## 2 Clouds Properties from Infrared and Microwave Satellite Observations
### Microwave Observations Only  
Due to its long wavelength, microwave radiation can penetrate entire cloud layers and is particularly responsive to large particles; consequently, observations are more sensitive to water vapor, clouds, and precipitation. Furthermore, given the different effects of liquid/ice water clouds on the measured brightness temperature (BT), the peculiarities of microwaves allow the detection of different types of clouds at different frequencies. For example, observations in the 23.8 and 89 GHz channels are more influenced by liquid than cirrus clouds. On the other hand, observations in frequencies close to the 157 and 183 GHz channels are more influenced by the presence of ice clouds. In the literature, several works exploit the knowledge acquired from MW radiometric applications consolidated since the late 1980s, which have seen the development of various methods to retrieve meteorological quantities based on physical and empirical methods. The most used methods are reviewed below; they generally exploit different channels (23.8, 31.4, 90, and 183 GHz) to find information on cloud liquid water and atmospheric water vapor, and the 90 and 150 GHz and higher-frequency window channels to obtain information on ice clouds and the rain rate. Early attempts to retrieve cloud microphysical parameters from MW radiance were mainly concerned with the knowledge of ocean surface winds [9], total precipitable water (TPW) [10], CIWP, CLWP and precipitation [11], and CLWP and cloud frequency [12] using the Special Sensor Microwave/Imager (SSM/I) measurements. Subsequently, various algorithms have been developed to meet the specifications of increasingly new sensors. This is, for example, the case for [13], where the development of a regression scheme to retrieve TPW and CLWP over the ocean using the channels at 23.8 and 31.4 GHz of the multichannel MW radiometer Advanced Microwave Sounding Unit (AMSU) is described. This algorithm, although based on the same TPW and CLWP retrieval theoretical background from the SSM/I, differs from the latter due to the geometric and spectral particularities of the two instruments. Several comprehensive comparisons between TPW and CLWP retrievals from AMSU and other satellite instruments (including SSM/I and Tropical Rainfall Measurement Mission Microwave Imager--TMI) or ground-based radiometers [14] can be found in the literature. Many works also demonstrate good agreement between AMSU TPW and radiosonde measurements, showing root mean square differences (RMSDs) of less than 3 kg/m\({}^{2}\) and biases of less than 1 kg/m\({}^{2}\) in the range between 5 and 60 kg/m\({}^{2}\). AMSU radiances have also been used for CLWP retrieval, for which it has been shown that AMSU time series and ground-based sensors follow each other in the range of 0-0.5 kg/m\({}^{2}\). As shown by [14], a comparison with other satellite measurements also yields good results despite a bias between AMSU and TMI for CLWP greater than 0.5 kg/m\({}^{2}\). The measurements at the frequencies of 89 and 150 GHz of the microwave sounder AMSU were also used by [15] to formulate an algorithm for the retrieval of CIWP and D\({}_{\rm e}\) to exploit the ice particle scattering parameters. In the same work, a careful analysis of the main sources of error that can influence the retrieval process and a screening procedure to distinguish the scattering signatures between atmospheric clouds and surface materials were performed. It was shown that the recovered D\({}_{\rm e}\) is mainly influenced by two error terms, related to the particle bulk volume density (which, in this work, is assumed to be constant and equal to 600 kg/m\({}^{3}\)) and the cloud base (BT) estimate at 89 and 150 GHz. Overall, these two terms impact the D\({}_{\rm e}\) value by 10% (considering a 3% error in the BT temperature) and between 5% and 20% (considering a 30% uncertainty in the bulk volume density of the particles).