Abstract
Abstract. To study climate change on multi-millennial timescales or to explore a model's parameter space, efficient models with simplified and parameterised processes are required. However, the reduction in explicitly modelled processes can lead to underestimation of some atmospheric responses that are essential to the understanding of the climate system. While more complex general circulations are available and capable of simulating a more realistic climate, they are too computationally intensive for these purposes. In this work, we propose a multi-level Gaussian emulation technique to efficiently estimate the outputs of steady-state simulations of an expensive atmospheric model in response to changes in boundary forcing. The link between a computationally expensive atmospheric model, PLASIM (Planet Simulator), and a cheaper model, EMBM (energy–moisture balance model), is established through the common boundary condition specified by an ocean model, allowing for information to be propagated from one to the other. This technique allows PLASIM emulators to be built at a low cost. The method is first demonstrated by emulating a scalar summary quantity, the global mean surface air temperature. It is then employed to emulate the dimensionally reduced 2-D surface air temperature field. Even though the two atmospheric models chosen are structurally unrelated, Gaussian process emulators of PLASIM atmospheric variables are successfully constructed using EMBM as a fast approximation. With the extra information gained from the cheap model, the multi-level emulator of PLASIM's 2-D surface air temperature field is built using only one-third the amount of expensive data required by the normal single-level technique. The constructed emulator is shown to capture 93.2 % of the variance across the validation ensemble, with the averaged RMSE of 1.33 °C. Using the method proposed, quantities from PLASIM can be constructed and used to study the effects introduced by PLASIM's atmosphere.
Highlights
Complex computer simulations are used in climate research to improve our understanding of the climate system
A clearer distinction between the ocean and the continents is modelled in PLASIM as shown in the standard deviation plot of Fig. 2
We explore the relationship between the “climate sensitivities” of the EMBM and PLASIM atmospheres, both forced by Grid ENabled Integrated Earth system modelling framework (GENIE)–EMBM sea surface temperature (SST) as discussed above, before considering how our approach could in future be extended to the fully coupled system
Summary
Complex computer simulations are used in climate research to improve our understanding of the climate system. Depending on the nature of the questions asked, these lower fidelity models might be insufficient To address this issue, an emulator is often employed to provide a statistical estimation of the expensive model’s response without the need to perform a new simulation. While the high-fidelity (complex) model is computationally expensive, the low-fidelity (simple) model is cheaper to evaluate and can be sampled more finely across the input space, providing extra information where expensive data are sparse The models forming this hierarchy can be structurally related or structurally unrelated. The emulators provide estimates of simulation results, at untried combinations of the inputs, as finely as needed, at a low cost This enables statistical methods such as history matching (Holden et al, 2010; Edwards et al, 2011) and sensitivity/uncertainty analysis (Rougier et al, 2009). Apart from above, the emulators of 2-D surface fields similar to the one constructed here can potentially be used to provide the fields needed for coupling with other climate models or components of climate models
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