Satellite analysis of cloud characteristics at different temporal and spatial scales using visible and infrared wavelengths.

Clouds play a complex role in the earth systems at different temporal and spatial scales interacting with other components of the systems through a wide range of physical processes. In particular they affect the energetic balance as regulators of the atmospheric radiative fluxes and source/sink of latent heat. Moreover, they take part in the hydrological cycle storing water and producing precipitation. Finally, they are a meteorological phenomenon at local scale that impacts on the ground in different ways. Although this relevant role, a satisfactory level of knowledge of the mechanisms taking place inside clouds has not yet reached, and the cloud parameterisation is still a major source of uncertainties in climatic models. During last decades observations from space have been made possible more detailed studies of cloud structure, increasing the cloud knowledge in order to improve the meteorological forecast and to evaluate the role of clouds in the climate systems. The activity of this PhD has been devoted to set up a set of tools using satellite data to study the main characteristics of cloud systems at different spatial and temporal scales. In this work new retrieval algorithms to estimate cloud characteristics are presented. In order to analyse cloud systems that develop at different spatial and temporal scales two sets of clouds parameters at two different resolutions are drawn. At first, a set of high resolution cloud parameters (cloud mask, cloud optical thickness, cloud particles effective radius and cloud top temperature) have been estimated from data by Spinning Enhanced Visible and Infrared Imager (SEVIRI) on board Meteosat 8-9 at 3-5 km of spatial resolution and 15 minutes of temporal resolution. A cloud mask algorithm for SEVIRI has been developed taking advantage from its high temporal resolution using the temperature rate of change as measured at 10.8 mm. The comparison of this product to MODerate resolution Imaging Spectroradiometer cloud mask has shown a good agreement. Moreover, cloud optical thickness (t), and effective radius (re) are retrieved using SEVIRI data and simulations of radiative transfer by the model Santa Barbara Disort Atmospheric Radiative Transfer. The retrieval algorithm is based on the matching of measured reflectance by sensor with entries in look-up tables of simulated reflection functions generated by different t and re combinations. An application of high resolution parameters (t, re and cloud top temperature) has been following the temporal development of convective systems highlighting the convective life stages from satellite point of view. We have found that the cloud microphysical structure evolves in agreement with the conceptual model developed primarily by radar observations. In the last part of the work an analysis of wide scale cloud systems at synoptic/sub-synoptic scale has been carried out by using cloud parameter at lower temporal and spatial resolution (3 hours and 30 km) derived by International Satellite Cloud Climatology Project (ISCCP) database. The main characteristics of the cloud covers associated to Cut-off Low (COL) systems detected from 1992 to 2001 during the warm seasons in European/Mediterranean areas have been described. It has been found that the cloudiness size is related to the vertical structure of the COL: large size mostly characterize those systems that show a corresponding depression at the ground, small size or no cloudiness characterize systems dynamically confined to upper tropospheric levels. Moreover, the NAO index differently affects the cloud cover associated to COLs that develop in Mediterranean area from those that develop in Atlantic area.