Net Radiative Loss Research Articles

Plasma diagnostics and Alfvén wave heating of solar prominences by multiwavelength observations

Abstract Solar prominences are cool and dense plasma supported by the magnetic field in the solar corona. They are composed of fine-scale structures called threads. Prominences are heated by the incident radiation from the solar atmosphere, but previous studies have shown that additional heating is necessary to maintain the temperature of prominences of about 104 K. This study aims to investigate quantitatively the mechanical heating of the prominences from observations. We performed spectroscopic observations of prominences in Hα (6563 Å), Hβ (4861 Å), and Ca ii IR (8542 Å) with the Domeless Solar Telescope at Hida observatory. The plasma parameters of the prominences are estimated by fittings of line profiles using the single-slab model and by solving the statistical equilibrium equation. By assuming that the turbulent motion is the manifestation of propagating Alfvén waves, net radiative losses and Alfvén wave energy flux in the prominences are estimated from these parameters. As a result, it was found that the energy flux by Alfvén waves is sufficient to compensate for the net radiative losses in most regions of the central part of the prominences. On the other hand, Alfvén wave energy flux tends to be insufficient at the outer part of prominences. In such regions, another heating mechanism such as enthalpy flux of mixing with coronal plasma needs to be considered.

Assessing requirements for modeling radiation in diffusion flames using an analytical, non-local model

An analytical approach for the calculation of radiative self-absorption in flames is presented, with the objective of providing an effective and computationally inexpensive tool for selecting radiation modeling methods for combustion applications. This one-dimensional non-local model approximates the flame structure as an infinitely long cylinder where all properties vary only along the radial direction. Flamelet solutions are then mapped to the radial direction and the radiative source terms, in particular the absorption, are computed analytically. The model is validated for a laminar jet flame and a turbulent pool fire, with results for the net radiative loss and the flame self-absorption showing good agreement to those of multidimensional, coupled simulations. To illustrate its applicability, the model is employed to study the characteristics of radiation in high-pressure combustion, in fire suppression and in hydrogen combustion. For the pressurized case, increased significance of radiation self-absorption is observed compared to an atmospheric flame, and accounting for the non-gray nature of radiation is found to be critical. In the fire suppression case, comparisons of the S-curves show that the choice of radiation model modifies the lower extinction limit, which becomes more important with reduced oxygen content. Lastly, for the hydrogen flame, the Planck-mean absorption coefficient is found to be more uniformly distributed in the mixture fraction space, and the optical thickness is smaller than a hydrocarbon flame at comparable conditions, indicating that radiation modeling requirements for that flame are likely less stringent. In general, the findings regarding radiation characteristics made on the basis of the non-local model are consistent with observations reported in the literature, but without the need for complex coupled combustion-radiation calculations. Therefore, the model provides a viable approach for selecting appropriate radiation models in combustion simulations.

Spatio-temporal analysis of chromospheric heating in a plage region

Context. Our knowledge of the heating mechanisms that are at work in the chromosphere of plage regions remains highly unconstrained from observational studies. While many heating candidates have been proposed in theoretical studies, the exact contribution from each of them is still unknown. The problem is rather difficult because there is no direct way of estimating the heating terms from chromospheric observations. Aims. The purpose of our study is to estimate the chromospheric heating terms from a multi-line high-spatial-resolution plage dataset, characterize their spatio-temporal distribution and set constraints on the heating processes that are at work in the chromosphere. Methods. We used nonlocal thermodynamical equilibrium inversions in order to infer a model of the photosphere and chromosphere of a plage dataset acquired with the Swedish 1-m Solar Telescope (SST). We used this model atmosphere to calculate the chromospheric radiative losses from the main chromospheric cooler from H I, Ca II, and Mg II atoms. In this study, we approximate the chromospheric heating terms by the net radiative losses predicted by the inverted model. In order to make the analysis of time-series over a large field of view computationally tractable, we made use of a neural network which is trained from the inverted models of two non-consecutive time-steps. We have divided the chromosphere in three regions (lower, middle, and upper) and analyzed how the distribution of the radiative losses is correlated with the physical parameters of the model. Results. In the lower chromosphere, the contribution from the Ca II lines is dominant and predominantly located in the surroundings of the photospheric footpoints. In the upper chromosphere, the H I contribution is dominant. Radiative losses in the upper chromosphere form a relatively homogeneous patch that covers the entire plage region. The Mg II also peaks in the upper chromosphere. Our time analysis shows that in all pixels, the net radiative losses can be split in a periodic component with an average amplitude of amp̅Q = 7.6 kW m−2 and a static (or very slowly evolving) component with a mean value of −26.1 kW m−2. The period of the modulation present in the net radiative losses matches that of the line-of-sight velocity of the model. Conclusions. Our interpretation is that in the lower chromosphere, the radiative losses are tracing the sharp lower edge of the hot magnetic canopy that is formed above the photosphere, where the electric current is expected to be large. Therefore, Ohmic current dissipation could explain the observed distribution. In the upper chromosphere, both the magnetic field and the distribution of net radiative losses are room-filling and relatively smooth, whereas the amplitude of the periodic component is largest. Our results suggest that acoustic wave heating may be responsible for one-third of the energy deposition in the upper chromosphere, whereas other heating mechanisms must be responsible for the rest: turbulent Alfvén wave dissipation or ambipolar diffusion could be among them. Given the smooth nature of the magnetic field in the upper chromosphere, we are inclined to rule out Ohmic dissipation of current sheets in the upper chromosphere.

Imaginary couplings in non-Hermitian coupled-mode theory: Effects on exceptional points of optical resonators

Exceptional point (EP) degeneracies in coupled cavities with gain and loss provide on-chip photonic devices with unconventional features and performance. However, such systems with realistic structures often miss the exact EPs even in simulation, and the mechanism of this EP disruption has yet to be thoroughly identified. Here, we extend the coupled-mode theory of one-dimensional non-Hermitian resonator arrays to study the effects of the imaginary part of the inter-cavity coupling, which is a second-order term and attributed to material amplification, absorption, and radiation. By taking an appropriate gauge for the model, we clarify that the imaginary coupling components have a symmetric form in the effective Hamiltonian and hence represent non-Hermiticity. These additional factors can lift the gain- and loss-based EP degeneracies. However, they are proportional to the sum of the imaginary permittivities for involved cavity pairs. Thus, when the amplification and absorption of adjacent cavities are balanced, their contribution to the imaginary coupling is canceled, and the EP singularity can be restored. Radiation-induced imaginary couplings measure the change in net radiation loss by the interference between cavity modes. Their impact on the EP can also be counteracted by small cavity resonance detuning even in loss-biased cases. We show and analyze eligible simulation examples based on photonic crystal nanocavities, and highlight design of an ideal EP degeneracy that is protected by generalized PT symmetry and induced by radiation.

External controls on the transition between stable boundary‐layer turbulence regimes

AbstractThe mean wind speed at which the stable boundary layer (SBL) experiences a turbulence regime transition (Vr) depends on other flow characteristics, such as its thermal stability. Here, Vr variability is examined both at a single site and across sites using three multiple‐level datasets: Santa Maria, Cooperative Atmosphere–Surface Exchange Study–1999 (CASES‐99), and Fluxes over Snow Surfaces (FLOSS II). A method to determine Vr is introduced and validated. It is taken as the mean wind speed at which the vertical gradient of the turbulence velocity scale switches sign. Emphasis is given to the control exerted on Vr by quantities that are external to the SBL, such as radiation, roughness, and soil properties. In each of the experiments, Vr increases with net radiative loss at the surface (Rn) at a rate that is site‐dependent. It also increases for smaller roughness lengths, as indicated by its wind direction dependence. No conclusive relationship has been found between Vr and downward longwave radiation observed at the surface. The across‐site comparison indicates that soil heat capacity influences the rate at which Vr increases with Rn.

Clouds damp the radiative impacts of polar sea ice loss

Abstract. Clouds play an important role in the climate system: (1) cooling Earth by reflecting incoming sunlight to space and (2) warming Earth by reducing thermal energy loss to space. Cloud radiative effects are especially important in polar regions and have the potential to significantly alter the impact of sea ice decline on the surface radiation budget. Using CERES (Clouds and the Earth's Radiant Energy System) data and 32 CMIP5 (Coupled Model Intercomparison Project) climate models, we quantify the influence of polar clouds on the radiative impact of polar sea ice variability. Our results show that the cloud short-wave cooling effect strongly influences the impact of sea ice variability on the surface radiation budget and does so in a counter-intuitive manner over the polar seas: years with less sea ice and a larger net surface radiative flux show a more negative cloud radiative effect. Our results indicate that 66±2% of this change in the net cloud radiative effect is due to the reduction in surface albedo and that the remaining 34±1 % is due to an increase in cloud cover and optical thickness. The overall cloud radiative damping effect is 56±2 % over the Antarctic and 47±3 % over the Arctic. Thus, present-day cloud properties significantly reduce the net radiative impact of sea ice loss on the Arctic and Antarctic surface radiation budgets. As a result, climate models must accurately represent present-day polar cloud properties in order to capture the surface radiation budget impact of polar sea ice loss and thus the surface albedo feedback.

Numerical analysis of turbulence radiation interaction effect on radiative heat transfer in a swirling oxyfuel furnace

With the application of oxyfuel combustion technology, the status of radiative heat transfer in oxyfuel flames is prominent, and accurate description of it is essential. This study simulates the turbulence radiation interaction (TRI) in a swirling gas-fired furnace to understand the thermal radiation behavior under oxycombustion conditions. Employing the non-gray weighted sum of gray gases model calculates the radiative properties of the participating media and the TRI is modeled. The effects of TRI on the radiative heat transfer in the furnace under different oxycombustion conditions and swirl numbers are analyzed. The results show that the effects of thermal radiation on temperature fields are critical in the gas-fired furnace, especially for oxycombustion condition with high oxygen concentration, and the same is true for TRI effect. With the increase of O2 concentration in O2/CO2 oxidizer, the radiative heat flux on the chamber wall, the radiative heat flux contribution to total heat flux, and the net radiative heat loss are increased. Increasing swirl number can weaken the TRI effect. It is observed that with 35% O2 in oxidizer, the radiation contribution to heat flux along the entire wall and local increase of radiative heat flux due to TRI effect are almost than 45% and 18%, respectively.

Monitoring thermal anomaly and radiative heat flux using thermal infrared satellite imagery – A case study at Tuzla geothermal region

Geothermal energy, which is renewable, reliable and environmentally friendly, is one of the most important energy resources. Thus, it is crucial to explore geothermal areas in order to reduce the use of other energy sources that are detrimental to the environment and ecology. Thermal Infrared (TIR) remote sensing is an effective way to detect thermal anomalies in geothermal areas and volcanoes, since it is cost and time effective, and offers to work on a large scale compared to geophysical methods. The aim of this study is to investigate thermal anomalies in Tuzla geothermal region using daytime and nighttime TIR data with reference to Land Surface Temperature (LST) and Radiative Heat Flux (RHF). Many geophysical studies have been conducted in this region; however, it can also be studied with TIR remote sensing for further exploration. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data, acquired on 15.06.2012 and 15.07.2017 as daytime image and 15.09.2013 and 31.12.2017 as nighttime image, were utilized as satellite imagery. In addition to ASTER data, we proposed a multi-sensor based LST retrieval for nighttime using Landsat 8 data for emissivity acquisition. In order to evaluate the accuracy of LST images, cross-validation method was utilized with reference to Moderate Resolution Imaging Spectroradiometer (MODIS) LST products. The coefficient of determination (r2) and Root Mean Square Error (RMSE) were considered as statistical metrics and the lowest result was obtained as 90% and 1.76 K, respectively. As a result of the analyses, it was observed that nighttime LST presented better results for thermal anomalies in that geothermal area than daytime LST. Considering geothermal anomaly, the geothermal area had higher LST values even though it held identical or same NDVI values as compared to non-geothermal surroundings. In addition, the net radiative heat loss values were calculated as 17.83 MW and 121.28 MW for 2013 and 2017, respectively. The obtained results proved that TIR remote sensing could be utilized in the studies of geothermal area exploration.

Prediction of turbulence radiation interactions of CH4[sbnd]H2/air turbulent flames at atmospheric and elevated pressures

Predicting thermal radiation for turbulent combustion highlights the significance of turbulence radiation interactions (TRI). Thermal radiation behaviors of methane/hydrogen flames under elevated pressures are investigated numerically using the developed TRI module integrated into CFD codes. The updated non-gray weighted sum of gray gases model is used to calculate the radiative properties of participating media. TRI effects have been analyzed with 0%–50% volumetric fraction of hydrogen in the methane/hydrogen blended fuels under 1–5 atm working pressures. Employing the radiation model considering TRI achieves closer predicted consistency to the experimental data. Only thermal radiation makes the flame temperature dropped about 60–140 K, while the predicted radiative source term calculated with TRI is higher than that without TRI, which results in a colder flame (approximately 13–60 K lower). The impact of TRI on the radiation behavior is enhanced in hydrogen-enriched high-pressure flame as the predicted radiation heat flux and radiative source term are increased above 25% than that without TRI. On account of TRI effect, the net radiative heat loss increases almost 50% at elevated pressure. The strong radiation of participating media in methane/hydrogen flames under elevated pressures emphasizes the importance of TRI effect on accurate predictions of thermal radiation and NO emission.

Analysis of the performance of an alkali metal thermoelectric converter (AMTEC) based on a lumped thermal-electrochemical model

The alkali metal thermoelectric converter (AMTEC) is a regenerative concentration cell for direct conversion of heat to electrical energy. Optimization of the components in AMTECs has been gained wide interest to improve power output and cell efficiency. A lumped thermal-electrochemical model has been developed in this work based on some previous efforts by Tournier et al. (1997). The main contribution of the present model is to simplify the calculation of radiation heat exchange by rendering the BASE, the condenser and the cell walls as a system of three closed-surfaces. The net radiative heat loss from the BASE is determined using a network method of radiation. For model validation purposes, the lumped model is applied to a PX-3A type cell. Good agreements have been obtained between numerical results and experimental data on variations of voltage, current, power output and cell efficiency within a wide range of external loads. Based on the numerical results, the role and separate influence of each component are quantitatively analyzed, including temperature levels for hot and cold ends, BASE properties, electrode materials, heat reduction methods and conductor leads. Optimum parameters are suggested in the analyses. With the recent advances in BASE and electrode materials, an integrated optimization is made to the AMTEC aiming for extra-terrestrial application. In the new module, the surface area of electrodes is increased, new electrodes materials are used and thermal radiation losses are minimized. With stable parameters, the peak power of the cell has been over 8.0 We. The stable voltage is over 3.0 V when RL > 1.2 Ω and the cell efficiency is over 20% when RL is within 1.2–4.0 Ω.

A Multi-layer Radiation Model for Urban Neighbourhoods with Trees

A neighbourhood-scale multi-layer urban canopy model of shortwave and longwave radiation exchange that explicitly includes the radiative effects of tall vegetation (trees) is presented. Tree foliage is permitted both between and above buildings, and mutual shading, emission and reflection between buildings and trees are included. The basic geometry is a two-dimensional canyon with leaf area density profiles and probabilistic variation of building height. Furthermore, the model accounts for three-dimensional path lengths through the foliage. Ray tracing determines the receipt of direct shortwave irradiance by building and foliage elements. View factors for longwave and shortwave diffuse radiation exchange are computed once at the start of the simulation using a Monte Carlo ray tracing approach; for subsequent model timesteps, matrix inversion rapidly solves infinite reflections and interception of emitted longwave between all elements. The model is designed to simulate any combination of shortwave and longwave radiation frequency bands, and to be portable to any neighbourhood-scale urban canopy geometry based on the urban canyon. Additionally, the model is sufficiently flexible to represent forest and forest-clearing scenarios. Model sensitivity tests demonstrate the model is robust and computationally feasible, and highlight the importance of vertical resolution to the performance of urban canopy radiation models. Full model evaluation is limited by the paucity of within-canyon radiation measurements in urban neighbourhoods with trees. Where appropriate model components are tested against analytic relations and results from an independent urban radiation transfer model. Furthermore, system response tests demonstrate the ability of the model to realistically distribute shortwave radiation among urban elements as a function of built form, solar angle and tree foliage height, density and clumping. Separate modelling of photosynthetically-active and near-infrared shortwave bands is shown to be important in some cases. Increased canyon height-to-width ratio and/or tree cover diminishes the net longwave radiation loss of individual canyon elements (e.g., floor, walls), but, notably, has little effect on the net longwave loss of the whole urban canopy. When combined with parametrizations for the impacts of trees on airflow and hydrological processes in the urban surface layer, the new radiation model extends the applicability of urban canopy models and permits more robust assessment of trees as tools to manage urban climate, air quality, human comfort and building energy loads.

The Minimum Wind Speed for Sustainable Turbulence in the Nocturnal Boundary Layer

AbstractThe collapse of turbulence in the nocturnal boundary layer is studied by means of a simple bulk model that describes the basic physical interactions in the surface energy balance. It is shown that for a given mechanical forcing, the amount of turbulent heat that can be transported downward is limited to a certain maximum. In the case of weak winds and clear skies, this maximum can be significantly smaller than the net radiative loss minus soil heat transport. In the case when the surface has low heat capacity, this imbalance generates rapid surface cooling that further suppresses the turbulent heat transport, so that eventually turbulence largely ceases (positive feedback mechanism). The model predicts the minimum wind speed for sustainable turbulence for the so-called crossing level. At this level, some decameters above the surface, the wind is relatively stationary compared to lower and higher levels. The critical speed is predicted in the range of about 5–7 m s−1, depending on radiative forcing and surface properties, and is in agreement with observations at Cabauw. The critical value appears not very sensitive to model details or to the exact values of the input parameters. Finally, results are interpreted in terms of external forcings, such as geostrophic wind. As it is generally larger than the speed at crossing height, a 5 m s−1 geostrophic wind may be considered as the typical limit below which sustainable, continuous turbulence under clear-sky conditions is unlikely to exist. Below this threshold emergence of the very stable nocturnal boundary layer is anticipated.

CONDITIONS FOR PHOTOSPHERICALLY DRIVEN ALFVÉNIC OSCILLATIONS TO HEAT THE SOLAR CHROMOSPHERE BY PEDERSEN CURRENT DISSIPATION

A magnetohydrodynamic model that includes a complete electrical conductivity tensor is used to estimate conditions for photospherically driven, linear, non-plane Alfvenic oscillations extending from the photosphere to the lower corona to drive a chromospheric heating rate due to Pedersen current dissipation that is comparable to the net chromospheric net radiative loss of $\sim 10^7$ ergs-cm$^{-2}$-sec$^{-1}$. The heating rates due to electron current dissipation in the photosphere and corona are also computed. The wave amplitudes are computed self-consistently as functions of an inhomogeneous background (BG) atmosphere. The effects of the conductivity tensor are resolved numerically using a resolution of 3.33 m. The oscillations drive a chromospheric heating flux $F_{Ch} \sim 10^7 - 10^8$ ergs-cm$^{-2}$-sec$^{-1}$ at frequencies $\nu \sim 10^2 - 10^3$ mHz for BG magnetic field strengths $B \gtrsim 700$ G and magnetic field perturbation amplitudes $\sim 0.01 - 0.1$ $B$. The total resistive heating flux increases with $\nu$. Most heating occurs in the photosphere. Thermalization of Poynting flux in the photosphere due to electron current dissipation regulates the Poynting flux into the chromosphere, limiting $F_{Ch}$. $F_{Ch}$ initially increases with $\nu$, reaches a maximum, and then decreases with increasing $\nu$ due to increasing electron current dissipation in the photosphere. The resolution needed to resolve the oscillations increases from $\sim 10$ m in the photosphere to $\sim 10$ km in the upper chromosphere, and is proportional to $\nu^{-1/2}$. Estimates suggest that these oscillations are normal modes of photospheric flux tubes with diameters $\sim 10-20$ km, excited by magnetic reconnection in current sheets with thicknesses $\sim 0.1$ km.

Radiation characteristics and turbulence–radiation interactions in sooting turbulent jet flames

A comprehensive modeling strategy including detailed chemistry, soot and radiation models coupled with state-of-the-art closures for turbulence–chemistry interactions and turbulence–radiation interactions is applied to various luminous turbulent jet flames. Six turbulent jet flames are simulated with Reynolds numbers varying from 6700 to 15,000, two fuel types (pure ethylene, 90% methane–10% ethylene blend) and different oxygen concentrations in the oxidizer stream (from 21% O2 to 55% O2). All simulations are carried out with a single set of physical and numerical parameters (model constants). A Lagrangian particle Monte Carlo method is used to solve a modeled joint probability density function (PDF) transport equation, which allows accurate closure for turbulence–chemistry interactions including nonlinear soot subprocesses. Radiation is calculated using a particle-based photon Monte Carlo method that is coupled with the PDF method to accurately account for both emission and absorption turbulence–radiation interactions (TRI). Line-by-line databases are used for accurate spectral radiative properties of CO2 and H2O; soot radiative properties also are modeled as nongray. For the flames that have been investigated, soot emission can be almost 45% of the total emission, even when the peak soot volume fraction is of the order of a few parts-per-million (ppm) and up to 99% of soot emission can escape the domain without re-absorption. Turbulence–radiation interactions have a strong effect on the net radiative heat loss from these sooting flames. For a given temperature, species and soot distribution, TRI increases emission from the flames by 30–60%, and the net heat loss from the flame increases by 45–90% when accounting for TRI. This is higher than the corresponding increase in radiative heat loss due to TRI in nonsooting flames. Absorption TRI was found to be negligible in these laboratory-scale sooting flames with soot levels on the order of a few ppm, but may be important in larger industrial-scale flames and in higher-pressure systems.

SIMULATION OF MAGNETOHYDRODYNAMIC SHOCK WAVE GENERATION, PROPAGATION, AND HEATING IN THE PHOTOSPHERE AND CHROMOSPHERE USING A COMPLETE ELECTRICAL CONDUCTIVITY TENSOR

An electrical conductivity tensor is used in a 1.5D magnetohydrodynamic (MHD) simulation to describe how MHD shock waves may form, propagate, and heat the photosphere and chromosphere by compression and resistive dissipation. The spatial resolution is 1 km. A train of six shock waves is generated by a sinusoidal magnetic field driver in the photosphere with a period T = 30 s, mean of 500 G, and variation of 250 G. The duration of the simulation is 200 s. Waves generated in the photosphere evolve into shock waves at a height z ~ 375 km above the photosphere. The transition of the atmosphere from weakly to strongly magnetized with increasing height causes the Pedersen resistivity η P to increase to ~2000 times the Spitzer resistivity. This transition occurs over a height range of a few hundred kilometers near the temperature minimum of the initial state at z ~ 500 km. The initial state is a model atmosphere derived by Fontenla et al., plus a background magnetic field. The increase in η P is associated with an increase in the resistive heating rate Q. Shock layer thicknesses are ~10-20 km. They are nonzero due to the presence of resistive dissipation, so magnetization-induced resistivity plays a role in determining shock structure, and hence the compressive heating rate Qc . At t = 200 s the solution has the following properties. Within shock layers, Q maximum ~ 1.4-7 erg cm–3 s–1, and Q c,maximum ~ 10-103 Q maximum. Between shock waves, and at some points within shock layers, Qc < 0, indicating cooling by rarefaction. The integrals of Q and Qc over the shock wave train are F ~ 4.6 × 106 erg cm–2 s–1 and Fc ~ 1.24 × 109 erg cm–2 s–1. A method based on the thermal, mechanical, and electromagnetic energy conservation equations is presented for checking the accuracy of the numerical solution, and gaining insight into energy flow and transformation. The method can be applied to higher dimensional simulations. It is suggested that observations be performed to map out the transition region across which the transition from weakly ionized, weakly magnetized plasma to weakly ionized, strongly magnetized plasma occurs, and to correlate it with net radiative loss.

Chromospheric changes in K stars with activity

We study the differences in chromospheric structure induced in K stars by stellar activity, to expand our previous work for G stars, including the Sun as a star. We selected six stars of spectral type K with 0.82 < B−V < 0.90, including the widely studied Epsilon Eridani and a variety of magnetic activity levels. We computed chromospheric models for the stars in the sample, in most cases in two different moments of activity. The models were constructed to obtain the best possible match with the Ca ii K and the Hβ observed profiles. We also computed in detail the net radiative losses for each model to constrain the heating mechanism that can maintain the structure in the atmosphere. We find a strong correlation between these losses and SCa II, the index generally used as a proxy for activity, as we found for G stars.

Evening Temperature Rises on Valley Floors and Slopes: Their Causes and Their Relationship to the Thermally Driven Wind System

AbstractAt slope and valley floor sites in the Owens Valley of California, the late afternoon near-surface air temperature decline is often followed by a temporary temperature rise before the expected nighttime cooling resumes. The spatial and temporal patterns of this evening warming phenomenon, as seen in the March/April 2006 Terrain-Induced Rotor Experiment, are investigated using a widely distributed network of 51 surface-based temperature dataloggers. Hypotheses on the causes of the temperature rises are tested using heavily instrumented 34-m meteorological towers that were located within the datalogger array. The evening temperature rise follows the development of a shallow temperature deficit layer over the slopes and floor of the valley in which winds blow downslope. Background winds within the valley, freed from frictional deceleration from the earth’s surface by this layer, accelerate. The increased vertical wind shear across the temperature deficit layer eventually creates shear instability and mixes out the layer, creating the observed warming near the ground. As momentum is exchanged during the mixing event, the wind direction near the surface gradually turns from downslope to the background wind direction. After the short period of warming associated with the mixing, ongoing net radiative loss causes a resumption of the cooling.

The large‐scale energy budget of the Arctic

This paper synthesizes a variety of atmospheric and oceanic data to examine the large‐scale energy budget of the Arctic. Assessment of the atmospheric budget relies primarily on the ERA‐40 reanalysis. The seasonal cycles of vertically integrated atmospheric energy storage and the convergence of energy transport from ERA‐40, as evaluated for the polar cap (defined by the 70°N latitude circle), in general compare well with realizations from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis over the period 1979–2001. However, shortcomings in top of atmosphere radiation, as compared to satellite data, and the net surface flux, contribute to large energy budget residuals in ERA‐40. The seasonal cycle of atmospheric energy storage is strongly modulated by the net surface flux, which is also the primary driver of seasonal changes in heat storage within the Arctic Ocean. Averaged for an Arctic Ocean domain, the July net surface flux from ERA‐40 of −100 W m−2 (i.e., into the ocean), associated with sea ice melt and oceanic sensible heat gain, exceeds the atmospheric energy transport convergence of 91 W m−2. During winter (for which budget residuals are large), oceanic sensible heat loss and sea ice growth yield an upward surface flux of 50–60 W m−2, complemented with an atmospheric energy convergence of 80–90 W m−2 to provide a net radiation loss to space of 175–180 W m−2.

Experimental and analytical study of radiation-recrystallized near-surface facets in snow

Abstract Radiation recrystallization occurs when surface snow metamorphoses into faceted crystals due to a radiation exchange that can occur near the surface of a snowpack. In particular, large temperature gradients and associated vapor pressure gradients can occur near the surface when absorbed solar radiation is coupled with cooling effects from longwave radiation and turbulent fluxes. An experimental study on the formation of radiation-recrystallized near-surface facets in snow was performed in an environmental chamber. A metal-halide lamp was used to mimic solar radiation, which penetrates the snow allowing thermal energy to be absorbed at depth. In addition, the ceiling was cooled to simulate a cold sky, thus inducing a net longwave radiation loss at the snow surface. Air temperature in the chamber was maintained at − 10 °C, while turbulent flux parameters, including relative humidity and wind velocity, were measured. Forty centimeter thick snow samples with insulated lateral sides were placed in the chamber on a constant temperature plate also at − 10 °C. The study focused on the significance of radiation exchange and snow density on recrystallization near the snow surface. Imposed boundary conditions led to the formation of facets of varying size at and near the snow surface. Faceting was observed when an applied solar flux between 350 and 1100 W/m 2 was combined with longwave and turbulent exchanges for snow with densities ranging from 170 to 410 kg/m 3 . To better understand the dominant factors, a thermodynamic model was used to extrapolate from the experimental results. The model incorporated meteorological inputs and was used to calculate a snowpack temperature profile using relevant snow parameters. Both experimental and model analyses indicate that radiation exchange and snow density are significant factors in the growth of radiation-recrystallized near-surface facets in snow.

Chromospheric models of solar analogues with different activity levels

We computed chromospheric models of the Sun as a star and of nine solar analogues. The atmospheric models were constructed to obtain the best possible match with the Ca II K and Hβ lines, including the asymmetry of the lines due to macroscopic velocity fields. The stars were chosen with 0.62 < B − V < 0.68 (the solar B − V = 0.65) and have a wide variety of magnetic activity levels, which allows us to study the differences in atmospheric structures induced by activity. For the less active stars we found that the changes with activity are in the region of the temperature minimum, while the most active stars show changes all along their atmospheric structures, mainly in the upper chromosphere. Regarding the macroscopic velocity fields, we can distinguish between the two groups. The most active group has a velocity field in the temperature-minimum region, and the other group in the chromospheric plateau. We also computed the net radiative losses for each model, and found that they depend linearly on the usual index of chromospheric activity, S CaII.