Diurnal asymmetry of climatic response to increased CO2 and aerosols: Forcings and feedbacks

To examine the causes of the observed diurnally asymmetrical climate change over land, the roles of different physical mechanisms are evaluated using a radiative-convective model of the diurnal cycle. This model explicitly calculates a complete set of physical processes, including the water vapor distribution, clouds, transports in the turbulent boundary layer, and convection. Calculations were carried out for midlatitude summer and winter and for tropical spring conditions taking into account the most important climate forcings: CO2 increase, tropospheric aerosol pollution, and the combined case with simultaneous CO2 and aerosol effects. We rind that feedbacks in the climate system are more important than forcings in producing diurnal asymmetry. The water vapor shortwave feedback dominates the diurnal distribution of the response. For all cases with warming, the diurnal temperature range (DTR) decreases, not due to the greenhouse effect of water vapor, but as a result of more intensive absorption of the solar radiation in the near infrared by water vapor and cloud water in a warmer, wetter climate independent of the type of forcing. Aerosol reflection and absorption of solar radiation cool the surface and decrease DTR directly, but the negative daytime water vapor feedback virtually cancels out the diurnal asymmetry. In the combined case, with a 50% CO2 content increase combined with tropospheric aerosol pollution, which is not far from the current observed conditions over land, the greenhouse warming raises the temperature enough that the direct aerosol effect decreases the DTR. In all cases the time and spatial redistribution of clouds have a significant impact on the climate sensitivity and diurnal cycle. As in the observations, increasing of cloudiness and water vapor content occurs with decreasing of the DTR. In our model the cloudiness and water vapor changes are produced by the same forcings that lower the DTR; they are not independent causes of changes of the DTR, but rather are important internal feedback mechanisms.