Abstract
Abstract. A global data set of vertical profiles of polar stratospheric cloud (PSC) volume density has been derived from Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) space-borne infrared limb measurements between 2002 and 2012. To develop a well characterized and efficient retrieval scheme, systematic tests based on limb-radiance simulations for PSCs from in situ balloon observations have been performed. The finally selected wavenumber range was 831–832.5 cm−1. Optical constants of nitric acid trihydrate (NAT) have been used to derive maximum and minimum profiles of volume density which are compatible with MIPAS observations under the assumption of small, non-scattering and larger, scattering PSC particles. These max/min profiles deviate from their mean value at each altitude by about 40 %–45 %, which is attributed as the maximum systematic error of the retrieval. Further, the retrieved volume density profiles are characterized by a random error due to instrumental noise of 0.02–0.05 µm3 cm−3, a detection limit of about 0.1–0.2 µm3 cm−3 and a vertical resolution of around 3 km. Comparisons with coincident observations by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) satellite showed good agreement regarding the vertical profile shape. Quantitatively, in the case of supercooled ternary solution (STS) PSCs, the CALIOP dataset fits to the MIPAS retrievals obtained under the assumptions of small particles. Unlike for STS and NAT, in the case of ice PSCs the MIPAS retrievals are limited by the clouds becoming optically thick in the limb-direction. In these cases, the MIPAS volume densities represent lower limits. Among other interesting features, this climatology helps to study quantitatively the on-set of PSC formation very near to the South Pole and the large variability of the PSC volume densities between different Arctic stratospheric winters.
Highlights
Polar stratospheric clouds (PSCs) form when temperatures fall below about 195 K in the Arctic and Antarctic stratosphere during wintertime
We have performed the first quantitative retrievals of particle volume density of PSC observations for the whole Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) operational period. This is an extension of the available MIPAS datasets on PSC existence, cloud-top and composition as, e.g., published by Spang et al (2018)
The retrieval of volume densities of PSCs is complicated through the fact that the particles of nitric acid trihydrate crystals (NAT) and ice PSCs can be so large that the scattered contribution to the detected radiance cannot be neglected
Summary
Polar stratospheric clouds (PSCs) form when temperatures fall below about 195 K in the Arctic and Antarctic stratosphere during wintertime. In a first step we have produced a test dataset by simulating MIPAS limb radiances of PSCs including scattering of tropospheric radiation for different tropospheric cloud scenarios These synthetic observations provide the basis for simulations to optimize the retrieval settings with respect to wavelength range and refractive index set to derive PSC volume density profiles. This parameter set is subsequently used to process the entire MIPAS dataset on basis of which comparisons with measurements by CALIOP has been performed. In the following these different steps are presented in more detail
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