Far-infrared spectral radiance studies: Application to water vapour and cirrus

• Main Author: Fox, Cathryn
• Format: Text
• Language: unknown
• Published: Imperial College London 2015
• Subjects: Barrow; Alaska
• Online Access: https://dx.doi.org/10.25560/25752; http://spiral.imperial.ac.uk/handle/10044/1/25752

This thesis presents the analysis of high-resolution spectral radiances recorded using the Imperial College Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) instrument, which uniquely has the ability to record high resolution in-situ measurements, both up-welling and down-welling, in the far-infrared spectral region 80-800cm-1. The TAFTS data used in this work has been recorded during two diverse field campaigns, one ground-based, and one onboard the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 research aircraft. The far-infrared spectral region has been shown to have a dominant contribution to the radiative cooling of the planet to space. Atmospheric water vapour is known to absorb radiation in a pure rotation band in the far-infrared, and also exhibits a background slowly-varying absorption known as the water vapour continuum. Observations of far-infrared radiances made by TAFTS during the Radiative Heating in Underexplored Bands Campaign (RHUBC) in Barrow, Alaska, are compared to simulations using line-by-line radiative transfer models in order to validate the current water vapour continuum parameterisation. Results presented here show that the current MT-CKD v2.5 parameterisation is accurately representing the water vapour continuum at wavenumbers between 350-500 cm-1. This thesis also considers the radiative effect of cirrus clouds in the far-infrared. These clouds have an important influence on current climate, and are known to scatter and absorb radiation in the infrared, however currently there have been relatively few measurements of the cirrus radiative effect in the far-infrared spectral region. Presented here are mid- and far-infrared observations of the radiative signature of cirrus recorded using the TAFTS and the UK Met Office Airborne Research Interferometer Evaluation System (ARIES) instruments, during the first phase of the aircraft-based Cirrus Coupled Cloud-Radiation Experiment (CIRCCREX), which flew out of Prestwick, Scotland in November 2013. These spectral measurements were compared to simulations using the LBLDIS model, which utilises cirrus scattering databases developed by Baum et al. at the SSEC, University of Wisconsin-Madison. This work represents the first time that the consistency of these databases have been tested across the entire infrared region. It was found that the model was underestimating brightness temperatures in the region 330-600 cm-1, and that further work is required in this region.