Abstract. Far-infrared (FIR: 100cm-1<wavenumber, ν<667 cm−1) radiation emitted by the Earth and its
atmosphere plays a key role in the Earth's energy budget. However, because of
a lack of spectrally resolved measurements, radiation schemes in climate
models suffer from a lack of constraint across this spectral range.
Exploiting a method developed to estimate upwelling far-infrared radiation
from mid-infrared (MIR: 667cm-1<ν<1400 cm−1) observations, we explore the possibility of
inferring zenith FIR downwelling radiances in zenith-looking observation
geometry, focusing on clear-sky conditions in Antarctica. The methodology
selects a MIR predictor wavenumber for each FIR wavenumber based on the
maximum correlation seen between the different spectral ranges. Observations
from the REFIR-PAD instrument (Radiation Explorer in the Far Infrared –
Prototype for Application and Development) and high-resolution radiance
simulations generated from co-located radio soundings are used to develop and
assess the method. We highlight the impact of noise on the correlation
between MIR and FIR radiances by comparing the observational and theoretical
cases. Using the observed values in isolation, between 150 and 360 cm−1, differences between the “true” and
“extended”
radiances are less than 5 %. However, in spectral bands of low signal,
between 360 and 667 cm−1, the impact of instrument
noise is strong and increases the differences seen. When the extension of the
observed spectra is performed using regression coefficients based on
noise-free radiative transfer simulations the results show strong biases,
exceeding 100 % where the signal is low. These biases are reduced to just a
few percent if the noise in the observations is accounted for in the
simulation procedure. Our results imply that while it is feasible to use this
type of approach to extend mid-infrared spectral measurements to the
far-infrared, the quality of the extension will be strongly dependent on the
noise characteristics of the observations. A good knowledge of the
atmospheric state associated with the measurements is also required in order
to build a representative regression model.
Can downwelling far-infrared radiances over Antarctica be estimated from mid-infrared information?