Michael Schönhuber

Drop Shapes, Model Comparisons, and Calculations of Polarimetric Radar Parameters in Rain

Abstract Drop shapes derived from a previously conducted artificial rain experiment using a two-dimensional video disdrometer (2DVD) are presented. The experiment involved drops falling over a distance of 80 m to achieve their terminal velocities as well as steady-state oscillations. The previous study analyzed the measured axis ratios (i.e., ratio of maximum vertical to maximum horizontal chord) as a function of equivolumetric spherical drop diameter (Deq) for over 115 000 drops ranging from 1.5 to 9 mm. In this paper, the actual contoured shapes of the drops are reported, taking into account the finite quantization limits of the instrument. The shapes were derived from the fast line-scanning cameras of the 2DVD. The drops were categorized into Deq intervals of 0.25-mm width and the smoothed contours for each drop category were superimposed on each other to obtain their most probable shapes and their variations due to drop oscillations. The most probable shapes show deviation from oblate spheroids for De...

A wideband, directional model for the satellite‐to‐indoor propagation channel at S‐band

This paper presents a hybrid empirical-statistical model for the satellite-to-indoor propagation channel at S-band derived from measurements using a helicopter to simulate the satellite. The measurements have been carried out by means of a wideband, circularly polarized channel sounder in a SIMO (Single Input Multiple Output) configuration, allowing the derivation of entry loss, wideband and spatial model parameters. It is hoped that this paper will provide relevant information on the satellite-to-indoor channel given the large amount of experimental data available, covering a significant member of different buildings of various types, and the amount of measurement configurations: elevations and azimuths. Copyright

Attenuation of Spruce, Pine, and Deciduous Woodland at C-Band

The presence of vegetation in the radio path significantly attenuates centimeter and millimeter radio waves. The attenuation depends on the type of vegetation, its density, the season, and the frequency. Attenuation models exist for the horizontal path through vegetation and the slant foliage path through roadside trees. In this study, slant foliage paths through roadside woodland are investigated. This letter presents results for the slant path attenuation loss at 5.2-GHz C-band frequency at three elevation angles (20°, 40°, and 60°) and three types of vegetation (spruce, pine, and deciduous woodland). Differences between horizontal and slant path results are discussed, and cumulative distribution statistics of the attenuation in the various vegetation types are given.

Building entry loss and delay spread measurements on a simulated HAP-to-indoor link at S-band

Results from a measurement campaign emulating the high altitude platform (HAP)-to-indoor communication channel at S-band are presented in this paper. A link was established between a transmitter carried by a helicopter, representing the HAP, and a receiver placed at several locations in different building types including an airport, an office building, a shopping mall, a residential house, and a skyscraper. A wideband, directive channel sounder was used to measure building entry loss and time delay spread. Results of the building entry loss are presented as a function of building type, elevation, and building entry angle. Results of delay spread for each building are also provided.

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.

Estimation of Millimeter Wave Attenuation Due to Rain Using 2D Video Distrometer Data in Malaysia

Hilbert Vibration Decomposition (HVD) is introduced to the voltage flicker analysis. When voltage flicker accompanies with high order harmonics, the instantaneous frequency of its analytic signal in principle consists of two different parts, power frequency and a rapidly varying asymmetrical oscillating part. The important property of the instantaneous frequency offers a direct way to estimate the power frequency using a low-pass filter and remove the high order harmonics without pre-treatment procedures. Corresponding voltage flicker envelope is estimated using synchronous detection. The HVD method does not involves basic functions that the wavelet transform method needs. It can also adaptively estimate the frequency and amplitude of every modulation frequency component. Simulation results prove that the proposed method could accurately detect voltage flicker with high order harmonics. It has higher calculation efficiency and detection precision than wavelet transform method. Experimental results show that the new algorithm is feasible and efficient.

Drop shape and size distributions in rain from 2-D video disdrometer and propagation calculations at 20 GHz

Drop shapes derived from the 2-dimensional video disdrometer are presented in terms of probability contours. They show good agreement with the Beard-Chuang model for equivalent diameters up to 6 mm. A fitted equation is used for T-matrix calculations to derive the specific attenuation for horizontal and vertical polarizations at 20 GHz. When compared with the calculations using approximated oblate spheroids, good agreement is obtained for most cases. However, unusual drop size distributions (DSDs) with large median volume diameter can give rise to some differences and for such cases, the fitted equation would need to be used. Also evaluated is the impact on cross-polar discrimination versus co-polar attenuation variation. For the earth-space scenario considered here, no significant difference is observed between the calculations using the contoured shapes and their oblate spheroid approximations. Both show good agreement with 1-year of beacon measurements at 19.7 GHz.

Improved characterisation of precipitation type and intensity with polarisation diversity radar

Examples of data acquired with the frequency-agile dual-polarisation C-band weather radar Graz/Hilmwarte are presented. Having been developed and built by IAS for ESA/ESTEC, this radar is in use for systematic studies of precipitation-induced microwave propagation impairments on Earth-satellite links. Striking results regarding ice-water discrimination, bright band analysis and radar-derived rainfall intensity are given.

Averaging aspects in short term forecasts of tropospheric attenuation

At cruising altitude airborne satellite connectivity is unproblematic since no clouds or precipitation occur above the aircraft. The critical portions of the flight are directly after take-off and before landing, where especially rain can severely influence microwave signals. In short term forecasts of precipitation high rain peaks may not be resolved due to the inherent averaging of forecasts. Here we present an empirical study to illuminate the deviations of forecasted data from high resolution rain rate measurements of a distrometer situated in Graz, Austria. The forecasts are simulated by averaging over the measured data to create ideal conditions. The focus lies on the averaging effects only; the forecast algorithm itself is not examined. The integration time is adapted to reflect available forecast data in Austria. In addition, potential limits for the deviations between high resolution time series of rain rates and average values of whole periods are discussed.

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

AbstractThe 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 2DV...