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We present direct eddy covariance measurements of the surface heat flux in sea ice over a wide range of conditions across the Arctic Ocean made during two research cruises. Photographic imagery of the surface around the ship provides a local, in situ estimate of the ice fraction. Aerodynamically rough conditions prevail for the majority of the time in the consolidated pack ice. The results are analyzed in the framework of a recently-developed parameterization scheme in which the exchange coefficients over ice are functions of a roughness Reynolds number, R*, hence account for aerodynamic roughness variability. This parameterization accurately represents the measured fluxes under all conditions, while under aerodynamically rough conditions the existing parameterizations from both the Met Office Unified Model, and ECMWF Integrated Forecast System overestimate the fluxes. The results corroborate those of a previous airborne study over the marginal ice zone, and encompass a wider range of atmospheric stability conditions.

Uncertainties in representing land–atmosphere interactions can substantially influence regional climate simulations. Among these uncertainties, the surface exchange coefficient C h is a critical parameter, controlling the total energy transported from the land surface to the atmosphere. Although it directly impacts the coupling strength between the surface and atmosphere, it has not been properly evaluated for regional climate models. This study assesses the representation of surface coupling strength in a stand-alone Noah-MP land surface model and in coupled 4-km Weather Research and Forecasting (WRF) model simulations. The data collected at eight FLUXNET sites of the Canadian Carbon Program and seven AMRIFLUX sites are used to evaluate the offline Noah-MP simulations. Nine of these FLUXNET sites are used for the evaluation of the coupled WRF simulations. These sites are categorized into three land use types: grassland, cropland, and forest. The surface exchange coefficients derived using three formulations in Noah-MP simulations are compared to those calculated from observations. Then, the default C zil  = 0 and new canopy-height dependent C zil are used in coupled WRF simulations over the spring and summer in 2006 to compare their effects on surface heat flux, temperature, and precipitation. When the new canopy-height dependent C zil scheme is used, the simulated C h exchange coefficient agrees better with observation and improves the daily maximum air temperature and heat flux simulation over grassland and cropland in the US Great Plains. Over grassland, the modeled C h shows a different diurnal cycle than that for observed C h , which makes WRF lag behind the observed diurnal cycle of sensible heat flux and temperature. The difference in precipitation between the two schemes is not as clear as the temperature difference because the impact of changing C h is not local.

In this study, the effects of surface exchange coefficients on simulations of Super Typhoon Megi (2010) are investigated using a fully coupled ocean-atmosphere-wave model. Several experiments are conducted using different parameterization schemes for the drag ( C D ) and enthalpy exchange ( C K ) coefficients. For the selected case, considering only the leveling-off of C D at high wind speeds does not effectively improve the simulated typhoon track, intensity, or size. We found that increasing C K monotonically with wind speed (Komori et al. , 2018) yields stronger winds and deeper pressures by enhancing latent and sensible heat fluxes, but typhoon intensity remains underestimated. We propose a new higher C K than that from Komori et al. (2018) based on the theory of Emanuel (1995). This approach produces a greater modeled typhoon intensity that is in good agreement with the best track data and effectively improves the track error for the simulation. Improved accuracy for modeled typhoon intensity is achieved with the new coefficient because C K / C D reaches the threshold of about 0.75 predicted by Emanuel (1995). The new proposed C K also results in a reasonably accurate modeled sea surface temperature. However, typhoon size and surface wave height are overestimated. This finding implies that more numerical tests for tropical cyclones of different nature (such as strong, weak, dissipating, rapidly intensifying, or weakening tropical cyclones) should be studied, and more physical processes should be explored in future coupled models.

AbstractAttention is drawn to the unexplored mechanism of generation of density currents in stratified media. Horizontal inhomogeneities of the exchange coefficients in a stratified medium lead to inhomogeneities of the vertical diffusion flow of buoyancy and its horizontal distribution and, consequently, to the emergence of horizontal inhomogeneities of hydrostatic pressure and the generation of currents. The appearance of ordered flows in a temperature (density) stratified turbulent medium in a gravity field near an inclined surface is considered as an example. This is due to the existence of a region of weakened turbulent exchange near the solid surface. In this case, horizontal components of the temperature, density and, consequently, pressure gradients appear near the inclined surface. This, in turn, leads to the emergence of an average (nonturbulent) slope current even in the absence of heat and momentum sources/sinks.

To examine the impact of air-sea exchange coefficients on the structure and intensity of tropical cyclones, sensitivity experiments are performed using the Weather Research and Forecasting (WRF) Model. A typhoon-like idealized vortex embedded in a calm environment is simulated using combinations of three different values of air-sea momentum and enthalpy exchange coefficients. In order to investigate the clear roles of each exchange coefficient, the experiments are designed that the two exchange coefficients change independently. For example, while the enthalpy exchange coefficient varies, the momentum exchange coefficient keeps its original value and vice versa. The track and intensity of sensitivity tests are analyzed to check the validity of the experiments. Results show that the track of the idealized vortices under a calm environment remains stationary, while the intensity varies with different air-sea exchange coefficients. The results indicate that changes in storm intensity with different enthalpy exchange coefficients are mainly related to an alteration in the amount of energy input to the storm, whereas the intensity changes with different momentum exchange coefficient values mostly result from angular momentum conservation due to changes in the size of the vortex. There is little change in the net energy input to the storms when the values of the momentum exchange coefficient are changed. Calculations of the volume-integrated angular momentum at the radius of the maximum winds confirm the different intensifying mechanisms found in this study.

A global “power law − 2” of long-term natural changes in total volume of the terrestrial water mass and of the natural eustatic changes in the global mean sea level has been proposed using coastal gauges data and satellite altimetry. A “power law − 1/2” of the dependence of the coefficient of variation of the annual river discharge on the runoff depth was formulated based on analysis of more than 2000 time series of “natural” rivers’ annual runoff. Additionally, a law of “statistical invariance” of the water exchange coefficient of drainage lakes vs. average runoff through the lakes was hypothesized using data from 249 lake systems of the world. The considered regularities were justified with the use of the climate system stochastic models concept by Klaus Hasselmann and for the reasons of dimensions of its hydrological objects (rivers, lakes, glaciers) participating in the processes of moisture accumulation and transfer on the land surface.

Apatite inclusions hosted by zircon offer a means to probe the magmatic history of granitic rocks and better constrain the volatile budgets of crystallising granitic melts. Building on recently developed F–Cl–OH partitioning models for apatite and coexisting melt, we outline an approach for estimating the melt concentrations of F and Cl from the composition of apatite inclusions in zircon, constrained by Ti-in-zircon crystallisation temperatures. The melts in equilibrium with apatite inclusions in zircon for the ‘I-type’ Jindabyne, Why Worry and Cobargo granitic suites of the Lachlan Orogen (eastern Australia), have Cl concentrations of 20–2880 ppm and F concentrations of 65–575 ppm. Variations in melt Cl and F concentrations between the granitic suites is attributed to differences in source compositions, specifically the relative contribution of F-rich turbiditic sediments and Cl-rich juvenile arc magmas. Within individual granitic suites, the calculated melt F and Cl concentrations decrease with magmatic differentiation and falling melt temperatures, and this appears to reflect the partitioning of Cl and F into biotite and hornblende, and into exsolving aqueous fluids. This study demonstrates that apatite-melt exchange coefficients for F, Cl and OH can be applied to apatite inclusions in zircon to quantify the F and Cl content of the melt, without additional context from the host rock samples.

Air–sea flux of carbon dioxide (CO2) is a critical component of the global carbon cycle and the climate system with the ocean removing about a quarter of the CO2 emitted into the atmosphere by human activities over the last decade. A common approach to estimate this net flux of CO2 across the air–sea interface is the use of surface ocean CO2 observations and the computation of the flux through a bulk parameterization approach. Yet, the details for how this is done in order to arrive at a global ocean CO2 uptake estimate vary greatly, enhancing the spread of estimates. Here we introduce the ensemble data product, SeaFlux (Gregor and Fay, 2021, 10.5281/zenodo.5482547​​​​​​​,https://github.com/luke-gregor/pySeaFlux, last access: 9 September 2021​​​​​​​); this resource enables users to harmonize an ensemble of products that interpolate surface ocean CO2 observations to near-global coverage with a common methodology to fill in missing areas in the products. Further, the dataset provides the inputs to calculate fluxes in a consistent manner. Utilizing six global observation-based mapping products (CMEMS-FFNN, CSIR-ML6, JENA-MLS, JMA-MLR, MPI-SOMFFN, NIES-FNN), the SeaFlux ensemble approach adjusts for methodological inconsistencies in flux calculations. We address differences in spatial coverage of the surface ocean CO2 between the mapping products, which ultimately yields an increase in CO2 uptake of up to 17 % for some products. Fluxes are calculated using three wind products (CCMPv2, ERA5, and JRA55). Application of a scaled gas exchange coefficient has a greater impact on the resulting flux than solely the choice of wind product. With these adjustments, we present an ensemble of global surface ocean pCO2 and air–sea carbon flux estimates. This work aims to support the community effort to perform model–data intercomparisons which will help to identify missing fluxes as we strive to close the global carbon budget.

Several key issues in the simple time-dependent theories of tropical cyclone (TC) intensification developed in recent years remain, including the lack of a closure for the pressure dependence of saturation enthalpy at sea surface temperature (SST) under the eyewall and the definition of environmental conditions, such as the boundary layer enthalpy in TC environment and the TC outflow-layer temperature. In this study, some refinements to the most recent time-dependent theory of TC intensification have been accomplished to resolve those issues. The first is the construction of a functional relationship between the surface pressure under the eyewall and the TC intensity, which is derived using the cyclostrophic wind balance and calibrated using full-physics axisymmetric model simulations. The second is the definition of TC environment that explicitly includes the air–sea temperature difference. The third is the TC outflow-layer temperature parameterized as a linear function of SST based on global reanalysis data. With these refinements, the updated time-dependent theory becomes self-contained and can give both the intensity-dependent TC intensification rate (IR) and the maximum potential intensity (MPI) under given environmental thermodynamic conditions. It is shown that the pressure dependence of saturation enthalpy at SST can lead to an increase in the TC MPI and IR by about half of that induced by dissipative heating due to surface friction. Results also show that both MPI and IR increase with increasing SST, surface enthalpy exchange coefficient, environmental air–sea temperature difference, and decreasing environmental boundary layer relative humidity, but the maximum IR is insensitive to surface drag coefficient.

In addition to initial conditions, uncertainty in model physics can also influence the practical predictability of tropical cyclones. In this study, the influence that various magnitudes of uncertainty in the surface exchange coefficients of momentum ( C d ) and enthalpy ( C k ) can have on an otherwise highly predictable major hurricane (Hurricane Patricia) is compared with that resulting from climatological environmental initial condition uncertainty and the intrinsic limit for this case. As the systematic uncertainty in C d and C k is reduced from 40% to 1%, the simulated uncertainty in the intensity and structure is substantially reduced and approaches the intrinsic limit when uncertainty is reduced to 1%. In addition, the forecasted intensity and structure uncertainty only becomes less than that resulting from climatological environmental initial condition uncertainty once the systematic uncertainty in C d and C k is reduced to ∼10%, highlighting the strong influence of model error in limiting TC predictability. If C d and C k are perturbed stochastically, instead of systematically, it is shown that the influence on the simulated intensity and structure is negligible and nearly identical to the intrinsic limit, regardless of the magnitude of the stochastic C d and C k perturbations. While the magnitude of the stochastic C d and C k perturbations are comparable to the systematic perturbations, the stochastic perturbations are shown to not substantially perturb the time-integrated inner-core fluxes of momentum or enthalpy that predominantly determine simulated tropical cyclone intensity. Last, it is shown that the kinetic energy error growth behavior varies with the radius, azimuthal wavenumber, and ensemble design.