The “Grey Zone” cold air outbreak global model intercomparison: A cross evaluation using large‐eddy simulations

Abstract

A stratocumulus‐to‐cumulus transition as observed in a cold air outbreak over the North Atlantic Ocean is compared in global climate and numerical weather prediction models and a large‐eddy simulation model as part of the Working Group on Numerical Experimentation “Grey Zone” project. The focus of the project is to investigate to what degree current convection and boundary layer parameterizations behave in a scale‐adaptive manner in situations where the model resolution approaches the scale of convection. Global model simulations were performed at a wide range of resolutions, with convective parameterizations turned on and off. The models successfully simulate the transition between the observed boundary layer structures, from a well‐mixed stratocumulus to a deeper, partly decoupled cumulus boundary layer. There are indications that surface fluxes are generally underestimated. The amount of both cloud liquid water and cloud ice, and likely precipitation, are under‐predicted, suggesting deficiencies in the strength of vertical mixing in shear‐dominated boundary layers. But also regulation by precipitation and mixed‐phase cloud microphysical processes play an important role in the case. With convection parameterizations switched on, the profiles of atmospheric liquid water and cloud ice are essentially resolution‐insensitive. This, however, does not imply that convection parameterizations are scale‐aware. Even at the highest resolutions considered here, simulations with convective parameterizations do not converge toward the results of convection‐off experiments. Convection and boundary layer parameterizations strongly interact, suggesting the need for a unified treatment of convective and turbulent mixing when addressing scale‐adaptivity.

Uncertainties related to the representation of momentum transport in shallow convection

“…LESs are performed using the UCLA‐LES [see Tomassini et al ., ]. (The setup and the forcing data for the LES are documented at http://appconv.metoffice.com/cold_air_outbreak/constrain_case/crm_setup.html.)…”

Exploring the convective grey zone with regional simulations of a cold air outbreak

“…Current global NWP models typically do not yet include grey zone convection schemes. The companion intercomparison study with global NWP models suggests that conventional convection parametrizations remove atmospheric instability too easily and prevent models from resolving part of the vertical overturning explicitly even at high resolutions (Tomassini et al , 2017). Another important aspect in the context of the Grey Zone parametrization problem is the issue of physical parametrization interferences.…”

Turbulent Transport in the Gray Zone: A Large Eddy Model Intercomparison Study of the CONSTRAIN Cold Air Outbreak Case

“…Satellite images and aircraft observations collected in the last part of the trajectory on which the case is based show a gradual transition from overcast stratocumulus off the coast of Greenland to a boundary layer dominated by shallow cumulus clouds north of the United Kingdom. The present work is accompanying two recent studies by Tomassini et al (2017) and Field et al (2017) who explored the representation of the CAO case in simulations performed with global and LAMs that operated at gray zone resolutions, with which is meant that the horizontal grid spacing is potentially fine enough (<10 km) to resolve some fraction of the convective transport. LES models are designed to resolve turbulence, and in the present study they were employed to study the temporal evolution of the CAO in addition to its sensitivity to cloud microphysics in terms of prescribed cloud droplet number concentrations and ice microphysics.…”

“…Tomassini et al () investigated the representation of the CONSTRAIN CAO in global models. In particular, they assessed to what extent current convection and boundary layer parameterizations behave in a scale‐adaptive manner in situations where the horizontal grid spacing of global models approaches the scale of convection.…”

“…A striking feature of CAOs is the presence of so-called cloud streets (e.g., Brümmer, 1999;Gryschka & Raasch, 2005), which organization is controlled by the ratio of shear to buoyancy production of turbulent kinetic energy (TKE) in a nontrivial way. Tomassini et al (2017) investigated the representation of the CONSTRAIN CAO in global models. In particular, they assessed to what extent current convection and boundary layer parameterizations behave in a scale-adaptive manner in situations where the horizontal grid spacing of global models approaches the scale of convection.…”