Search

The infrared window region (780–1,250 cm−1, 12.8 to 8.0 μm) is of great importance to Earth's climate due to its high transparency and thermal energy. We present here a new investigation of the transparency of this spectral region based on observations by interferometers of downwelling surface radiance at two DOE Atmospheric Radiation Measurement program sites. We focus on the dominant source of absorption in this region, the water vapor continuum, and derive updated values of spectral absorption coefficients for both the self and foreign continua. Our results show that the self continuum is too strong in the previous version of Mlawer‐Tobin_Clough‐Kneizys‐Davies (MT_CKD) water vapor continuum model, a result that is consistent with other recent analyses, while the foreign continuum is too weak in MT_CKD. In general, the weaker self continuum derived in this study results in an overall increase in atmospheric transparency in the window, although in atmospheres with low amounts of water vapor the transparency may slightly decrease due to the increase in foreign continuum absorption. These continuum changes lead to a significant decrease in downwelling longwave flux at the surface for moist atmospheres and a modest increase in outgoing longwave radiation. The increased fraction of surface‐leaving radiation that escapes to space leads to a notable increase (∼5–10%) in climate feedback, implying that climate simulations that use the new infrared window continuum will show somewhat less warming than before. This study also points out the possibly important role that aerosol absorption may play in the longwave radiative budget. Plain Language Summary: The spectral region in the infrared from 780 to 1,250 cm−1 (12.8–8.0 μm) is referred to as a window due to its transparency; in this region, thermal radiation emitted by the surface can pass relatively unimpeded through the atmosphere, allowing Earth to cool. The limited amount of atmospheric absorption that does occur in this region is primarily due to water vapor, in particular an absorption mechanism termed the water vapor continuum. The strength of water vapor continuum absorption in the infrared window therefore has important consequences for Earth's climate. This study provides a new evaluation of water vapor continuum absorption in the infrared window from an analysis of spectrally resolved measurements of downwelling surface radiances. Our results indicate that for most atmospheres the strength of water vapor continuum absorption is less than had been previously thought due to reduced absorption related to the interactions of water vapor molecules with other water vapor molecules, that is, the water vapor self continuum. The derived water vapor continuum changes allow the Earth to cool ∼5–10% better than had previously been thought, and climate simulations that use the revised infrared window continuum will show somewhat less warming than before. Key Points: Analysis of ground‐based radiance observations indicates that the infrared window region is more transparent than had been thought.The derived water vapor self continuum is 10%–30% weaker than previously thought, while the foreign continuum is substantially stronger.The revised H2O continuum results in a 5%–10% increase in climate feedback and a large change to the radiative budget for moist atmospheres. [ABSTRACT FROM AUTHOR]

Ocean surface albedo (OSA) is an important factor for the transfer of radiation in the coupled atmosphere-ocean system. By resolving the spectral variations of the reflective properties for incident direct and diffuse solar radiation, we develop an OSA computational scheme to study the impact of ocean biogeochemistry on the air-sea boundary condition of solar radiative transfer in the atmosphere. The new scheme is implemented for the General Circulation Model applications of the shortwave rapid radiative transfer model RRTMG_SW, a radiative transfer model used extensively in regional and global models. We show that a number of OSA schemes lead to underestimated results in comparison with in-situ measurements obtained at a site 25 km east of Virginia Beach. The scheme developed in this study considers multiple influential factors and is robust in terms of the mean absolute percentage error (MAPE) and the root mean square error in comparison with in-situ measurements. Furthermore, the new simulations are highly consistent with the Clouds and the Earth's Radiant Energy System (CERES) OSA distribution on a global scale. However, the theoretical results show slight differences compared with the CERES OSA under all sky conditions and overestimate the OSA in the subpolar Southern Ocean under clear sky conditions. The assumption of a uniform phase function, which neglects the spatial variability of the optical properties of oceanic particles, is largely responsible for the primary source of uncertainties in an OSA scheme. [ABSTRACT FROM AUTHOR]

Volcanic explosions are the most critical replenishing mechanism of the stratospheric aerosol Junge layer. A fresh volcanic cloud comprises mostly sulfur‐bearing gases, volcanic ash, and water vapor. It is commonly assumed that only sulfate aerosols remain in an aged volcanic cloud. Accurate simulation of the initial evolution of multicomponent fresh volcanic clouds is largely missing due to insufficient spatial resolution and a lack of relevant physics in global climate models. However, this initial stage is essential, as the vertical structure, composition, and altitude of a freshly developed volcanic cloud affect its long‐term evolution. To fill this gap, we modified a regional WRF‐Chem model to study the dispersion of a Pinatubo‐size volcanic cloud in the equatorial belt with a 25 km grid spacing explicitly accounting for the SO2, ash, sulfate, water vapor, and hydrometeors radiative effects. The model best reproduces the observed evolution of the Pinatubo optical depth when eruptive products are injected above the cold tropical tropopause at 17 km. During the first week, the volcanic cloud in our simulations rises 1 km/day. Ash is primarily responsible for the heating and lofting of the volcanic products. Radiative heating of SO2 is weaker than that of ash and sulfate but is sufficient to position the core of the SO2 layer 1–2 km above the sulfate layer. Utilizing a more realistic description of the volcanic cloud's initial stage potentially improves overall volcanic cloud predictability. It might also be essential to designing geoengineering technologies based on the injection of aerosol precursors in the lower stratosphere. Key Points: The model better reproduces the observed evolution of the Pinatubo optical depth when eruptive products are injected at 17 kmThe short‐lived ash is primarily responsible for the heating of the volcanic cloudRadiative heating of SO2 is weaker than that of ash and sulfate aerosols but still is significant to position the core of the SO2 cloud 1–2 km above the sulfate layer [ABSTRACT FROM AUTHOR]

Changes in concentrations of greenhouse gases lead to changes in radiative fluxes throughout the atmosphere. The value of this change, the instantaneous radiative forcing, varies across climate models, due partly to differences in the distribution of clouds, humidity, and temperature across models and partly due to errors introduced by approximate treatments of radiative transfer. This paper describes an experiment within the Radiative Forcing Model Intercomparision Project that uses benchmark calculations made with line‐by‐line models to identify parameterization error in the representation of absorption and emission by greenhouse gases. Clear‐sky instantaneous forcing by greenhouse gases is computed using a set of 100 profiles, selected from a reanalysis of present‐day conditions, that represent the global annual mean forcing from preindustrial times to the present day with sampling errors of less than 0.01 W m−2. Six contributing line‐by‐line models agree in their estimate of this forcing to within 0.025 W m−2 while even recently developed parameterizations have typical errors 4 or more times larger, suggesting both that the samples reveal true differences among line‐by‐line models and that parameterization error will be readily identifiable. Agreement among line‐by‐line models is better in the longwave than in the shortwave where differing treatments of the water vapor continuum affect estimates of forcing by carbon dioxide and methane. The impacts of clouds on instantaneous radiative forcing are estimated from climate model simulations, and the adjustment due to stratospheric temperature changes estimated by assuming fixed dynamical heating. Adjustments are large only for ozone and for carbon dioxide, for which stratospheric cooling introduces modest nonlinearity. Key Points: Mean clear‐sky instantaneous radiative forcing by greenhouse gases is computed with six benchmark models using 100 atmospheric profilesSampling error is several times smaller than the level of disagreement among models, which is itself smaller than parameterization errorThe impacts of clouds and stratospheric adjustment are roughly estimated; adjustments are large only for carbon dioxide and ozone [ABSTRACT FROM AUTHOR]

Knowledge of gas spectroscopy is still questionable at these frequencies but is important to fully exploit upcoming meteorological satellite measurements. The first of the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Polar System (EPS) Second Generation (EPS-SG) meteorological satellites will be launched in the 2023 timeframe and will include the Ice Cloud Imager (ICI), a passive conical-scanning radiometer observing in the microwave to submillimeter wave range of the spectrum with 11 channels from 183 to 664 GHz. Brightness temperature simulations calculated at the ICI viewing angle for a midlatitude summer scenario in the frequency range covered by ICI. Conversely, fast RTMs use parameterized transmittance, trained with statistical fit against line-by-line transmittance for diverse atmospheres and for specific instrument characteristics. [Extracted from the article]

This paper describes the initial implementation of a new toolbox that seeks to balance accuracy, efficiency, and flexibility in radiation calculations for dynamical models. The toolbox consists of two related code bases: Radiative Transfer for Energetics (RTE), which computes fluxes given a radiative transfer problem defined in terms of optical properties, boundary conditions, and source functions; and RRTM for General circulation model applications--Parallel (RRTMGP), which combines data and algorithms to map a physical description of the gaseous atmosphere into such a radiative transfer problem. The toolbox is an implementation of well-established ideas, including the use of a k-distribution to represent the spectral variation of absorption by gases and the use of two-stream, plane-parallel methods for solving the radiative transfer equation. The focus is instead on accuracy, by basing the k-distribution on state-of-the-art spectroscopy and on the sometimes-conflicting goals of flexibility and efficiency. Flexibility is facilitated by making extensive use of computational objects encompassing code and data, the latter provisioned at runtime and potentially tailored to specific problems. The computational objects provide robust access to a set of high-efficiency computational kernels that can be adapted to new computational environments. Accuracy is obtained by careful choice of algorithms and through tuning and validation of the k-distribution against benchmark calculations. Flexibility with respect to the host model implies user responsibility for maps between clouds and aerosols and the radiative transfer problem, although comprehensive examples are provided for clouds. [ABSTRACT FROM AUTHOR]

Cloud longwave scattering is generally neglected in general circulation models (GCMs), but it plays a significant and highly uncertain role in the atmospheric energy budget as demonstrated in recent studies. To reduce the errors caused by neglecting cloud longwave scattering, two new radiance adjustment methods are developed that retain the computational efficiency of broadband radiative transfer simulations. In particular, two existing scaling methods and the two new adjustment methods are implemented in the Rapid Radiative Transfer Model (RRTM). The results are then compared with those based on the Discrete Ordinate Radiative Transfer model (DISORT) that explicitly accounts for multiple scattering by clouds. The two scaling methods are shown to improve the accuracy of radiative transfer simulations for optically thin clouds but not effectively for optically thick clouds. However, the adjustment methods reduce computational errors over a wide range, from optically thin to thick clouds. With the adjustment methods, the errors resulting from neglecting cloud longwave scattering are reduced to less than 2W-22 for the upward irradiance at the top of the atmosphere and less than 0.5W-22 for the surface downward irradiance. The adjustment schemes prove to be more accurate and efficient than a four-stream approximation that explicitly accounts for multiple scattering. The neglect of cloud longwave scattering results in an underestimate of the surface downward irradiance (cooling effect), but the errors are almost eliminated by the adjustment methods (warming effect). [ABSTRACT FROM AUTHOR]

Abstract: General circulation models (GCMs) are extensively used to estimate the influence of clouds on the global energy budget and other aspects of climate. Because radiative transfer computations involved in GCMs are costly, it is typical to consider only absorption but not scattering by clouds in longwave (LW) spectral bands. In this study, the flux and heating rate biases due to neglecting the scattering of LW radiation by clouds are quantified by using advanced cloud optical property models, and satellite data from Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), CloudSat, Clouds and the Earth's Radiant Energy System (CERES), and Moderate Resolution Imaging Spectrometer (MODIS) merged products (CCCM). From the products, information about the atmosphere and clouds (microphysical and buck optical properties, and top and base heights) is used to simulate fluxes and heating rates. One‐year global simulations for 2010 show that the LW scattering decreases top‐of‐atmosphere (TOA) upward flux and increases surface downward flux by 2.6 and 1.2 W/m2, respectively, or approximately 10% and 5% of the TOA and surface LW cloud radiative effect, respectively. Regional TOA upward flux biases are as much as 5% of global averaged outgoing longwave radiation (OLR). LW scattering causes approximately 0.018 K/d cooling at the tropopause and about 0.028 K/d heating at the surface. Furthermore, over 40% of the total OLR bias for ice clouds is observed in 350–500 cm−1. Overall, the radiative effects associated with neglecting LW scattering are comparable to the counterpart due to doubling atmospheric CO2 under clear‐sky conditions. [ABSTRACT FROM AUTHOR]

The vector radiative transfer equation is decomposed into two components: a forward component and a diffuse component. The forward component is analytically solved with a small-angle approximation. The solution of the forward component becomes the source for the diffuse component. In the present study, the diffuse component is solved using the successive order of scattering method. The strong anisotropy of the scattering of radiation by a medium is confined to the forward component for which a semianalytical solution is given; consequently, the diffuse component slowly varies as a function of scattering angle once the forward-scattering peak is removed. Moreover, the effect on the diffuse component induced by the forward component can be interpreted by including the low orders of the generalized spherical function expansion of the forward component or even replaced by the Dirac delta function. As a result, the computational effort can be significantly reduced. The present two-component method is validated using the benchmarks related to predefined aerosol and cloud layers with a totally absorbing underlying surface. As a canonical application, the optical properties of water clouds and ice clouds used for the Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 6 cloud-property retrieval products are used for radiative transfer simulations under cloudy conditions. [ABSTRACT FROM AUTHOR]

Solar heating of the relative humidity (RH) probe on Vaisala RS92 radiosondes results in a large dry bias in the upper troposphere. Two different algorithms (Miloshevich et al., 2009, MILO hereafter; and Wang et al., 2013, WANG hereafter) have been designed to account for this solar radiative dry bias (SRDB). These corrections are markedly different with MILO adding up to 40% more moisture to the original radiosonde profile than WANG; however, the impact of the two algorithms varies with height. The accuracy of these two algorithms is evaluated using three different approaches: a comparison of precipitable water vapor (PWV), downwelling radiative closure with a surface-based microwave radiometer at a high-altitude site (5.3 km m.s.l.), and upwelling radiative closure with the space-based Atmospheric Infrared Sounder (AIRS). The PWV computed from the uncorrected and corrected RH data is compared against PWV retrieved from groundbased microwave radiometers at tropical, midlatitude, and arctic sites. Although MILO generally adds more moisture to the original radiosonde profile in the upper troposphere compared to WANG, both corrections yield similar changes to the PWV, and the corrected data agree well with the groundbased retrievals. The two closure activities - done for clear-sky scenes - use the radiative transfer models MonoRTM and LBLRTM to compute radiance from the radiosonde profiles to compare against spectral observations. BothWANG- and MILOcorrected RHs are statistically better than original RH in all cases except for the driest 30% of cases in the downwelling experiment, where both algorithms add too much water vapor to the original profile. In the upwelling experiment, the RH correction applied by the WANG vs. MILO algorithm is statistically different above 10 km for the driest 30% of cases and above 8 km for the moistest 30% of cases, suggesting that the MILO correction performs better than the WANG in clear-sky scenes. The cause of this statistical significance is likely explained by the fact the WANG correction also accounts for cloud cover - a condition not accounted for in the radiance closure experiments. [ABSTRACT FROM AUTHOR]

Cloud phase and microphysical properties control the radiative effects of clouds in the climate system and are therefore crucial to characterize in a variety of conditions and locations. An Arctic-specific, ground-based, multisensor cloud retrieval system is described here and applied to 2 yr of observations from Barrow, Alaska. Over these 2 yr, clouds occurred 75% of the time, with cloud ice and liquid each occurring nearly 60% of the time. Liquid water occurred at least 25% of the time, even in winter, and existed up to heights of 8 km. The vertically integrated mass of liquid was typically larger than that of ice. While it is generally difficult to evaluate the overall uncertainty of a comprehensive cloud retrieval system of this type, radiative flux closure analyses were performed in which flux calculations using the derived microphysical properties were compared with measurements at the surface and the top of the atmosphere. Radiative closure biases were generally smaller for cloudy scenes relative to clear skies, while the variability of flux closure results was only moderately larger than under clear skies. The best closure at the surface was obtained for liquid-containing clouds. Radiative closure results were compared with those based on a similar, yet simpler, cloud retrieval system. These comparisons demonstrated the importance of accurate cloud-phase and cloud-type classification, and specifically the identification of liquid water, for determining radiative fluxes. Enhanced retrievals of liquid water path for thin clouds were also shown to improve radiative flux calculations. [ABSTRACT FROM AUTHOR]

The Radiative Heating in Underexplored Bands Campaign (RHUBC-II) project was held from August to October 2009 in the Atacama Desert in Chile at 5320-m altitude. Observations from this experiment and a high-resolution numerical simulation with the Weather Research and Forecasting Model (WRF) were used to understand the structure and evolution of the atmosphere over a region with complex terrain and extremely dry environmental conditions. The mechanisms driving the local circulations during synoptically unperturbed conditions at the field site were studied. The study suggests that the field site is mainly affected by a mountain-scale and a plateau-scale thermally driven circulation. The latter seems to dominate. The advection of warm air by downslope flows from higher heights during nighttime may be the mechanism that counteracts the longwave radiative cooling at the surface, causing a small decrease of near-surface temperature during the night. WRF represents the near-surface and upper atmosphere reasonably well above the RHUBC-II site. Important orographic features are misrepresented in the model terrain, which may cause the observed differences in near-surface winds. The zonal pressure gradient between both sides of the mountain and the static stability of the air mass on the windward side of the terrain control the local circulations over the field site. Consequently, a misrepresentation of these mechanisms in the model may cause differences between the simulated winds and observations. [ABSTRACT FROM AUTHOR]

Ice clouds influence the climate system by changing the radiation budget and large-scale circulation. Therefore, climate models need to have an accurate representation of ice clouds and their radiative effects. In this paper, new broadband parameterizations for ice cloud bulk scattering properties are developed for severely roughened ice particles. The parameterizations are based on a general habit mixture that includes nine habits (droxtals, hollow/solid columns, plates, solid/hollow bullet rosettes, aggregate of solid columns, and small/large aggregates of plates). The scattering properties for these individual habits incorporate recent advances in light-scattering computations. The influence of ice particle surface roughness on the ice cloud radiative effect is determined through simulations with the Fu-Liou and the GCM version of the Rapid Radiative Transfer Model (RRTMG) codes and the National Center for Atmospheric Research Community Atmosphere Model (CAM, version 5.1). The differences in shortwave (SW) and longwave (LW) radiative effect at both the top of the atmosphere and the surface are determined for smooth and severely roughened ice particles. While the influence of particle roughening on the single-scattering properties is negligible in the LW, the results indicate that ice crystal roughness can change the SW forcing locally by more than 10 W m−2 over a range of effective diameters. The global-averaged SW cloud radiative effect due to ice particle surface roughness is estimated to be roughly 1-2 W m−2. The CAM results indicate that ice particle roughening can result in a large regional SW radiative effect and a small but nonnegligible increase in the global LW cloud radiative effect. [ABSTRACT FROM AUTHOR]

The accurate modeling of continuum absorption is crucial for the so-called window regions of the spectrum, the relatively transparent regions between lines. The window regions in the microwave are of critical importance for Earth remote sensing and data assimilation. Presented in this paper is an evaluation of the widely used Mlawer, Tobin, Clough, Kneizys, and Davis (MT_CKD) water vapor continuum model in the microwave region, performed using measurements from ground-based radiometers operated by the Department of Energy's Atmospheric Radiation Measurement Program at sites in Oklahoma, USA, and the Black Forest, Germany. The radiometers used were the Radiometrics 23.8/31.4-GHz microwave radiometers (MWRs), the Radiometer Physics GmbH 90/150-GHz MWR at high frequencies (MWRHF), and the Radiometrics 183 GHz G-band vapor radiometer profiler (GVRP). Radiometer measurements were compared with brightness temperatures calculated using radiosonde temperature and humidity profiles input to the monochromatic radiative transfer model (MonoRTM), which uses the MT_CKD continuum model. Measurements at 23.8 GHz were used to correct for biases in the total precipitable water vapor (PWV) from the radiosondes. The long-term 31.4 GHz data set, with a range of PWV values spanning from 0.15 to 5 cm, allowed the separation of uncertainties in the self- and foreign-broadened components of the water vapor continuum. The MT_CKD model has been updated in the microwave region to provide improved agreement with the measurements. MonoRTM has been updated accordingly. The results for the different instruments and frequencies were consistent, providing high confidence in the continuum updates. The estimated uncertainties on the updated continuum coefficients in MT_CKD are 4% on the foreign-broadened water vapor continuum and 4% on the self-broadened water vapor continuum. [ABSTRACT FROM PUBLISHER]

The article offers information on the new paradigm for global climate models (GCMs) radiative transfer (RT) code intercomparison in the U.S. It highlights the need for Continual Intercomparison of Radiation Codes (CIRC) in assessing code performance of radiation measurements. It notes that the reference calculations' quality of CIRC is based on atmospheric radiation measurement from the Department of Energy. CIRC seeks to provide documentation standards for radiation code performance.

During the Radiative Heating in Underexplored Bands Campaign (RHUBC), held in February-March 2007, three millimeter-wave radiometers were operated at the Atmospheric Radiation Measurement Program's site in Barrow, Alaska. These radiometers contain several channels located around the strong 183.31-GHz water vapor line, which is crucial for ground-based water-vapor measurements in very dry conditions, typical of the Arctic. Simultaneous radiosonde observations were carried out during conditions with very low integrated-water-vapor (IWV) content (< 2mm). Observations from the three instruments are compared, accounting for their different design characteristics. The overall agreement during RHUBC among the three instruments and between instruments and forward model is discussed quantitatively. In general, the instrument cross-validation performed for sets of channel pairs showed agreement within the total expected uncertainty. The consistency between instruments allows the determination of the IWV to within around 2% for these dry conditions. Comparisons between these data sets and forward-model simulations using radiosondes as input show spectral features in the brightness-temperature residuals, indicating some degree of inconsistency between the instruments and the forward model. The most likely cause of forward-model error is systematic errors in the radiosonde humidity profiles. [ABSTRACT FROM AUTHOR]

Air-broadened half-widths of the 22- and 183-GHz water-vapor lines and associated uncertainties have been determined using comparisons between ground-based radiometric measurements from Atmospheric Radiation Measurement sites in Oklahoma and Alaska, and MonoRTM, a radiative transfer model. Values of the widths obtained using the measurements are 0.0900 cm-1/atm with 1.6% uncertainty for the 22-GHz line and 0.0992 cm-1/atm with 2.4% uncertainty for the 183-GHz line. Also presented are spectroscopic parameters for these lines from new calculations performed using the complex implementation of the Robert-Bonamy theory (CRB). The CRB values of the air-broadened widths are 0.0913 cm-1/atm with 3% uncertainty and a temperature exponent of 0.755 for the 22-GHz line and 0.0997 cm-1/atm with 3% uncertainty and a temperature exponent of 0.769 for the 183-GHz line. The values for the air-broadened half-widths derived from the measurement/model comparisons show good agreement with the new CRB calculations. For future versions of MonoRTM, width values of 0.0900 and 0.0997 cm-1/atm are to be adopted with temperature dependences of 0.76 and 0.77 for the 22- and 183-GHz lines, respectively. [ABSTRACT FROM AUTHOR]

The Tropospheric Emission Spectrometer (TES) is a high-resolution spaceborne sensor that is capable of observing tropospheric species. In order to exploit fully TES's potential for tropospheric constituent retrievals, an accurate and fast operational forward model was developed for TES. The forward model is an important component of the TES retrieval model, the Earth Limb and Nadir Operational Retrieval (ELANOR), as it governs the accuracy and speed of the calculations for the retrievals. In order to achieve the necessary accuracy and computational efficiency, TES adopted the strategy of utilizing precalculated absorption coefficients generated by the line-by-line calculations provided by line-by-line radiation transfer modeling. The decision to perform the radiative transfer with the highest monochromatic accuracy attainable, rather than with an accelerated scheme that has the potential to add algorithmic forward model error, has proven to be very successful for TES retrievals. A detailed description of the TES forward model and Jacobians is described. A preliminary TES observation is provided as an example to demonstrate that the TES forward model calculations represent TES observations. Also presented is a validation example, which is part of the extensive forward model validation effort. [ABSTRACT FROM AUTHOR]