Turbulent Fluxes Research Articles

Northeast Pacific Marine Heatwave Mechanism Inferred from Adjoint Sensitivities

Abstract A classic example of a marine heatwave (MHW) was the 2014–16 warm event that spread across the northeastern Pacific (NEP) Ocean. We use an adjoint sensitivity approach to shed new light on potential causes for such reoccurring NEP MHW events. The study is based on the Massachusetts Institute of Technology General Circulation Model (MITgcm) and its adjoint, for which the mean top 100-m potential temperature during different target years was set as the objective function, separately for the two target regions (145°–160°W, 48°–56°N) and (130°–145°W, 40°–48°N). Resulting adjoint sensitivities show that during MHW years, local turbulent surface heat flux is the dominant atmospheric driver, with air temperature, specific humidity, and longwave radiation leading to up to 80% of the temperature anomaly of the NEP; during normal years, this is only about 60%. In contrast, increased wind typically does not lead to an MHW occurrence as it is associated with a deepening of the mixed layer. We find the horizontal temperature advection, i.e., the impact of the basinwide ocean circulation, to be less important during an MHW year, but it could act as a preconditioning of MHW through its role in climate oscillations. Response analysis shows that atmospheric forcing anomalies occurring within 3 months (from October to December) prior to an MHW year play a critical role in driving the MHW. The reconstruction using various sensitivity periods suggests that the leading 6-month atmospheric conditions should have potential predictive skills for the next year. Reconstruction that includes leading 36-month atmospheric conditions performs better than persistence.

Assessing the Combined Impact of Land Surface Temperature and Droughts to Heatwaves over Europe Between 2003 and 2023

The increasing frequency, intensity, and duration of heatwaves and droughts pose significant societal and environmental challenges across Europe. This study analyzes land surface temperature (LST) observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) between 2003 and 2023 to identify thermal anomalies associated with heatwaves. Additionally, this study examines the role of different land cover types in modulating heatwave impacts, employing turbulent flux observations from micrometeorological towers. The interaction between heatwaves and droughts is further explored using the Standardized Precipitation Evapotranspiration Index (SPEI) and soil moisture data, highlighting the amplifying role of water stress through land–atmosphere feedbacks. The results reveal a statistically significant upward trend in LST-derived thermal anomalies, with the 2022 heatwave identified as the most extreme event, when approximately 75% of Europe experienced strong positive anomalies. On average, 91% of heatwave episodes identified in reanalysis-based air temperature records coincided with LST-defined anomaly events, confirming LST as a robust proxy for heatwave detection. Flux tower observations show that, during heatwaves, evergreen coniferous and mixed forests predominantly enhance sensible heat fluxes (mean anomalies during midday of 74 W/m2 and 62 W/m2, respectively), while grasslands exhibit increased latent heat flux (89 W/m2). Notably, under extreme compound heat–drought conditions, this pattern reverses for grassed sites due to rapid soil moisture depletion. Overall, the findings underscore the combined influence of surface temperature and drought in driving extreme heat events and introduce a novel, multi-source approach that integrates satellite, reanalysis, and ground-based data to assess heatwave dynamics across scales.

Formation and Circulation of Dense Water From a Two‐Year Moored Record in the Northwestern Iceland Sea

AbstractThe recent sea‐ice retreat in the western Nordic Seas has exposed the ocean to the atmosphere in winter, thereby facilitating dense‐water formation. Here, we present a 2‐year long record (2016–2018) of ocean stratification and currents from the northwestern Iceland Sea, which was obtained from a mooring deployed in Eggvin Offset, an approximately 1,500 m deep passage between the Greenland and Iceland Seas. The trajectory of an Argo float deployed in winter 2017/2018 indicates a connection between Eggvin Offset and the slope north of Iceland, where the boundary currents that supply the overflows east and west of Iceland originate. However, the low transport of potential overflow water ( 27.8 kg m−3) through Eggvin Offset demonstrates that it is not a major passage for the exchange of dense water. Dense‐water formation occurred during both winter 2016/2017 and winter 2017/2018; the mixed layer cooled and deepened mainly as an integrated response to a succession of cold‐air outbreaks. Greater turbulent heat fluxes and a more even distribution of cold‐air outbreaks at the mooring site in winter 2016/2017 resulted in mixed‐layer depths reaching a maximum of approximately 450 m, compared to 350 m the following winter. The water formed in Eggvin Offset those recent winters attained densities similar to those of water formed in the central Iceland Sea four decades ago. This supports the notion that in a warming climate, the locus of dense‐water formation has shifted from the interior basin to the western Iceland Sea.

Influence of winter Arctic sea ice anomalies on the following autumn Indian Ocean Dipole development

Abstract This study shows a close relationship between winter Arctic sea ice concentration (WASIC) anomalies in the Barents-Greenland Seas and the subsequent autumn Indian Ocean Dipole (IOD) based on the observational analysis and numerical simulations. Particularly, more (less) WASIC in the Barents-Greenland Seas tends to lead to a positive (negative) IOD in the following autumn. Above-normal WASIC in the Barents-Greenland Seas results in reduction of the upward turbulent heat flux and induces tropospheric cooling over the Arctic. This tropospheric cooling triggers an atmospheric teleconnection extending from the Eurasian Arctic to the subtropical North Pacific. Numerical experiments with both the linear barotropic model and atmospheric general circulation model can well capture the atmospheric teleconnection associated with the WASIC anomalies. The subtropical atmospheric anomalies generated by the WASIC anomalies then result in subtropical sea surface temperature (SST) warming, which sustains and expands southward to the equatorial central Pacific during the following summer via a wind-evaporation-SST feedback. The resulting equatorial central Pacific SST warming anomalies induce local atmospheric heating and trigger an anomalous Walker circulation with descending motion and low-level anomalous southeasterly winds over the southeastern tropical Indian Ocean. These anomalous southeasterly winds trigger positive air-sea interaction in the tropical Indian Ocean and contribute to the development of the IOD. The close connection of the WASIC anomalies with the subsequent IOD and the underlying physical processes can be reproduced by the coupled climate models participated in the CMIP6. These results indicate that the condition of WASIC is a potential effective precursor of IOD events.

Urban fluxes for free: Estimating urban turbulent surface fluxes from crowdsourced meteorological canyon layer observations

Urban fluxes for free: Estimating urban turbulent surface fluxes from crowdsourced meteorological canyon layer observations

Satellite observations reveal the impact of changing sea ice on wintertime surface turbulent fluxes over the Arctic Ocean

Satellite observations reveal the impact of changing sea ice on wintertime surface turbulent fluxes over the Arctic Ocean

The Near‐Surface Boundary Layer of Hurricane Laura (2020) at Landfall

AbstractWhile challenging, quantification of the near‐surface landfalling hurricane wind field is necessary for understanding hurricane intensity changes and damage potential. Using single‐ and dual‐Doppler Doppler on Wheels and in situ anemometer data, the wind structure of the very near‐surface boundary layer of Hurricane Laura (2020) is characterized. Small‐scale hurricane boundary layer (HBL) rolls (HBLRs) with a median size of approximately 400 m are present throughout much of the landfall, but are most vigorous in the eyewall. The maximum turbulent kinetic energy (TKE) and momentum flux associated with HBLRs occur in the eyewall and are much larger than previously documented at landfall. DOW‐derived and anemometer‐derived TKE values are comparable. Observed maximum surface gusts were consistent with the maximum radar wind speeds aloft, suggesting the importance of vertical transport within the HBL by sub‐kilometer scale structures for the enhancement of surface wind speeds.

Meso-scale photospheric convection during chromospheric fan-shaped surge on light bridge

Recently, intermittent and aperiodic fan-shaped chromospheric surges have attracted significant attention, though their related photospheric dynamic signals remain unclear. This study examines seven such surges and their potential photospheric signals along a light bridge (LB) in NOAA AR 12371 over a period of 100 min using BBSO/GST observations. Each surge displays as dark, jet-like structures with nearly uniform amplitudes, aligned closely along the LB to form a long smooth upper edge. Simultaneously, the photospheric LB exhibits dynamic grains, including bright points and granules. Adjacent grains brighten, expand, and merge into a ‘grain group’ (GG), which span LB’s cross-section. As GG moves along the LB, its leading edge develops an arched structure. During GG formation, the local horizontal magnetic field direction undergoes significant deflection. Within the 100-min interval, 10 GGs were recorded, occurring intermittently and aperiodically. Notably, three photospheric GG-free intervals corresponded to three chromospheric surge-free intervals, with a temporal delay (80–712 s) between GG reappearance and surge recurrence. Our findings suggest that meso-scale photospheric GGs, larger than individual granules but smaller than the full extent of the LB, are closely related to chromospheric surges. A conceptual model integrating inverse turbulent cascades and flux tube interactions is proposed, unifying multi-scale energy transfer from photospheric convection to chromospheric reconnection.

Role of ocean coupling in the weakening of the extratropical storm tracks from Arctic sea ice loss

Abstract Within the changing climate, the Northern Hemisphere storm tracks have been projected to shift poleward and expand eastward. Changes in the Northern Hemisphere storm tracks arise from a variety of sometimes competing effects on the region’s baroclinicity. Arctic amplification weakens the meridional temperature gradient, hence weakening the storm track, while low-latitude warming has the opposite effect, and surface-amplified warming at high latitudes reduces static stability. To determine the mechanisms driving these competing changes, we use a hierarchy of models with different levels of ocean–atmosphere coupling, using forcings from the Polar Amplification Model Intercomparison Project (PAMIP) to assess the role of Arctic sea ice loss on the Northern Hemisphere storm tracks. We find that ocean–atmosphere coupling enhances and sharpens the weakening of both the North Atlantic and North Pacific storm tracks in response to Arctic sea ice loss, that surface turbulent heat flux modulates the intensity of the weakening, and that local ocean dynamics controls the meridional location of the response. A moist isentropic diagnostic of the atmospheric overturning circulation shows that most of the weakening of the storm tracks and its associated changes in atmospheric heat transport (AHT) arise from a weakening of the transient eddy mass flux. Poleward of the storm tracks, the AHT also decreases, but through weakened effective stratification rather than weakening mass fluxes, an effect which occurs even in the absence of ocean dynamical coupling.

Investigating the influence of changing ice surfaces on gravity wave formation impacting glacier boundary layer flow with large-eddy simulations

Abstract. Mountain glaciers are located in highly complex terrain, and their local microclimate is influenced by mountain boundary layer processes and dynamically induced gravity waves. Previous observations from turbulence flux towers, as well as large-eddy simulations, over the Hintereisferner (HEF) glacier in the Austrian Alps have shown that down-glacier winds are often disturbed by cross-glacier flow from the north-west associated with gravity waves. In this work, we explore how changing the ice surface coverage upstream of HEF influences this gravity wave formation and intensity and the feedback that this has on boundary layer flow over HEF. In semi-idealized large-eddy simulations, we explore the impact of changing surface properties on HEF's microclimate by removing the upstream glaciers only (NO_UP) and removing all ice surfaces (NO_GL). Simulations suggest that removing the upstream glaciers (which causes a change in boundary layer stratification from stable to unstable) leads to a weaker gravity wave that breaks earlier than in the reference simulation, resulting in enhanced turbulent mixing over HEF. As a consequence, this leads to higher temperatures over the HEF tongue. Removing all glaciers results – as expected – in higher temperatures of up to 5 K over the missing ice surfaces, while the gravity wave pattern is similar to that in the NO_UP simulation, indicating that the upstream boundary layer exerts dominant control over downstream responses in such highly dynamic conditions. Furthermore, the results show that the upstream glaciers have a stabilizing effect on the boundary layer, impacting gravity wave formation, downslope windstorm intensity, and their feedback on the flow structure in valleys downstream. This case study shows that a single glacier tongue is not isolated from its environment under strong synoptic forcing and that surrounding glaciers and local topography have to be taken into account when studying atmosphere–cryosphere exchange processes.

Impact of improved air quality during complete and partial lockdowns on surface energetics and atmospheric boundary layer.

Impact of improved air quality during complete and partial lockdowns on surface energetics and atmospheric boundary layer.

Simulating Atmospheric Processes in Earth System Models and Quantifying Uncertainties With Deep Learning Multi‐Member and Stochastic Parameterizations

AbstractDeep learning is a powerful tool to represent subgrid processes in climate models, but many application cases have so far used idealized settings and deterministic approaches. Here, we develop stochastic parameterizations with calibrated uncertainty quantification to learn subgrid convective and turbulent processes and surface radiative fluxes of a superparameterization embedded in an Earth System Model (ESM). We explore three methods to construct stochastic parameterizations: (a) a single Deep Neural Network (DNN) with Monte Carlo Dropout; (b) a multi‐member parameterization; and (c) a Variational Encoder Decoder with latent space perturbation. We show that the multi‐member parameterization improves the representation of convective processes, especially in the planetary boundary layer, compared to individual DNNs. The respective uncertainty quantification illustrates that methods (b) and (c) are advantageous compared to a dropout‐based DNN parameterization regarding the spread of convective processes. Hybrid simulations with our best‐performing multi‐member parameterizations remained challenging and crash within the first days. Therefore, we develop a pragmatic partial coupling strategy relying on the superparameterization for condensate emulation. Partial coupling reduces the computational efficiency of hybrid Earth‐like simulations but enables model stability over 5 months with our multi‐member parameterizations. However, our hybrid simulations exhibit biases in thermodynamic fields and differences in precipitation patterns. Despite this, the multi‐member parameterizations enable improvements in reproducing tropical extreme precipitation compared to a traditional convection parameterization. Despite these challenges, our results indicate the potential of a new generation of multi‐member machine learning parameterizations leveraging uncertainty quantification to improve the representation of stochasticity of subgrid effects.

Turbulence flux condition in Jakarta during Cross Equatorial Northerly Surge (CENS)

Turbulence flux condition in Jakarta during Cross Equatorial Northerly Surge (CENS)

Southern Ocean Carbon Export Revealed by Backscatter and Oxygen Measurements From BGC‐Argo Floats

AbstractThe Southern Ocean (south of 30°S) contributes significantly to global ocean carbon uptake through the solubility, physical and biological pumps. Many studies have estimated carbon export to the deep ocean, but very few have attempted a basin‐scale perspective, or accounted for the sea‐ice zone (SIZ). In this study, we use an extensive array of BGC‐Argo floats to improve previous estimates of carbon export across basins and frontal zones, specifically including the SIZ. Using a new method involving changes in particulate organic carbon and dissolved oxygen along the mesopelagic layer, we find that the total Southern Ocean carbon export from 2014 to 2022 is 2.69 ± 1.23 PgC y−1. The polar Antarctic zone contributes the most (41%) with 1.09 ± 0.46 PgC y−1. Conversely, the SIZ contributes the least (8%) with 0.21 ± 0.09 PgC y−1 and displays a strong shallow respiration in the upper 200 m. However, the SIZ contribution can increase up to 14% depending on the depth range investigated. We also consider vertical turbulent fluxes, which can be neglected at depth but are important near the surface. Our work provides a complementary approach to previous studies and is relevant for work that focuses on evaluating the biogeochemical impacts of changes in Antarctic sea‐ice extent. Refining estimates of carbon export and understanding its drivers ultimately impacts our comprehension of climate variability at the global ocean scale.

Improved machine learning estimation of surface turbulent flux using interpretable model selection and adaptive ensemble algorithms over the Horqin Sandy Land area

Improved machine learning estimation of surface turbulent flux using interpretable model selection and adaptive ensemble algorithms over the Horqin Sandy Land area

A Simple Model of the Tropical Cyclone Boundary Layer at Landfall

Abstract As a tropical cyclone approaches landfall, it progressively experiences asymmetric friction that directly modifies the boundary layer flow, since the land surface is rougher than the sea. An idealized model of the boundary layer flow within a stationary cyclone, located exactly on the coast, is presented. This model is linearized and utilizes simple representations of the turbulent fluxes. These simplifications enable analytical solutions, which represent the flow as the sum of three components: a symmetric component and two asymmetric components of azimuthal wavenumber 1, which rotate with height. The stronger asymmetric component rotates anticyclonically with height and the weaker asymmetric component rotates cyclonically. The solution is thereby similar to that for a moving cyclone over sea, except that (i) the symmetric component has larger amplitude because the azimuthal-mean surface roughness is greater and (ii) the amplitudes and phases of the asymmetric components are different because of the different surface boundary condition. The linear solutions are compared to simulations using a nonlinear model with more sophisticated representations of the turbulent fluxes, and it is shown how to extract the two asymmetric components from the flow in this latter model. This study complements the analysis of the observed wind asymmetry in Tropical Cyclone Veronica in a companion paper. Significance Statement Landfall is the time at which tropical cyclones generally are most dangerous, so understanding the processes that cause structure change during landfall helps to mitigate impact. We derive a simple model of the near-surface flow in a tropical cyclone over a coastline and compare the results from this model to those from a more complete model, leading to an improved physical understanding of the effects of asymmetric friction on a tropical cyclone.

Coriolis effects on wind turbine wakes across neutral atmospheric boundary layer regimes

Wind turbines operate in the atmospheric boundary layer (ABL), where Coriolis effects are present. As wind turbines with larger rotor diameters are deployed, the wake structures that they create in the ABL also increase in length. Contemporary utility-scale wind turbines operate at rotor diameter-based Rossby numbers, the non-dimensional ratio between inertial and Coriolis forces, of $\mathcal {O}(100)$ where Coriolis effects become increasingly relevant. Coriolis forces provide a direct forcing on the wake, but also affect the ABL base flow, which indirectly influences wake evolution. These effects may constructively or destructively interfere because both the magnitude and sign of the direct and indirect Coriolis effects depend on the Rossby number, turbulence and buoyancy effects in the ABL. Using large eddy simulations, we investigate wake evolution over a wide range of Rossby numbers relevant to offshore wind turbines. Through an analysis of the streamwise and lateral momentum budgets, we show that Coriolis effects have a small impact on the wake recovery rate, but Coriolis effects induce significant wake deflections which can be parsed into two regimes. For high Rossby numbers (weak Coriolis forcing), wakes deflect clockwise in the northern hemisphere. By contrast, for low Rossby numbers (strong Coriolis forcing), wakes deflect anti-clockwise. Decreasing the Rossby number results in increasingly anti-clockwise wake deflections. The transition point between clockwise and anti-clockwise deflection depends on the direct Coriolis forcing, pressure gradients and turbulent fluxes in the wake. At a Rossby number of 125, Coriolis deflections are comparable to wake deflections induced by ${\sim} 20^{\circ }$ of yaw misalignment.

Effects of large density variations on near-wall turbulence and heat transfer in channel flow at supercritical pressure

The real-fluid effect induced by large density variation at supercritical pressure (SCP) modulates the turbulent dynamics and heat transfer, and poses challenges to existing turbulence models that are based on ideal-gas conditions. This study conducts direct numerical simulations of fully developed channel flows at SCP, with the upper and lower channel walls being isothermally heated and cooled, respectively. Emphasis is placed on examining the effects of various levels of density variations on near-wall turbulence as well as turbulent heat transfer by changing wall temperatures. The results show that the density fluctuation significantly impacts both first-order and second-order turbulence statistics near the heated wall owing to the close vicinity of pseudo-boiling point. Such real-fluid impact increases substantially with increasing density ratio, and tends to weaken the turbulent kinetic energy by damping turbulence production, while simultaneously inducing an additional turbulent mass flux that partially offsets this reduction. Detailed quadrant analysis reveals that the ‘ejection’ events dominate diverse effects of density fluctuation on Reynolds shear stresses, with density fluctuation contributing positively on the cooled wall side, and negatively on the heated wall side. Regarding the turbulent heat transfer, density fluctuation enhances the enthalpy–pressure–gradient correlation, tending to weaken the turbulent heat flux, which is slightly compensated by additional terms induced by density fluctuations. The overall negative contribution of density fluctuation to turbulent heat flux stems primarily from ‘hot ejection’ motions. Instantaneous flow characteristics provide additional support for these findings. Additionally, the mechanisms by which density fluctuations affect Reynolds shear stress and turbulent heat flux could also be extended to the skin friction coefficient and Nusselt number, respectively.

Predictions of core plasma performance for the Infinity Two Fusion Pilot Plant

Transport characteristics and predicted confinement are shown for the Infinity Two fusion pilot plant baseline plasma physics design, a high field stellarator concept developed using modern optimization techniques. Transport predictions are made using high fidelity nonlinear gyrokinetic turbulence simulations along with drift kinetic neoclassical simulations. A pellet fueled scenario is proposed that enables supporting an edge density gradient to substantially reduce ion temperature gradient turbulence. Trapped electron mode turbulence is minimized through the quasi-isodynamic configuration that has been optimized with max-J. A baseline operating point with deuterium-tritium fusion power of Pfus,DT = 800 MW with high fusion gain Qfus = 40 is demonstrated, respecting the Sudo density limit and magnetohydrodynamic stability limits. Additional higher power operating points are also predicted, including a fully ignited (Qfus = ∞) case with Pfus,DT = 1.5 GW. Pellet ablation calculations indicate it is plausible to fuel and sustain the desired density profile. Impurity transport calculations indicate turbulent fluxes dominate neoclassical fluxes deep into the core, and it is predicted that impurity peaking will be smaller than assumed in the transport simulations. A path to access large radiation fraction needed to satisfy exhaust requirements while sustaining core performance is also discussed.

Modelling the influence of soil moisture on the Turkana jet

AbstractLow‐level jets (LLJs) are sensitive to continental‐scale pressure gradients. Soil moisture influences these gradients by altering turbulent flux partitioning and near‐surface temperatures, thereby affecting LLJ characteristics. The Turkana jet, a strong southeasterly LLJ flowing through a channel between the Ethiopian and East African Highlands, is an important feature of the East African water cycle. Previous work has shown that the jet is sensitive to soil‐moisture‐induced pressure gradients driven by the Madden–Julian oscillation. Here, we build on this finding through using convection‐permitting UK Met Office Unified Model simulations to isolate the role of soil moisture in shaping jet characteristics. Modelling experiments reveal that the Turkana jet is highly sensitive to soil‐moisture‐induced temperature gradients across the channel's exit. Prescribing realistic dry soils intensifies the local surface‐induced thermal low and strengthens the jet. A maximum jet sensitivity of up to occurs when comparing dry and wet surface states within 750 km downstream of the exit, highlighting the significant influence of soil moisture on jet dynamics, given typical speeds of 8–. The impact of soil moisture on the jet is most pronounced when synoptic forcing is weak and skies are clear. Notably, despite a substantial impact on LLJ strength, we find a minor sensitivity of the vertically integrated moisture transport. We speculate that this minimal sensitivity is linked to model errors in the representation of boundary‐layer turbulence, which affects midtropospheric moisture and the strength of elevated nocturnal inversions. This study highlights that the Turkana channel is a hotspot for surface–jet interactions, due to the strong sensitivity of surface fluxes to soil moisture near a topographically constrained LLJ. Future research should continue examining surface‐driven predictability, particularly in regions where land–atmosphere interactions influence dynamical atmospheric conditions, and evaluate such processes in weather prediction models.