The atmosphere — radiative transfer

Radiative transfer describes the transfer of electromagnetic energy in the atmosphere.

In order to make weather predictions and conduct climate simulations our forecast models need to represent the processes of radiative transfer and their correct distribution across the globe, and with height in the atmosphere.

Radiative transfer — an overview

The Earth, atmosphere and oceans are driven by the energy reaching the Earth from the sun. This incoming solar radiation is balanced by thermal emission to outer space.

However, the distribution of the incoming solar radiation on the planet is not uniform because the Earth is a sphere and it’s axis of rotation is tilted. It can also be affected by clouds, aerosols and the albedo of the surface (the extent to which it diffusely reflects light from the sun) — all of which vary significantly across the globe.

The thermal emission from the Earth atmosphere system to space is also not evenly distributed with surface properties — clouds and aerosols have an effect.

The amount of energy arriving at the top of the atmosphere from the sun varies according to wavelength (or colour) of the light. The peak in intensity is in the visible part of the spectrum where our eyes have evolved to be most sensitive. This range of energy with wavelength is called a spectrum of energy. Different surfaces, cloud types, gases in the atmosphere and chemical species of aerosols affect different parts of the solar spectrum in different ways.

The thermal emission from the planet back to space varies across the spectrum of wavelength — the peak in the emission is at longer wavelengths than the incoming solar spectrum and is not visible to the human eye.

This graphic shows some of the key interactions between solar radiation (black lines) and the outgoing thermal radiation (red lines).