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 addressed this issue through advancement of understanding of non-linear and small-scale ocean-atmosphere exchange processes and, in parallel, investigated their representation in coupled climate models of the CMIP Earth System Models (ESMs) family. EUREC4A-OA made use of, and contributed to, the ElUcidating the RolE of Clouds-Circulation Coupling in ClimAte (EUREC4A) initiative 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 was collected using cutting-edge technology on airplanes, ships, autonomous vehicles, augmented with the Barbados Cloud Observatory time series. EUREC4A-OA added 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 was on meso- and submesoscale ocean dynamics and related atmospheric boundary layer processes. EUREC4A-OA was 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. We used this to investigate the effect of resolution on the representation of atmospheric boundary layer processes, including the formation of low-level clouds and deep convection. We use data assimilation in the ocean and spectral nudging in the atmosphere to 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. We developed an offline data assimilation for parameter estimation methodology that we used to investigate the sensitivity of the low-level cloud and boundary layer physics scheme to unknown parameters, and therefore to identify the largest sources of model bias. We found that the low level cloud cover is highly sensitive to two of the sixteen parameters we considered, the most important one controls the updrafts within low level clouds. A second round of simulations confirmed that by fixing these two parameters we could remove the bias in low level cloud in model runs with an assimilated ocean. 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 found 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.

The EUREC4A-OA partnership provided factsheets summarizing the lessons learnt from the emerging constraints on model parameterization and new metrics; these are available online (project website) but were also sent to meteorological organizations involved in the provision of climate services. EUREC4A-OA co-organized a conference together with sister-projects TRIATLAS and PIRATA. This provided a shared venue for a broad community of researchers working on issues of tropical ocean-atmosphere coupling, model bias, and cloud-circulation relationships. Several of the key scientific findings of the project, including the mitigation of the widely-known double-ITCZ bias and the parameter sensitivity analysis of the CLUBB scheme for addressing model biases in low level cloud, were presented here and at other conferences. These results are likely to be taken up by other climate modelling groups who have faced the same challenges. The EUREC4A-OA project has had a significant impact on climate modeling, particularly through the development of a high-resolution Norwegian Earth System Model (NorESM) with reduced biases. The mitigation of the double-ITCZ bias in the new 1/4 degree resolution model represents a substantial improvement, enabling more reliable and accurate climate predictions. High-resolution climate data are crucial for detailed climate impact studies, which are essential for informing societal actions and policy decisions in Norway and beyond. The enhanced resolution and accuracy of the NorESM model will support a wide range of applications, from forecasting extreme weather events to long-term climate change projections, thus providing valuable insights for climate adaptation and mitigation strategies. In terms of dissemination and utilization, the project has already yielded several publications in scientific journals, with plans for additional papers addressing the biases in low-level cloud representation linked to the CLUBB scheme and of the need for scale dependent bulk formula for surface flux computations. These publications are expected to garner broad interest within the climate modeling community, as many contemporary models incorporate the CLUBB scheme. Furthermore, the model simulations generated during the project have been, or will soon be, uploaded to NorStore, ensuring their availability for future use by researchers in the climate science field. The innovative data assimilation methodology for parameter estimation developed during the project is set to be widely adopted within the NorESM community, facilitating faster model tuning and a deeper understanding of model sensitivity to unknown parameters. This will enhance the overall robustness and reliability of future climate models, benefiting both the scientific community and society at large.

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.