## 2 Clouds Properties from Infrared and Microwave Satellite Observations
### Infrared Observations Only
In contrast, the clouds' reflectance at an absorbing wavelength (1.6 or 3.8 \(\upmu\)m) is mainly related to the particle size. Consequently, using operational calibrations, SEVIRI and AVHRR cloud properties differ significantly; this recalibration improved the differences between the cloud properties retrieved via SEVIRI and AVHRR, bringing the values below 5%. Ref. [53] addresses the problem of retrieval of cloud properties (effective radius, optical thickness, and cloud temperature) from MODIS infrared imagery using an NN approach. The multilayer perceptron (MLP) architecture was trained with supervised backpropagation with a momentum algorithm. To evaluate the robustness of the method, several sources of uncertainties were analyzed, such as the measured radiance, cloud temperature specifications, and lower boundary conditions. The total error lies between 0.25 and 0.45 \(\upmu\)m for the effective radius and between 0.05 and 0.88 for the COT when simultaneous uncertainties are considered. According to the authors, these uncertainties dominate the uncertainty budget over the errors introduced via the NN inversion, estimated within 1%. Ref. [54] presents the Integrated Cloud Analysis System (ICAS) algorithm for retrieving the macroscopic, physical, and optical properties of clouds. ICAS was developed based on the optimal estimation approach, and it exploits MODIS observations in Thermal InfraRed (TIR) bands. Four TIR bands centered at 8.6, 10.4, 11.2, and 12.4 \(\upmu\)m are identified in the atmospheric window to retrieve parameters such as cloud-top height (CTH), COT, and the effective radius, D\({}_{\text{e}}\)/2, which are sensitive to the properties of clouds. Performance is tested using retrieval simulations, showing that ice cloud properties are inferred with high accuracy for COT between 0.1 and 10. Cloud-top pressure is retrieved with an uncertainty of less than 10% when COT is greater than 0.3. ICAS is also applied to the multiband observations of the Himawari-8 instrument [55]. A sensitivity study is performed to demonstrate that the addition of the single CO\({}_{2}\) band of Himawari-8 is effective for CTH estimation. For validation purposes, a systematic comparison is conducted between the retrieved cloud properties and collocated active remote sensing counterparts with small time delays. Reasonable agreement is found for single-layer clouds, while multilayer cloud systems with optically thin upper clouds overlapping lower clouds are the main source of error. The variance in measurement model differences within an area of (10 km)\({}^{2}\) is used to estimate measurement noise. Comparisons are performed between ICAS-derived data and the raDAR/liDAR (DARDAR) cloud product to validate the retrievals. The DARDAR project provides cloud mask vertical profiles and ice cloud properties, including the ice water content, D\({}_{\text{e}}\), and extinction coefficient obtained from combined MODIS observations, CloudSat radar, and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar, as well as TIR measurements, with a vertical resolution of 60 m and a horizontal spatial resolution of 1.1 km. Data from high-spectral-resolution infrared sounders can provide consistent microphysical cloud properties, and the benefits of their use have been recognized since the 1970s [56]. Subsequently, applications to cloud properties were explored for data from both aircraft (e.g., [57; 58]) and satellites (e.g., [59; 60]). A technique for inferring the microphysical and radiative properties of water and cirrus clouds is presented in [57]. This technique, validated using theoretical calculations, is applied to ground-based and NASA aircraft-based Hyperspectral Imaging Sensor (HIS) instrument observations, along with cloud lidar backscatter and in situ atmospheric temperature and humidity observations. This study describes the cloud optical properties' spectral variability within the window region.