Scattering Calculations at C-Band for Asymmetric Raindrops Reconstructed from 2D Video Disdrometer Measurements

Abstract

The distribution of raindrop shapes is well known to be important in deriving retrieval algorithms for drop size distribution parameters (such as the mass-weighted mean diameter) and rain rate, as well as for attenuation correction using the differential propagation phase constraint. While past work has shown that in the vast majority of rain events the most “probable” shapes conform to those arising primarily from the axisymmetric (2,0) oscillation mode, a more recent event analysis has shown that drop collisions can give rise to mixed-mode oscillations and that for high collision rate scenarios, a significant percentage of drops can become “asymmetric” at any given instant. As a follow-up to such studies, this study involved performing scattering calculations for 3D-reconstructed shapes of asymmetric drops using the shape measurements from a 2D video disdrometer (2DVD) during the above-mentioned rain event. A recently developed technique is applied to facilitate the 3D reconstruction from the 2DVD camera data for these asymmetric drops. The reconstruction requires a specific technique to correct for the drop image distortions due to horizontal velocities. Scattering calculations for the reconstructed asymmetric drops have been performed using a higher-order method of moments solution to the electric and magnetic field surface integral equations. Results show that the C-band scattering amplitudes of asymmetric drops are markedly different from those of oblate spheroids. The intention for future studies is to automate the entire procedure so that more realistic simulations can be performed using the 2DVD-based data, particularly for cases where collision-induced drop oscillations give rise to considerable numbers of asymmetric drops.

Analysis of Raindrop Shapes and Scattering Calculations: The Outer Rain Bands of Tropical Depression Nate

“…Regarding (i), even though it is assumed (e.g., [24]) that horizontal velocities of precipitation particles do not differ from prevalent wind velocities (e.g., [24,32]) during steady conditions, it is only the 2DVD instrument which is capable of providing direct observational evidence of this in natural rain. It is consistent with previous studies [11,33] from a line convection event and a hurricane event which showed excellent agreement of the 2DVD-based drop horizontal velocities at near-ground level with the wind measurements at 10 m height. Currently, a 3D sonic anemometer has been installed at the UAH site to measure the ambient winds and turbulent intensity…”

“…Because both the H and V radar cross sections for any given drop will vary with the look angle, its differential reflectivity will also have a φ variation. This has been illustrated in [11] in terms of the complex scattering amplitudes for both polarizations (see their Figure 7 where φ was varied from 0 to 360° in the horizontal plane).…”

“…It is the only disdrometer that gives the drop images in two orthogonal planes and the fall speed for each drop that falls in the 10 cm × 10 cm measurement area. Recently, the contoured shapes in the two orthogonal planes were used to derive the 3-dimensional particle shapes which have been described in [8,9,11]. The procedure involved in the drop contour derivation also includes a 'deskewing' technique which not only yields the corrected contoured shapes for each drop but also enables the magnitude and direction of the horizontal drop velocity to be determined.…”

“…A relatively recent advance has been the use of 2DVD in reconstructing the 3D shapes of natural rain drops even if they are asymmetric [8,9] and further to calculate the scattering matrices of such asymmetric drops using advanced electromagnetic scattering codes [10,11]. This enables simulation of what is termed 'drop-by-drop' integration to arrive at the radar reflectivity, the differential reflectivity and the copolar correlation coefficient [12].…”

Analysis of Raindrop Shapes, Fall Velocities, and Scattering Calculations during Tropical Storm Nate

“…A relatively recent advance has been the use of 2DVD in reconstructing the 3D shapes of natural rain drops even if they are asymmetric [6], [7], and further to calculate the scattering matrices of such asymmetric (and the detected symmetric shape) drops using advanced electromagnetic scattering codes [8], [9]. This enables simulation of what is termed 'drop-by-drop' integration to arrive at the radar reflectivity, the differential reflectivity and the copolar correlation coefficient [10].…”