Fig. 1 Top view of the Liquid Water Path of a Large Eddy simulation (within yellow box) at 100 meter resolution and of a Storm Resolving Simulation ( outside yellow box) at 2,5 km resolution.

Introduction

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 organisation over the subtropical ocean.
  • Understanding the underlying dynamical processes  that lead to these mesoscale cloud patterns
  • Assessing the radiative response of this abundant cloud 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 spatio-temporal domains and resolution:

  • Storm Resolving Model (SRM) simulations with resolutions in the range of 0.5~2.5 km over a domain of preferably 3000 x 4000 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 250 X 400 km for the period Februari 1-11 2020.

The simulations will be carried for under present day conditions such as observed during the EUREC4A field campaign and additionally for future weather conditions by using a  Pseudo Global Warming perturbation. 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

  • For the set up for present day EUREC4A simulations for Storm Resolving Models proceed here
  • For the set up for present day EUREC4A simulations for Large Eddy Models proceed here
  • For the set up for future weather EUREC4A simulations using the Pseudo-Global Warming (PGW) framework go here

 

 

Fig. 2. Preferred Storm-Resolving Model domain (Black Box), Typical used Storm Resolving Model domain Blue Box)  and Large Eddy Model domain (Green Box).  The orange box indicates the common domain of analysis for the Storm-Resolving Models

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