Achieving consistency between in-situ and remotely sensed optical and microphysical properties of Arctic cirrus: the impact of far-infrared radiances

This paper describes the first retrieval of cirrus optical and microphysical properties from ground-based measurements simultaneously with co-located measurements from aircraft. In particular, the present effort exploits infrared radiances spanning the mid to far-infrared spectral regime based on co-located in-situ aircraft sampling and ancillary ground-based remote sensing. Spectrally resolved radiances covering the range 400–1500 cm−1, in-situ measurements of cirrus particle sizes and habits, backscatter ceilometer observations of cloud vertical structure and microwave inferred temperature and humidity profiles are used to investigate whether we can obtain consistency between the derived cloud properties and atmospheric state from these independent sources of data. The primary focus of this study is on the sensitivity of the retrieved cloud particle size to the assumed crystal habit. Excellent consistency of the retrieved cloud parameters is achieved both with the ceilometer derived optical depth and the size distribution measured by the aircraft by assuming the crystal habit to be comprised of bullet rosettes. The averaged values of the effective diameter and optical depth obtained from radiometric measurements are (26.3±0.5) μm and (0.130±0.004) in comparison with the values derived from in-situ and ceilometer measurements equal to (31.5±5.0) μm and (0.120±0.004), respectively. Furthermore, we demonstrate that the radiance information contained within the far-infrared (wavenumbers < 650 cm−1) spectrum is critical to achieving this level of agreement with the in-situ aircraft observations. The results emphasize why it is vital to expand the currently limited, database of measurements encompassing the far-infrared spectrum, particularly in the presence of cirrus.