## 2 Clouds Properties from Infrared and Microwave Satellite Observations
### Microwave Observations Only
The CLWP product is validated against both ground radiometric measurements (ARM site in Azores, Portugal) and a satellite reference dataset (NASA GPROF). In this work, the collocation method for the ground-based comparisons considers observations within 30 min from the satellite overpass and an FOV within a 3 km radius centered on the site. The dataset includes data collected over 3 years (for a total of 1535 match-ups), and the results show an overall all-season correlation coefficient, bias, and standard deviation of 0.59, \(-\)0.065 kg/m\({}^{2}\), and 0.2 kg/m\({}^{2}\), respectively. In detail, the seasonal analysis shows good correlation coefficients (greater than 0.5 for all seasons), with a maximum value for autumn (0.66) and a minimum for summer (0.51). Similarly, as regards the bias, the values are between \(-\)0.052 kg/m\({}^{2}\) (found in autumn) and \(-\)0.076 kg/m\({}^{2}\) (found in summer). Finally, the comparison with GPROF shows a correlation coefficient, bias, and standard deviation equal to 0.71, 0.005 kg/m\({}^{2}\), and 0.07 kg/m\({}^{2}\), respectively. Ref. [30] uses a neural network approach to retrieve the snow ice water path, liquid-water path, and integrated water vapor from millimeter and submillimeter brightness temperatures from airborne radiometers (ISMAR and MARSS). The database for training neural networks was built with ICON-NWP atmospheric profiles and Atmospheric Radiative Transfer Simulator (ARTS) simulations. Comparing the retrieved integrated water vapor (IWV) values with those measured via the 12 dropsondes reveals a mean difference between them of 0.5 kg/m\({}^{2}\) and an RMSD of 0.8 kg/m\({}^{2}\). Recently, the progress of technology, combined with the advancement of MW radiometry, has allowed the use of cm-sized mini-satellites (cubesat) for Earth observation. Time-Resolved Observations of Precipitation Structure and Storm Intensity with a Constellation of Smallsats (TROPICS) and the Temporal Experiment for Storms and Tropical Systems Technology-Demonstration (TEMPEST-D) are two NASA-funded missions that exploit MW radiometers on board the cubesat modules. The aim of TROPICS [31] is to provide profiles of water vapor and temperature, as well as the liquid/ice rain rate, utilizing seven channels near the oxygen absorption line at 118.75 GHz, three channels near the line of the water vapor absorption at 183 GHz, and single channels near 90 and 205 GHz. TEMPEST-D [32] aims to provide measurements of water vapor profiles, CLWP, and CIWP with an accuracy of 0.1 kg/m\({}^{2}\), exploiting frequencies between 89 and 183 GHz. Note that some of these channels (e.g., the 118 GHz band and the 205 GHz channel) have never been tested in space and could serve as proxies for channels foreseen for the MicroWave Imager (MWI), 118.75 GHz, and MicroWave Sounder (MWS), 229 GHz. The TEMPEST-D measurements appear to be of similar quality to the MHS measurements, as highlighted in [33], both for the consistency between the TEMPEST-D and the Microwave Humidity Sounder (MHS) brightness temperatures and for that of the atmospheric parameters retrieved from TEMPEST-D via the Colorado State University (CSU) 1D-VAR algorithm with those retrieved from MHS. This applies to both MIRS retrievals and CSU 1D-VAR retrievals. Table 1 summarizes the main information for each study reviewed in Section 2.1, the sensors and channels used, and the products investigated.