Methodology

In pseudo-global warming (PGW) experiments,  selected changes in the climate system are imposed on a Limited Area Model (LAM) simulation by modifying the spatial boundary conditions. In simple mathematical terms, the PGW concept can be expressed as:

PGW = HIST + \Delta

Where HIST and PGW represent the boundary conditions of the two LAM simulations of the past and the future climate and \Delta are the future changes . \Delta can be computed from separate climate projections as:

\Delta= SCEN - CTRL

Where SCEN is a future time slice of a climate projection and CTRL is the corresponding historical time slice coming from a GCM (see Fig 5.2). The changes cover thermodynamic and dynamic aspects and are designed to have a seasonal cycle, but they neglect potential changes in interannual variability.

 

 

The perturbation fields \Delta are meant to be based on 30-year monthly means of the driving GCM. The required GCM fields include three-dimensional fields of temperature, specific humidity and wind (T, qv, u, v), as well as the geopotential height field (f) on at least one pressure level. Monthly mean changes of these fields are linearly interpolated to the target date, and then merged with ERA5 information.

In order to prepare PGW simulations for the EUREC4A MIP, perturbation fields D have been derived from AMIP and AMIP-p4K experiments. The perturbations fields D are derived from different GCMs  (GFDL, HadGEM3, IPSL and NorESM) that have different Equilibrium Climate Sensitivities (ECS). The proposal is to rerun the present-day SRM and LEM simulations on their respective domains and duration and use the results to assess how the various cloud patterns will respond to the warming and  to quantify the associated change in cloud radiative effect (CRE). This should fill an important gap in our understanding and our assessment of the role of mesoscale organization on low-cloud feedback.

 


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