Response to comments on The global aerosol-climate model ECHAM6.3-HAM2.3 – Part 2: Cloud evaluation, aerosol radiative forcing and climate sensitivity, Geosci. Model Dev. Discuss., gmd-2018-307

The global aerosol-climate model ECHAM6.3-HAM2.3 (E63H23) and the previous model versions ECHAM5.5HAM2.0 (E55H20) and ECHAM6.1-HAM2.2 (E61H22) are evaluated using global observational datasets for clouds and 15 precipitation. In E63H23 low cloud amount of low clouds, liquid and ice water path and cloud radiative effects are more realistic than in previous model versions. E63H23 has a more physically based aerosol activation scheme, improvements in the cloud cover scheme, changes in detrainment of convective clouds, changes in the sticking efficiency for accretion of ice crystals by snow, consistent ice crystal shapes throughout the model, changes in mixed phase freezing and an inconsistency in ice crystal number concentration (ICNC) in cirrus clouds was removed. Biases that were identifiedCommon biases in 20 ECHAM and in E63H23 (and in previous modelECHAM-HAM versions) are a too low cloud amount in stratocumulus regions, and that deep convective clouds inover the Atlantic and Pacific oceans form too close to the continents and there(while tropical land precipitation is underestimated). There are indications that ICNCs are overestimated in E63H23. Since clouds are important for effective radiative forcing due to aerosol-radiation and aerosol-cloud interactions (ERFari+aci) and equilibrium climate sensitivity (ECS), also differences in ERFari+aci and ECS between the model versions were analyzed. 25 ERFari+aci is weaker in E63H23 (-1.0 W m) than in E61H22 (-1.2 W m) (or E55H20; -1.1 W m). This is caused by the weaker shortwave ERFari+aci (new aerosol activation scheme and sea salt emission parameterization in E63H23, more realistic simulation of cloud water) overcompensating the weaker longwave ERFari+aci (removal of an inconsistency in ICNC in cirrus clouds in E61H22). The decrease in ECS in E63H23 (2.5 K) compared to E61H22 (2.8 K) is due to changes in the entrainment rate for shallow 30 convection (affecting the cloud amount feedback) and a stronger cloud phase feedback. Experiments with minimum cloud droplet number concentrations (CDNCmin) of 40 cm or 10 cm show that a higher value of CDNCmin reduces ERFari+aci as well as ECS in E63H23.