Geostatistical simulation of two-dimensional fields of raindrop size distributions at the meso-¿ scale

The large variability of the raindrop size distribution (DSD) in space and time must be taken into account to improve remote sensing of precipitation. The ability to simulate a large number of 2-D fields of DSDs sharing the same statistical properties provides a very useful simulation framework that nicely complements experimental approaches based on DSD ground measurements. These simulations can be used to investigate radar beam propagation through rain and to evaluate different radar retrieval techniques. The proposed approach uses geostatistical methods to provide structural analysis and stochastic simulation of DSD fields. First, the DSD is assumed to follow a Gamma distribution with three parameters. As a consequence, 2-D fields of DSDs can be described as a multivariate random function. The parameters are normalized using a Gaussian anamorphosis and simulated by taking advantage of fast Gaussian simulation algorithms. Variograms are used to characterize the spatial structure of the DSD fields. The generated fields have identical spatial structure and are consistent with the observations. Because intermittency cannot be simulated using this technique, the size of the simulation domain is limited to the meso-¿ scale (2-20 km). To assess the proposed approach, the method is applied to data collected during intense Mediterranean rainfall. Taylor's hypothesis is invoked to convert time series into 1-D range profiles. The anisotropy of the fields is derived from radar measurements. Simulated and measured reflectivity fields are in good agreement with respect to the mean, the standard deviation, and the spatial structure, demonstrating the promising potential of the proposed stochastic model of DSD field

Modelling of road surface pollution buildup and washoff using rainfall simulator

Water quality management of nonpoint source (NPS) pollution is still being confronted with identification and assessment. The extent of pollution due to NPS in tropics is not yet affirmed, and the relative influences of its associated sources were not yet totally understood. This study explored the significance of road as a NPS unit in tropical region of persistent rainfall, and investigated the possible sources of heavy metals in urban areas. To achieve the objectives of this study, the natural rainfall dynamic of the study area was appraised using the flour pellet method. The information was used as a basis for developing a highly efficient Rainfall Simulator (RS) that was used to investigate pollutant washoff process under different rainfall depth and intensities. A total of 30 buildup samples were collected from five chosen roads of varying characteristics, and fractionated into 10 classes of particle sizes each. For quantitative analysis, 60 samples were analysed for dissolved Zn, Fe, Cd, Pb, Cu, Ni, Mn, Al, and Cr concentrations. A multivariate principal component and factor analyses were used to investigate the likely sources of these heavy metals. Three sources were identified, the indigenous, geogenic and scavenge. The natural raindrop sizes were found to vary from less than 1.2 mm to as big as 7.0 mm with median raindrop diameters (D50) of 2.51 mm and a mean of 2.56 mm. These raindrops have an average rain kinetic energy content of 30 J m??2 mm??1. The developed RS can satisfactorily simulate rain intensity similar to natural rainfall, with an average kinetic energy content of 42 J m??2 mm??1 and a D50 between 2.41 and 2.64 mm. An advanced principal component and cluster analysis identified TDS as a surrogate for measuring dissolved metals pollution among eight physicochemical parameters considered, and was therefore used in the modelling of the washoff process. The developed models suggested that the rain intensity plays a more prominent role in the occurrence of first flush, while the rain depth plays a central role in the total washoff event. This research demonstrated that the influence of sediment to retain mass loading did not necessarily translate to higher pollution loading of heavy metals, and the residency of heavy metals in different particle classes cannot be generalised

FREE SPACE OPTICS LINKS AFFECTED BY OPTICAL TURBULENCE: CHANNEL MODELING, MEASUREMENTS AND CODING TECHNIQUES FOR ERROR MITIGATION

FSO is an optical wireless line-of-sight communication system able to offer good broadband performance, electromagnetic interference immunity, high security, license-free operation, low power consumption, ease of relocation, and straightforward installation. It represents a modern technology, significantly functional when it is impossible, expensive or complex to use physical connections or radio links. Unfortunately, since the transmission medium in a terrestrial FSO link is the air, these communications are strongly dependent on various atmospheric phenomena (e.g., rain, snow, optical turbulence and, especially, fog) that can cause losses and fading. Therefore, in worst-case conditions, it could be necessary to increase the optical transmission power, although, at the same time, it is needed to comply to safety regulations. The effects of the already mentioned impairments are: scattering (i.e., Rayleigh and Mie) losses, absorption and scintillation. The first two can be described by proper attenuation coefficients and increase if the atmospheric conditions get worst. As regards scintillation, it is a random phenomenon, appreciable even under clear sky. Because of scintillation, in FSO links, the irradiance fluctuates and could drop below a threshold under which the receiver is not able to detect the useful signal. In this case, communications suffer from erasure errors, which cause link outages. This phenomenon becomes relevant at high distance, but it can also be observed in 500m-long FSO links. Moreover, the optical turbulence intensity can change of an order of magnitude during the day: it reaches its maximum around midday (when the temperature is the highest) and, conversely, it is lower during the night. In order to reduce or eliminate these impairments, different methods (both hardware and software) were studied and reported in literature. Hardware solutions focus on aperture averaging effects to reduce irradiance fluctuations, in particular by using a bigger detector or multi-detector systems. On the other hand, software techniques mostly focus on transmission codes. Rateless codes are an innovative solution, suitable for channels affected by erasure or burst errors. They add a redundant coding (also settable on the fly) to the source data, allowing the receiver to successfully recover the whole payload that, otherwise, would be corrupted or partially lost. To test rateless codes, recovery capabilities in FSO channels, detailed information about the occurring signal fading are needed: in particular, its depth, temporal duration and statistics. For this reason, I have implemented a time-correlated channel model able to generate an irradiance time-series at the receiver side, at wide range of turbulence conditions (from weak to strong). The time-series represents a prediction of temporal irradiance fluctuations caused by scintillation. In this way, I was able to test the recovery capabilities of several types of rateless codes. I have performed measurement campaigns in order to characterize Free Space Optics links affected by the optical turbulence. In particular, I have used three different setups placed in the Laboratory of Optics of the University of Palermo and in the Optical Communication Laboratory of the Northumbria University. Thanks to an in-depth post-processing of the collected data, I was able to extract useful information about the FSO link quality and the turbulence strength, thus proving the effectiveness of the Gamma-Gamma model under several turbulence conditions. In Chapter 1, I will introduce the theory of optical wireless communications and, in particular, of Free Space Optics communications. In detail, I will describe the advantages and the impairments that characterize this kind of communication and discuss about its applications. In Chapter 2, the adopted channel models are presented. In particular, these models are able to predict irradiance fluctuations at the detector in Free Space Optics links and were designed for terrestrial and space-to-ground communications at different link specifications, turbulence conditions and temporal covariance. Firstly, a brief description of the employed irradiance distribution and of the irradiance covariance functions is presented. The details of the above mentioned channel model implementation and the performance are then described. Finally, in order to detail the channel model features, several examples of irradiance fluctuation predictions are depicted. In Chapter 3, the details of a measurement campaign, focused on the analysis of optical turbulence effects in a FSO link, will be treated. Three different measurement setups composed of different typologies of laser sources, detectors and turbulent channels will be described. Data post-processing will be discussed. Moreover, a performance evaluation of the terrestrial channel model described in Chapter 2 will be discussed. In Chapter 4, rateless codes will be presented. These codes introduce a redundancy by means of repair symbols, associated to the source data, and, in case of losses, they are able to recover the source data without any need for retransmission. They can also manage large amounts of data and offer very interesting features for erasure channels and multicast/broadcast applications. Three different classes of rateless codes will be described and, in particular: Luby Transform, Raptor and RaptorQ codes. Moreover, the performance of the rateless codes in Free Space Optics links will be investigated. The implemented simulators are based on the channel models presented in Chapter 2 and focus on the study of rateless codes recovery capabilities when erasure errors due to fadings occur. The results on the performance of three rateless codes typologies, in two different FSO links, will be illustrated. All the research work was supported by the European Space Agency (grant no. 5401001020). Experimental activities were performed in collaboration with the Optical Communications Research Group of the Northumbria University and within the COST IC1101 European Action

Terahertz wireless communication through atmospheric atmospheric turbulence and rain

This dissertation focusses on terahertz (THz) wireless communication technology in different weather conditions. The performance of the communication links is mainly studied under propagation through atmospheric turbulence and rain. However, as real outdoor weather conditions are temporally and spatially varying, it is difficult to obtain reproducible atmospheric conditions to verify results of independent measurements making it a challenge to measure and analyze the impact of outdoor atmospheric weather on communication links. Consequently, dedicated indoor weather chambers are designed to produce controllable weather conditions to emulate the real outdoor weather as closely as possible. To emulate turbulent air conditions, an enclosed chamber is developed into which air with controllable airspeeds and temperatures are introduced to generate a variety of atmospheric turbulence for beam propagation. To emulate varying rain conditions, an enclosed chamber is built in which pressurized air forces drops of water through an array of 30 gauge needles. In order to study and compare propagation features of THz links with infrared (IR) links under identical weather conditions, a THz and IR communications lab setup with a maximum data rate of 2.5 Gb/s at 625 GHz carrier frequency and 1.5 μm wavelength, are developed. A usual non return-to-zero (NRZ) format is applied to modulate the IR channel but a duobinary coding technique is used for driving the multiplier chain-based 625 GHz source, which enables signaling at high data rate and higher output power. The power and bit-error rate (BER) on the receiver side are measured, which can be used to analyze the signal performance. To analyze the phase change in the turbulence chamber due to the refractive index change induced by turbulence, a Mach-Zehnder Interferometer with He-Ne laser at 632.8nm is developed. In the same weather conditions, the impact on THz in comparison with IR link is not equivalent due to the spectral dependence on atmospheric turbulence and rain. In the experiment, after THz (625 GHz) and IR (1.5 μm) beams propagate through the same condition, performance of both channels is analyzed and compared. Kolmogrov theory is employed to simulate the atmospheric turbulence which leads to attenuation of THz and IR signals. Mie scattering theory is employed to simulate the attenuation of THz and IR beams due to rain. Under identical turbulence conditions, THz links are superior to IR links. However, the performance of THz and IR links are comparable under identical rain conditions

Variability of the Rain Drop Size Distribution:Stochastic Simulation and Application to Telecommunication Microwave Links

Precipitation is an important component of the Earth' water cycle and needs to be carefully monitored. Its large variability over a wide range of spatial and temporal scales must be taken into account. For example, hydrological models require accurate rainfall estimates at high spatial and temporal resolutions (e.g., 1 km and 5 min or higher). Obtaining accurate rainfall estimates at these scales is known to be difficult. So far, the only instruments capable of measuring rainfall over extended domains at such resolutions are weather radars. Their estimates are, however, affected by large errors and uncertainties partly due to the spatial and temporal variability of the drop size distribution (DSD). Major progress in the field is slowed down by the lack of knowledge about the spatial and temporal variability of DSD at scales that are relevant in remote sensing. This lack of reference data can be addressed through two different methods : (1) experimental investigations and (2) stochastic simulation. In this thesis, a comprehensive framework for the stochastic simulation of DSD fields at high spatial and temporal resolutions is proposed. The method is based on Geostatistics and uses variograms to describe the spatial and temporal structures of the DSD. The simulator' ability to generate large numbers of DSD fields sharing the same statistical properties provides a very useful theoretical framework that nicely complements experimental approaches based on large networks of weather sensors. To illustrate its potential, the simulator is applied to different rain events and validated using data from a network of disdrometers at EPFL. The results show that the simulator is able to reproduce realistic spatial and temporal structures that are in adequacy with ground measurements. The second part of this thesis focuses on the simulation and parametrization of intermittency (i.e., the alternating between dry/rainy periods). Simple scaling functions that can be used to downscale/upscale intermittency at different spatial and temporal resolutions are proposed and used to parametrize a new disaggregation method that includes the DSD as an output. Finally, different methods to identify dry and rainy periods and to quantify rainfall intermittency using telecommunication microwave links are proposed. The false dry/wet classification error rates of each method are estimated using data from a new and innovative experimental set-up located in Dübendorf, Switzerland. The results show that the dry/wet classification is significantly improved when data from multiple channels are used