The second version of the atmospheric radiative transfer simulator, ARTS, is introduced. This is a general software package for long wavelength radiative transfer simulations, with a focus on passive microwave observations. The core part provides a workspace environment, in line with script languages. New for this version is an agenda mechanism that gives a high degree of modularity. The framework is intended to be as general as possible: the polarisation state can be fully described, the model atmosphere can be one-(1D), two-(2D) or three-dimensional (3D), a full description of geoid and surface is possible, observation geometries from the ground, from satellite, and from aeroplane or balloon are handled, and surface reflection can be treated in simple or complex manners. Remote sensing applications are supported by a comprehensive and efficient treatment of sensor characteristics. Jacobians can be calculated for the most important atmospheric variables in non-scattering conditions. Finally, the most prominent feature is the rigorous treatment of scattering that has been implemented in two modules: a discrete ordinate iterative approach mainly used for 1D atmospheres, and a Monte Carlo approach which is the preferred algorithm for 3D atmospheres. ARTS is freely available, and maintained as an open-source project.

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ABSTRACT:A passive satellite radiometer operating at submillimetre wavelengths can measure cloud ice water path (IWP), ice particle size, and cloud altitude. The paper first discusses the scientific background for such measurements. Formal scientific mission requirements are derived, based on this background and earlier assessments. The paper then presents a comprehensive prototype instrument and mission concept, and demonstrates that it meets the requirements.

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[1] Retrieval and validation of upper tropospheric ice water content (IWC) measurements with the Aura Microwave Limb Sounder (MLS) are described. The MLS version 2.2 (V2.2) IWC, derived from 240-GHz cloud-induced radiances (Tcir) at high tangent heights, is scientifically useful at 215-83 hPa. The V2.2 IWC represents a bulk cloud property averaged over a $300 Â 7 Â 4 km 3 volume near the pointing tangent height. Precision, accuracy, and spatial resolution of the V2.2 IWC are determined through model simulations and comparisons with CloudSat observations. Comparisons of MLS V2.2 and CloudSat R03 IWCs are made for the months of January and July in terms of normalized probability density function (PDF). The differences between MLS and CloudSat IWC PDFs are generally less than 50% over the IWC range where the MLS technique is valid. At pressures <177 hPa and extratropical latitudes, the MLS V2.2 IWC exhibits a slightly low bias compared to CloudSat, part of which can be attributed to systematic errors in the MLS retrieval. Cloud inhomogeneity and particle size distribution are the leading sources of uncertainties in the V2.2 IWC.