In this work we proposed a technique to retrieve optical depth for optically thin clouds (clouds that enable the transmission of solar radiation through them). This method complements the current performance of AERONET for optically thick cloud measurements using data eliminated by the cloud-screening algorithm that are not useful to derive aerosol properties, therefore inexpensively increasing the capabilities of the sun-photometers. It is based on the computation of apparent cloud optical depths and a forward scattering correction method that exploits state-of-the-art ice cloud scattering models. This complementary procedure is applied to Cimel sun-photometer measurements performed at the Évora Geophysics Centre (Portugal, 38.6ºN, 7.9ºW, 293 m asl) in order to obtain a climatology of optical depths for optically thin clouds over middle-latitude regions. A comparison with MODIS retrievals is presented. Main features regarding annual variability from 2007 to 2010 are also reported.

This work intends to study the Saharan desert dust storms effects on clouds. This is done through the estimation of the cloud radiative forcing in the presence of mineral desert dust aerosols during a strong desert dust event that occured in the end of May 2006. The assessment of the cloud radiative forcing is made at a regional scale both at the top of the atmosphere (TOA) and at the surface levels.
The results are obtained from numerical simulations with a mesoscale atmospheric model (MesoNH) over Portugal area and nearby Atlantic Ocean.
From the results obtained it is possible to observe that, for all days under study, a cooling effect is always found both at the TOA and surface levels. Also, for these two levels and for clouds developing in a dusty atmosphere, a more pronounced cooling effect (more negative cloud radiative forcing values) is found compared with the corresponding cloud radiative forcing values for clouds developing in a dust free atmosphere.

Modifications in the climate are expected to be noticeable in different climate variables including precipitation and surface air temperature. Changes in precipitation and air temperature variability and extremes can be studied by inspecting trends in specific indices defined for daily occurrences. This approach is used in this study, which explores recent modifications in the climatology of extreme events in precipitation and surface air temperature (maximum and minimum) using data from various locations in mainland Portugal. The study is conducted by analysing trends and variations in selected specific indices derived for daily observations; this is done for different time periods and different seasons. The precipitation data are from 57 measuring stations and cover the period 1941-2007. For air temperature, data are from 23 stations and the sub-periods investigated are 1945-1975 (cooling period) and 1976-2006 (warming period). The study aims mostly at understanding variations in the intensity, frequency and duration of extreme events and pays special attention to regional differences and seasonality.

Madeira Island, Portugal, faced an intense heavy rainfall on 20 February 2010 which lost more than 40 lives and caused great damage. This event is recorded as one of the major flash flood events during the past three decades. Keeping this in view, in the present study a series of numerical experiments using the Weather Research Forecasting model and MESO-NH model at very high resolution of 1 km are performed. Lateral and boundary conditions are updated every 6 hours using NCEP FNL data for WRF model and with ECMWF IFS data for MESO-NH model, available at 1 degree and 0.25 degree resolutions, respectively. The WRF model is designed with four nested domains and an inner domain with 1 km resolution centered on the island, which is the area of interest. MESO-NH model is integrated with 3 domains and inner domain at 1 km resolution located over Madeira Island.
Both models show that the extreme rainfall event was originated by the effect of orography on the prevailing large scale flow of a conditionally unstable moist air. Experiments for other heavy rainfall events over Madeira show the same mechanism, indicating the capability of high resolution mesoscale models in reproducing this type of events. At higher resolution, convection parameterization schemes are not so important to resolve mesoscale cloud features but microphysics schemes are important. In this aspect we performed a series of sensitivity experiments with different microphysics schemes. Different cloud microphysical properties are examined and discussed. These cloud microphysical parameters are compared with satellite retrievals. Results indicate that the model is sensitive to different microphysics schemes.