Shallow Cumulus over the subtropical oceans is the most abundant cloud type  in our climate system and its radiative response to global warming is highly uncertain. Previous MIPs on shallow cumulus convection have primarily explored the representation of spatially unorganised shallow cumulus convection and their response to climate warming.

It has become clear however over the last decade that marine shallow cumulus convection has a natural strong tendency to develop into mesoscale organised cloud structures and that unorganised shallow cumulus convection is the exception rather than the rule.

It is particular challenging to realistically simulate these mesoscale cloud patterns. On the one hand, this requires turbulence resolving resolutions to represent the small-scale vertical convective mixing processes while at the same time domains of several hundreds of kilometres are needed to represent the observed mesoscale cloud structures. It is only recently that the computational capability is allowing us to simulate these rich structures.

It is for this reason that we propose a MIP on shallow cumulus convection over the Northern Atlantic subtropical ocean such as observed during the EUREC4A field campaign in January-February 2020.

The main objectives of this MIP are:

  • Assessing the simulation capability of the observed shallow cloud mesoscale organisaiton over the subtropical ocean.
  • Understanding the underlying dynamical processes leading to the mesoscale organisaitonal patterns
  • Assessing the radiative response of this weather regime to climate warming using the Pseudo-Global Warming(PGW) framework

Ideally this requires models that simulate the atmosphere at turbulent resolving resolutions of 100 meter on domains of several thousands of kilometres. At present this is not yet a numerically feasible option. We therefore propose  two different model approaches in this MIP that concentrate on different spatiotemporal domains and resolution:

  • Storm Resolving Model (SRM) simulations with resolutions in the range of 0.5~2.5 km over a domain of typical 2000 X 2000 km2 for the whole EUREC4A period of Jan-Feb 2020.
  • Large Eddy Model (LEM) simulations with resolutions in the range of 50~250 meter over a domain of 300 X 400 km for the period Februari 1-11 2020.

The simulations will be carried under a Present Day Climate and a Pseudo Warming Experiment setting. For the latter we propose to do a +4K SST AMIP run in which the perturbed lateral boundary conditions will be provided from climate models with a high ( HADGEM3, IPSL) , low (NORESM2) and a medium (GFDL) climate sensitivity

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