A Prospectus for Constraining Rapid Cloud Adjustments in General Circulation Models

Abstract

Rapid cloud adjustments are an important component of the atmosphere's total response to increased CO2 concentrations. Unfortunately, scientific understanding of rapid shortwave cloud adjustments is rather poor. State‐of‐the‐art 5th Coupled Model Intercomparison Project models showed large uncertainty in regard to rapid cloud adjustments. This study determines whether large‐eddy simulations may, in principle, be used as a reference, thanks to their ability to resolve cloud dynamics and thermodynamics, to constrain rapid shortwave cloud adjustments in general circulation models. This is an open question since large‐eddy models can only be run over limited domains, for a short period of time, and are influenced by boundary conditions. Using the Icosahedral Non‐hydrostatic global climate model—Atmospheric component (ICON‐A), we examine shortwave rapid cloud adjustments over central Europe, which is found to be representative of shortwave rapid cloud adjustments over Northern Hemispheric global continents in the 5th Coupled Model Intercomparison Project models. This work finds (i) a couple of days of simulation is sufficient to get a clear signal in the net top‐of‐atmosphere radiative balance to emerge after a 4xCO2 perturbation and (ii) use of present‐day meteorological and CO2 concentrations for boundary conditions in global simulations is not an issue for short lead times, up to ∼36 hr. We also found that atmospheric processes influencing shortwave rapid cloud adjustments over central Europe are largely thermodynamically driven changes in local cloud dynamics and are rather independent of the synoptic‐scale and circulation effects on short timescales (<2 days). These results imply that high‐resolved large‐eddy simulations over a limited area can be instructive for assessing and constraining global rapid cloud adjustments.

Absorbing aerosol decreases cloud cover in cloud‐resolving simulations over Germany

“…Nam et al . (2018) further argued that mechanisms that lead to rapid cloud adjustments over Central Europe are representative for the continental Northern Hemisphere and that such high‐resolution simulations like ours can be helpful for assessing and constraining global rapid cloud adjustments. Another shortcoming is that the applied aerosol perturbation leads to a transient atmospheric response that has not reached equilibrium within the short integration time and over the limited domain size.…”

“…Although the case offered a good mixture of convective and stratiform clouds, it is not clear to what extent similar responses can be found for different weather situations. However, it has been discussed by Nam et al (2018) that atmospheric processes influencing short-wave rapid cloud adjustments over Central Europe are mainly caused by local cloud dynamics and are, for shorter time periods, rather independent of the synoptic-scale circulations. Nam et al (2018) further argued that mechanisms that lead to rapid cloud adjustments over Central Europe are representative for the continental Northern Hemisphere and that such high-resolution simulations like ours can be helpful for assessing and constraining global rapid cloud adjustments.…”

“…However, it has been discussed by Nam et al (2018) that atmospheric processes influencing short-wave rapid cloud adjustments over Central Europe are mainly caused by local cloud dynamics and are, for shorter time periods, rather independent of the synoptic-scale circulations. Nam et al (2018) further argued that mechanisms that lead to rapid cloud adjustments over Central Europe are representative for the continental Northern Hemisphere and that such high-resolution simulations like ours can be helpful for assessing and constraining global rapid cloud adjustments. Another shortcoming is that the applied aerosol perturbation leads to a transient atmospheric response that has not reached equilibrium within the short integration time and over the limited domain size.…”

“…However, it has been discussed by Nam et al . (2018) that atmospheric processes influencing short‐wave rapid cloud adjustments over Central Europe are mainly caused by local cloud dynamics and are, for shorter time periods, rather independent of the synoptic‐scale circulations. Nam et al .…”

“…Therefore, energy and moisture need to be freely exchanged across the boundaries of the limited‐area domain. Feedbacks onto synoptic‐scale circulation systems will become more relevant for time‐scales longer than a few days (Nam et al ., 2018).…”