reeuwijk M, Vassilicos JC, Craske J. JFM 908 (2021)5. Fossa’ L., Abdunabiev S., Golshan M., Tordella D., Physics of fluids 34, (2022)6. Turbulence_&_Diffusion_Note_288, Met Office, UK (2003)