Abstract
The vertical temperature structure in the tropics is primarily set by convection and therefore follows a moist adiabat to first order. However, tropical upper-tropospheric temperatures differ among climate models and observations, as atmospheric convection remains poorly understood. Here, we quantify the variations in tropical lapse rates in CMIP6 models and explore reasons for these variations. We find that differences in surface temperatures weighted by the regions of strongest convection cannot explain these variations and, therefore, we hypothesize that the representation of convection itself and associated small-scale processes are responsible. We reproduce these variations in perturbed physics experiments with the global atmospheric model ICON-A, in which we vary autoconversion and entrainment parameters. For smaller autoconversion values, additional freezing enthalpy from the cloud water that is not precipitated warms the upper troposphere. Smaller entrainment rates also lead to a warmer upper troposphere, as convection and thus latent heating reaches higher. Furthermore, we show that according to most radiosonde datasets all CMIP6 AMIP simulations overestimate recent upper-tropospheric warming. Additionally, all radiosonde datasets agree that climate models on average overestimate the amount of upper-tropospheric warming for a given lower-tropospheric warming. We demonstrate that increased entrainment rates reduce this overestimation, likely because of the reduction of latent heat release in the upper troposphere. Our results suggest that imperfect convection parameterizations are responsible for a considerable part of the variations in tropical lapse rates and also part of the overestimation of warming compared to the observations.
Significance Statement
A major criticism of climate model simulations has been their overestimation of warming in the tropical upper troposphere, between 8- and 13-km altitude, compared to observations. We show that climate models already disagree on the mean upper-tropospheric temperatures, even before warming. We demonstrate that the process of how much a convective cloud mixes with its surroundings, so-called entrainment, significantly influences upper-tropospheric temperatures and their rate of warming. Increasing entrainment decreases the heat released by condensation, which in turn reduces upper-tropospheric warming to resemble the observed warming. Improving the representation of this process in climate models, as well as other aspects of convection, should therefore be beneficial for the simulation of upper-tropospheric temperatures.
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