Search Results

You are looking at 1 - 3 of 3 items for

  • Author or Editor: Ana Maria Silva x
  • All content x
Clear All Modify Search
Maria João Costa, Ana Maria Silva, and Vincenzo Levizzani

Abstract

A method based on the synergistic use of low earth orbit (LEO) and geostationary earth orbit (GEO) satellite data for aerosol-type characterization, as well as aerosol optical thickness (AOT) retrieval and monitoring over the ocean, is presented. These properties are used for the estimation of the direct shortwave aerosol radiative forcing at the top of the atmosphere. The synergy serves the purpose of monitoring aerosol events at the GEO time and space scales while maintaining the accuracy level achieved with LEO instruments. Aerosol optical properties representative of the atmospheric conditions are obtained from the inversion of high-spectral-resolution measurements from the Global Ozone Monitoring Experiment (GOME). The aerosol optical properties are input for radiative transfer calculations for the retrieval of the AOT from GEO visible broadband measurements, avoiding the use of fixed aerosol models available in the literature. The retrieved effective aerosol optical properties represent an essential component for the aerosol radiative forcing assessment. A sensitivity analysis is also presented to quantify the effects that changes on the aerosol model may have on modeled results of spectral reflectance, AOT, and direct shortwave aerosol radiative forcing at the top of the atmosphere. The impact on modeled values of the physical assumptions on surface reflectance and vertical profiles of ozone and water vapor are analyzed. Results show that the aerosol model is the main factor influencing the investigated radiative variables. Results of the application of the method to several significant aerosol events, as well as their validation, are presented in a companion paper.

Full access
Maria João Costa, Vincenzo Levizzani, and Ana Maria Silva

Abstract

A method based on the synergistic use of low earth orbit and geostationary earth orbit satellite data for aerosol-type characterization and aerosol optical thickness (AOT: τa) retrieval and monitoring over the ocean is presented in Part I of this paper. The method is now applied to a strong dust outbreak over the Atlantic Ocean in June 1997 and to two other relevant transport events of biomass burning and desert dust aerosol that occurred in 2000 over the Atlantic and Indian Oceans, respectively. The retrievals of the aerosol optical properties are checked against retrievals from sun and sky radiance measurements from the ground-based Aerosol Robotic Network (AERONET). The single-scattering albedo values obtained from AERONET are always within the error bars presented for Global Ozone Monitoring Experiment (GOME) retrievals, resulting in differences lower than 0.041. The retrieved AOT values are compared with the independent space–time-collocated measurements from the AERONET, as well as to the satellite aerosol official products of the Polarization and Directionality of the Earth Reflectances (POLDER) and the Moderate Resolution Imaging Spectroradiometer (MODIS). A first estimate of the AOT accuracy derived from comparisons with AERONET data leads to ±0.02 ± 0.22τa when all AOT values are retained or to ±0.02 ± 0.16τa for aerosol transport events (AOT > 0.4). The upwelling flux at the top of the atmosphere (TOA) was computed with radiative transfer calculations and used to estimate the TOA direct shortwave aerosol radiative forcing; a comparison with space–time-collocated measurements from the Clouds and the Earth's Radiant Energy System (CERES) TOA flux product was also done. It was found that more than 90% of the values differ from CERES fluxes by less than ±15%.

Full access
Vanda Salgueiro, Maria João Costa, Ana Maria Silva, and Daniele Bortoli

Abstract

The shortwave cloud radiative forcing is calculated from surface measurements taken in Évora from 2003 to 2010 with a multifilter rotating shadowband radiometer (MFRSR) and with an Eppley black and white pyranometer. A new approach to estimate the clear-sky irradiance based on radiative transfer calculations is also proposed.

The daily-mean values of the cloud radiative forcing (absolute and normalized) as well as their monthly and seasonal variabilities are analyzed. The study shows greater variability of radiative forcing during springtime with respect to the other seasons. The mean daily cloudy periods have seasonal variation proportional to the seasonal variation of the cloud radiative forcing, with maximum values also occurring during springtime. The minimum values found for the daily-mean cloud radiative forcing are −139.5 and −198.4 W m−2 for MFRSR and Eppley data, respectively; the normalized values present about 40% of sample amplitude, both for MFRSR and Eppley. In addition, a quantitative relationship between the MFRSR and Eppley cloud radiative forcings applicable to other locations is proposed.

Full access