Far‐infrared Spectral Radiance Observations and Modeling of Arctic Cirrus: Preliminary Results From RHUBC

Abstract

Recent studies have highlighted the important contribution of the far‐infrared (electromagnetic radiation with wavelengths greater than 12 μm) to the Earth’s radiative energy budget. In a cloud‐free atmosphere, a significant fraction of the Earth’s cooling to space from the mid‐ and upper troposphere takes place via the water vapor pure rotational band between 17 and 33 μm. Cirrus clouds also play an important role in the Earth’s outgoing longwave radiation. The effect of cirrus on far‐infrared radiation is of particular interest, since the refractive index of ice depends strongly on wavelength in this spectral region. The scattering properties of ice crystals are directly related to the refractive index, so consequently the spectral signature of cirrus measured in the FIR is sensitive to the cloud microphysical properties [1, 2]. By examining radiances measured at wavelengths between the strong water vapor absorption lines in the FIR, the understanding of the relationship between cirrus microphysics and the radiative transfer of thermal energy through cirrus may be improved. Until recently, very few observations of FIR spectral radiances had been made. The Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) was developed by Imperial College to address this lack of observational data. TAFTS observes both zenith and nadir radiances at 0.1 cm−1 resolution, between 80 and 600 cm−1. During February and March 2007, TAFTS was involved in RHUBC (the Radiative Heating in Under‐explored Bands Campaign), an ARM funded field campaign based at the ACRF‐North Slope of Alaska site near Barrow, situated at 71° latitude. Infrared zenith spectral observations were taken by both TAFTS and the AERI‐ER (spectral range 400–3300 cm−1) from the ground during both cloud‐free and cirrus conditions. A wide range of other instrumentation was also available at the site, including a micropulse lidar, 35 GHz radar and the University of Colorado/NOAA Ground‐based Scanning Radiometer (GSR). Data from these instruments, as well as from frequently launched radiosondes, were used to characterize the atmospheric state needed as input for line‐by‐line radiative transfer calculations. By comparing these calculations with the TAFTS and AERI‐ER observations, it is possible to test the effectiveness of ice crystal size distribution parameterizations (which are generally derived from mid‐latitude and tropical in‐situ observations) when applied to Arctic cirrus. The influence of the assumed single scattering properties (here calculated for ice aggregates by A. Baran of the UK Met Office) on the calculated spectra is also considered in this work.