An emulator approach to stratocumulus susceptibility

Glassmeier, Franziska; Hoffmann, Fabian; Johnson, Jill S.; Yamaguchi, Takanobu; Carslaw, Ken S.; Feingold, Graham

The climatic relevance of aerosol–cloud interactions depends on the sensitivity of the radiative effect of clouds to cloud droplet number N, and liquid water path LWP. We derive the dependence of cloud fraction CF, cloud albedo AC, and the relative cloud radiative effect rCRE=CFAC on N and LWP from 159 large-eddy simulations of nocturnal stratocumulus. These simulations vary in their initial conditions for temperature, moisture, boundary-layer height, and aerosol concentration but share boundary conditions for surface fluxes and subsidence. Our approach is based on Gaussian-process emulation, a statistical technique related to machine learning. We succeed in building emulators that accurately predict simulated values of CF, AC, and rCRE for given values of N and LWP. Emulator-derived susceptibilities ln⁡rCRE/ln⁡N and ln⁡rCRE/ln⁡LWP cover the nondrizzling, fully overcast regime as well as the drizzling regime with broken cloud cover. Theoretical results, which are limited to the nondrizzling regime, are reproduced. The susceptibility ln⁡rCRE/ln⁡N captures the strong sensitivity of the cloud radiative effect to cloud fraction, while the susceptibility ln⁡rCRE/ln⁡LWP describes the influence of cloud amount on cloud albedo irrespective of cloud fraction. Our emulation-based approach provides a powerful tool for summarizing complex data in a simple framework that captures the sensitivities of cloud-field properties over a wide range of states.



Glassmeier, Franziska / Hoffmann, Fabian / Johnson, Jill S. / et al: An emulator approach to stratocumulus susceptibility. 2019. Copernicus Publications.


Rechteinhaber: Franziska Glassmeier et al.

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