Abstract
Tropical storm Nate, which was a powerful hurricane prior to landfall along the US Gulf coast, traversed north and weakened considerably to a tropical depression as it moved near an instrumented site in Hunstville, AL. The outer rain bands lasted 18 h (03:00 to 21:00 UTC on 08 October 2017) and a 2D-video disdrometer (2DVD) captured the event which was shallow at times and indicative of pure warm rain processes. The 2DVD measurements are used for 3D reconstruction of drop shapes (including the rotationally asymmetric drops) and the drop-by-drop scattering matrix has been computed using Computer Simulation Technology integral equation solver for drop sizes >2.5 mm. From the scattering matrix elements, the polarimetric radar observables are simulated by integrating over 1 min consecutive segments of the event. These simulated values are compared with dual-polarized C-band radar data located at 15 km range from the 2DVD site to evaluate the contribution of the asymmetric drop shapes, specifically to differential reflectivity. The drop fall velocities and drop horizontal velocities in terms of magnitude and direction, all being derived from each drop image from two orthogonal cameras of the 2DVD, are also considered.
Highlights
One of the important applications of polarimetric radar is the measurement of rainfall whose accuracy depends critically on the assumed drop shape model [1]
In panel (c), we show the percentage of deviation of the drop fall velocities from the expected terminal fall speeds of Gunn–Kinzer [25], again for all drops >2 mm
A plane excitation source was defined with linear polarization and a frequency of 5.625 GHz
Summary
One of the important applications of polarimetric radar is the measurement of rainfall whose accuracy depends critically on the assumed drop shape model [1]. The wind-tunnel experiments of Szakáll et al [3] showed that larger drops (>2.0 mm) oscillate primarily in the axisymmetric oblate-prolate mode with smaller amplitude asymmetric modes mixed in These wind-tunnel data (based on high speed camera images of different sized suspended drops for a few seconds) were consistent with the earlier 80 m fall ‘artificial rain’ experiment of drop shapes imaged as individual ‘snap shots’ with a 2D-video disdrometer (2DVD) [4,5,6]. 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].
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