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Readdressing Convective-Surface Interaction in Global Climate Models

Periodic Reporting for period 1 - COGNAC (Readdressing Convective-Surface Interaction in Global Climate Models)

Reporting period: 2015-10-01 to 2017-09-30

A large part of our knowledge on Earth's climate is provided by Global Climate Models (GCMs), numerical models based on the time integration of the prognostic equations of the atmospheric and oceanic fluid-dynamics. Such models are able to produce simulations of the climate for both present and future scenarios. However, the skill and the predictive power of current state-of-the-art GCMs is limited by the low horizontal resolution and by the imprecise representation of several physical processes. Among them, the simulation of the coupling between the atmospheric circulation and the hydrological cycle, and more specifically the representation of clouds, is still to date one of the largest issues. Indeed, clouds controls the precipitation and the radiation budget of the Earth: COGNAC has been a fundamental project framed in this context. It aimed at re-addressing the representation of convection and clouds in GCMs. In this project we addressed specifically the physical processes leading to the formation and controlling the dynamics of stratocumulus clouds.
Those clouds are ubiquitous over the globe and have a probably the most important impact on Earth’s radiation budget. The fractional coverage of those low-level clouds is controlled by different physical processes, including the dynamics of the boundary layer (henceforth STBL for Stratocumulus Topped Boundary Layer). The dynamics of the STBL are challenging for two main reasons: 1) the concurrence of several thermodynamical and turbulent processes in action and 2) the relative thin region over which the most of these processes take place.
The STBL and the related stratocumulus cloud cover are in the climate community spotlight since they represent a large source of uncertainty in GCM simulations. Increasing in horizontal resolution, due to the augmented available computational power, will not alleviate the issue. Weather prediction simulations are now run at 9 km, and further grid refinement up to 5 km is expected before 2025: higher resolution would be needed to resolve the formation of stratocumulus clouds (hundreds of meters at least), preventing a numerical resolution of the STBL for a few decades at least.
In order to provide a reasonable representation of the stratocumulus clouds dynamics in GCMs, an accurate parameterization of the STBL is mandatory. And in order to do so, a comprehensive knowledge of the dynamics and more specifically of the turbulent fluxes within the STBL must be achieved. To this day, no systematic study has investigated updrafts, downdrafts and entrainment in a unique framework and has evaluated each contribution to the overall turbulent transport of the STBL. Nonetheless, the identification of convective structures - including the role of entrainment - is of key importance to improve our knowledge and correctly parametrize stratocumulus clouds.
COGNAC has been developed during these two years in this direction: aiming at assessing how the turbulent fluxes evolve and control the dynamics of the STBL, which is the key to govern stratocumulus cloud cover. This has been done in order to put the ground for the development of a new parametrization of STBL convection able to reduce the current bias and incertitude in the GCMs simulation of a such important element of the climate system.
The investigation has been done using Large Eddy Simulations (LES), i.e. numerical integration of fluid-dynamics solving small scale feature, at high horizontal (up to 10 meters) and vertical resolution (up to 1 meter).
COGNAC has chosen the University of Los Angeles Large Eddy Simulation model, UCLA-LES, as the principal investigation tool for the rest of the project. Once the first simulations have been obtained, the core part of the project has been related to the development of a method able to track and quantify the turbulent transport within the STBL. This work has been following the previous work done by Dr. Seungbu Park and Prof. Pierre Gentine (Columbia University), including two visits to their institution in order to enhance the collaboration. The method uses two different passive tracers added into the LES model. A few months have been spent into the effort of tune and optimize the methodology developed, which is based on the coherent structures approach including an octant analysis.
All the efforts provided a novel robust and adaptive method that has been presented in two different conferences in 2016 and 2017 (AGU 2016 and EGU 2017) and has been recently published on Journal Atmospheric Sciences.
Further work has been developed, mainly following two streams of research 1) the analysis of the sensitivity of STBL to the different forcings that characterize the large scale of stratocumulus circulation, as surface fluxes, low tropospheric stability, radiative cooling, etc. 2) given the interesting results emerged from the analysis of the fluxes, a parallel stream of research has been opened focusing on the fundamental dynamics of the STBL. Specifically, the role of evaporative cooling causing the buoyancy reversal, and more in general the turbulence generation at the cloud top has been investigated.
Aside of the main goal of the project COGNAC the previous scientific production of the Experienced Research (ER) has been carried out. This includes the publication of 2 papers as first author on atmospheric blocking dynamics and modelling, with the collaboration of scientists from the Host Institution, 1 paper as first author involving GCM numerical modeling and other 3 peer-reviewed publication as coauthor.
The main findings resulting from COGNAC highlighted that a new parametrization of stratocumulus convection should be urgently developed, and that such parameterization must take into account for the non-negligible contribution of downdrafts.
Indeed, most of the present-day parametrization of the STBL in both weather and climate models typically include a eddy-diffusion scheme and possibly a non-local mass-flux component to characterize an ensemble of updrafts. Nonetheless the contribution of downdrafts is on the same order of magnitude of updrafts and it should be added as well in order to reproduce correctly the STBL dynamics, as widely demonstrated during COGNAC.
Furthermore, the relative contributions of the coherent structures - especially to heat and moisture turbulent fluxes - are likely dependent on external forcing (as the strength of the radiative cooling or the intensity of the surface fluxes) and on the mean state (as temperature and moisture profiles). As can be easily imagined, two different structures can be affected in different ways by different forcing, putting further attention on the parametrization of the entrainment itself.
COGNAC thus paves the way to further studies that are actually already under way: it represents a step towards a parametrization of the STBL that is not merely based on eddy-diffusion scheme between the boundary layer and the free troposphere, but that can take into account the different contributions of each coherent structure, potentially implying extraordinary benefit to the simulations of low level clouds in Global Climate Models.
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