On the opportunistic use of geostationary satellite signals to estimate rain rate in the purpose of radar calibration

International audienceRain gauge networks are often used for radar calibration. However, the deployment of such networks is relatively complex and expensive. The present study deals with the development of a new low cost microwave device devoted to radar calibration. The principle of this device is to use rain atmospheric attenuation along microwave links to deduce the path averaged rain rate. In order to perform a feasibility study of such a device, a measurement campaign is performed for two years near Paris. Measurements of atmospheric attenuation over an earth-space link have been carried out by receiving TV channels from different geostationary satellites in Ku-band. These links are characterized by an aperture angle of 2° and a 30° elevation angle corresponding more or less to a 6 km path length through troposphere. The aim of this paper is to propose an algorithm to retrieve rain rate from the measured signal and to quantify the expected averaged rain rate accuracy along the earth-space link. In practice, the received signal is sensitive to rain as well as to many other fluctuations due to atmospheric scintillations, clouds, water vapor, small changes in the satellite orbit, power fluctuations

Estimation par assimilation de données variationnelle de champs de pluie à petite échelle à partir d'atténuations de micro-ondes de satellites TV

International audienceLes deux moyens principaux pour observer les précipitations sont les radars météorologiques et les réseaux de pluviomètres. Cependant les premiers sont chers et ne sont pas disponibles partout, et les seconds ne fournissent que des mesures ponctuelles. Or la pluie est un phénomène naturel présentant une forte variabilité spatio-temporelle. Nous présentons ici une méthode permettant de reconstituer des champs de pluie à petite échelle à partir de l'affaiblissement par la pluie des ondes radio en bande Ku (10-12 GHz) en provenance de satellites de télévision. les mesures d'affaiblissements sont couplées à un modèle numérique d'évolution des champs de pluie (advection) via un algorithme d'assimilation de données variationnelle (AD-VAR). cette méthode est appliquée sur des données enregistrées en 2013 dans l'Ardèche dans le cadre de la campagne européenne HyMeX (Hydrological cycle in mediterranean Experiment). Les cartes de précipitations obtenues à petite échelle (10s, 0,5 *0.5km2) sont comparées à des observations radar et pluviomètres colocalisées

Short-term prediction of rain attenuation level and volatility in Earth-to-Satellite links at EHF band

This paper shows how nonlinear models originally developed in the finance field can be used to predict rain attenuation level and volatility in Earth-to-Satellite links operating at the Extremely High Frequencies band (EHF, 20–50 GHz). A common approach to solving this problem is to consider that the prediction error corresponds only to scintillations, whose variance is assumed to be constant. Nevertheless, this assumption does not seem to be realistic because of the heteroscedasticity of error time series: the variance of the prediction error is found to be time-varying and has to be modeled. Since rain attenuation time series behave similarly to certain stocks or foreign exchange rates, a switching ARIMA/GARCH model was implemented. The originality of this model is that not only the attenuation level, but also the error conditional distribution are predicted. It allows an accurate upper-bound of the future attenuation to be estimated in real time that minimizes the cost of Fade Mitigation Techniques (FMT) and therefore enables the communication system to reach a high percentage of availability. The performance of the switching ARIMA/GARCH model was estimated using a measurement database of the Olympus satellite 20/30 GHz beacons and this model is shown to outperform significantly other existing models. The model also includes frequency scaling from the downlink frequency to the uplink frequency. The attenuation effects (gases, clouds and rain) are first separated with a neural network and then scaled using specific scaling factors. As to the resulting uplink prediction error, the error contribution of the frequency scaling step is shown to be larger than that of the downlink prediction, indicating that further study should focus on improving the accuracy of the scaling factor

Monitoreo de servicios ecosistémicos en un observatorio de cafetales agroforestales. Recomendaciones para el sector cafetalero

Ocho años de estudio de la ecofisiología del café, a través de experimentación y de modelación y el monitoreo de los servicios del ecosistema (SE) en una gran finca cafetalera en Costa Rica, revelaron varias recomendaciones prácticas para los agricultores y los formuladores de políticas. El sistema de cultivo estudiado dentro de nuestro observatorio colaborativo (Coffee-Flux), corresponde a un sistema agroforestal (SAF) a base de café bajo la sombra de grandes árboles de Erythrina poeppigiana (16% de la cubierta del dosel). Una gran cantidad de SE y limitantes dependen de las propiedades locales del suelo (en este caso Andisoles), especialmente de la erosión/infiltración, el agua/carbono y la capacidad de almacenamiento de nutrientes. Por lo tanto, para la evaluación de SE, el tipo de suelo es crucial. Una densidad adecuada de árboles de sombra (bastante baja aquí por la condición de libre crecimiento), redujo la severidad de las enfermedades de las hojas con la posibilidad de reducir el uso de plaguicidas y fungicidas. Un inventario simple del área basal en el collar de las plantas de café permitió estimar la biomasa subterránea y la edad promedio de la plantación, para juzgar su valor de mercado y decidir cuándo reemplazarla. Las fincas de café probablemente estén mucho más cerca de la neutralidad de C que lo indicado en el protocolo actual de C-neutralidad, que solo considera árboles de sombra, no los cafetos ni el suelo. Se proponen evaluaciones más completas, que ncluyen árboles, café, hojarasca, suelo y raíces en el balance C del SAF. Los árboles de sombra ofrecen muchos SE si se gestionan adecuadamente en el contexto local. En comparación con las condiciones a pleno sol, los árboles de sombra pueden (i) reducir la erosión laminar en un factor de 2; (ii) aumentar la fijación de N y el % de N reciclado en el sistema, reduciendo así los requisitos de fertilizantes; (iii) reducir la severidad de enfermedades de las hojas; (iv) aumentar el secuestro de C; (v) mejorar el microclima y (vi) reducir sustancialmente los efectos del cambio climático. En nuestro estudio de caso, no se encontró ningún efecto negativo sobre el rendimiento del café

Rôle des précipitations dans l'interaction ondes atmosphère dans la bande 10-50 GHz

In the troposphere, the microwave electromagnetic radiation interacts with matter according to three processes: absorption by gas molecules and water, scattering due to the deflection of the waves by hydrometeors, and the emission of a natural radiation.My work has a common basis for these processes of interaction of electromagnetic waves with the atmosphere, and relates to two distinct areas: Earth-satellite telecommunications and small scales studies of precipitations.I will present measurement campaigns and modeling works performed in relation with the development of new Fade Mitigation Techniques for Earth – satellite telecommunications. In this context I will show the interest to use rain microphysics properties to improve frequency scaling models. A model of short term prediction of rain attenuation level and its volatility in Earth-to-Satellite links will be also presented. This model uses a statistical model based on GARCH model which is able to well represents periods of high volatility and is particular well designed to model rain variability.Finally, multifractal properties of rain will be presented within the framework of universal multifractals.Les ondes électromagnétiques dans le domaine des micros ondes interagissent avec l'atmosphère terrestre suivant des processus d'absorption, de diffusion et d'émission. Les hydrométéores et notamment la pluie contribuent largement à ces interactions. Dans le domaine des télécommunications, ces phénomènes sont en général néfastes et il convient de les compenser. A contrario en télédétection ils permettent de caractériser le milieu.Les travaux présentés comportent plusieurs volets liés aux précipitations à petites échelles: - la mesure des précipitations aux moyens de diverses techniques (radiomètre micro ondes, liaisons radio point à point, mesure ponctuelle au sol). - Les travaux de modélisation, notamment dans le cadre de la mise au point de nouvelles techniques de lutte contre les affaiblissements (Fade Mitigation Technique). Les originalités de ces travaux reposent d'une part sur la prise en compte de la microphysique de la pluie dans les modèles et d'autre part dans l'utilisation de méthodes statistiques développées originellement dans le domaine de la finance : modélisations statistiques du type TARIMA GARCH, qui s'attachent, plutôt que de prévoir le comportement moyen du processus, à prévoir la variabilité et donc le risque d'un comportement extrême.- La modélisation des propriétés d’invariance d’échelle des précipitations dans un contexte multifractal et les problèmes liés à l’alternance pluie – non pluie dans l’estimation des paramètres

Rainfall variability measurements using geostationary satellite signals

The present study investigates the development of a low cost microwave device devoted to rainfall measurement at a spatial resolution of about one square kilometre and at a temporal resolution in order of a few minutes. The principle of this device is to estimate rain attenuation along earth-space links to deduce the path average rain rate. A measurement campaign has been performed for four month near Paris. Measurements of atmospheric attenuation over earth-space links have been carried out by receiving TV channels from different geostationary satellites in Ku-band (10-12 GHz). These links are characterized by a 30° elevation angle that corresponds to a 6 km tropospheric path length. In practice, the proposed device only measures the received power. A critical issue in our approach is thus the ability to distinguish between rain feature and many other fluctuations of the received signal due to atmospheric scintillations or changes: in water vapour concentration, cloud water content, emitted power, satellite orbit, and temperature. First part of the study deals with the estimation of rain attenuation observed on a unique earthspace link. We propose an algorithm based on an artificial neural network to identify and separate dry and rainy periods and to suppress the variability of the received signal due to no-rain effects. Taking into account the height of the rain layer, rain attenuation is then inverted to obtain path average rain rate. Obtained rainfall rate are compared with co-located rain gauges and radar measurements. On one hand accumulated rainfall and quantile - quantile plots of hourly accumulated rainfall obtained on the whole experiment period are compared, on the other hand the most significant rainy events are analyzed. In a second part, the possibility of estimating rain field is discussed using measurements performed from four earth-space links associated with tomographic techniques. Simulations are performed using synthetics rain maps. Different geometric configurations and various meteorological situations are considered. In conclusion, so as to evaluate if this features in terms of sensing local rainfall may lead to a better assessment of flood risk, the properties of the proposed device in comparison with meteorological radar or rain gauge network are analysed in terms of cost, maintenance constraint, measurement accuracy , spatial and temporal resolution, dimension of sensing area

Vertical evolution of raindrop size distribution: Impact on the shape of the DSD

International audienceModels of coalescence and breakup lead to equilibrium of the raindrop size distribution (DSD) after a fall through sufficient vertical height. At equilibrium, the DSD no longer evolves, and its shape is unique whatever the rain rate or Liquid Water Content (LWC). This implies that the DSD is known, to within a multiplication constant. In the past, numerous measurements using disdrometers revealed that the slope of the DSD tail is close to 20-22 cm-1 when equilibrium is reached, whereas models based on the Low and List experiment predict values of approximately 65 cm-1. The present paper proposes a simple modification of the coalescence efficiency in the Low and List parameterization, leading to a DSD tail with a slope of 24 cm-1. To evaluate the relevance of this modification, some of the DSD parameters such as slope, mean volume diameter, and the relationship between moments are calculated, and compared with experimental DSD. The modified parameterization is then used to study the evolution of an initially gamma-like DSD in a 1D vertical rain shaft

Comparison between natural Rain drop size distributions and corresponding models near equilibrium state during warm rain

The study of the vertical evolution of raindrop size distributions (DSDs) during rainfall, from the freezing level isotherm to ground level, is a key to improving our understanding of the microphysics of rain. In numerous domains such as remote sensing, telecommunications, soil erosion, and the study of the rain's efficiency in 'washing' the atmosphere, the DSD plays an important role. Among the different processes affecting the evolution of DSD, breakup and coalescence are two of the most significant. Models of coalescence and breakup lead to equilibrium of the raindrop size distribution (DSD) after a fall through sufficient vertical height. At equilibrium, the DSD no longer evolves, and its shape is unique whatever the rain rate or LWC. This implies that the DSD is known, to within a multiplication constant. These models based on experimental measurements have been developed over the past 40 years. The Low and List (1982a,b) parameterization (hereinafter LL82) and the Greg M. McFarquhar (2004) model are both based on the same laboratory experiments, which lead to an equilibrium drop size distribution (EDSD) with two or three peaks, and an exponential tail with a slope of approximately Λ=65 cm-1. Numerous measurements using disdrometer collected in different climatic areas: Paris, France (Mars to October 2000), Iowa-City (April to October 2002), and Djougou (Benin June to September 2006) corresponding to 537 hours of rain period have shown that for high rain rates, close to a state of equilibrium, this slope lies between Λ=20 - 22 cm-1. This latter value is corroborated by others measurements found in the literature (Hu & Srivastava, 1995). Hu & Srivastava suggested that the Low and List parameterization may overestimate the effects of the breakup process. This hypothesis is in adequation with recent laboratory experiments (A.P. Barros 2008) in which the authors conclude that the number of fragments droplets produced when small drops and large drops collide is overestimate. As new parameterization of LL82 is not possible due to the lack of new sufficient large experimental dataset, we have simply tried in the present study to 'compensate' the problem previously mentioned by replacing the coalescence/breakup model proposed in LL82 by another one in which the breakup process is less dominant. In order to evaluate the relevance of this modification, some of the DSD parameters such as slope, mean volume diameter, and relation between moments are calculated, and comparisons with experimental DSD are made. Simulations at equilibrium lead to a DSD tail with a slope of 23 cm-1 and a mean volume diameter equal to 2.5 mm. These values are in good agreements with experimental data. Similarly, the linear relationship between No* and the rainfall rate is also in good agreement. In the last part, the modified parameterization is then used to study the evolution of an initially gamma-like DSD in a 1D vertical rain shaft