Marzuki

Diurnal Variation of Rain Attenuation Obtained From Measurement of Raindrop Size Distribution in Equatorial Indonesia

The measured rain rate, raindrop size distribution (DSD), and the ITU-R model over the frequency range from 1-100 GHz have been used to elucidate the cumulative rainfall rate and the variability of rain attenuation at Kototabang. Rain rate and DSD are recorded from ground-based optical rain gauge and disdrometer measurements, respectively. Considerable differences between the recorded data and the ITU-R model are observed at small time percentage. The specific rain attenuation obtained from the DSD measurement shows diurnal variation with the largest attenuation observed in the morning hours. This characteristic is due to the raindrop spectra of rain events in this period containing more small-sized drops (<2 mm) than at others as described by the largest contribution of these drops on the specific rain attenuation. The diurnal variation is serious for frequencies higher than 60 GHz especially in very extreme rain.

Raindrop Size Distribution Parameters of Distrometer Data With Different Bin Sizes

A 2D video distrometer (2DVD) provides raindrop size distribution (DSD) at nominal drop diameters that correspond to the mean of the bin sizes. Selection of bin width may influence the shape of DSD. Therefore, we investigated the effect of binning on the DSD parameter estimates. First, we studied the effect of binning by examining their ability to recover known parameters of simulated DSD. Second, real DSD data collected in the equatorial region by 2DVD were analyzed. We compared the DSD parameters calculated from binned DSD with those calculated from a drop-by-drop data basis. Both simulated and real DSDs were binned at 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, and 0.50 mm. In general, the DSD parameters increased with increasing bin width. With very large number of raindrop which should be accompanied by heavy rain, the bias due to bin width selection is small. However, the bias is significant in the opposite case. The average fractional error between a mass-weighted mean diameter (Dm) calculated from DSD and that derived from drop-by-drop data was relatively small for all rainfall rates. A rather high error was observed in the median volume diameter (D0) which may be due to moment method and interpolation error. Finally, using small bin widths (0.20-0.30 mm) may be the best choice because the DSD parameters of these bin widths were very close to those obtained from drop-by-drop data.

Complex Permittivity Measurements of Rainwater in the 0.5–26.5 GHz Frequency Range

Modeling electromagnetic wave propagation in rain requires knowledge of the complex permittivity of rainwater. In response, we measured the complex permittivity of rainwater in the 0.5-26.5 GHz frequency range using an Agilent Technologies 85070E Dielectric Probe Kit and an Agilent N5242A-400 Vector Network Analyzer. Rainwater samples were collected in Graz (Austria) and Kototabang (Indonesia). The results obtained were found to differ slightly from those of Rays and Liebes models. However, the difference in the complex permittivity of rainwater between the measurement and model results exhibits very small biases in the Mie extinction coefficients ( <; 0.01%).

Estimation of Raindrop Size Distribution Parameters Using Rain Attenuation Data from a Ku-Band Communications Satellite

The rain attenuation of down-link radio wave signals from the Superbird-C satellite and surface rainfall data have been used to estimate the parameters of exponential raindrop size distribution (DSD) at Koto Tabang (100.32 ° E, 0.20 ° S), West Sumatra, Indonesia. Prior to analyzing the measured data, the ability of the method to recover the parameters of known DSDs from which the samples were taken was examined. It was found that the method can accurately retrieve the input parameter of the sample. Only six case studies are presented here, so the results are representative rather than definitive. The method successfully estimated the DSD parameters of a stratiform case with steady intensity and deep convective rains of a short duration. This can be inferred from the small difference between the parameters derived from rain attenuation data and those derived from a 2D video disdrometer. The poor performance of the method was observed for a stratiform case with strong rain intensity fluctuation and shallow convective rains with very low rain top height. This phenomenon is probably due to the bias that may be inherent in the estimation of specific rain attenuation, such as the assumption of a constant path length throughout the rain.