KLIMAFORSK-Stort program klima

To initiate and support societal actions as a response to climate change, future projections of the climate system require high-resolution coupled climate model simulations. An important challenge for high-resolution modelling is the need to resolve processes that have typically been parameterized in coarse-grid simulations. The exchange of heat, water and gas at the air-sea interface is key to regulating the state and evolution of our climate. Large exchanges can occur at the air-sea interface on short time and small spatial scales. There is therefore an urgent need to understand the processes governing these exchanges, so that we can quantitatively evaluate model predictions and projections and understand why different models give different answers. EUREC4A-OA will address this issue through advancement of understanding of non-linear and small-scale ocean-atmosphere exchange processes and, in parallel, investigate their representation in coupled climate models of the CMIP Earth System Models (ESMs) family. EUREC4A-OA makes use of, and contributes to, the ElUcidating the RolE of Clouds- Circulation Coupling in ClimAte (EUREC4A) initiative (Bony et al. 2017) that aims to advance understanding of the interplay between clouds, convection and circulation, and their role in climate change. The core of EUREC4A is a one-month (Jan/Feb 2020) field study in the western tropical North Atlantic Ocean where high-resolution, synchronized observational data will be collected using cutting-edge technology on airplanes, ships, autonomous vehicles, augmented with the Barbados Cloud Observatory time series. EUREC4A-OA will add the ocean component to EUREC4A by investigating heat, momentum and CO2 exchange across the air/sea interface using innovative high-resolution ocean observations and a hierarchy of numerical simulations. Our focus is on meso- and submesoscale ocean dynamics and related atmospheric boundary layer processes. EUREC4A-OA is focused on the tropics where the primary external time scale affecting air-sea exchange is the diurnal cycle. We have developed a new, high-resolution version of the Norwegian coupled global climate model, NorESM, that has ¼ degree resolution in both the ocean and atmosphere. This model setup includes a framework for testing higher vertical resolution in the atmosphere to better resolve the atmospheric boundary layer processes, including the formation of low-level clouds, which are crucial for surface exchanges between ocean and atmosphere. We have designed our model experiments to use data assimilation in the ocean and spectral nudging in the atmosphere so that we can constrain both the slow ocean processes and the large-scale atmospheric circulation. This allows us to focus on how well the NorESM can reproduce the ocean-atmosphere coupling in the tropical Atlantic on short timescales. Our next step will be to perform sensitivity analysis of the cloud and boundary layer physics schemes to identify the largest sources of model error, and the sensitivity of this performance to vertical resolution. We completed a semi-idealized simulation with our ¼ degree resolution version of NorESM forced by ¼ degree OISST sea surface temperature data from January to February in 2020, which is the period of the EURECA observation campaign. With a simple method of meso-scale SST eddy detection, we conducted eddy composite analysis and found that the planetary boundary layer height and low-level cloud formation are higher and more frequent over the warm ocean eddies. Although the relationship between surface winds and sea surface temperature seems similar to what has been revealed in other oceanic regions, the surface wind anomaly associated with the eddies is much weaker than other regions. This might be because the eddies in the EUREC4A-OA region are relatively small. We evaluated the ¼° model NorESM1.3 in which the well-known “double-ITCZ problem” in the Pacific is mitigated. However, excessive precipitation is produced in the northern branch of the ITCZ. The excessive precipitation is consistent with too high latent heat flux in the tropical ocean. Further analysis shows that in NorESM1.3, the latent heat flux is too sensitive to the surface wind. The increased sensitivity in the ¼° model is partly due to small-scale air-sea interaction. The sensitivity of latent heat flux to surface wind, at scales finer than 2.5°, is up to 40 (Wm-2 / ms-1), which is almost twice that found at scales coarser than 2.5°. This study helps to understand extra air-sea interaction resolved by higher-resolution models, and helps to tune and correct the related model bias. We have run experiments to test the sensitivity of the NorESM results to the values of parameters in the cloud and boundary layer parameterization schemes. Initial analysis shows that the low level cloud cover is highly sensitive to two of the sixteen parameters we considered. We will use these sensitivities to reduce the model bias in low clouds.

The exchange of heat, water and gas at the air/sea interface is key to regulating the state and evolution of our climate. Sizeable air-sea exchanges of energy and ocean-atmosphere boundary layer processes can occur on short time and small spatial scales. To initiate and support societal actions as a response to climate change, future projections of the climate system require high-resolution coupled climate model simulations. A generic challenge for high-resolution modelling is the need to resolve processes that have typically been parameterized in coarse-grid simulations. EUREC4A-OA will address this issue thorough advancement of understanding of non-linear and small-scale ocean-atmosphere exchanges processes and investigate their representation in the CMIP Earth System Models (ESMs) family. EUREC4A-OA will leverage from, and contribute to, the ElUcidating the RolE of Clouds-Circulation Coupling in ClimAte (EUREC4A) initiative (Bony et al. 2017) that aims to advance understanding of the interplay between clouds, convection and circulation, and their role in climate change. The core of EUREC4A is a one-month (Jan/Feb 2020) field study in the western tropical North Atlantic Ocean where high-resolution, synchronized observational data will be collected using cutting-edge technology on airplanes, ships, autonomous vehicles, augmented with the Barbados Cloud Observatory time series. EUREC4A-OA will add the ocean component to EUREC4A by investigating heat, momentum and CO2 exchange across the air/sea interface using innovative high-resolution ocean observations and a hierarchy of numerical simulations. Our focus is on meso- and submesoscale ocean dynamics and related atmospheric boundary layer processes. EUREC4A-OA is focused on the tropics where the primary external time scale affecting air-sea exchange is the diurnal cycle. However, the internal ocean and atmosphere dynamics convolute the diurnal, seasonal and longer time scales to climate variability.