EVOLUTION OF THE SNOW COVERED SURFACE IN EMILIA ROMAGNA THROUGH MODIS L1B DATA
The activity carried out during 2005-2006 and 2006-2007 winters
This article describes the activity carried out by the remote monitoring laboratory (Arpa Emilia Romagna) in monitoring the size of the snow cover through the analysis of data remotely collected by the Modis (MOD02-L1B) sensor mounted on the Terra and Aqua satellites. Detection is based on the calculation of the Ndsi index (normalized difference snow index) and on the implementation of two decision trees that discriminate both the sun and clouds. The dates on the bulletins are chosen based on the temporal nearness with snowfalls and the best weather conditions. The bulletins produced are then uploaded in the Internet under the remote monitoring page of the Arpa website. From the website it is then possible to download the bulletin in PDF format and the corresponding Modis image in false colours in JPG format with the related georeferencing file to be uploaded in the GIS environment. The snow, if present, is shown in red-orange colour, a colour that sometimes is also that of multilayer clouds, which are characterised by very low temperatures or shading. The service turned out to be a reliable system for estimating the size of the snow cover, as this is effectively discriminated by the presence of optimal spectral bands, as well as a useful instrument for the drawing up of the landslide risk bulletins processed by the functional centre of the civil defence service of Regione Emilia Romagna. As a rule, though no calculations to estimate the error percentage have been made, the algorithm can be said to partially tend to underestimate the size of the snow cover in case of wet snow, shady areas or in closed forest areas. The underestimate percentage is instead drastically reduced in the plain areas, mainly with fresh or compact snow.