Abstract
A single column version of the CNRM-CM6-1 global climate model has been developed to ease development and validation of the boundary layer physics and air-sea coupling in a simplified environment. This framework is then used to assess the ability of the coupled model to represent the sea surface temperature (SST) diurnal cycle. To this aim, the atmospheric-ocean single column model (AOSCM), called CNRM-CM6-1D, is implemented on a case study derived from the Cindy-Dynamo field campaign over the Indian Ocean, where large diurnal SST variabilities have been well documented. Comparing the AOSCM and its uncoupled components (atmospheric SCM and oceanic SCM, called OSCM) highlights that the impact of coupling in the atmosphere results both from the possibility to take in to account the diurnal variability of SST, not usually available in forcing products, and from the change in mean state SST as simulated by the OSCM, the ocean mean state not being heavily impacted by the coupling. This suggests that coupling feedbacks are more due to advection processes in the 3D model than to the model physics. Additionally, a sub-daily coupling frequency is needed to represent the SST diurnal variability but the choice of the coupling time-step between 15 min and 3 h does not impact much on the diurnal temperature range simulated. The main drawback of a 3-h coupling being to delay the SST diurnal cycle by 5 h in asynchronous coupled models. Overall, the diurnal SST variability is reasonably well represented in the CNRM-CM6-1 with a 1 h coupling time-step and the upper ocean model resolution of 1 m. This framework is shown to be a very valuable tool to develop and validate the boundary layer physics and the coupling interface. It highlights the interest to develop other atmosphere-ocean coupling case studies.
Highlights
Because of the many interactions and feedbacks occurring in a general circulation model (GCM), either between parameterizations or between the parameterized subgrid processes and the resolved dynamics, understanding their behaviour or the origin of their systematic errors is often complex
Comparing the atmospheric-ocean single column model (AOSCM) and its uncoupled components 15 highlights that the impact of coupling in the atmosphere results both from the possibility to take in to account the diurnal variability of sea surface temperature (SST), not usually available in forcing products, and from the change in mean state SST as simulated by the Oceanic SCM (OSCM), the ocean mean state not being heavily impacted by the coupling
The reduced set of interactions and feedbacks in single column model (SCM) compared to the host GCM, and the possibility to output many diagnostics at model time steps on the model vertical grid help the modeller to better tackle the cause-and-effect relationship among the parameterized processes and thereby identify model deficiencies
Summary
Because of the many interactions and feedbacks occurring in a general circulation model (GCM), either between parameterizations or between the parameterized subgrid processes and the resolved dynamics, understanding their behaviour or the origin of their systematic errors is often complex. We take advantage of the AOSCM framework to better understand the ability of the CNRM-CM61 atmospheric and ocean vertical physics to represent diurnal oceanic warm layers in the tropics. In this regard, an AOSCM case study based on the Cooperative Indian Ocean Experiment on Intraseasonal Variability in the Year 2011 75 (CINDY2011)/Dynamics of the MJO (DYNAMO) field campaign (Yoneyama et al, 2013) is developed and serves to highlight the features of the model configuration key to properly capture the SST diurnal cycle. To enable a fair comparison of the Oceanic SCM (OSCM) with the AOSCM, we have run the OSCM jointly with the SURFEX platform so as to ensure a common flux computation using the COARE version 3.0 bulk scheme 120 (Fairall et al, 2003)
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