A51E-0106
Energetically consistent large-eddy simulations of subtropical cloud-topped boundary layers under idealized climate change
Energetically consistent large-eddy simulations of subtropical cloud-topped boundary layers under idealized climate change
Friday, 18 December 2015
Poster Hall (Moscone South)
Abstract:
How subtropical marine boundary layer (MBL) clouds respond to climate change is one of the central uncertainties in climate projections. Large-eddy simulations (LES) have been used to study this response. However, the sea surface temperature (SST) or surface fluxes are usually prescribed explicitly in these simulations, and the surface energy budget may not be closed.We present LES experiments that satisfy energetic and large-scale dynamical constraints. The SST is allowed to evolve interactively with radiative and heat fluxes at sea surface so that the surface energy budget is closed. The free troposphere is relaxed towards a radiative-convective equilibrium profile representing the deep tropics, and the subsidence velocity is set to be consistent with the dynamical constraints of the tropical circulations. The LES is coupled with the RRTMG radiation scheme and a warm-rain microphysics scheme. Representative cases of cumulus and stratocumulus under current climate conditions are tested and compared with reanalysis data, validating the capacity of this LES configuration to realistically represent the subtropical MBL cloud regimes.
We have investigated the steady-state subtropical MBL cloud responses over a wide range of climates with this energetically consistent LES configuration and have compared the results to those from simulations with prescribed SST increase of 4K/2xCO2. While both sets of simulations show dissipation of stratocumulus under warming induced by quadrupling CO2, the simulated cumulus responses are different. With prescribed SST, surface latent heat flux (LHF) increases so strongly that the surface energy balance constraint is violated, resulting in a deepening of the cumulus layer. With the energetically consistent configuration, the increase of LHF is modest and the cumulus layer shallows with reduced liquid water path (LWP). The cumulus shortwave response to warming differs correspondingly.
The contrasting results highlight the importance of closing the surface energy budget and formulating large-scale forcing consistently in LES experiments, as well as the importance of taking into account large-scale dynamics and energetic constraints in developing a mechanistic understanding of the MBL cloud response to warming.