Walter L. Randeu

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.

2007 Special Issue: Improving weather radar estimates of rainfall using feed-forward neural networks

In this paper an approach is described to improve weather radar estimates of rainfall based on a neural network technique. Other than rain gauges which measure the rain rate R directly on the ground, the weather radar measures the reflectivity Z aloft and the rain rate has to be determined over a Z-R relationship. Besides the fact that the rain rate has to be estimated from the reflectivity many other sources of possible errors are inherent to the radar system. In other words the radar measurements contain an amount of observation noise which makes it a demanding task to train the network properly. A feed-forward neural network with Z values as input vector was trained to predict the rain rate R on the ground. The results indicate that the model is able to generalize and the determined input-output relationship is also representative for other sites nearby with similar conditions.

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.

A novel method for determining the mutual inductance for 13.56MHz RFID systems

A main parameter of a 13.56 MHz RFID system is the magnetic coupling factor k respectively the mutual inductance M. In this paper a new method for determining the mutual inductance is presented. This method is based on a two-port consideration to obtain the elements of the impedance matrix of the coupling system. The mutual inductance is calculated from the impedance matrix and the coilpsilas equivalent circuit values using a physical inductor model. Furthermore the system model presented in this paper includes the capacitive coupling between the two coils. Present methods and their disadvantages are described and the new method and its improvements are explained in detail. Finally, measurement results of a present method and of the new method are compared to EM simulation results.

Weather Radar Estimates of Rainfall Adjusted to Rain Gauge Measurements Using Neural Networks

Other than rain gauges which measure the rain rate R directly on the ground, the weather radar measures the reflectivity Z aloft and the rain rate has to be determined over a Z-R relationship. Besides the fact that the rain rate has to be calculated from the reflectivity many other sources of possible errors are inherent to the radar system. Worth mentioning are especially errors caused by the vertical profile of reflectivity (VPR). In this paper an approach is described to estimate ground rainfall using radar data based on a neural network technique. The results indicate that the relationship determined by the neural network model between VRP and rain rate measured on the ground, is also representative for sites nearby.

Enhancing the physical significance of rainfall breakpoints through two‐dimensional video distrometer data

[1] The temporal resolution of any rainfall model limits the degree to which it can physically represent the rainfall process. The recently developed hidden semi-Markov model (HSMM) of rainfall is a statistical model based on breakpoint data in which the progression of rainfall is well approximated by a succession of periods with steady rain rates. These periods are not of any fixed length, such as an hour, but can vary from being sub-minute for brief heavy showers to being many days for the dry period between events. Since the breakpoint periods have a duration precision of the order of seconds, the HSMM can follow changes at that timescale and so provide a physical representation of rainfall. This inference fails if the breakpoint data themselves have no physical significance which raises the question concerning what occurs at a breakpoint to cause a sudden change in the rain rate. The hypothesis advanced was that a breakpoint occurs at a change in the (rain) drop-size distribution (DSD) when the rate of rainfall also changes. The Wilcoxon statistic can be used to test for differences in distribution between two samples and time series of this statistic were extracted from data collected by the two-dimensional video distrometer (2DVD) on the size, shape, speed, and timing of individual raindrops. Changes in the DSD were located at significant values of the Wilcoxon statistic and these locations compared to those of the breakpoints extracted from the same data sets. Over the 96 daily data sets, the median time gap between a significant Wilcoxon and a breakpoint was about 30 s, and Monte Carlo methods showed that for nearly half of the data sets the median time gaps would only be smaller for 1% of all random re-distributions of the breakpoints. Thus the overall degree of the match between the significant Wilcoxon statistics and the breakpoints occurring by chance is negligible, and the hypothesis that breakpoints occur at DSD changes acceptable. Furthermore, a hierarchical view of the rainfall process is advanced in which the top levels can be attributed to the changing dynamics of the atmosphere while the breakpoint level is a reflection of the physical processes that form raindrops.

Cumulative Distributions of Rainfall Rate over Sumatra

The microwave radio links above 5 GHz suffer from attenuation due to precipitation. The need for employing higher frequencies has therefore encouraged research into rain attenuation due to precipitation. The natural variations of tropical precipitation occur in a wide range of time-scales, so does probably the behavior of radio communication links. This paper examines the variations of cumulative distribution of rainfall in Sumatra from an optical rain gauge measurement with a near continuous record of operation over eleven consecutive years (2002-2012). The worst month statistics were also examined and all results were compared with the ITU-R model. Of some natural variations of rainfall rate investigated, the diurnal variation had the most significant effect on the cumulative distribution of rainfall rate. The ITU-R model overestimated the rainfall rate for the first half of the day (00:00-11:59 LT) whereas it underestimated the rainfall rate until 0.01% of time for the second half of the day (12:00-23:59 LT) before the model starts to overestimate. The ITU model overestimated 52.85% of rainfall rate at 0.01% of time for the first half of the day and underestimates 7.59% for the second half. Considerable differences between the recorded data and the ITU-R model for the annual, seasonal, and intreaseasonal variations are only significant at small time percentage (≤ 0.01%). The relationship of worst month statistics was also slightly different from the ITU-R model. This result reinforces the previous studies on the limitation of the ITU-R model for the tropical region.

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%).

Near field antenna development for metal and semiconductor detection

Semiconductor detectors have been used in counter surveillance for many years. The principle should now be used for the detection of semiconductors (in electronic devices like mobile phones, transmitters, or tape recorders) carried by persons into or out of security areas. Therefore known system elements and components are to be integrated into walkthrough sluices. First outcomes and a prototype are described in this paper.

New Weather Radar Techniques: Ready for Operational Use?

Relevant new weather radar techniques are reviewed with respect to their operation principles, expected benefits and acceptance for use in operational networks.