Shortwave Radiation Budget

Background

Solar radiation provides the energy that drives the Earth's weather and climate. Approximately two thirds of the solar radiant energy incident on the Earth is absorbed, heating the Earth's surface until it radiates nearly as much energy back to space as it absorbs from the sun. The remaining one third of the incident solar energy is reflected back into space. Both the absorbed and reflected solar radiation are in the shortwave part of the electromagnetic spectrum, while the Earth's radiation emitted back to space is longwave or thermal infrared radiation. The balance between the solar input and the thermal output is known as the Earth's Radiation Budget. This radiation budget is affected by the nature of the Earth's surface and its atmosphere, particularly clouds.

Product Description

The Reflected Shortwave Radiation product will measure the total amount of shortwave radiation that exits the Earth through the top of the atmosphere. The algorithm will use several spectral channels in both the visible and infrared spectrum to measure the Reflected Shortwave Radiation. Information from this product will provide an integral piece of the Earth's radiation budget, aiding in climate modeling and prediction.

The Downward Shortwave Radiation (DSR) product is an estimate of the total amount of shortwave radiation (both direct and diffuse) that reaches the Earth's surface. The product algorithm uses spectral channels in both the visible and the infrared in addition to data regarding albedo and atmospheric composition to compute the Downward Shortwave Radiation at the Earth's surface.

Improvements and Benefits

The shortwave radiation algorithms have many applications both in the general and applied sciences. As one of the components of the surface energy budget, it is needed in climate studies. Used together with cloud and aerosol properties it provides estimates of cloud and aerosol effects (forcing). It is also used in surface energy budget models, land surface assimilation models such as those used at NOAA NCEP, NASA LDAS, and ocean assimilation models either as an input (providing observationally-based forcing term), or as an independent data source to evaluate model performance. DSR data are also employed in estimating heat flux components over the coastal ocean to drive ocean circulation models. In agriculture, DSR is used as input in crop modeling. In hydrology, it is used in watershed and run-off analysis, which is important for determining flood risks and dam monitoring. The solar energy industry also needs estimates of DSR for both real-time and short-term forecasts for building energy usage modeling and optimization. Since high irradiance values result in surface drying, DSR is also used in monitoring fire risk.

How does it work? - Algorithm

The ABI SRB algorithm is developed as a hybrid algorithm based on the NASA Clouds and the Earth's Radiant Energy System (CERES)/Surface and Atmospheric Radiation Budget (SARB) algorithm (Charlock and Alberta, 1996) and on the UMD SRB algorithm (Pinker and Laszlo, 1992). The primary algorithm is the NASA algorithm, which mostly uses ABI products (e.g., surface albedo; cloud optical depth, particle size, and top height; and aerosol optical depth and single scattering albedo) to estimate the SW fluxes in a forward calculation, hence dubbed as the "direct path". When these ABI products used as inputs are not available the algorithm switches to an appropriately modified version of the UMD algorithm developed at NOAA/NESDIS/STAR to perform the SW flux retrieval. The latter algorithm, being an inversion scheme in nature, termed as the "indirect path". This path requires the ABI observed channel reflectance as major inputs with minimum reliance on the derived Level 2 ABI products. It uses SW broadband TOA albedo estimated from the spectral NTB transformation and angular correction to estimate atmospheric transmittance. Even though they estimate the fluxes in different schemes, both paths use the same lookup table to accomplish the flux retrieval.