Microwave remote sensing and hydrological modelling of snow melting cycle Pampaloni P., G. Macelloni, S. Paloscia, P. Poggi IFAC-CNR- via Panciatichi 64 50127 I Florence, Italy e-mail P.Pampaloni @ifac.cnr.it S. Zecchetto ISAC-CNR, Padua, Italy R. Ranzi, University of Brescia, Dept. Civil Eng. Brescia, Italy A.Crepaz Centro Valanghe Arabba, Italy Abstract— A study of the melting cycle of snow was carried out by combining microwave active and passive measurements with meteorological data and snow modelling. The experiment took place in the eastern Italian Alps from mid February to late May 2003. Brightness temperature at C-, Ku- and Ka- bands (vertical and horizontal polarizations) and backscattering coefficient at Ku-band (VV), were continuously measured (24h/day) with ground based sensors. Remote sensing observations were supported by meteorological data, and snow measurements. A continuous simulation of the snow temperature, depth, and liquid water content was performed for the entire monitoring period by means of a physically based distributed snowmelt model. Both hydrological and remote sensing approaches gave useful and coherent results in describing the snow melting and refreezing cycles. Microwave active and passive data were consistent each other. During the melting cycle, the presence of liquid water caused an increase of absorption with a consequent increase of the brightness temperature and a decrease of the backscattering coefficient. Keywords- snow melting; microwave radiometry; scatterometric measurements; hydrological cycle I. INTRODUCTION Snow cover constitutes the largest component of the cryosphere and plays a significant role in the global climate and climate response to global changes. Microwave remote sensing has proved to be an efficient technique in monitoring snow parameters and seasonal variations in snow cover. Radiation emitted or scattered at the lower frequencies of the microwave band by soil covered with dry snow is mostly influenced by the soil conditions. At frequencies higher than about 10 GHz, however, the role played by volume scattering increases, and microwave emission and scattering becomes sensitive to snow water equivalent, SWE [e.g. 1-3]. Indeed, microwave emission from dry snow increases as snow depth (SD) increases and backscattering decreases. If snow melts, the presence of liquid water in the surface layer causes an increase in emission [e.g. 4-5] and a decrease in backscattering. The monitoring of the melting cycle of snow is fundamental to the management of water resources, as well as for flood and avalanche forecasting. During the melting process, typical grains of dry snow are transformed into large rounded grains. During the night-refreezing phase, which mainly involves the upper layer of snow cover, the crystals aggregate in polycrystalline grains, and may form surface crusts. This transformation impacts on the emission and scattering properties of snow, which change in accordance with the daily melting and refreezing cycles. A study of the melting cycle of snow was carried out by combining microwave active and passive measurements with meteorological data and snow modelling. II. THE EXPERIMENT The experiment took place in the eastern Italian Alps on a site located at an altitude of 2030 m a.s.l. This site was characterized by a relatively smooth topography, the absence of forests, the availability of historical data and the relatively easy access. Nivological, micrometeorological and microwave remote sensing measurements were carried out from mid February to late May 2003, in the framework of the European project ENVISNOW. At the beginning of the experiment, the snow cover, 70 cm deep, was characterized by dry snow and base layers of old snow with solid faceted particles (classified as F = 4 a in [6]; average grain size = 2-3 mm) and rounded This work was partly supported by the European Commission ENVISNOW project EVG2-2001-00018 and by the Italian Space Agency within the ENVI- ALGO grant mw radiometers scatterometer I. R Fig.1 The Microwave station