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Studies of raindrop shapes, oscillation modes, and implications for radio wave propagation are presented. Drop shape measurements in natural rain using 2-D video disdrometers (2DVDs) are discussed. As a representative exception to vast majority of the cases where the “most probable” shapes conform to the axisymmetric (2,0) oscillation mode, an event with a highly organized line convection embedded within a larger rain system is studied. Measurements using two collocated 2DVD instruments and a C-band polarimetric radar clearly show the occurrence of mixed-mode drop oscillations within the line, which in turn is attributed to sustained drop collisions. Moreover, the fraction of asymmetric drops determined from the 2DVD camera data increases with the calculated collision probability when examined as time series. Recent wind-tunnel experiments of drop collisions are also discussed. They show mixed-mode oscillations, with (2,1) and (2,2) modes dramatically increasing in oscillation amplitudes, in addition to the (2,0) mode, immediately upon collision. The damping time constant of the perturbation caused by the collision is comparable to the inverse of the collision frequency within the line convection. Scattering calculations using an advanced method of moments numerical technique are performed to accurately and efficiently determine the pertinent parameters of electrically large oscillating raindrops with asymmetric shapes needed for radio wave propagation. The simulations show that the scattering matrix and differential reflectivity of drops are dependent on the particular oscillation modes and different time instants within the oscillation cycle. The technique can be utilized in conjunction with 3-D reconstruction of drop shapes from 2DVD data.

[1] Individual drop images in natural rain from a two-dimensional video disdrometer (2DVD) have been processed to obtain their shapes, sizes, and orientation angles, which, in turn, have been used to derive their complex forward scattering amplitudes at 19.701 GHz. Using these calculations, the specific attenuation for horizontal and vertical polarizations and the specific differential attenuation and the specific differential phase of all drops over 1 min are determined. A point-to-path scaling is used to derive the total rain attenuation for both polarizations as well as the total differential attenuation and the total differential phase. A 19.701 GHz experimental Earth-space link is simulated to determine the variation of cross-polar discrimination versus copolar attenuation (XPD-CPA) and the calculations are compared with 2 years of joint probability XPD-CPA beacon measurements. The comparisons indicate clearly that the variations in drop shape and drop orientation can give rise to significant spread in XPD.

[1] This paper presents the results and analyses of Doppler radar measurements taken as part of a long-term campaign conducted in Singapore. Eight months of data taken in the zenith-pointing mode have been analyzed, in order to examine the validity of the assumptions underlying the retrieval algorithm proposed for the precipitation radar on board the TRMM satellite. Stratiform and convective events were analyzed separately. The former exhibits a clear melting layer in the 4–5 km height region, defined by an enhancement in the radar reflectivity, an increase in the mean fall velocity from 1–2 m/s in the snow region just above the melting layer to over 8 m/s in the rain region just below it. The corresponding width increases from less than 1 m/s to greater than 1 m/s. The variation in all three parameters can be accurately represented by the Noncoalescence-Nonbreakup melting layer model used for the satellite radar retrieval algorithms. Also examined was the monthly variation of the reflectivity-height profiles. In the rain region below the melting layer, a consistent decrease in the radar reflectivity with decreasing height is seen, indicating that the drop size distribution (DSD) cannot be assumed constant over all heights. However, if only the parameter N0 of the DSD is assumed constant, then good agreement is obtained between the mean velocity estimated from the reflectivity data and the measured velocity throughout the height profile. The agreement is also obtained in the spectrum width profile in the rain region. Convective precipitation on the other hand, does not exhibit a melting layer, and its averaged dBZ and velocity profiles cannot be characterized even under the constant N0 assumption. Its averaged reflectivity-height profiles show more variability than stratiform precipitation. For such cases, factors such as up and down drafts and nonstandard atmosphere need to be taken into account. The implication for the so-called N0 adjustment method is that while it can be applied to stratiform precipitation, the convective events need additional information, such as dual-frequency radar measurements, in order to improve the accuracy of the rainfall estimates.