Simple Radiative Transfer Model Research Articles

Limb Temperature Observations in the Stratosphere and Mesosphere Derived from the OMPS Sensor

Molecular scattering (Rayleigh scattering) has been extensively used from the ground with lidars and from space to observe the limb, thereby deriving vertical temperature profiles between 30 and 80 km. In this study, we investigate how temperature can be measured using the new Ozone Mapping and Profiler Suite (OMPS) sensor, aboard the Suomi NPP and NOAA-21 satellites. The OMPS consists of three instruments whose main purpose is to study the composition of the stratosphere. One of these, the Limb Profiler (LP), measures the radiance of the limb of the middle atmosphere (stratosphere and mesosphere, 12 to 90 km altitude) at wavelengths from 290 to 1020 nm. This new data set has been used with a New Simplified Radiative Transfer Model (NSRTM) to derive temperature profiles with a vertical resolution of 1 km. To validate the method, the OMPS-derived temperature profiles were compared with data from four ground-based lidars and the ERA5 and MSIS models. The results show that OMPS and the lidars are in agreement within a range of about 5 K from 30 to 80 km. Comparisons with the models also show similar results, except for ERA5 beyond 50 km. We investigated various sources of bias, such as different attenuation sources, which can produce errors of up to 120 K in the UV range, instrumental errors around 0.8 K and noise problems of up to 150 K in the visible range for OMPS. This study also highlighted the interest in developing a new miniaturised instrument that could provide real-time observation of atmospheric vertical temperature profiles using a constellation of CubeSats with our NSRTM.

Hyperfine excitation of NH and ND by molecular hydrogen

The NH and ND radicals are of key importance in the comprehension of nitrogen chemistry and the enhancement of deuterated molecules in the interstellar medium. Observations by space telescopes yield spectra that can resolve the fine and hyperfine structure of these radicals, a consequence of the electronic and magnetic interactions of nitrogen, hydrogen, and deuterium nuclei. Accurate rate coefficients, induced by collisions with H2, are required to interpret spectra of these radicals. We report the first rate coefficients for fine and hyperfine transitions of NH and ND in collision with both ortho- and para-H2. Based on a recent four-dimensional potential energy surface, fine-structure resolved cross sections and rate coefficients are computed with the time-independent close-coupling method over a temperature range of 5–300 K. Our calculations include the first 25 energy levels of NH and ND. Hyperfine resolved cross sections and rate coefficients are determined using the infinite-order sudden (IOS) approximation between 5 and 200 K for NH and 100 K for ND. We consider the first 71 and 105 energy levels of NH and ND, respectively. General propensity rules are discussed. We found a significant isotopic substitution effect in the rate coefficients. In addition, the rate coefficients for collisions with H2 are larger than those with He by a factor of up to 5, leading to lower critical densities for collisional excitation with H2 than He. The impact of the new set of collisional data has been investigated in simple radiative transfer models of the NH emission seen toward the Orion Bar and the ejecta of the η Carinae binary star. We observed significant differences by a factor of 5 between the presently determined column densities for NH compared to those from the literature using He as a collider.

Evolution of Meteosat Solar and Infrared Spectra (2004–2022) and Related Atmospheric and Earth Surface Physical Properties

The evolution of atmospheric and Earth surface physical properties over a period of 15 years (based on data from the longer period from 2004 to 2022) is analyzed through the radiance fluxes measured by the Meteosat second generation (MSG) satellite series. The results show significant changes in the solar (−2.6% to −1.2%) and infrared (+0.4% to +1.0%) domains, with −3.9% for the CO2 absorption band (near 13.4 µm), all variations consistent with results from similar studies of radiation fluxes. Whereas the variation at 13.4 μm radiation is explained by the increase in the CO2 concentration in the atmosphere, the flux increase towards the satellite in the remainder of the infrared spectra measured by MSG corresponds to surface warming (as documented in external sources like the IPCC, the Intergovernmental Panel on Climate Change). The solar outgoing flux decrease exposes a recent reduction in the Earth’s cloud cover under the nominal field of view of Meteosat at 0 degrees longitude (MFOV). Radiance evolution at 6.2 µm and 7.3 µm, a spectral region of intense absorption by water vapor, is interpreted in terms of sensitivity to the humidity content in the middle and upper troposphere by means of a simple radiation transfer model.

Aerosol mass scattering efficiencies and single scattering albedo under high mass loading in Chiang Mai valley, Thailand

Aerosols amend the Earth's radiation budget by absorbing and scattering the incoming solar radiation. Severe air pollution episodes with high aerosol loading have continually occurred in northern Thailand for a long decade. This study comprehensively investigates the light scattering mechanism contingent on synchronous measurement of aerosol physicochemical and optical properties at Chiang Mai, the second largest city in Thailand per population, for two months during the spring of 2016. Daily mean values of the aerosol light scattering coefficient (σsca) at 550 nm ranged between 146 and 501 Mm−1. The scattering Ångström exponent (SAE450/700) ranged from 1.7 to 2.1, indicating a predominant contribution of finer particles to total light scattering. The mean mass scattering efficiency (MSE) of (NH4)2SO4, NH4NO3, organic matter (OM), dust, and others at 550 nm, estimated using a multilinear regression model were 3.3, 1.4, 4.4, 1.8, and 4.0 m2 g−1, respectively. The mean contribution of individual components to σsca was in the order of OM (66%) > Others (12%) > Dust (11%) > (NH4)2SO4 (9%) > NH4NO3 (2%). The mean (±SD) value of aerosol single scattering albedo at 550 nm wavelength was 0.80 ± 0.02 (range; 0.75–0.84), highlighting the presence of strongly absorbing aerosols. The total light absorption was primarily due to the mixture of organic and elemental carbons. The mean (±SD) MSE and mass absorption efficiency (MAE) of composite aerosols were 3.4 ± 0.4 m2 g−1 and 0.9 ± 0.1 m2 g−1, respectively. Notwithstanding the absorption, a simple radiative transfer model quantifies a negative top-of-atmosphere radiative forcing over Chiang Mai. These findings shed light on the impact of ambient aerosols on light scattering and urban climate. Moreover, our study offers a basic dataset to assess the visibility impairment in Chiang Mai.

The excitation of CO in CO-dominated cometary comae

ABSTRACTAn abundance of CO significantly surpassing the abundance of H2O is observed in the comae of comets at large heliocentric distances. In these environments, CO molecules can be the most abundant species and they may be therefore the dominant projectiles inducing collisional excitation of the cometary molecules. It is thus of high interest to investigate the excitation of CO by CO. This article provides a new set of CO–CO collisional rate coefficients for temperatures up to 150 K and for CO rotational levels j1 up to 10. These data are obtained from quantum scattering calculations using the coupled states approximation. They are used in a simple radiative transfer model in order to test their impact on the excitation of cometary CO. Because mutual (de-)excitations of the target and projectile are important, the CO projectile was assumed to be thermalized at the kinetic temperature. We found that the non-local thermodynamical equilibrium regime extends for CO densities in the range 103–107 cm−3. We also observed that as soon as the CO/H2O ratio is larger than 70 per cent/30 per cent, the contribution of H2O collisions can be neglected. Similarly, the excitation of CO by CO may be ignored for relatively low CO/H2O density ratios (≤30 per cent/70 per cent). Finally, when the coma is a ∼50 per cent/50 per cent mixture of CO and H2O, the contribution of both colliders is similar and has to be considered.

Estimating leaf mass per area with leaf radiative transfer model

Leaf mass per area (LMA) is an important leaf trait but challenging to be accurately estimated. This article proposes a simple leaf radiative transfer model called ISPECT. It explains the difference in optical properties observed on the adaxial (upper) and abaxial (lower) sides of leaves, i.e., their dorsiventrality, with a limited number of structural parameters. The performance of ISPECT in estimating LMA is compared to that of five other leaf radiative transfer models (FASPECT, DLM, PROSPECT-D, PROSPECT-5B, and Leaf-SIP). We tested six experimental datasets with 962 leaf samples and two spectral ranges: the solar domain (0.4–2.5 µm) and the shortwave infrared (1.7–2.4 µm). Results show that PROSPECT-D and PROSPECT-5B accurately estimate LMA using the shortwave infrared spectra, while ISPECT and FASPECT perform well in both spectral ranges. Further analysis demonstrates that leaf dorsiventrality is likely to be an influential factor for LMA estimation: thus ISPECT can accurately estimate LMA in the solar and shortwave infrared domains, with NRMSE of 26.0% and 28.8%, respectively. This motivates further studies on LMA mapping from spaceborne imaging spectrometers (e.g., PRISMA, GaoFen-5, EnMAP) by coupling ISPECT with canopy radiative transfer models.

STMRT: A simple tree canopy radiative transfer model for outdoor mean radiant temperature

Tree canopy shading can significantly decrease the outdoor mean radiant temperature (MRT). However, canopy radiative transfer is quite complex, making it challenging to evaluate the MRT under a tree canopy. In this study, a simple tree canopy radiative transfer model (STMRT) was developed to determine the MRT under a tree canopy. In STMRT, the direct and diffuse solar radiation transfers are calculated based on the leaf inclination angle distribution, and the longwave radiation emitted from a canopy is evaluated based on the simplification that the canopy surface temperature is the same as the surrounding air temperature. The STMRT's accuracy was evaluated by measurement and compared to that of 4SAIL. The results demonstrate that the MRT differences (ΔMRT) in STMRT are typically less than 2.3 °C; in Shanghai, China, ΔMRT caused by the differences in direct solar radiation transmissivity between STMRT and 4SAIL are less than 2.0 °C, unless the leaf area index is less than 2 and the solar zenith angle is greater than 70°. According to STMRT, it is found that ΔMRT generated by the leaf inclination angle variances across various tree species can reach 9.5 °C; ΔMRT caused by a uniform distribution simplification for leaf inclination angle can reach 5.7 °C. STMRT will help plan the outdoor thermal environment by evaluating the MRT under tree canopies.

Mapping potential surface contributions to reflected solar radiation

Modifying Earth’s albedo is one of the strategies considered to reduce its energy imbalance and slow global warming by reflecting solar energy. Atmospheric contributions to reflected solar radiation through stratospheric aerosols or cloud brightening have received considerable attention; however, the efficacy of surface interventions is less understood. We address this gap by estimating the potential for surface contributions to reflected solar radiation at approximately 30 km resolution using a simple radiative transfer model. Long-term average annual-mean incoming and outgoing top-of-atmosphere and surface solar fluxes are input to determine atmospheric shortwave optical properties (i.e., transmittance, absorptance, and reflectance), which can be used with surface albedo to estimate surface-reflected outgoing solar radiation. A comparison of reanalysis- and satellite-based input datasets shows good agreement. The results indicate global annual-mean surface-reflected outgoing solar radiation potential of 109 Wm−2, nearly a factor of five larger than the actual value, and local areas where it could be increased above 200 Wm−2 with surface albedo enhancement. Regions with particularly strong potential include Andean South America, the Middle East, southwestern North America, southwestern Africa, Australia, and the sub-equatorial tropical oceans. Future research could extend the methods to account for seasonal variations and the potential to mitigate extreme heat events in particular.

An Optical Spectrum of the Diffuse Galactic Light from BOSS and IRIS

We present a spectrum of the diffuse Galactic light (DGL) between 3700 and 10,000 Å, obtained by correlating optical sky intensity with far-infrared dust emission. We use nearly 250,000 blank-sky spectra from BOSS/SDSS-III together with IRIS-reprocessed maps from the IRAS satellite. The larger sample size compared to SDSS-II results in a factor-of-2 increase in signal to noise. We combine these data sets with a model for the optical/far-infrared correlation that accounts for self-absorption by dust. The spectral features of the DGL agree remarkably well with the features present in stellar spectra. There is evidence for a difference in the DGL continuum between the regions covered by BOSS in the northern and southern Galactic hemispheres. We interpret the difference at red wavelengths as the result of a difference in stellar populations, with mainly old stars in both regions, but a higher fraction of young stars in the south. There is also a broad excess in the southern DGL spectrum over the prediction of a simple radiative transfer model, without a clear counterpart in the north. We interpret this excess, centered at ∼6500 Å, as evidence for luminescence in the form of extended red emission. The observed strength of the 4000 Å break indicates that at most ∼7% of the dust-correlated light at 4000 Å can be due to blue luminescence. Our DGL spectrum provides constraints on dust scattering and luminescence, independent of measurements of extinction.

SSolar-GOA v1.0: a simple, fast, and accurate Spectral SOLAR radiative transfer model for clear skies

Abstract. The aim of this work is to describe the features of and to validate a simple, fast, accurate, and physically based spectral radiative transfer model in the solar wavelength range under clear skies. The model, named SSolar-GOA (the first “S” stands for “spectral”), was developed to evaluate the instantaneous values of spectral solar irradiances at ground level or at a given altitude of the atmosphere. The model requirements are designed based on the simplicity of the analytical expressions for the transmittance functions in order to be easily replicated and applied by a wide community of users for many different applications (atmospheric and environmental research studies, satellite remote sensing, solar energy, agronomy and forestry, ecology, and others). Although spectral, the model runs quickly and has sufficient accuracy for the evaluation of solar irradiances with a spectral resolution of 1–10 nm. The model assumes a single mixed molecule–aerosol scattering layer where the original Ambartsumian method of “adding layers” in a one-dimensional medium is applied, obtaining a parameterized expression for the total transmittance of scattering. Absorption by the different atmospheric gases follows “band model” parameterized expressions. The input parameters must be realistic and easily available since the spectral aerosol optical depth (AOD) is the main driver of the model. The validation of the SSolar-GOA model has been carried out through comparison with simulated irradiance data from the libRadtran package and with direct and global spectra measured by spectroradiometers. Thousands of spectra under clear skies have been compared for different atmospheric conditions and solar zenith angles (SZA). The SSolar-GOA is validated by a quantitative comparison with libRadtran, showing that it underestimates direct normal, global, and diffuse spectral components with relative differences of +1 % (RMSE % = 4.6–8), +3 % (RMSE % = 5.3–8), and 8 % (RMSE % = 9.3–9.6), respectively, when the SZA varies from 6 to 60∘. Compared with the measured irradiance data of the LI-1800 and ASD spectroradiometers, the relative differences of direct normal and global components are within the overall experimental error, about ±2 %–12 % (RMSE % = 5–8.3), with underestimated or overestimated values. The diffuse component presents the highest degree of relative difference that can reach ±20 %–30 % and RMSE of 25 %–50 %. The relative differences depend strongly on the spectral solar region analysed and the SZA, but the high values of RMSE are due to the artifice generated by the different spectral resolution of the absorption coefficients of both models. Model approach errors combined with calibration instrument errors may explain the observed differences. The SSolar-GOA v1.0 is implemented in Python and open-source licensing.

Multi-line characterization of whole molecular clouds using stratified random sampling

We have developed a new technique to characterize the multi-line emission from molecular clouds using statistical sampling. Our method uses available extinction maps to select a relatively small sample of cloud positions that cover the full range of column densities in the cloud, and that can be observed with only a modest investment of telescope time. Here we present the first results of applying this technique to the Perseus molecular cloud, a nearby star-forming region that contains a population of both isolated and clustered protostars. We have used the IRAM 30m telescope to observe a sample of 100 random positions that cover two orders of magnitude in H2 column density. These positions have been observed over the full 3mm wavelength band together with selected portions of the 2 and 1mm bands. We find that the emission properties of most species are strongly correlated with the column density over the whole cloud, and that they can be reproduced using a relative simple radiative transfer model.

Humic-like substances (HULIS) in springtime aerosols at a high-altitude background station in the western North Pacific: Source attribution, abundance, and light-absorption

Atmospheric humic-like substances (HULIS) are important components of biomass-burning (BB) emissions and highly associated with light-absorbing organic aerosols (often referred to as brown carbon). This study highlights the importance of BB-emitted HULIS aerosols in peninsular Southeast Asian outflow to the subtropical western North Pacific. We determined various key light-absorbing characteristics of HULIS i.e. mass absorption cross-section (MACHULIS), absorbing component of the refractive index (kHULIS), and absorption Ångström exponent (AAEHULIS) based on ground-based aerosol light absorption measurements along with HULIS concentrations in springtime aerosols at Lulin Atmospheric Background Station (LABS; 2862 m above mean sea level), which is a representative high-altitude remote site in the western North Pacific. Daily variations of HULIS (0.58–12.92 μg m−3) at LABS were mostly linked with the influence from incoming air-masses, while correlations with BB tracers and secondary aerosols indicated the attribution of primary and secondary sources. Stronger light absorption capability of HULIS was clearly evident from MACHULIS and kHULIS values at 370 nm, which were about ~1.5 times higher during BB-dominated days (1.16 ± 0.75 m2 g−1 and 0.05 ± 0.03, respectively) than that during non-BB days (0.77 ± 0.89 m2 g−1 and 0.03 ± 0.04, respectively). Estimates from a simple radiative transfer model showed that HULIS absorption can add as much as 15.13 W g−1 to atmospheric warming, and ~46% more during BB-dominated than non-BB period, highlighting that HULIS light absorption may significantly affect the Earth–atmosphere system and tropospheric photochemistry over the western North Pacific.

Bi-hemispherical Canopy Reflectance Model with Surface Heterogeneity Effects for the Estimation of LAI and fAPAR from MODIS White-Sky Spectral Albedo Data

Bi-hemispherical reflectance (BHR), in the land surface research community also known as “white-sky albedo”, is independent of the directions of incidence and viewing. For vegetation canopies, it is also nearly independent of the leaf angle distribution, and therefore it can be considered an optical quantity that is only dependent on material properties. For the combination leaf canopy and soil background, the most influential material properties are the canopy LAI (leaf area index), optical properties of the leaves, and soil brightness. When the leaf and soil optical properties are known or assumed, one may estimate the canopy LAI from its white-sky spectral albedo. This is also because a simple two-stream radiative transfer (RT) model is available for the BHR of the leaf canopy and soil combination. In this contribution, crown clumping and lateral linear mixing effects are incorporated in this model. A new procedure to estimate soil brightness is introduced here, even under a moderate layer of green vegetation. The procedure uses the red and NIR spectral bands. A MODIS white-sky albedo product at a spatial resolution of 0.05° is used as a sample input to derive global maps of LAI, soil brightness, and fAPAR at the local moments of minimum and maximum NDVI over a 20-year period. These maps show a high degree of spatial coherence and demonstrate the possible utility of products that can be generated with little effort by using a direct LUT technique.

Theoretical Uncertainty Analysis of Satellite Retrieved Aerosol Optical Depth Associated with Surface Albedo and Aerosol Optical Properties

Deriving aerosol optical depth (AOD) from space-borne observations is still challenging due to uncertainties associated with sensor calibration drift, cloud screening, aerosol type classification, and surface reflectance characterization. As an initial step to understanding the physical processes impacting these uncertainties in satellite AOD retrievals, this study outlines a theoretical approach to estimate biases in the satellite aerosol retrieval algorithm affected by surface albedo and prescribed aerosol optical properties using a simplified radiative transfer model with a traditional error propagation approach. We expand the critical surface reflectance concept to obtain the critical surface albedo (CSA), critical single scattering albedo (CSSA), and critical asymmetry parameter (CAP). The top-of-atmosphere (TOA) reflectance is not sensitive to significant variability in aerosol loading (AOD) at the critical value; thus, the AOD cannot be determined. Results show that 5% bias in surface albedo (A), single scattering albedo (SSA), or asymmetry parameter (g) lead to large retrieved AOD errors, especially high under conditions when A, SSA, or g are close to their critical values. The results can be useful for future research related to improvements of satellite aerosol retrieval algorithms and provide a preliminary framework to analytically quantify AOD uncertainties from satellite retrievals.

Assessment of spectral effects on outdoor characterization of PV modules using silicon reference cells with spectral filters

This article describes outdoor temperature-controlled measurement of various photovoltaic (PV) modules fabricated using different technologies and using reference cells with spectral filters. Short-circuit current measurements of all PV modules and reference cells were recorded simultaneously using 16-bits Keysight digitizers and high-resolution Agilent multimeters. The spectral incompatibility factor was calculated considering the spectral response of each device and the spectrum measured outdoors. Corresponding spectra were also generated by the simple model of atmospheric radiative transfer of the sun SMARTS2 and were additionally used to compare the spectral incompatibility factors. The validation of the measurement method was demonstrated by comparing the incompatibility factor calculated with variation of the short-circuit current relationship between the device under test and the reference cell over time. Additionally, it was evaluated how the relationship between spectral variation of irradiance and the spectral response of different technologies affects the productivity in thin-film modules. Main observation from measurements (performed in Labsol) was that a-Si/μ-Si, CdTe, and triple a-Si junction converted 0.21%, 2.47%, and 3.65% more irradiation into electric current (in summer), respectively, compared to c-Si technology. The PV CIGS module converted on average 0.26% more irradiation into electric current in winter compared to c-Si technology. The results help to understand the spectral effects on the working of outdoor PV module and the relationships between measured and modeled spectra, as well as errors introduced by the spectral incompatibility factor.

A Novel Cloud Detection Algorithm Based on Simplified Radiative Transfer Model for Aerosol Retrievals: Preliminary Result on Himawari-8 Over Eastern China

Aerosol particles affect the Earth's radiative balance and represent one of the largest uncertainties in climate research. The removal of clouds is the first step and critical in the aerosol retrievals. However, the cloud detection is still challenging. Here, a novel simplified cloud detection algorithm (SCDA) is proposed to identify the cloud and clear-sky over land and based on a simplified radiative transfer model (RTM). The fewer input bands, dynamic thresholds, and only one parameter to be modified are the main advantages of the algorithm, which can be applied to different satellite sensors. In this article, we apply SCDA to the Himawari-8 data in 2016 for preliminary analysis. The detection results are validated using Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) vertical feature mask (VFM) data and the National Centers for Environmental Information (NCEI) ground-based observation data. We also compare the results with the Himawari-8 cloud products from the Japan Aerospace Exploration Agency (JAXA). Compared with CALIPSO VFM data and NCEI ground-based observation data, the correct rate of SCDA cloud detection result is 86.08% and 79.86%, which are higher than that of Himawari-8 cloud products (85.71% and 78.89%). The correct rate of SCDA clear-sky detection result is 88.33% and 87.85%, which are close to the correct rate of Himawari-8 clear-sky products (90.54% and 88.63%). The overall performance of the SCDA is comparable to that of the threshold method for JAXA Himawari-8 cloud products. Therefore, the SCDA can provide accurate cloud mask with only one threshold to be modified and few input parameters.

High-resolution optical remote sensing for coastal benthic habitat mapping: A case study of the Suape Estuarine-Bay, Pernambuco, Brazil

The benefits of high resolution remote sensing techniques and data for the mapping of coastal environments include their ability to capture the spatial variability of the rich and diverse sedimentary landforms and habitats of coastal systems as well as gather and manage different types of data and information using the same platform for multi-scale and multi-layer analysis. This work analyses the efficacy of high resolution satellite imagery for mapping benthic habitat and the seascape characteristics of a coastal lagoon (Suape Bay - 40 km south of Recife, Pernambuco, Brazil) subjected to high anthropogenic impact and exposed to different conditions of tides, suspended sediment regime, and wave dynamics, using different types of processing and classification methods. We analyzed two World View 2 images acquired in 2010 and 2015 in similar tidal conditions and low water turbidity. Preprocessing included the application of radiometric and atmospheric correction, sunglint removal, and land/sea areas masking. To extract the depth information, a Band Ratio and Simple Radiative Transfer Model were employed. The model was corrected and calibrated by using a set of 150 control points collected by echo sounding surveying. A benthic terrain model was developed to identify and map topographic features at broad and fine scales. Supervised Pixel and Object-Based classification methods were applied to visible bands of WV2 and bathymetric data to produce benthic habitat maps of the lagoon area. The analyses were validated by ground truthing based on bottom sediment sampling, underwater photographs, and side scan sonar surveys.

Ice Concentration Retrieval from the Analysis of Microwaves: Evaluation of a New Methodology Optimized for the Copernicus Imaging Microwave Radiometer

A new methodology has been described in Kilic et al. (Ice Concentration Retrieval from the Analysis of Microwaves: A New Methodology Designed for the Copernicus Imaging Microwave Radiometer, Remote Sensing 2020, 12, 1060, Part 1 of this study) to estimate Sea Ice Concentration (SIC) from satellite passive microwave observations between 6 and 36 GHz. The Ice Concentration Retrieval from the Analysis of Microwaves (IceCREAM) algorithm is based on an optimal estimation, with a simple radiative transfer model derived from satellite observations at 0% and 100% SIC. Observations at low and high frequencies have different spatial resolutions, and a scheme is developed to benefit from the low errors of the low frequencies and the high spatial resolutions of the high frequencies. This effort is specifically designed for the Copernicus Imaging Microwave Radiometer (CIMR) project, equipped with a large deployable antenna to provide a spatial resolution of ∼5 km at 18 and 36 GHz, and ∼15 km at 6 and 10 GHz. The algorithm is tested with Advanced Microwave Scanning Radiometer 2 (AMSR2) observations, for a clear scene over the north polar region, with collocated Moderate Resolution Imaging Spectroradiometer (MODIS) estimates and the Ocean Sea Ice—Satellite Application Facilities (OSI SAF) operational products. Several algorithm options are tested, and the study case shows that both high spatial resolution and low errors are obtained with the IceCREAM method. It is also tested for the full polar regions, winter and summer, under clear and cloudy conditions. Our method is globally applicable, without fine-tuning or further weather filtering. The systematic use of all channels from 6 to 36 GHz makes it robust to changes in ice surface conditions and to weather interactions.

Visible and near-infrared spectro-interferometric analysis of the edge-on Be star o Aquarii

Aims. We present a detailed visible and near-infrared spectro-interferometric analysis of the Be-shell star o Aquarii from quasi-contemporaneous CHARA/VEGA and VLTI/AMBER observations. Methods. We analyzed spectro-interferometric data in the Hα (VEGA) and Brγ (AMBER) lines using models of increasing complexity: simple geometric models, kinematic models, and radiative transfer models computed with the 3D non-LTE code HDUST. Results. We measured the stellar radius of o Aquarii in the visible with a precision of 8%: 4.0 ± 0.3 R⊙. We constrained the circumstellar disk geometry and kinematics using a kinematic model and a MCMC fitting procedure. The emitting disk sizes in the Hα and Brγ lines were found to be similar, at ~10–12 stellar diameters, which is uncommon since most results for Be stars indicate a larger extension in Hα than in Brγ. We found that the inclination angle i derived from Hα is significantly lower (~15°) than the one derived from Brγ: i ~ 61.2° and 75.9°, respectively. While the two lines originate from a similar region of the disk, the disk kinematics were found to be near to the Keplerian rotation (i.e., β = −0.5) in Brγ (β ~ −0.43), but not in Hα (β ~ −0.30). After analyzing all our data using a grid of HDUST models (BeAtlas), we found a common physical description for the circumstellar disk in both lines: a base disk surface density Σ0 = 0.12 g cm−2 and a radial density law exponent m = 3.0. The same kind of discrepancy, as with the kinematic model, is found in the determination of i using the BeAtlas grid. The stellar rotational rate was found to be very close (~96%) to the critical value. Despite being derived purely from the fit to interferometric data, our best-fit HDUST model provides a very reasonable match to non-interferometric observables of o Aquarii: the observed spectral energy distribution, Hα and Brγ line profiles, and polarimetric quantities. Finally, our analysis of multi-epoch Hα profiles and imaging polarimetry indicates that the disk structure has been (globally) stable for at least 20 yr. Conclusions. Looking at the visible continuum and Brγ emission line only, o Aquarii fits in the global scheme of Be stars and their circumstellar disk: a (nearly) Keplerian rotating disk well described by the viscous decretion disk (VDD) model. However, the data in the Hα line shows a substantially different picture that cannot fully be understood using the current generation of physical models of Be star disks. The Be star o Aquarii presents a stable disk (close to the steady-state), but, as in previous analyses, the measured m is lower than the standard value in the VDD model for the steady-state regime (m = 3.5). This suggests that some assumptions of this model should be reconsidered. Also, such long-term disk stability could be understood in terms of the high rotational rate that we measured for this star, the rate being a main source for the mass injection in the disk. Our results on the stellar rotation and disk stability are consistent with results in the literature showing that late-type Be stars are more likely to be fast rotators and have stable disks.

A radiative transfer model for solar induced fluorescence using spectral invariants theory

Solar Induced chlorophyll Fluorescence (SIF) shows promise as an approach for estimating gross primary production (GPP) remotely. However, sun-target-sensor geometry and within-canopy absorption of SIF can alter the relationship between measured SIF and GPP, because sensors can only retrieve some unknown fraction of the total emitted SIF. Radiative transfer models that allow for variation in canopy structure and sensor angles are therefore needed to properly interpret SIF measurements. Spectral invariants allow decoupling of the wavelength-independent canopy structure and the wavelength-dependent leaf and soil spectrum in the radiative transfer process. Here we develop a simple analytical Fluorescence Radiative Transfer model based on Escape and Recollision probability (FluorRTER) to investigate the impact of canopy structure and sun-target-sensor geometry on SIF emissions. SIF simulations using the FluorRTER model agreed well the one-dimensional Soil-Canopy Observation of Photochemistry and Energy balance (SCOPE) model and the three-dimensional Fluorescence model with Weighted Photon Spread (FluorWPS) model. The fractional vegetation cover (FVC) and clumping effect have a large influence the SIF emission of 3D discontinuous canopies. For a moderate solar zenith angle (30°) and a clumped canopy (FVC = 0.6), the difference between the directional observed SIF of a 3D discontinuous canopy and a 1D homogeneous canopy was as large as 43.2% and 38.4% for Photosystem I + II fluorescence at 685 nm and at 740 nm, respectively. By bridging the gap between observed SIF and total emitted SIF over 3D heterogeneous vegetation canopies, the FluorRTER model can assist with the angular normalization of SIF measurements and enable the more robust interpretation of how variations in SIF from directional and hemispherical in-situ, airborne and satellite observations relate to leaf and whole-canopy physiological processes.