The global aerosol–climate model ECHAM6.3–HAM2.3 – Part 2: Cloud evaluation, aerosol radiative forcing, and climate sensitivity

Abstract

Abstract. The global aerosol–climate model ECHAM6.3–HAM2.3 (E63H23)\nas well as the previous model versions ECHAM5.5–HAM2.0 (E55H20) and ECHAM6.1–HAM2.2\n(E61H22) are evaluated using global observational datasets for clouds and\nprecipitation. In E63H23, the amount of low clouds, the liquid and ice water path, and\ncloud radiative effects are more realistic than in previous model versions.\nE63H23 has a more physically based aerosol activation scheme, improvements\nin the cloud cover scheme, changes in the detrainment of convective clouds,\nchanges in the sticking efficiency for the accretion of ice crystals by snow,\nconsistent ice crystal shapes throughout the model, and changes in mixed-phase\nfreezing; an inconsistency in ice crystal number concentration (ICNC) in\ncirrus clouds was also removed. Common biases in ECHAM and in E63H23 (and in\nprevious ECHAM–HAM versions) are a cloud amount in stratocumulus\nregions that is too low and deep convective clouds over the Atlantic and Pacific oceans\nthat form too close to the continents (while tropical land precipitation is\nunderestimated). There are indications that ICNCs are overestimated in\nE63H23. Since clouds are important for effective radiative forcing due to\naerosol–radiation and aerosol–cloud interactions (ERF ari+aci ) and\nequilibrium climate sensitivity (ECS), differences in ERF ari+aci \nand ECS between the model versions were also analyzed. ERF ari+aci is weaker\nin E63H23 ( −1.0 \u2009W\u2009m −2 ) than in E61H22 ( −1.2 \u2009W\u2009m −2 ) (or E55H20;\n −1.1 \u2009W\u2009m −2 ). This is caused by the weaker shortwave ERF ari+aci \n(a new aerosol activation scheme and sea salt emission parameterization in\nE63H23, more realistic simulation of cloud water) overcompensating for the\nweaker longwave ERF ari+aci (removal of an inconsistency in ICNC in\ncirrus clouds in E61H22). The decrease in ECS in E63H23 (2.5\u2009K) compared to E61H22 (2.8\u2009K) is due to\nchanges in the entrainment rate for shallow convection (affecting the cloud\namount feedback) and a stronger cloud phase feedback. Experiments with minimum cloud droplet number concentrations (CDNCmin) of\n40\u2009cm −3 or 10\u2009cm −3 show that a higher value of CDNCmin reduces\nERF ari+aci as well as ECS in E63H23.