Scattering calculations for rain drops undergoing asymmetric, mixed mode, oscillations and their impact on polarimetric radar variables

Calculations of polarimetric radar variables for rain often assume ‘fixed' rain drop shapes depending only on the equi-volume drop diameter, Deq. Whilst such calculations are adequate when drops conform to the most-probable shapes, they need to be examined if and when significant fraction of drops (particularly the large ones) have shapes which are substantially different, for example, as a result of frequent and sustained drop collisions. A recent study using 2D video disdrometer (2DVD) data and polarimetric radar observations for one particular event found that the fraction of moderate-to-large drops (Deq > 3 mm) that do not possess rotational symmetry increases with collision probability. Additionally, collision-induced drop oscillations were also investigated using an advanced wind-tunnel facility which clearly highlighted mixed mode, large amplitude oscillations immediately upon collision which can last up to several hundred milliseconds. Details of such investigations including field experiments and wind-tunnel measurements can be found in Thurai et al. (2014).

As a follow-up to such studies, we are currently involved in performing scattering calculations for (a) simulated drop shapes and (b) 3D-reconstructed drop shapes, with particular emphasis on asymmetric drop shapes. For (a), different analytical oscillation modes specified in Thurai et al. (2014) are combined in a randomized fashion in order to represent collision-induced drop oscillations. For (b), the shape measurements from the 2DVD for the abovementioned rain event are used. Note that, in order to reconstruct the shapes for drops which do not possess symmetry axis, we first need to know the horizontal velocity vector for each of these drops. To this end, we output all known horizontal velocities (from both 2DVD cameras) for drops of the same size that do have symmetry axis and for which the data processing algorithm can determine these velocities, and this is then interpolated in time for the asymmetric drops. The velocity vectors so derived are then used for correcting the measured contours in the x-z and the y-z planes for each individual drop, and the corrected contours are subsequently used to construct the corresponding 3D shapes.

In this paper, we will present both the theoretically simulated shapes and the 3D reconstructed shapes from the 2DVD measurements in natural rain. Scattering calculations for such drops will also be presented, together with the implications for polarimetric radar variables. The scattering simulations were performed using the efficient and accurate higher order method of moments in the surface integral equation formulation (Djordjević and Notaroš, 2004).

References:

Thurai, M., V. N. Bringi, A. B. Manić, N. J. Šekeljić, and B. M. Notaroš, 2014: ‘Investigating raindrop shapes, oscillation modes, and implications for radio wave propagation', Radio Sci., 49, 921–932.

Djordjević, M., Notaroš, B. M., ‘Double higher order method of moments for surface integral equation modeling of metallic and dielectric antennas and scatterers', IEEE Transactions on Antennas and Propagation, 2004, 52, (8), 2118-2129. 04, 52, (8), 2118-2129.