## 3 Combining Infrared and Microwave Observations  
Exploiting the knowledge acquired so far, we can certainly state that IR and MW radiation are complementary. First, spaceborne radiometers detect IR radiation coming mainly from the top of the clouds, due to the strong absorption of IR wavelengths via cloud-forming hydrometeors, unlike MW radiation, which is only slightly affected as it passes through non-precipitating clouds. Furthermore, it is widely known that IR radiation is more responsive to small particles and rather low CIWP cirrus clouds. On the other hand, MW radiation is responsive to larger ice crystals and thicker cirrus cloud layers. These peculiarities of MW and IR radiation are schematically summarized in Figure 2, and they seem to suggest that a proper combination of MW and IR multispectral measurements could help determine the vertical structure and composition of clouds, especially in the case of multilayered clouds. Figure 2: Sketches of the interaction between MW (**top**) and IR (**bottom**) radiation with thick convective cloud (**left**), low liquid cloud (**middle**), and cirrus cloud (**right**). Wave transparency indicates the radiation intensity after cloud interaction qualitatively. Also reported are the cloud phase and dominant cloud–radiation interaction (emission, absorption, and scattering). In this context, ref. [103] proposed the new Microwave Visible and Infrared (MVI) technique. MVI allows the estimation of the frequency of multilayered clouds and the effective radius of droplets in water clouds, combining the possibility of deriving the CLWP and the water cloud temperature (Tw) from MW observations. This allows validation with radiosoundings and retrieval using a combined VIS and IR retrieval method. In [104], the authors reported the results of a series of tests on the MVI technique using Meteosat and SSM/I data collected in the framework of the Atlantic Stratocumulus Transition Experiment. The combination of satellite and ground-based radar data demonstrates the ability of the MVI technique to detect multilayered clouds, especially when low water clouds underlie higher ice clouds. The CLWP resulting from this method was evaluated against the corresponding product obtained from ground observation. The results show a correlation coefficient of 0.54 and a mean difference ground-based versus SSM/I CLWP estimation of 0.034 kg/m\({}^{2}\) or approximately 30% of the mean value with a maximum in the case of ground observations of locally high CLWP values (\(>\)0.2 kg/m\({}^{2}\)). The benefits of the synergy between IR and MW are evident in the work of [105]. They address the cloud base height problem with a combination of MW observations from SSM/I and Special Sensor Microwave Water Vapor Sounder (SSM/T-2) on the Defense Meteorological Satellite Program (DMSP) satellite from a theoretical point of view. However, they find that only across the ocean is the signal strong. Retrievability over a significant range in CLWP improves significantly using the synergy of MW and IR data acquired from the same satellite (AMSU and AVHRR on NOAA-K). This is due to the ability to satisfy IR observations' cloud-top temperature constraint. The potential for synergy between MW and IR is also clear in 1D-Var approaches, as in the paper of [106]. In this study, cloud-influenced observations are analyzed with a 1D-Var approach including both IR and MW sensors (HIRS/3 and AMSU-A aboard NOAA-15). A fast radiative transfer model, based on RTTOV 10.1 (see [72], Section 2.6.1), is presented, taking into account the specificities of both IR radiation (treating the cloud cover as a single semitransparent layer, defined by the cloud-top pressure and its effective amount) and MW (computing the cloud absorption from the profiles of cloud cover and liquid/ice water on any vertical pressure level). The developed scheme is tested with synthetic observations on individual profiles and in the real case of cloud retrieval from observations on 15 March 2001.