Investigating water vapour and cirrus clouds using far infrared observations and simulations

Theoretical models suggest that globally half the radiation emitted by Earth back to space is at wavelengths greater than 15micrometer. This spectral region is referred to as the far infrared (FIR) and is strongly influenced by water vapour and high altitude clouds. Despite its significance in Earth's greenhouse effect, there have previously been no global high-resolution measurements of the FIR radiation at the TOA due to technical limitations. This is set to change with the launch of the ESA's Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) satellite in 2027. In preparation for the launch of FORUM, more FIR experimental data is needed. This motivated Imperial College to build the Far INfrarEd Surface Emissivity Spectrometer (FINESSE) measuring across the infrared spectrum. In early 2023, FINESSE measured clear sky downwelling radiation spectra during its first field campaign at the ALOMAR Observatory in Norway. In the cold and dry Arctic conditions, the FIR 'dirty window' between 400-600 cm-1 opens up allowing for the measurement of radiation emitted from higher in the atmosphere which is sensitive to water vapour concentrations at these altitudes. These downwelling radiance measurements may allow improved characterisation of the lower-mid tropospheric water vapour profile. In this PhD project, I will investigate how well these clear sky observations match radiative transfer simulations that rely on atmospheric profiles from local instruments (e.g Radiosonde). In early 2024, FINESSE took part in the WHAFFFERS Campaign in Canada. FINESSE measured in clear sky and cirrus cloud conditions with other ground based and aircraft instruments. In this project, we will attempt to match downwelling radiance measurements in the clear sky cases and also in the presence of cirrus clouds to simulations and investigate the key sensitivities. Another avenue to explore is the differences between the radiative signature of natural cirrus and contrails.