Today, the CALIOP lidar and CloudSat radar have collected more than seven years of observations, and will hopefully still operate in 2016, after the EarthCARE-ATLID/CPR launch. Lidars and Radars in space provide cutting edge information on the detailed vertical structure of clouds: a key element for both the evaluation of the description of clouds in climate models, and the survey of the clouds inter-annual evolution in various climatic conditions (El Nino, variation of North Atlantic Oscillations, polar regions, etc). For this purpose, the observations collected by CALIOP and by ATLID as well as CloudSat and EarthCARE CPR need to be merged into a long-term (15 years) cloud climatology. Here, we examine the possibility of building such a climatology, with the aim of defining its accuracy and relevance for cloud inter-annual studies. We examine the differences between the instruments (wavelengths, satellite’s altitudes, telescope fields of view, multiple scattering processes, spatial resolutions) and their ability to detect the same clouds consistently. Then, we define a set of cloud detection thresholds for ATLID, CALIOP, CloudSat and EarthCARE-CPR and test against synthetic cloud scenes (cirrus and shallow cumulus) over small areas (about 200km) produced by a lidar and radar instrument simulator (ECSIM) running on Large Eddy Simulations. Doing so, we verify that the fourth instruments will be able to detect the same clouds despite their differences (e.g. their sensitivities to noise). Finally, we use the COSP lidar and radar simulator to predict the global scale cloud cover that ATLID, CALIOP, CloudSat and EarthCARE CPR would observe if they were overflying the same atmosphere predicted by a GCM. Our results suggest that a merged CALIOP/ATLID and CloudSat/CPR cloud climatology could be to be useful for clouds inter-annual studies, if the post-launch sensitivity of EarthCARE instruments is in line with what is predicted today.