Surface fogging is a common phenomenon that can have significant and detrimental effects on surface transparency and visibility. It affects the performance in a wide range of applications including windows, windshields, electronic displays, cameras, mirrors, and eyewear. A host of ongoing research is aimed at combating this problem by understanding and developing stable and effective antifogging coatings that are capable of handling a wide range of environmental challenges "passively" without consumption of electrical energy. Here we introduce an alternative approach employing sunlight to go beyond state-of-the-art techniques, such as superhydrophilic and superhydrophobic coatings, by rationally engineering solar absorbing metasurfaces that maintain transparency, while upon illumination induce localized heating to significantly delay the onset of surface fogging or decrease defogging time. For the same environmental conditions, we demonstrate that our metasurfaces are able to reduce defogging time by up to 4-fold and under supersaturated conditions inhibit the nucleation of condensate outperforming conventional state-of-the-art approaches in terms of visibility retention. Our research illustrates a durable and environmentally sustainable approach to passive antifogging and defogging for transparent surfaces. This work opens up the opportunity for large-scale manufacturing that can be applied to a range of materials, including polymers and other flexible substrates.

Ice nucleation from vapor presents a variety of challenges across a wide range of industries and applications including refrigeration, transportation, and energy generation. However, a rational comprehensive approach to fabricating intrinsically icephobic surfaces for frost formation-both from water condensation (followed by freezing) and in particular from desublimation (direct growth of ice crystals from vapor)-remains elusive. Here, guided by nucleation physics, we investigate the effect of material composition and surface texturing (atomically smooth to nanorough) on the nucleation and growth mechanism of frost for a range of conditions within the sublimation domain (0 °C to -55 °C; partial water vapor pressures 6 to 0.02 mbar). Surprisingly, we observe that on silicon at very cold temperatures-below the homogeneous ice solidification nucleation limit (<-46 °C)-desublimation does not become the favorable pathway to frosting. Furthermore, we show that surface nanoroughness makes frost formation on silicon more probable. We experimentally demonstrate at temperatures between -48 °C and -55 °C that nanotexture with radii of curvature within 1 order of magnitude of the critical radius of nucleation favors frost growth, facilitated by capillary condensation, consistent with Kelvin's equation. Our findings show that such nanoscale surface morphology imposed by design to impart desired functionalities-such as superhydrophobicity-or from defects can be highly detrimental for frost icephobicity at low temperatures and water vapor partial pressures (<0.05 mbar). Our work contributes to the fundamental understanding of phase transitions well within the equilibrium sublimation domain and has implications for applications such as travel, power generation, and refrigeration.

Separating petroleum hydrocarbons from water is an important problem to address in order to mitigate the disastrous effects of hydrocarbons on aquatic ecosystems. A rational approach to address the problem of marine oil-water separation is to disperse the oil with the aid of surfactants in order to minimize the formation of large slicks at the water surface and to maximize the oil-water interfacial area. Here we investigate the fundamental wetting and transport behavior of such surfactant-stabilized droplets and the flow conditions necessary to perform sieving and separation of these stabilized emulsions. We show that, for water-soluble surfactants, such droplets are completely repelled by a range of materials (intrinsically underwater superoleophobic) due to the detergency effect; therefore, there is no need for surface micro-/nanotexturing or chemical treatment to repel the oil and prevent fouling of the filter. We then simulate and experimentally investigate the effect of emulsion flow rate on the transport and impact behavior of such droplets on rigid meshes to identify the minimum pore opening (w) necessary to filter a droplet with a given diameter (d) in order to minimize the pressure drop across the mesh-and therefore maximize the filtering efficiency, which is strongly dependent on w. We define a range of flow conditions and droplet sizes where minimum droplet deformation is to be expected and therefore find that the condition of w ≈ d is sufficient for efficient separation. With this new understanding, we demonstrate the use of a commercially available filter-without any additional surface engineering or functionalization-to separate oil droplets (d < 100 μm) from a surfactant-stabilized emulsion with a flux of ∼11,000 L m-2 h-1 bar-1. We believe these findings can inform the design of future oil separation materials.

An analysis of atmospheric energy transport in 22 years (1980-2001) of the 40-yr ECMWF Re-Analysis (ERA-40) is presented. In the analyzed budgets, there is a large cancellation between divergences of dry static and latent energy such that the total energy divergence is positive over all tropical oceanic regions except for the east Pacific cold tongue, consistent with previous studies. The west Pacific and Indian Oceans are characterized by a balance between diabatic sources and mean advective energy export, with a small eddy contribution. However, in the central and eastern Pacific convergence zone, total energy convergence by the mean circulation is balanced by submonthly eddies, with a small diabatic source. Decomposing the mean advective tendency into terms due to horizontal and vertical advection shows that the spatial variation in the mean advection is due largely to variations in vertical advection; these variations are further attributed to variations in the vertical profile of the vertical velocity. The eddy energy export, due almost exclusively to eddy moisture export, does not exhibit any significant seasonal variation.The relationship between the eddies and the mean circulation is examined. Large-scale moisture diffusion is correlated with eddy moisture export on (500 km)(2) spatial scales, implying that eddy activity preferentially dries narrow convergence zones over wide ones. Eddy moisture export is further linked to the depth of mean convection in large-scale convergence zones with larger eddy export associated with shallower circulations. This suggests a mechanism that could contribute to the observed variation in mean divergence profiles across the northern tropical Pacific whereby sea surface temperature gradients set the width of convergence zones and eddy activity modulates the tropospheric relative humidity and divergence profile. The importance of variations in the vertical profile of the vertical velocity and eddies in closing the energy budget implies that simple models of the mean tropical circulation should include these effects.

In simulations of a wide range of circulations with an idealized general circulation model, clear scaling laws of dry atmospheric macroturbulence emerge that are consistent with nonlinear eddy–eddy interactions being weak. The simulations span several decades of eddy energies and include Earth-like circulations and circulations with multiple jets and belts of surface westerlies in each hemisphere. In the simulations, the eddy available potential energy and the barotropic and baroclinic eddy kinetic energy scale linearly with each other, with the ratio of the baroclinic eddy kinetic energy to the barotropic eddy kinetic energy and eddy available potential energy decreasing with increasing planetary radius and rotation rate. Mean values of the meridional eddy flux of surface potential temperature and of the vertically integrated convergence of the meridional eddy flux of zonal momentum generally scale with functions of the eddy energies and the energy-containing eddy length scale, with a few exceptions in simulations with statically near-neutral or neutral extratropical thermal stratifications. Eddy energies scale with the mean available potential energy and with a function of the supercriticality, a measure of the near-surface slope of isentropes. Strongly baroclinic circulations form an extended regime in which eddy energies scale linearly with the mean available potential energy. Mean values of the eddy flux of surface potential temperature and of the vertically integrated eddy momentum flux convergence scale similarly with the mean available potential energy and other mean fields. The scaling laws for the dependence of eddy fields on mean fields exhibit a regime transition between a regime in which the extratropical thermal stratification and tropopause height are controlled by radiation and convection and a regime in which baroclinic entropy fluxes modify the extratropical thermal stratification and tropopause height. At the regime transition, for example, the dependence of the eddy flux of surface potential temperature and the dependence of the vertically integrated eddy momentum flux convergence on mean fields changes—a result with implications for climate stability and for the general circulation of an atmosphere, including its tropical Hadley circulation.

Based on reanalysis data for the years 1980–2001 from the European Centre for Medium-Range Weather Forecasts (ERA-40 data), a climatology of tropospheric zonal-mean water vapor fields and fluxes in isentropic coordinates is presented. In the extratropical free troposphere, eddy fluxes dominate the meridional flux of specific humidity along isentropes. At all levels, isentropic eddy fluxes transport water vapor from the deep Tropics through the subtropics into the extratropics. Isentropic eddy fluxes of specific humidity diverge near the surface and in the tropical and subtropical free troposphere; they converge in the extratropical free troposphere. Isentropic mean advective fluxes of specific humidity play a secondary role in the meridional water vapor transport in the free troposphere; however, they dominate the meridional flux of specific humidity near the surface, where they transport water vapor equatorward and, in the solstice seasons, across the equator. Cross-isentropic mean advective fluxes of specific humidity are especially important in the Hadley circulation, in whose ascending branches they moisten and in whose descending branches they dry the free troposphere. Near the minima of zonal-mean relative humidity in the subtropical free troposphere, the divergence of the cross-isentropic mean advective flux of specific humidity in the descending branches of the Hadley circulation is the dominant divergence in the mean specific humidity balance; it is primarily balanced by convergence of cross-isentropic turbulent fluxes that transport water vapor from the surface upward. Although there are significant isentropic eddy fluxes of specific humidity through the region of the subtropical relative humidity minima, their divergence near the minima is generally small compared with the divergence of cross-isentropic mean advective fluxes, implying that moistening by eddy transport from the Tropics into the region of the minima approximately balances drying by eddy transport into the extratropics. That drying by cross-isentropic mean subsidence near the subtropical relative humidity minima is primarily balanced by moistening by upward turbulent fluxes of specific humidity, likely in convective clouds, suggests cloud dynamics may play a central role in controlling the relative humidity of the subtropical free troposphere.

It is generally held that atmospheric macroturbulence can be strongly nonlinear. Yet weakly nonlinear models successfully account for scales and structures of baroclinic eddies in Earth's atmosphere. Here a theory and simulations with an idealized GCM are presented that suggest weakly nonlinear models are so successful because atmospheric macroturbulence organizes itself into critical states of weak nonlinear eddy–eddy interactions. By modifying the thermal structure of the extratropical atmosphere such that its supercriticality remains limited, macroturbulence inhibits nonlinear eddy–eddy interactions and the concomitant inverse energy cascade from the length scales of baroclinic instability to larger scales. For small meridional surface temperature gradients, the extratropical thermal stratification and tropopause height are set by radiation and convection, and the supercriticality is less than one; for sufficiently large meridional surface temperature gradients, the extratropical thermal stratification and tropopause height are modified by baroclinic eddies such that the supercriticality does not significantly exceed one. In either case, the scale of the energy-containing eddies is similar to the scale of the linearly most unstable baroclinic waves, and eddy kinetic and available potential energies are equipartitioned. The theory and simulations point to fundamental constraints on the thermal structures and global circulations of the atmospheres of Earth and other planets, for example, by providing limits on the tropopause height and estimates for eddy scales, eddy energies, and jet separation scales.

An idealized GCM is used to investigate how the strength and meridional extent of the Hadley circulation depend on the planet radius, rotation rate, and thermal driving. Over wide parameter ranges, the strength and meridional extent of the Hadley circulation display clear scaling relations with regime transitions, which are not predicted by existing theories of axisymmetric Hadley circulations. For example, the scaling of the strength as a function of the radiative-equilibrium equator-to-pole temperature contrast exhibits a regime transition corresponding to a regime transition in scaling laws of baroclinic eddy fluxes. The scaling of the strength of the cross-equatorial Hadley cell as a function of the latitude of maximum radiative-equilibrium temperature exhibits a regime transition from a regime in which eddy momentum fluxes strongly influence the strength to a regime in which the influence of eddy momentum fluxes is weak. Over a wide range of flow parameters, albeit not always, the Hadley circulation strength is directly related to the eddy momentum flux divergence at the latitude of the streamfunction extremum. Simulations with hemispherically symmetric thermal driving span circulations with local Rossby numbers in the horizontal upper branch of the Hadley circulation between 0.1 and 0.8, indicating that neither nonlinear nearly inviscid theories, valid for Ro → 1, nor linear theories, valid for Ro → 0, of axisymmetric Hadley circulations can be expected to be generally adequate. Nonlinear theories of axisymmetric Hadley circulations may account for aspects of the circulation when the maximum radiative-equilibrium temperature is displaced sufficiently far away from the equator, which results in cross-equatorial Hadley cells with nearly angular momentum-conserving upper branches. The dependence of the Hadley circulation on eddy fluxes, which are themselves dependent on extratropical circulation characteristics such as meridional temperature gradients, suggests that tropical circulations depend on the extratropical climate.

The nonlinear behavior of planetary waves excited by midlatitude topography is considered in an atmospheric GCM. The GCM is run at standard resolution (T42) and includes all of the complexity normally associated with a GCM. Only two simplifications are made to the model. First, it is run in perpetual January mode, so that the solar radiation takes the diurnally varying value associated with 15 January. Second, the lower boundary is simplified so that it is entirely ocean with zonally symmetric SSTs. Planetary waves are excited by Gaussian-shaped topography centered at 45°N, 90°W. As in earlier studies, the excited wave train propagates toward low latitudes where, for sufficiently large forcing amplitude (i.e., height of topography), the wave will break. Several different experiments are run with different mountain heights. Each experiment is run for a total of 4015 days. The response of the model depends on the height of the mountain. For the small-amplitude mountain (500 m), the wave is dissipated at low latitudes near its critical latitude. For large-amplitude mountains (2000, 3000, and 4000 m), wave breaking and nonlinear reflection out of the wave breaking region is observed. The spatial character of the reflected wave train is similar to that detected in earlier studies with more idealized models.

The nonlinear behaviour of quasi-stationary planetary waves that are excited by mid-latitude orographic forcing is considered in a global shallow-water model taken to represent the upper troposphere. The waves propagate toward low latitudes where the background flow is normally weak and the waves are therefore likely to break. Nonlinear pseudomomentum conservation relations are used to quantify the absorption-reflection behaviour of the wave-breaking region. Two different flow scenarios are represented: (i) initial states without a representation of the Hadley circulation, but where the axisymmetric equatorial background flow changes from being weak and easterly to moderate Westerly; (ii) initial states that include a representation of the Hadley circulation and that have weak equatorial easterlies.Based on linear arguments, both (i) and (ii) are expected to influence the progression of the wave train. The nonlinear behaviour in the presence of low-latitude westerly background flows is different from linear predictions. For large-amplitude forcing, wave breaking takes place even though there is no zero-wind line in the initial state, and the cross-equatorial wave propagation that took place for small-amplitude forcing is stopped before it can reach the equator. Nonlinear reflection is found to take place back into the hemisphere of origin but not across to the other hemisphere.In the presence of a Hadley circulation representative of winter conditions, the nonlinear reflection takes longer to get established, i.e. it requires more forcing, but a reflected wave train is still present in the numerical simulations, both for a longitudinally symmetric forcing and for the more realistic case of an isolated forcing. A summer Hadley circulation allows wave activity to get to the winter hemisphere. As the forcing is increased, wave breaking occurs and eventually nonlinear reflection.