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The impact of biomass burning aerosols (BBA) emitted in central Africa on the tropical African climate is studied using the ocean–atmosphere global climate model CNRM-CM, including prognostic aerosols. The direct BBA forcing, cloud feedbacks (semi-direct effects), effects on surface solar radiation, atmospheric dynamics and precipitation are analysed for the 1990–2014 period. During the June–July–August (JJA) season, the CNRM-CM simulations reveal a BBA semi-direct effect exerted on low-level clouds with an increase in the cloud fraction of ∼5 %–10 % over a large part of the tropical ocean. The positive effect of BBA radiative effects on low-level clouds is found to be mainly due to the sea surface temperature response (decrease of ∼0.5 K) associated with solar heating at 700 hPa, which increases the lower-tropospheric stability. Over land, results also indicate a positive effect of BBA on the low-cloud fraction, especially for the coastal regions of Gabon and Angola, with a potentially enhanced impact in these coupled simulations that integrates the response (cooling) of the sea surface temperature (SST). In addition to the BBA radiative effect on SST, the ocean–atmosphere coupled simulations highlight that the oceanic temperature response is noticeable (about -0.2 to -0.4 K) down to ∼80 m depth in JJA between the African coast and 10° W. In parallel to low-level clouds, reductions of ∼5 %–10 % are obtained for mid-level clouds over central Africa, mainly due to BBA-induced surface cooling and lower-tropospheric heating inhibiting convection. In terms of cloud optical properties, the BBA radiative effects induced an increase in the optical depth of about ∼2 –3 over the ocean south of the Equator. The result of the BBA direct effect and feedback on tropical clouds modulates the surface solar radiation over the whole of tropical Africa. The strongest surface dimming is over central Africa (∼-30 W m−2), leading to a large reduction in the continental surface temperature (by ∼1 to 2 K), but the solar radiation at the oceanic surface is also affected up to the Brazilian coast. With respect to the hydrological cycle, the CNRM-CM simulations show a negative effect on precipitation over the western African coast, with a decrease of ∼1 to 2 mm d−1. This study also highlights a persistent impact of BBA radiative effects on low-level clouds (increase in cloud fraction, liquid water content and optical depth) during the September–October–November (SON) period, mainly explained by a residual cooling of sea surface temperature over most of the tropical ocean. In SON, the effect on precipitation is mainly simulated over the Gulf of Guinea, with a reduction of ∼1 mm d−1. As for JJA, the analysis clearly highlights the important role of the slow response of the ocean in SON and confirms the need to use coupled modelling platforms to study the impact of BBA on the tropical African climate. [ABSTRACT FROM AUTHOR]

Due to their severity and lack of predictability, understanding and forecasting extreme precipitation events (EPEs) is critical for disaster risk reduction. The present work documents the large‐scale environment of tropical EPEs based on a 42‐year data set combining dense rain‐gauge networks that cover several tropical small islands and coastal regions. Approximately 10%–30% of EPEs are associated with a tropical storm or cyclone (TC), except for Reunion, for which its high topography makes it reach 55%. TCs multiply the EPE probability by a factor of 4–15, especially during TCs of category 1 or higher. A composite analysis demonstrates that the remaining large part of EPEs occurs within large‐scale and strong moist, convective, and cyclonic wind anomalies resulting from the superimposition of intraseasonal, seasonal‐to‐annual, and interannual timescales. These intense anomalies come essentially from intraseasonal variability, and lower frequencies improve the effect of intraseasonal events in creating a favorable environment for EPEs. Plain Language Summary: Floods and landslides, mainly caused by extreme precipitation events (EPEs), are severe disasters that affect populations and economies. However, their prediction, especially in tropical areas, is challenging. Here, we study the main atmospheric configurations leading to EPEs using rain‐gauge data densely sampling several tropical small islands and coastal regions, covering the period 1979–2021. In areas prone to tropical storms and cyclones (TCs), these systems account for 10%–30% of EPEs, except for Reunion, whose high topography increases it up to 55%. TC‐related EPEs are also more intense than non‐TC‐related EPEs. TCs multiply the EPE probability of occurrence by 4–15 times, depending on the region and the TC category. Non‐TC‐related EPEs are shown to occur within specific large‐scale weather patterns in which moisture is highly increased, and convective storms are more active. These patterns involve phenomena occurring at intraseasonal (2–90‐day periods), seasonal‐to‐annual (91–365‐day periods), and interannual (periods above one year) timescales. The intraseasonal variability helps the most to develop this environment favorable to the occurrence of EPEs. Slower signals build a background on which intraseasonal variability can have more effects. The real‐time monitoring of these different features should improve EPE forecasting capabilities and deliver more early warnings. Key Points: Over most of the studied tropical small islands, only 10%–30% of extreme precipitation events occur near a tropical storm or cycloneTropical storms and cyclones favor heavier rainfall and raise the probability of occurrence of extreme precipitation by a factor of 4–15The other largest part of events occur in a large‐scale, intense, moist, and convective anomaly driven mainly by the intraseasonal timescale [ABSTRACT FROM AUTHOR]

Precipitation and atmospheric circulation are the coupled processes through which tropical ocean surface temperatures drive global weather and climate.sup.1-5. Local sea surface warming tends to increase precipitation, but this local control is difficult to disentangle from remote effects of conditions elsewhere. As an example of such a remote effect, El Niño Southern Oscillation (ENSO) events in the equatorial Pacific Ocean alter precipitation across the tropics. Atmospheric circulations associated with tropical precipitation are predominantly deep, extending up to the tropopause. Shallow atmospheric circulations.sup.6-8 affecting the lower troposphere also occur, but the importance of their interaction with precipitation is unclear. Uncertainty in precipitation observations.sup.9,10 and limited observations of shallow circulations.sup.11 further obstruct our understanding of the ocean's influence on weather and climate. Despite decades of research, persistent biases remain in many numerical model simulations.sup.12-18, including excessively wide tropical rainbands.sup.14,18, the 'double-intertropical convergence zone problem'.sup.12,16,17 and too-weak responses to ENSO.sup.15. These biases demonstrate gaps in our understanding, reducing confidence in forecasts and projections. Here we use observations to show that seasonal tropical precipitation has a high sensitivity to local sea surface temperature. Our best observational estimate is an 80 per cent change in precipitation for every gram per kilogram change in the saturation specific humidity (itself a function of the sea surface temperature). This observed sensitivity is higher than in 43 of the 47 climate models studied, and is associated with strong shallow circulations. Models with more realistic (closer to 80%) sensitivity have smaller biases across a wide range of metrics. Our results apply to both temporal and spatial variation, over regions where climatological precipitation is about one millimetre per day or more. Our analyses of multiple independent observations, physical constraints and model data underpin these findings. The spread in model behaviour is further linked to differences in shallow convection, thus providing a focus for accelerated research to improve seasonal forecasts through multidecadal climate projections. The response of tropical precipitation to variation in sea surface temperature is stronger than in most climate models, with cool and warm ocean regions linked by strong shallow atmospheric circulations., Author(s): Peter Good [sup.1] , Robin Chadwick [sup.1] [sup.2] , Christopher E. Holloway [sup.3] , John Kennedy [sup.1] , Jason A. Lowe [sup.1] [sup.4] , Romain Roehrig [sup.5] , Stephanie [...]

The impact of biomass burning aerosols (BBA) emitted in Central Africa on the tropical African climate is studied using the ocean-atmosphere global climate model CNRM-CM, including prognostic aerosols. The direct BBA forcing, cloud feedbacks (semi-direct effects), effects on surface solar radiation, atmospheric dynamics and precipitation are analysed for the 1990-2014 period. During the June-July-August (JJA) season, the CNRM-CM simulations reveal a BBA semi-direct effect exerted on low-level clouds with an increase in cloud fraction of ∼5-10% over a large part of the tropical ocean. The positive feedback of BBA radiative effects on low-level clouds is found to be mainly due to the sea surface temperature response (decrease of ∼-0.5 K) associated with solar heating at 700 hPa, which increases the lower tropospheric stability. Over land, results also indicates a positive effect of BBA on the low cloud fraction especially for the coastal regions of Gabon and Angola with a potentially enhanced impact in these coupled simulations that integrate the response (cooling) of the SST. In addition to the BBA radiative effect on sea surface temperature, the ocean-atmosphere coupled simulations highlight that the oceanic temperature response is noticeable (about -0.2 to -0.4 K) down to ∼80 m depth in the JJA between the African coast and 10°W. In parallel to low-level clouds, reductions of ∼5-10% are obtained for mid-level clouds over central Africa, mainly due to BBA-induced surface cooling and lower tropospheric heating inhibiting convection. In terms of cloud optical properties, the BBA radiative effects induced an increase of the optical depth by about ∼2-3 south of the equator over the ocean. The result of the BBA direct effect and feedback on tropical clouds modulates the surface solar radiation over the whole Tropical Africa. The strongest surface dimming is over central Africa (∼-30 W m-2), leading to a large reduction of the continental surface temperature (by ∼-1 to -2 K), but the solar radiation at the oceanic surface is also affected up to the Brazilian coast. With respect to the hydrological cycle, the CNRM-CM simulations show a negative feedback on precipitation over theWest African coast with a decrease of ∼-1 to -2 mm per day. This study highlights also a persistent impact of BBA radiative effects on low-level clouds (increase in cloud fraction, liquid water content and optical depth) during the September-October-November (SON) period, mainly explained by a residual cooling of sea surface temperature over most of the tropical ocean. In SON, the feedback on precipitation is mainly simulated over the Gulf of Guinea with a reduction by ∼-1 mm per day. As for JJA, the analysis clearly highlights the important role of the slow response of the ocean in SON and confirms the need to use coupled modelling platforms to study the impact of BBA on tropical African climate. [ABSTRACT FROM AUTHOR]

Clouds affect the Earth climate with an impact that depends on the cloud nature (solid and/or liquid water). Although the Antarctic climate is changing rapidly, cloud observations are sparse over Antarctica due to few ground stations and satellite observations. The Concordia station is located on the eastern Antarctic Plateau (75 ∘ S, 123 ∘ E; 3233 m above mean sea level), one of the driest and coldest places on Earth. We used observations of clouds, temperature, liquid water, and surface irradiance performed at Concordia during four austral summers (December 2018–2021) to analyse the link between liquid water and temperature and its impact on surface irradiance in the presence of supercooled liquid water (liquid water for temperature less than 0 ∘ C) clouds (SLWCs). Our analysis shows that, within SLWCs, temperature logarithmically increases from -36.0 to -16.0 ∘ C when liquid water path increases from 1.0 to 14.0 g m -2. The SLWC radiative forcing is positive and logarithmically increases from 0.0 to 70.0 W m -2 when liquid water path increases from 1.2 to 3.5 g m -2. This is mainly due to the downward longwave component that logarithmically increases from 0 to 90 W m -2 when liquid water path increases from 1.0 to 3.5 g m -2. The attenuation of shortwave incoming irradiance (that can reach more than 100 W m -2) is almost compensated for by the upward shortwave irradiance because of high values of surface albedo. Based on our study, we can extrapolate that, over the Antarctic continent, SLWCs have a maximum radiative forcing that is rather weak over the eastern Antarctic Plateau (0 to 7 W m -2) but 3 to 5 times larger over West Antarctica (0 to 40 W m -2), maximizing in summer and over the Antarctic Peninsula. [ABSTRACT FROM AUTHOR]

Extreme precipitation events (EPE) are often associated with severe floods and significant damages in Central Sahel. To better understand their formation and improve their forecasts, we investigate the sub‐seasonal drivers of EPEs. A composite analysis reveals that moist, cyclonic and upper‐level divergence anomalies are found on average as a result of several tropical waves. The equatorial Rossby wave (ER) dominates at large scale providing a moist and convectively‐active anomaly over the northern Sahel together with a smaller‐scale African Easterly Wave (AEW). The Madden‐Julian Oscillation provides upper‐level divergence anomalies and a Kelvin wave increases convection during the EPE. Statistics show the prevalence of AEW and emphasize ER as a key driver of EPE. The co‐occurrences of several tropical waves, especially those involving AEW, ER, and Kelvin waves, increase the probability of EPE. Monitoring these tropical waves combinations could improve EPEs forecasts. Plain Language Summary: The drivers of extreme precipitation events (EPE) over the Central Sahel are studied at subseasonal scales. A statistical approach is adopted to build an average extreme rainfall event. EPEs occur within a large‐scale moist anomaly, an upper‐level divergence, and at shorter scales an intense vortex. These features are provided by multiple tropical meteorological systems, called tropical waves, that have typical spatio‐temporal scales greater than that of a convective storm. The four tropical waves studied here contribute to this favorable environment. However two of them—an Equatorial Rossby wave, rather slow, and an African Easterly Wave (AEW) (more rapid)—explain the largest part of the favorable conditions leading to an EPE. Statistics show that a single intense AEW or the combination of multiple tropical waves increases the probability of EPE. Monitoring these tropical waves combinations could improve EPEs forecasts. Key Points: Tropical waves are key to build the atmospheric environment conducive to an extreme precipitation eventEquatorial Rossby waves appear as a new driver of extreme rainfall in the Central Sahel when combined with African Easterly WavesThe combination of African Easterly Wave with an equatorial Rossby wave and/or Kelvin wave, increase the probability of extreme rainfall [ABSTRACT FROM AUTHOR]

Getting the right precipitation probability density function in the Mediterranean area is a challenge for most global and regional climate models with parametrized convection. Over land in particular, the intensity of heavy‐precipitating events is often underestimated. In the present study, we provide a process‐based analysis of the representation by the CNRM‐ALADIN63 regional climate model of one of these events, which occurred in the southeast of France on November 1–2, 2008. The CNRM‐ALADIN63 model, when run in a configuration where the large‐scale dynamics is nudged towards that of the ERA‐Interim reanalysis, is first shown to capture the location and intensity of the heavy‐precipitating event appropriately. Then, using a reference convection‐permitting simulation of the same event and a conditional sampling approach to identify and characterize convective updraughts, the ability of the model convection parametrization to capture further convective details is assessed. The model misses the occurrence of the updraught mass flux largest values, despite a significant and systematic overestimation of the updraught vertical velocity. The area covered by the convective updraught is in fact found to be severely underestimated, suggesting an inappropriate approach for convective closure: the event occurred along the foothills of the Massif Central, where the subgrid‐scale features of the topography interact strongly with the impinging large‐scale flow and thereby drive the position and large area fractions of convective updraughts, at least during the mature phase of the event. A preliminary topography‐based convective closure is proposed and implemented in the model to assess this hypothesis further. The results confirm that the parametrization deficiencies can be significantly reduced with a proper inclusion of subgrid‐scale topographic features. [ABSTRACT FROM AUTHOR]

We investigate the dependence of radiative feedback on the pattern of sea‐surface temperature (SST) change in 14 Atmospheric General Circulation Models (AGCMs) forced with observed variations in SST and sea‐ice over the historical record from 1871 to near‐present. We find that over 1871–1980, the Earth warmed with feedbacks largely consistent and strongly correlated with long‐term climate sensitivity feedbacks (diagnosed from corresponding atmosphere‐ocean GCM abrupt‐4xCO2 simulations). Post 1980, however, the Earth warmed with unusual trends in tropical Pacific SSTs (enhanced warming in the west, cooling in the east) and cooling in the Southern Ocean that drove climate feedback to be uncorrelated with—and indicating much lower climate sensitivity than—that expected for long‐term CO2 increase. We show that these conclusions are not strongly dependent on the Atmospheric Model Intercomparison Project (AMIP) II SST data set used to force the AGCMs, though the magnitude of feedback post 1980 is generally smaller in nine AGCMs forced with alternative HadISST1 SST boundary conditions. We quantify a "pattern effect" (defined as the difference between historical and long‐term CO2 feedback) equal to 0.48 ± 0.47 [5%–95%] W m−2 K−1 for the time‐period 1871–2010 when the AGCMs are forced with HadISST1 SSTs, or 0.70 ± 0.47 [5%–95%] W m−2 K−1 when forced with AMIP II SSTs. Assessed changes in the Earth's historical energy budget agree with the AGCM feedback estimates. Furthermore satellite observations of changes in top‐of‐atmosphere radiative fluxes since 1985 suggest that the pattern effect was particularly strong over recent decades but may be waning post 2014. Key Points: Post 1980 the Earth warmed with feedbacks uncorrelated with—and indicating much lower equilibrium climate sensitivity than—that expected for long‐term CO2 increaseSatellite observations of changes in top‐of‐atmosphere radiative fluxes since 1985 are in agreement with the modelsThe pattern effect may be waning post 2014 [ABSTRACT FROM AUTHOR]

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 in a case study derived from the CINDY2011/DYNAMO 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 the fact that the impact of coupling in the atmosphere results from both the possibility to take into account the diurnal variability of SST, which is not usually available in forcing products, and the change in mean state SST as simulated by the OSCM, with the ocean mean state not being heavily impacted by the coupling. This suggests that coupling feedbacks in the 3D model do not arise from the coupling of ocean and atmosphere vertical column physics but are more due to the large-scale dynamics resolved by the 3D model. 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 the diurnal temperature range simulated much. The main drawback of a 3 h coupling is delaying the SST diurnal cycle by 5 h in asynchronous coupled models. Overall, the diurnal SST variability is reasonably well represented in 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. [ABSTRACT FROM AUTHOR]

Clouds affect the Earth climate with an impact that depends on the cloud nature (solid/liquid water). Although the Antarctic climate is changing rapidly, cloud observations are sparse over Antarctica due to few ground stations and satellite observations. The Concordia station located on the East Antarctic Plateau (75°S, 123°E, 3233 m above mean sea level), one of driest and coldest places on Earth. We used observations of clouds, temperature, liquid water and surface radiation performed at Concordia during 4 austral summers (December 2018-2021) to analyse the link between liquid water and temperature and its impact on surface radiation the presence of supercooled liquid water (liquid water for temperature less than 0°C) clouds (SLWCs). Our analysis shows that, within SLWCs, temperature logarithmically increases from -36.0°C to -16.0°C when liquid water path increases from 1.0 to 14.0 g m-2, and SLWCs positively impact the net surface radiation, which logarithmically increases by 0.0 to 50.0 m-2 when liquid water path increases from 1.7 to 3.0 g m-2. We finally estimate that SLWCs have a great potential radiative impact over Antarctica whatever the season considered, up 5.0 W m-2 over the Eastern Antarctic Plateau and up to 30 W m-2 over the Antarctic Peninsula in summer. [ABSTRACT FROM AUTHOR]

Several studies have shown that most climate models underestimate cloud cover and overestimate cloud reflectivity, particularly for the tropical low‐level clouds. Here, we analyze the characteristics of low‐level tropical marine clouds simulated by six climate models, which provided COSP output within the CMIP6 project. CALIPSO lidar observations and PARASOL mono‐directional reflectance are used for model evaluation. It is found that the "too few, too bright" bias is still present for these models. The reflectance is particularly overestimated when cloud cover is low. Models do not simulate any optically thin clouds. They fail to reproduce the increasing cloud optical depth with increasing lower tropospheric stability as observed. These results suggest that most models do not sufficiently account for the effect of the small‐scale spatial heterogeneity in cloud properties or the variety of cloud types at the grid scale that is observed. Plain Language Summary: Low‐level clouds are ubiquitous in the tropics and play an important role in Earth's radiative balance. Climate models do not explicitly resolve the main low‐level cloud formation processes, which must therefore be parameterized. This modeling work is difficult and in the previous generation of models low‐level clouds had a systematically too low fraction and too large brightness. This models' deficiency is known as the "too few too bright bias." Here, we use six climate models of the latest generation that are compared to lidar and reflectance observations allowing for a detailed characterization of cloud properties. It is found that the too few too bright bias is still present for these models. Other common deficiencies in cloud simulation are revealed. At the daily time scale and models' grid scale, the lower the cloud cover, the greater the overestimation of the cloud brightness. Models do not simulate any thin clouds. They fail to reproduce the increasing cloud brightness with increasing stability of the lower troposphere as observed. The study suggests that most models do not sufficiently account for the variety of cloud properties and cloud types at the models' grid scale that is observed. Key Points: The "too few too bright" bias is still present in six CMIP6 models for low‐level cloudsThe overestimation of the low‐level cloud brightness gets higher as their cover is lowModels fail to reproduce the increasing cloud optical depth with increasing lower tropospheric stability as observed [ABSTRACT FROM AUTHOR]

The mesoscale enhancement of surface turbulent fluxes at the air–sea interface is driven by the mesoscale surface wind‐speed variability, especially the gustiness velocity and the mesoscale wind‐speed standard variation. This study proposes a parametrization of these two variables. A large dataset based on the operational 2.5‐km AROME convection‐permitting model is used in a coarse‐graining framework, to quantify various quantities that are subgrid at the scale of a 100‐km resolution global circulation model grid cell. This provides a learning dataset to help build the parametrization. The analysis of two case studies of intense wind‐speed mesoscale variability, combined with a literature review, provides a physically based set of 12 potential predictors, accounting for the convection activity and the large‐scale dynamics. The least absolute shrinkage and selection operator then frames a penalized multivariate linear regression approach to identify the most relevant predictors objectively. Five predictors are selected for predicting the gustiness velocity: the updraft mass flux at the lifting condensation level, the density‐current spreading velocity, the large‐scale horizontal shear and divergence, and the large‐scale wind speed. The parametrization of the mesoscale wind‐speed standard deviation requires an additional predictor, namely the cold‐pool object aggregation index. The proposed parametrization performs significantly better than the previously published parametrizations and is able to capture 80, 99, and 93%$$ \% $$ of the mesoscale enhancement of the momentum, sensible heat, and latent heat fluxes, respectively. From the perspective of a global circulation model implementation, in which some predictors may be unavailable, simpler versions of the parametrization, that is, involving fewer predictors, are also discussed. [ABSTRACT FROM AUTHOR]

Over the recent decades, extreme precipitation events (EPEs) have become more frequent over the Sahel. Their properties, however, have so far received little attention. In this study the spatial distribution, intensity, seasonality, and interannual variability of EPEs are examined, using both a reference dataset based on a high-density rain gauge network over Burkina Faso and 24 precipitation gridded datasets. The gridded datasets are evaluated in depth over Burkina Faso while their commonalities are used to document the EPE properties over the Sahel. EPEs are defined as the occurrence of daily accumulated precipitation exceeding the all-day 99th percentile over a 1° × 1° pixel. Over Burkina Faso, this percentile ranges between 21 and 33 mm day−1. The reference dataset show that EPEs occur in phase with the West African monsoon annual cycle, more frequently during the monsoon core season and during wet years. These results are consistent among the gridded datasets over Burkina Faso but also over the wider Sahel. The gridded datasets exhibit a wide diversity of skills when compared to the Burkinabe reference. The Global Precipitation Climatology Centre Full Data Daily version 1 (GPCC-FDDv1) and the Global Satellite Mapping of Precipitation Gauge Reanalysis version 6.0 (GSMaP-gauge-RNL v6.0) are the only products that properly reproduce all of the EPE features examined in this work. The datasets using a combination of microwave and infrared measurements are prone to overestimate the EPE intensity, while infrared-only products generally underestimate it. Their calibrated versions perform better than their uncalibrated (near-real-time) versions. This study finally emphasizes that the lack of rain gauge data availability over the whole Sahel strongly impedes our ability to gain insights in EPE properties. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We demonstrate a new approach for climate model tuning in a realistic situation. Our approach, the mathematical foundations and technical details of which are given in Part I, systematically uses a single‐column configuration of a global atmospheric model on test cases for which reference large‐eddy‐simulations are available. The space of free parameters is sampled running the single‐column model from which metrics are estimated in the full parameter space using emulators. The parameter space is then reduced by retaining only the values for which the emulated metrics match large eddy simulations within a given tolerance to error. The approach is applied to the 6A version of the LMDZ model which results from a long investment in the development of physics parameterizations and by‐hand tuning. The boundary layer is revisited by increasing the vertical resolution and varying parameters that were kept fixed so far, which improves the representation of clouds at process scale. The approach allows us to automatically reach a tuning of this modified configuration as good as that of the 6A version. We show how this approach helps accelerate the introduction of new parameterizations. It allows us to maintain the physical foundations of the model and to ensure that the improvement of global metrics is obtained for a reasonable behavior at process level, reducing the risk of error compensations that may arise from over‐fitting some climate metrics. That is, we get things right for the right reasons. Plain Language Summary: In view of the importance of global numerical models for the anticipation of future climate changes, their improvement is often considered too slow. We present a new approach that we believe could boost model improvement significantly. This approach promotes the use of machine learning techniques developed by the "uncertainty quantification" community for the adjustment of model free parameters, or tuning. These techniques are applied to physics improvement at process scale, represented through parameterizations. In this approach, the tuning of the global atmospheric model is preconditioned by calibration of the model free parameters on a series of well documented cloud scenes for which explicit very high resolution simulations are available. We demonstrate on a real example how the reduction of the parameter space with this approach allows us to save a large amount of computer resources and detract from the long and tedious by‐hand phase of model tuning. By automating part of the tuning process, the approach enables climate modeler expertize to focus on understanding and improving the model physics through parameterization. Key Points: We use an automatic tool to calibrate the parameterizations of a global climate modelWe show the benefit for global climate tuning of a preconditioning in single column modeWe show how this approach allows us to revisit a parameterization of boundary layer convection [ABSTRACT FROM AUTHOR]

The development of parameterizations is a major task in the development of weather and climate models. Model improvement has been slow in the past decades, due to the difficulty of encompassing key physical processes into parameterizations, but also of calibrating or "tuning" the many free parameters involved in their formulation. Machine learning techniques have been recently used for speeding up the development process. While some studies propose to replace parameterizations by data‐driven neural networks, we rather advocate that keeping physical parameterizations is key for the reliability of climate projections. In this paper we propose to harness machine learning to improve physical parameterizations. In particular, we use Gaussian process‐based methods from uncertainty quantification to calibrate the model free parameters at a process level. To achieve this, we focus on the comparison of single‐column simulations and reference large‐eddy simulations over multiple boundary‐layer cases. Our method returns all values of the free parameters consistent with the references and any structural uncertainties, allowing a reduced domain of acceptable values to be considered when tuning the three‐dimensional (3D) global model. This tool allows to disentangle deficiencies due to poor parameter calibration from intrinsic limits rooted in the parameterization formulations. This paper describes the tool and the philosophy of tuning in single‐column mode. Part 2 shows how the results from our process‐based tuning can help in the 3D global model tuning. Key Points: We apply uncertainty quantification to single‐column model/large‐eddy simulation comparison to calibrate free parametersWe revisit model development strategy with an emphasis on processes for model calibrationThe proposed tuning tool allows to formalize the complementary use of multicases with various metrics [ABSTRACT FROM AUTHOR]

The present work aims at better understanding regional climate–aerosol interactions by studying the relationships between aerosols and synoptic atmospheric circulation over the Euro-Mediterranean region. Two 40-year simulations (1979–2018) have been carried out with version 6.3 of the Centre National de Recherches Météorologiques (National Centre for Meteorological Research) – Aire Limitée Adaptation dynamique Développement InterNational (CNRM-ALADIN) regional climate model, one using interactive aerosols and the other one without any aerosol. The simulation with aerosols has been evaluated in terms of different climate and aerosol parameters. This evaluation shows a good agreement between the model and observations, significant improvements compared to the previous model version and consequently the relevance of using this model for the study of climate–aerosol interactions over this region. A first attempt to explain the climate variability of aerosols is based on the use of the North Atlantic Oscillation (NAO) index. The latter explains a significant part of the interannual variability, notably in winter for the export of dust aerosols over the Atlantic Ocean and the eastern Mediterranean, and in summer for the positive anomalies of anthropogenic aerosols over western Europe. This index is however not sufficient to fully understand the variations of aerosols in this region, notably at daily scale. The use of "weather regimes", namely persisting meteorological patterns, stable at synoptic scale for a few days, provides a relevant description of atmospheric circulation, which drives the emission, transport and deposition of aerosols. The four weather regimes usually defined in this area in winter and in summer bring significant information to answer this question. The blocking and NAO + regimes are largely favourable to strong aerosol effects on shortwave surface radiation and near-surface temperature, either because of higher aerosol loads or because of weaker cloud fraction, which reinforces the direct aerosol effect. Inversely, the NAO - and Atlantic Ridge regimes are unfavourable to aerosol radiative effects, because of weaker aerosol concentrations and increased cloud cover. This study thus puts forward the strong dependence of aerosol loads on the synoptic circulation from interannual to daily scales and, as a consequence, the important modulation of the aerosol effects on shortwave surface radiation and near-surface temperature by atmospheric circulation. The role of cloud cover is essential in this modulation as shown by the use of weather regimes. [ABSTRACT FROM AUTHOR]

Parametrizations of sea surface turbulent fluxes used in general circulation models (GCMs) assume horizontal homogeneity of atmospheric properties at the grid‐cell scale. The present study assesses the contribution of the meso‐scale (i.e., subgrid) to the grid‐scale surface fluxes, for GCM resolution ranging from 20 to 200 km, and thus quantifies the associated GCM surface flux error. A coarse‐graining method allows for an a priori analysis of the subgrid information. It is based on an atmospheric reference dataset produced by the convection‐permitting operational model AROME. The meso‐scale relative contribution to GCM‐scale fluxes exceeding 10% is shown to have large regional patterns, with large values (up to 90%) and high frequency of occurrence (up to 76% of the time). These meso‐scale motions are not necessarily due to convective activity but also occur frequently under dynamical perturbation conditions. Contributions to surface fluxes, at both the GCM scale and the meso‐scale, are disentangled through a Reynolds decomposition. It is found that temperature and humidity meso‐scale heterogeneities do not contribute much to the GCM‐scale fluxes. The subgrid dynamical processes are the main meso‐scale contribution and consist of two parts. The first one represents the wind magnitude and wind direction heterogeneities and corresponds to the so‐called gustiness approach. It is clarified that the contribution of the gustiness wind in the transfer coefficients cannot be neglected. The second part is the contribution of the wind speed subgrid variance, which contributes up to 10% of the GCM‐scale momentum flux. [ABSTRACT FROM AUTHOR]

The present study describes the atmospheric component of the sixth‐generation climate models of the Centre National de Recherches Météorologiques (CNRM), namely, ARPEGE‐Climat 6.3. It builds up on more than a decade of model development and tuning efforts, which led to major updates of its moist physics. The vertical resolution has also been significantly increased, both in the boundary layer and in the stratosphere. ARPEGE‐Climat 6.3 is now coupled to the new version (8.0) of the SURFace EXternalisée (SURFEX) surface model, in which several new features (e.g., floodplains, aquifers, and snow processes) improve the water cycle realism. The model calibration is discussed in depth. An amip‐type experiment, in which the sea surface temperatures and sea ice concentrations are prescribed, and following the CMIP6 protocol, is extensively evaluated, in terms of climate mean state and variability. ARPEGE‐Climat 6.3 is shown to improve over its previous version (5.1) by many climate features. Major improvements include the top‐of‐atmosphere and surface energy budgets in their various components (shortwave and longwave, total and clear sky), cloud cover, near‐surface temperature, precipitation climatology and daily‐mean distribution, and water discharges at the outlet of major rivers. In contrast, clouds over subtropical stratocumulus decks, several dynamical variables (sea level pressure, 500‐hPa geopotential height), are still significantly biased. The tropical intraseasonal variability and diurnal cycle of precipitation, though improved, remained area of concerns for further model improvement. New biases also emerge, such as a lack of precipitation over several tropical continental areas. Within the CMIP6 context, ARPEGE‐Climat 6.3 is the atmospheric component of CNRM‐CM6‐1 and CNRM‐ESM2‐1. Plain Language Summary: Since the early 1990s, the Centre National de Recherches Météorologiques (CNRM) has been developing a global atmosphere model for climate applications. The present work presents its latest version, ARPEGE‐Climat 6.3, as prepared for the sixth phase of the Coupled Model Intercomparison Project (CMIP6). It builds up on more than a decade of model development and tuning efforts. A CMIP6 amip‐type numerical experiment, in which the sea surface temperatures and sea ice concentrations are prescribed, is evaluated, in terms of climate mean state and variability. ARPEGE‐Climat 6.3 is shown to have better or similar skills compared to its previous version and to rank rather high among CMIP5 state‐of‐the‐art models by many mean‐state metrics. Major improvements include the top‐of‐atmosphere and surface energy budgets, cloud cover, near‐surface temperature, precipitation climatology and daily‐mean distribution, and water discharges at the outlet of major rivers. In contrast, clouds over the eastern part of ocean basins, and a few dynamical variables, such as sea level pressure, are still significantly biased. New biases also emerge, such as a lack of precipitation over several tropical continental areas. The remaining and new biases call for further understanding, especially whether they arise from calibration issues or model structural limits. Key Points: Version 6.3 of the ARPEGE‐Climat atmospheric model includes an increased vertical resolution and a major update of the moist physicsImprovements include radiation, cloud and precipitation climatology, daily rainfall distribution, and water discharge at major river outletsWeaknesses still include biases in low clouds and some dynamical fields, while the West African monsoon is a new model deficiency [ABSTRACT FROM AUTHOR]

Simulations are performed for the period 2000-2015 by two different regional climate models, ALADIN-Climat and RegCM, to quantify the direct and semi-direct radiative effects of biomass burning aerosols (BBA) in the Southeast Atlantic (SEA) region. The approach of using two different independent RCMs reinforces the robustness of the results. Different simulations have been performed using strongly absorbing BBA in accordance with recent in situ observations over the SEA. For the July-August-September (JAS) season, the single scattering albedo (SSA) and total aerosol optical depth (AOD) simulated by the ALADIN-Climat and RegCM models are consistent with the MACv2 climatology and MERRA-2 and CAMS-RA reanalyses near the biomass burning emission sources. However, the above-cloud AOD is slightly underestimated compared to satellite (MODIS and POLDER) data during the transport over the SEA. The direct radiative effect exerted at the continental and oceanic surfaces by BBA is significant in both models and the radiative effects at the top of the atmosphere indicate a remarkable regional contrast over SEA (in all-sky conditions), with a cooling (warming) north (south) of 10° S, which is in agreement with the recent MACv2 climatology. In addition, the two models indicate that BBA are responsible for an important shortwave radiative heating of ~ 0.5-1 K per day over SEA during JAS with maxima between 2 and 4 km above mean sea-level. At these altitudes, BBA increase air temperature by ~ 0.2-0.5 K, with the highest values being co-located with low stratocumulus clouds. Vertical changes in air temperature limit the subsidence over SEA creating a cyclonic anomaly. The opposite effect is simulated over the continent due to the increase in lower troposphere stability. The BBA semi-direct effect on the lower troposphere circulation is found to be consistent between the two models. Changes in the cloud fraction are moderate in response to the presence of smoke and the models differ over the Gulf of Guinea. Finally, the results indicate an important sensitivity of the direct and semi-direct effects to the absorbing properties of BBA. [ABSTRACT FROM AUTHOR]

A single‐column model approach conducted in the context of the Madden–Julian Oscillation through the CINDY2011/Dynamics of the Madden–Julian Oscillation field campaign is used to disentangle the respective role of the parameterizations of surface turbulent fluxes and of model atmospheric physics in controlling the surface latent heat flux. The major differences between the models used in this study occur during the suppressed phases of deep convection. They are attributed to differences in model atmospheric physics which is shown to control the near‐surface relative humidity and thereby the surface latent heat flux. In contrast, during active phases of deep convection, turbulent air‐sea flux parameterizations impact the latent heat flux through the drag coefficient and can represent two thirds of the divergence caused by the different atmospheric physics. The combined effects need to be accounted for to improve both the representation of latent heat flux and the atmospheric variables used to compute it. Key Points: A single‐column model approach is used in the context of the CINDY2011/DYNAMO field campaign to compare models results during a MJO eventNear‐surface relative humidity is a key constraint for the surface latent heat flux and the way it is simulated by different modelsCompared to atmospheric physics, air‐sea flux parameterization has a relatively large impact only during active convective phases [ABSTRACT FROM AUTHOR]

Estimates of the direct radiative effect (DRE) from absorbing smoke aerosols over the southeast Atlantic Ocean (SAO) require simulation of the microphysical and optical properties of stratocumulus clouds as well as of the altitude and shortwave (SW) optical properties of biomass burning aerosols (BBAs). In this study, we take advantage of the large number of observations acquired during the ObseRvations of Aerosols above Clouds and their intEractionS (ORACLES-2016) and Layered Atlantic Smoke Interactions with Clouds (LASIC) projects during September 2016 and compare them with datasets from the ALADIN-Climate (Aire Limitée Adaptation dynamique Développement InterNational) regional model. The model provides a good representation of the liquid water path but the low cloud fraction is underestimated compared to satellite data. The modeled total-column smoke aerosol optical depth (AOD) and above-cloud AOD are consistent (∼0.7 over continental sources and ∼0.3 over the SAO at 550 nm) with the Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2), Ozone Monitoring Instrument (OMI) or Moderate Resolution Imaging Spectroradiometer (MODIS) data. The simulations indicate smoke transport over the SAO occurs mainly between 2 and 4 km, consistent with surface and aircraft lidar observations. The BBA single scattering albedo is slightly overestimated compared to the Aerosol Robotic Network (AERONET) and more significantly when compared to Ascension Island surface observations. The difference could be due to the absence of internal mixing treatment in the ALADIN-Climate model. The SSA overestimate leads to an underestimation of the simulated SW radiative heating compared to ORACLES data. ALADIN-Climate simulates a positive (monthly mean) SW DRE of about +6 W m -2 over the SAO (20 ∘ S–10 ∘ N and 10 ∘ W–20 ∘ E) at the top of the atmosphere and in all-sky conditions. Over the continent, the presence of BBA is shown to significantly decrease the net surface SW flux, through direct and semi-direct effects, which is compensated by a decrease (monthly mean) in sensible heat fluxes (-25 W m -2) and surface land temperature (-1.5 ∘ C) over Angola, Zambia and the Democratic Republic of the Congo, notably. The surface cooling and the lower tropospheric heating decrease the continental planetary boundary layer height by about ∼200 m. [ABSTRACT FROM AUTHOR]

This work uses a network of GPS stations over Europe from which a homogenized integrated water vapor (IWV) dataset has been retrieved, completed with colocated temperature and precipitation measurements over specific stations to (i) estimate the biases of six regional climate models over Europe in terms of humidity; (ii) understand their origins; and (iii) finally assess the impact of these biases on the frequency of occurrence of precipitation. The evaluated simulations have been performed in the framework of HYMEX/Med-CORDEX programs and cover the Mediterranean area and part of Europe at horizontal resolutions of 50 to 12 km. The analysis shows that models tend to overestimate the low values of IWV and the use of the nudging technique reduces the differences between GPS and simulated IWV. Results suggest that physics of models mostly explain the mean biases, while dynamics affects the variability. The land surface–atmosphere exchanges affect the estimation of IWV over most part of Europe, especially in summer. The limitations of the models to represent these processes explain part of their biases in IWV. However, models correctly simulate the dependance between IWV and temperature, and specifically the deviation that this relationship experiences regarding the Clausius–Clapeyron law after a critical value of temperature (Tbreak). The high spatial variability of Tbreak indicates that it has a strong dependence on local processes which drive the local humidity sources. This explains why the maximum values of IWV are not necessarily observed over warmer areas, which are often dry areas. Finally, it is shown over the SIRTA observatory (near Paris) that the frequency of occurrence of light precipitation is strongly conditioned by the biases in IWV and by the precision of the models to reproduce the distribution of IWV as a function of the temperature. The results of the models indicate that a similar dependence occurs in other areas of Europe, especially where precipitation has a predominantly convective character. According to the observations, for each range of temperature, there is a critical value of IWV from which precipitation starts to increase. The critical values and the probability of exceeding them are simulated with a bias that depends on the model. Those models, which generally present light precipitation too often, show lower critical values and higher probability of exceeding them. [ABSTRACT FROM AUTHOR]

The representation of stratocumulus by the atmospheric component of the Centre National de Recherches Météorologiques model version 6 (CNRM‐CM6‐1) is assessed. An Atmospheric Model Intercomparison Project‐type simulation is first used to document the main model errors, namely, a large lack of stratocumulus over the eastern part of tropical ocean basins. Short‐term hindcasts, following the Transpose‐Atmospheric Model Intercomparison Project framework, are then used to better assess the timescales associated with the cloud bias growth and to highlight the processes leading to them. These biases are shown to appear within only a few hours, independently of errors in the large‐scale circulation that set up within a few days. Key processes underlying the low‐cloud formation are thus mainly local and, to the first order, do not imply any feedback between the model physics and the large‐scale dynamics. As a consequence, short‐term hindcasts provide a relevant framework to investigate whether the low‐cloud underestimate is related to errors in the large‐scale state variables or to errors in the model parameterizations. Sensitivity tests highlight that the involved processes arise (1) mostly from misrepresentation of subgrid effects on cloud formation and (2) partly from biases in drying induced by cloud‐top entrainment mixing. Improvements in the representation of stratocumulus in the CNRM‐CM6‐1 model might thus be expected by including a more realistic subgrid‐scale temperature and moisture distribution, that would link convective and turbulence processes. Finally, this study confirms the potential of short‐term hindcasts, which provide a trustworthy framework to evaluate and develop climate model parameterizations. Key Points: Short‐term hindcasts help to disentangle timescales of marine stratocumulus cloud bias growth in the CNRM‐CM6‐1 modelBiases in low clouds are mostly related to errors in the subgrid distribution of temperature and humidity and the cloud‐top entrainmentAlong with single‐columns models, short‐term hindcasts are a complementary testbed for cloud parameterization development [ABSTRACT FROM AUTHOR]

Estimates of the direct radiative forcing (DRF) from absorbing smoke aerosols over the Southeast Atlantic Ocean (SAO) requires simulation of the microphysical and optical properties of stratocumulus clouds (Sc) as well as of the altitude and shortwave (SW) optical properties of biomass burning aerosols (BBA). In this study, we take advantage of the large number of observations acquired during the ORACLES-2016 and LASIC projects during September 2016 and compare them with datasets from the ALADIN-Climate regional model. The model provides a good representation of the liquid water path (LWP) but the low cloud fraction (LCF) is underestimated compared to satellite data. The modeled total column smoke aerosol optical depth (AOD) and Above Cloud AOD (ACAOD) are consistent (~ 0.7 over continental sources and ~ 0.3 over SAO at 550 nm) with MERRA2, OMI or MODIS data. The simulations indicate smoke transport over SAO occurs mainly between 2 and 4 km, consistent with surface and aircraft lidar observations. The BBA single scattering albedo (SSA) is slightly overestimated compared to AERONET, and more significantly when compared to Ascension Island surface observations. The difference could be due to the absence of internal mixing treatment in the ALADIN-Climate model. The SSA overestimate leads to underestimate the simulated SW radiative heating compared to ORACLES data. For September 2016, ALADIN-Climate simulates a positive (monthly mean) SW DRF of about +6 W m-2 over SAO (20° S-10° N and 10° W-20° E) at the top of the atmosphere (TOA) and in all-sky conditions. Over the continent, the presence of BBA is shown to significantly decrease the net surface SW flux, through direct and semi-direct effects, which is compensated by a decrease (monthly mean) in sensible heat fluxes (-25 W/m-2) and surface land temperature (-1.5°C) over Angola, Zambia and Congo notably. The surface cooling and the lower tropospheric heating tends to decrease the continental planetary boundary layer (PBL) height by about ~ 200 m. [ABSTRACT FROM AUTHOR]

This work uses a network of GPS stations over Europe from which a homogenised integrated water vapor (IWV) dataset has been retrieved, completed with colocated temperature and precipitation measurements over specific stations to i) estimate the biases of six regional climate models over Europe in terms of humidity; ii) understand their origins; iii) and finally assess the impact of these biases on the frequency of occurrence of precipitation. The evaluated simulations have been performed in the framework of HYMEX/Med-CORDEX programs and cover the Mediterranean area and part of Europe at horizontal resolutions of 50 to 12km. The analysis shows that models tend to overestimate the low values of IWV and the use of the nudging technique reduces the differences between GPS and simulated IWV. Results suggest that physics of models mostly explain the mean biases, while dynamics affects the variability. The land surface/atmosphere exchanges affect the estimation of IWV over most part of Europe, especially in summer. The limitations of the models to represent these processes explain part of their baises in IWV. However, models correctly simulate the dependance between IWV and temperature, and specifically the deviation that this relationship experiences regarding the Clausius-Clapeyron law after a critical value of temperature (Tbreak). The high spatial variability of Tbreak indicates that it has a strong dependence on local processes which drive the local humidity sources. This explains why the maximum values of IWV are not necessarely observed over warmer area, that are often dry area. Finally, it is shown over SIRTA observatory (near Paris) that the frequency of occurrence of light precipitation is strongly conditioned by the biases in IWV and by the precision of the models to reproduce the distribution of IWV as a function of the temperature. The results of the models indicate that a similar dependence occurs in other areas of Europe, especially where precipitation has a predominantly convective character. According to the observations, for each range of temperature, there is a critical value of IWV from which precipitation picks up. The critical values and the probability to exceed them are simulated with a bias that depends on the model. Those models which present too often light precipitation generally show lower critical values and higher probability to exceed them. [ABSTRACT FROM AUTHOR]

Abstract: A single‐column model (SCM) approach is used to assess the CNRM climate model (CNRM‐CM) version 6 ability to represent the properties of the apparent heat source (Q1) and moisture sink (Q2) as observed during the 3 month CINDY2011/DYNAMO field campaign, over its Northern Sounding Array (NSA). The performance of the CNRM SCM is evaluated in a constrained configuration in which the latent and sensible heat surface fluxes are prescribed, as, when forced by observed sea surface temperature, the model is strongly limited by the underestimate of the surface fluxes, most probably related to the SCM forcing itself. The model exhibits a significant cold bias in the upper troposphere, near 200 hPa, and strong wet biases close to the surface and above 700 hPa. The analysis of the Q1 and Q2 profile distributions emphasizes the properties of the convective parameterization of the CNRM‐CM physics. The distribution of the Q2 profile is particularly challenging. The model strongly underestimates the frequency of occurrence of the deep moistening profiles, which likely involve misrepresentation of the shallow and congestus convection. Finally, a statistical approach is used to objectively define atmospheric regimes and construct a typical convection life cycle. A composite analysis shows that the CNRM SCM captures the general transition from bottom‐heavy to mid‐heavy to top‐heavy convective heating. Some model errors are shown to be related to the stratiform regimes. The moistening observed during the shallow and congestus convection regimes also requires further improvements of this CNRM‐CM physics. [ABSTRACT FROM AUTHOR]

The performance of updated versions of the four earth system models (ESMs) CNRM, EC-Earth, HadGEM, and MPI-ESM is assessed in comparison to their predecessor versions used in Phase 5 of the Coupled Model Intercomparison Project. The Earth System Model Evaluation Tool (ESMValTool) is applied to evaluate selected climate phenomena in the models against observations. This is the first systematic application of the ESMValTool to assess and document the progress made during an extensive model development and improvement project. This study focuses on the South Asian monsoon (SAM) and the West African monsoon (WAM), the coupled equatorial climate, and Southern Ocean clouds and radiation, which are known to exhibit systematic biases in present-day ESMs. The analysis shows that the tropical precipitation in three out of four models is clearly improved. Two of three updated coupled models show an improved representation of tropical sea surface temperatures with one coupled model not exhibiting a double Intertropical Convergence Zone (ITCZ). Simulated cloud amounts and cloud-radiation interactions are improved over the Southern Ocean. Improvements are also seen in the simulation of the SAM and WAM, although systematic biases remain in regional details and the timing of monsoon rainfall. Analysis of simulations with EC-Earth at different horizontal resolutions from T159 up to T1279 shows that the synoptic-scale variability in precipitation over the SAM and WAM regions improves with higher model resolution. The results suggest that the reasonably good agreement of modeled and observed mean WAM and SAM rainfall in lower-resolution models may be a result of unrealistic intensity distributions. [ABSTRACT FROM AUTHOR]

Ocean surface represents roughly 70% of the Earth's surface, playing a large role in the partitioning of the energy flow within the climate system. The ocean surface albedo (OSA) is an important parameter in this partitioning because it governs the amount of energy penetrating into the ocean or reflected towards space. The old OSA schemes in the ARPEGE-Climat and LMDZ models only resolve the latitudinal dependence in an ad hoc way without an accurate representation of the solar zenith angle dependence. Here, we propose a new interactive OSA scheme suited for Earth system models, which enables coupling between Earth system model components like surface ocean waves and marine biogeochemistry. This scheme resolves spectrally the various contributions of the surface for direct and diffuse solar radiation. The implementation of this scheme in two Earth system models leads to substantial improvements in simulated OSA. At the local scale, models using the interactive OSA scheme better replicate the day-to-day distribution of OSA derived from ground-based observations in contrast to old schemes. At global scale, the improved representation of OSA for diffuse radiation reduces model biases by up to 80% over the tropical oceans, reducing annual-mean model--data error in surface upwelling shortwave radiation by up to 7W m-2 over this domain. The spatial correlation coefficient between modeled and observed OSA at monthly resolution has been increased from 0.1 to 0.8. Despite its complexity, this interactive OSA scheme is computationally efficient for enabling precise OSA calculation without penalizing the elapsed model time. [ABSTRACT FROM AUTHOR]

This study presents a multi-scale analysis of an extreme rain event that occurred in Burkina Faso on 1 September 2009 with an absolute record of 263 mm rainfall observed at Ouagadougou. This high-impact weather system results from the combination of several favourable ingredients at different scales. The sea-surface temperature anomaly patterns in July-August 2009 of both the Atlantic cold tongue, the Tropical Atlantic Dipole and the Mediterranean Sea are favourable factors for the northward penetration of the West African monsoon. The intense convective activity of the last 10-day period in August is associated with the crossing of a convectively coupled Kelvin wave increasing the African easterly wave (AEW) activity, and of an equatorial Rossby wave. At the synoptic scale this event corresponds to the passage of a train of three AEWs with increasing magnitude. Behind the first AEW trough axis, an intense and deep southerly monsoon burst develops. It contributes to the amplification of the second AEW and its breaking is associated with the formation of an intense meso-vortex on the southern flank of the African easterly jet. Compared to the fast-moving squall line, the dominant type of precipitating weather system over the Sahel, the Ouagadougou precipitating system appears to be a moist vortex propagating slowly, allowing rainfall accumulation, without wind gusts or convective cold pools observed at the surface. The main precipitation area is located about 2° longitude downshear (westward due to the African easterly jet) of the centre of this strong meso-vortex. [ABSTRACT FROM AUTHOR]

Ocean surface represents roughly 70 % of the Earth surface, playing a large role in the partitioning of the energy flow within the climate system. The ocean surface albedo (OSA) is an important parameter in this partitioning because it governs the amount of energy penetrating into the ocean or reflected towards space. The old OSA schemes in the ARPEGE and LMDZ models only resolve the latitudinal dependence in an ad hoc way without an accurate representation of the solar zenith angle dependence. Here, we propose a new interactive OSA scheme suited for Earth system models, which gather contributions for relevant OSA processes published in the literature over the last decades. This scheme resolves spectrally the various contributions of the surface for direct and diffuse solar radiation. The implementation of this scheme in two Earth system models leads to substantial improvements in simulated OSA. At the local scale, models using the interactive OSA scheme better replicate the day-to-day distribution of OSA derived from ground-based observations in contrast to old schemes. At global scale, the improved representation of OSA for diffuse radiation reduces model biases by up to 80 % over the tropical oceans, reducing annual-mean model-data error in surface upwelling shortwave radiation by up to 7 W m-2 over this domain. The spatial correlation coefficient between modelled and observed OSA at monthly resolution has been increased from 0.1 to 0.8. Despite its complexity, this interactive OSA scheme is computationally efficient to enable precise OSA calculation without penalizing the model elapsed time. [ABSTRACT FROM AUTHOR]

This study compares the simulation of subseasonal tropical variability by a set of six state-of-the-art AGCMs in two experiments in aquaplanet configuration: a zonally symmetric experiment, and an experiment with a warm pool centered on the equator. In all six models, the presence of the warm pool generates zonal asymmetries in the simulated mean states in the form of a 'Gill-type' response, made more complex by feedbacks between moisture, convective heating and circulation. Noticeable differences appear from one model to another. Only half the models simulate mean low-level equatorial westerlies over the warm pool area. The presence of the warm pool can also favor the development of large-scale variability consistent with observed Madden-Julian Oscillation (MJO) characteristics, but this happens only in half the models. Our results do not support the idea that the presence of the warm pool and/or of mean low-level equatorial westerlies are sufficient conditions for MJO-like variability to arise in the models. Comparing spectral characteristics of the simulated Convectively Coupled Equatorial Waves (CCEWs) in the aquaplanet experiments and the corresponding coupled atmosphere-ocean (i.e., CMIP) and atmosphere-only (i.e., AMIP) simulations, we also show that there is more consistency for a given model across its configurations, than for a given configuration across the six models. Overall, our results confirm that the simulation of subseasonal variability by given model is significantly influenced by the parameterization of subgrid physical processes (most-likely cloud processes), both directly and through modulation of the mean state. [ABSTRACT FROM AUTHOR]

Several studies have pointed out the dependence of low-cloud feedbacks on the strength of the lower-tropospheric convective mixing. By analyzing a series of single-column model experiments run by a climate model using two different convective parametrizations, this study elucidates the physical mechanisms through which marine boundary-layer clouds depend on this mixing in the present-day climate and under surface warming. An increased lower-tropospheric convective mixing leads to a reduction of low-cloud fraction. However, the rate of decrease strongly depends on how the surface latent heat flux couples to the convective mixing and to boundary-layer cloud radiative effects: (i) on the one hand, the latent heat flux is enhanced by the lower-tropospheric drying induced by the convective mixing, which damps the reduction of the low-cloud fraction, (ii) on the other hand, the latent heat flux is reduced as the lower troposphere stabilizes under the effect of reduced low-cloud radiative cooling, which enhances the reduction of the low-cloud fraction. The relative importance of these two different processes depends on the closure of the convective parameterization. The convective scheme that favors the coupling between latent heat flux and low-cloud radiative cooling exhibits a stronger sensitivity of low-clouds to convective mixing in the present-day climate, and a stronger low-cloud feedback in response to surface warming. In this model, the low-cloud feedback is stronger when the present-day convective mixing is weaker and when present-day clouds are shallower and more radiatively active. The implications of these insights for constraining the strength of low-cloud feedbacks observationally is discussed. [ABSTRACT FROM AUTHOR]

This paper introduces the Tropical Rain belts with an Annual cycle and a Continent Model Intercomparison Project (TRACMIP). TRACMIP studies the dynamics of tropical rain belts and their response to past and future radiative forcings through simulations with 13 comprehensive and one simplified atmosphere models coupled to a slab ocean and driven by seasonally varying insolation. Five idealized experiments, two with an aquaplanet setup and three with a setup with an idealized tropical continent, fill the space between prescribed-SST aquaplanet simulations and realistic simulations provided by CMIP5/6. The simulations reproduce key features of present-day climate and expected future climate change, including an annual-mean intertropical convergence zone (ITCZ) that is located north of the equator and Hadley cells and eddy-driven jets that are similar to present-day climate. Quadrupling CO2 leads to a northward ITCZ shift and preferential warming in Northern high latitudes. The simulations show interesting CO2-induced changes in the seasonal excursion of the ITCZ and indicate a possible state dependence of climate sensitivity. The inclusion of an idealized continent modulates both the control climate and the response to increased CO2; for example, it reduces the northward ITCZ shift associated with warming and, in some models, climate sensitivity. In response to eccentricity-driven seasonal insolation changes, seasonal changes in oceanic rainfall are best characterized as a meridional dipole, while seasonal continental rainfall changes tend to be symmetric about the equator. This survey illustrates TRACMIP's potential to engender a deeper understanding of global and regional climate and to address questions on past and future climate change. [ABSTRACT FROM AUTHOR]

Future responses of cloud regimes are analyzed for five CMIP5 models forced with observed SSTs and subject to a patterned SST perturbation. Correlations between cloud properties in the control climate and changes in the warmer climate are investigated for each of a set of cloud regimes defined using a clustering methodology. The only significant (negative) correlation found is in the in-regime net cloud radiative effect for the stratocumulus regime. All models overestimate the in-regime albedo of the stratocumulus regime. Reasons for this bias and its relevance to the future response are investigated. A detailed evaluation of the models' daily-mean contributions to the albedo from stratocumulus clouds with different cloud cover fractions reveals that all models systematically underestimate the relative occurrence of overcast cases but overestimate those of broken clouds. In the warmer climate the relative occurrence of overcast cases tends to decrease while that of broken clouds increases. This suggests a decrease in the climatological in-regime albedo with increasing temperature (a positive feedback); this is opposite to the feedback suggested by the analysis of the bulk in-regime albedo. Furthermore we find that the inter-model difference in the sign of the in-cloud albedo feedback is consistent with the difference in sign of the in-cloud liquid water path response, and there is a strong positive correlation between the in-regime liquid water path in the control climate and its response to warming. We therefore conclude that further breakdown of the in-regime properties into cloud cover and in-cloud properties is necessary to better understand the behavior of the stratocumulus regime. Since cloud water is a physical property and is independent of a model's radiative assumptions, it could potentially provide a useful emergent constraint on cloud feedback. [ABSTRACT FROM AUTHOR]

The internal processes within an African Easterly Wave (AEW) system, involving mass, dynamic and water vapour fields are investigated using ERA-I reanalysis, in order to highlight the interactions between convection and AEWs. The budgets of heat, moisture and momentum are analysed during the different phases of AEWs detected using synoptic-scale precipitable water anomalies as proposed by Poan et al. (2013). The strong climatological meridional gradient of moisture present in the Sahel impacts the shape of the apparent heat source and humidity sink. AEW events over the Sahel are associated with a meridional shift of the intertropical convergence zone (ITCZ). Large exchanges of momentum by small-scale convective transport are also highlighted between the low- and mid-levels, contributing to the reinforcement of the AEW circulation at 600 hPa and the damping of the monsoon flow. This also appears as a possible mechanism for the vertical tilt of the meridional wind associated with AEWs. Heat budget computation, in the southern flank of the West African Heat-Low (HL) region where such AEWs occur, reveals that the heating anomalies are mainly driven by the horizontal advections. The vertical circulation acts as a precursor, which initiates the heat transport in the lower troposphere. However, weaker, turbulent mixing also participates in the development of these anomalies, especially in the boundary layer. These budgets ultimately allow the distinct contributions of diabatic and adiabatic processes to be determined. [ABSTRACT FROM AUTHOR]

We examine the diurnal cycle of marine cloud feedback using high frequency outputs in CFMIP-2 idealised uniform +4 K SST perturbation experiments from seven CMIP5 models. Most of the inter-model spread in the diurnal mean marine shortwave cloud feedback can be explained by low cloud responses, although these do not explain the model responses at the neutral/weakly negative end of the feedback range, where changes in mid and high level cloud properties are more important. All of the models show reductions in marine low cloud fraction in the warmer climate, and these are in almost all cases largest in the mornings when more cloud is present in the control simulations. This results in shortwave cloud feedbacks being slightly stronger and having the largest inter-model spread at this time of day. The diurnal amplitudes of the responses of marine cloud properties to the warming climate are however small compared to the inter-model differences in their diurnally meaned responses. This indicates that the diurnal cycle of cloud feedback is not strongly relevant to understanding inter-model spread in overall cloud feedback and climate sensitivity. A number of unusual behaviours in individual models are highlighted for future investigation. [ABSTRACT FROM AUTHOR]