Impact of thermodynamic surface properties on climate and climate change in a radiative-convective equilibrium version of ECHAM6

The comprehensive general circulation model ECHAM6 is used in a radiative-convective equilibrium (RCE) setup with the aim to enhance understanding of key processes concerning climate sensitivity and land-sea warming contrast. RCE is ensured by setting earth’s angular velocity to zero, specifying homogeneous insolation and defining a thermodynamic mixedlayer ocean (MLO) with no internal horizontal fluxes. The most important thermodynamic properties of the MLO, the heat capacity and the evaporative flux, are reduced in order to get a “land-like” climate. The ECHAM6-RCE climate resembles the tropical climate over the ocean in the realistic configuration of ECHAM6 remarkably well. This applies in particular to global mean temperature and precipitation as well as to the vertical atmospheric structure, including lapse rate, a bimodal relative humidity distribution and cloud and condensate profiles. Reducing the resolution from T63L47 to T31L47 does not change the main RCE climate features, though convection cluster size and time scale of vertical motions increase. The heat capacity is reduced by decreasing the MLO depth. The diurnal surface temperature range (DTR) increases, resulting in a shift of maximum precipitation from nightto daytime at an MLO depth of 0.1 m. This has a strong impact on atmospheric processes. For example, there is more convection on the daily average. Climate sensitivity remains approximately constant for sufficiently large MLO depths, but decreases by about a third when DTR starts influencing the atmosphere because boundary layer clouds dissipate in the control experiment. Subsequently, climate sensitivity increases due to a positive feedback mechanism involving 700 hPa cloud formation which encourages low SSTs and which is more effective with a less opaque atmosphere. The evaporative flux is restricted by introducing a new property, the evaporation resistance. An increase of evaporation resistance leads to a strong decrease of latent heat flux, which is primarily balanced by an increase of net longwave radiation from the surface and secondary by an increase of sensible heat flux, leading to “land-like” Bowen ratios. In response, surface temperatures increase until the atmosphere is so dry that longwave radiation is very efficient. With increasing evaporation resistance, a transition to a climate with only one convective cluster occurs. The self-aggregation leads to drier conditions in the subsidence regime. Concerning climate sensitivity, the experiments with this idealized land surface suggest that climate sensitivity is largest in areas where the negative lapse rate feedback is much smaller than over the ocean, but where the positive water vapor feedback is still large. Experiments in which a 0.1 m deep MLO is combined with increasing evaporation resistances confirm the previous results, especially concerning climate sensitivity, although surface temperatures increase even more and feedback factors change. Simulations with idealized continents show that surface inhomogeneities particularly influence the location where convection is triggered.