Rainfall at Urban Hydrological Scales : Opportunist use of TV-SAT Receivers Network
Résumé
Measurement of rainfall in urban area is an important issue in several domains such as urban hydrology, flash-flood early warning systems. The two main ways of measuring the rainfall rates are rain gauges networks and weather radars. The radars are limited by the reliefs, quite expensive and thus not available everywhere in the world. The traditional rain gauge networks are scarce or poorly maintained. This calls for alternative source of rainfall information. Various studies have recently shown that commercial microwave links from operational cellular communication networks may be used for rainfall monitoring. Despite their potential in rainfall observation the low availability of data or the inadequate frequencies used in some areas (<8 GHz) does not allow their use in all regions.
Previous studies have proved the possibility to rebuild rainfall in time and space at high resolution (a few hundred meters) in using a set of Satellite to Earth commercial microwave links. These links in Ku Band (10512 GHz) exist all over the world and allow estimating integrated rain attenuation along 55 7 km long links with a very high temporal resolution (10 seconds case).
A single Ku receiver deployed during Hymex experiment have produced local rainfall fields at resolution 10s and 0.5*0.5 km2 with features similar to radar maps ones at time resolution inferior to 5 minutes and spatial resolution around 1 km2.
Given the density of TV receivers in urban area operating in Ku-band, our goal is to study the quality of the rainfall fields that could be retrieved using this kind of receiver network.
As in the case of a single Ku receiver, the proposed method uses a variational assimilation method (4DVAR) with a dynamic advection model for rain cells evolution. This method integrates the observations from Ku5receivers in a numerical model and minimizes a cost function evaluating the gap between the initial rain field propagated through time by the model and the available observations.
This feasibility study of an opportunistic measurement system is made from simulations in using the following methodology: 1) A rain map simulator based on a rain support simulator associated with calibrated mutifractal simulations is used to generate numerous rain maps observations with actual statistical and spectral properties. 2) A set of rain attenuation times series corresponding to observations that could be made by a Ku-receivers network for corresponding rain maps are obtained. 3) A set of rain times series corresponding to observations that could be made by a realistic rain gauge network for corresponding rain maps are obtained. 4) A variational 4DVAR assimilation setting using an advection model is used to rebuild the rain fields from the attenuation time’s series.
Finally, the rain fields obtained with the Ku-band receiver network and with the one obtained from the rain gauge network are compared with the initial rain maps in terms of variability, temporal and spatial resolution.
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