Parameterization Of Absorption Research Articles

Optimized Wavelength Sampling for Thermal Radiative Transfer in Numerical Weather Prediction Models

In the thermal spectral range, there are millions of individual absorption lines of water vapor, CO2, and other trace gases. Radiative transfer calculations of wavelength-integrated quantities, such as irradiance and heating rate, are computationally expensive, requiring a high spectral resolution for accurate numerical weather prediction and climate modeling. This paper introduces a method that could highly reduce the cost of integration in the thermal spectrum by employing an optimized wavelength sampling method. Absorption optical thicknesses for various trace gases were calculated from the HITRAN 2012 spectroscopic dataset using the ARTS line-by-line model as input to a fast Schwarzschild radiative transfer model. Using a simulated annealing algorithm, different optimized sets of wavelengths and corresponding weights were identified, which allowed for accurate integrated quantities to be computed as a weighted sum, reducing the computational time by several orders of magnitude. For each set of wavelengths, a lookup table, including the corresponding weights and absorption cross-sections, is created and can be applied to any atmospheric setups for which it was trained. We applied the lookup table to calculate irradiances and heating rates for a large set of atmospheric profiles from the ECMWF 91-level short-range forecast. Ten wavelength nodes are sufficient to obtain irradiances within an average root mean square error (RMSE) of upward and downward radiation at any height below 1 Wm−2 while 100 wavelengths allowed for an RSME of below 0.05 Wm−2. The applicability of this method was confirmed for irradiances and heating rates in clear conditions and for an exemplary cloud at 3.2 km height. Representative spectral gridpoints for integrated quantities in the thermal spectrum (REPINT) is available as absorption parameterization in the libRadtran radiative transfer package, where it can be used as an efficient molecular absorption parameterization for a variety of radiative transfer solvers.

Effective parameterization of absorption by gaseous species and unknown UV absorber in 125–400 nm region of Venus atmosphere

We present an effective parameterization of molecular absorption of shortwave solar radiation in Venus atmosphere. It is addressed to general circulation modeling to accelerate radiative transfer calculations in the spectral interval 125 – 400 nm (25000 – 80000 cm−1). In F-UV and M-UV regions strong absorption of CO2 and SO2 enables to parameterize gaseous absorption with only three effective cross-sections. In N-UV region absorption of SO2 and unknown UV absorber are parameterized with six effective cross-sections for each species. For treatment of Rayleigh scattering and optical properties of Venus clouds eight effective spectral points are recommended. The parameterizations were validated by the original reference line-by-line Monte-Carlo radiative transfer model. The outcome of the validation shows discrepancy in fluxes less than 3%. Thus, it takes only eight-fold solution of radiative transfer equations to correctly describe solar fluxes and heating rates in the whole ultraviolet region.

Спектральные биооптические показатели вод Атлантического сектора Антарктики

Исследования изменчивости спектральных биооптических показателей вод Атлантического сектора Антарктики проводили в 79-м рейсе НИС «Академик Мстислав Келдыш» в период с 11.01.2020 по 04.02.2020. Получено, что концентрация хлорофилла a в сумме с феопигментами изменялась в исследованном слое на большинстве станций от 0,1 до 1,8 мг·м−3, за исключением двух станций, где достигала 2,2 и 4,4 мг·м−3. Установлена связь показателей поглощения света пигментами фитопланктона с концентрацией хлорофилла a на длинах волн, соответствующих основным максимумам спектра: aph(438) = 0,044 × Ca1,2, r2 = 0,84 (n = 117); aph(678) = 0,021 × Ca1,1, r2 = 0,89 (n = 117). Спектры показателей поглощения света неживым взвешенным веществом и окрашенным растворённым органическим веществом описаны экспоненциальной зависимостью. Подобраны коэффициенты параметризации: (1) показатели поглощения света неживым взвешенным (0,001–0,027 м−1) и растворённым органическим веществом (0,016–0,19 м−1) на длине волны 438 нм; (2) коэффициенты наклона экспоненты спектров этих компонент (0,005–0,016 и 0,009–0,022 нм−1 соответственно).

Modeling the Near‐Surface Diurnal Cycle of Sea Surface Temperature in the Mediterranean Sea

AbstractThe diurnal cycle of sea surface temperature (SST) is an important component of the ocean‐atmosphere system and is necessary for accurately computing air‐sea heat fluxes. Ocean temperatures in the near‐surface are highly sensitive to atmospheric conditions and can vary significantly depending on time of day. Ocean general circulation models are unable to fully capture the near‐surface diurnal SST variability, because they do not possess the necessary vertical structure and resolution. Furthermore, SST observations come from a number of sources that represent the temperature at various near‐surface depths. This presents difficulties when assimilating SST observations as well as constructing robust climate records of SST. In this study we model the fine‐scale near‐surface structure allowing SST comparisons between foundation SST, SST at depth, subskin SST, and skin SST. Hourly model results, forced and initialized using readily available reanalysis data, are from a 2‐year period, 2013–2014, over the Mediterranean Sea. Various solar absorption parameterizations are examined, and the resulting SSTs are compared to Spinning Enhanced Visible and InfraRed Imager‐derived observations of the skin temperature.

Short-term forecasts of GHI and DNI for solar energy systems operation: assessment of the ECMWF integrated forecasting system in southern Portugal

Solar energy is key factor in the demand for clean energy development and management. In particular, global horizontal irradiance (GHI) and direct normal irradiance (DNI) are the foremost important solar resource components that need to be well characterized in order to seek an efficient operation of photovoltaic and concentrated solar power plants, respectively. The objective of the present work is to assess the quality of short-term (24 h) forecasts from a global Numerical Weather Prediction (NWP) model regarding the GHI and DNI components for solar energy applications. Forecast accuracy for the Integrated Forecasting System (IFS), the global model of the European Centre for Medium-Range Weather Forecasts (ECMWF), was verified through the comparison of the predicted hourly values with the corresponding ground-based measurements in southern Portugal. In this study, results from one year of IFS data are analysed, yielding a general good agreement between model and four ground-based measuring stations. High correlations occur particularly for GHI whilst DNI simulations are predominantly hindered by cloud and aerosol representation (i.e. the radiative effects of clouds tend to be underestimated by the model and the radiative effects of the aerosols are overestimated by the model under very clear atmospheric conditions), being closely linked to the parameterization of absorption and scattering phenomena as function of cloud and aerosol type and dimension. Relative differences of annual availabilities for GHI are found between ∼0.16% to ∼2.12% whilst for DNI values ranging from ∼7% to ∼12% are found. The respective correlations coefficients are around 0.95 for GHI and between 0.65 and 0.77 for DNI. Regional irradiation maps of GHI and DNI are presented, showing that NWP predictions are an important tool for the operation of electricity generation systems based on solar energy.

The libRadtran software package for radiative transfer calculations (version 2.0.1)

Abstract. libRadtran is a widely used software package for radiative transfer calculations. It allows one to compute (polarized) radiances, irradiance, and actinic fluxes in the solar and thermal spectral regions. libRadtran has been used for various applications, including remote sensing of clouds, aerosols and trace gases in the Earth's atmosphere, climate studies, e.g., for the calculation of radiative forcing due to different atmospheric components, for UV forecasting, the calculation of photolysis frequencies, and for remote sensing of other planets in our solar system. The package has been described in Mayer and Kylling (2005). Since then several new features have been included, for example polarization, Raman scattering, a new molecular gas absorption parameterization, and several new parameterizations of cloud and aerosol optical properties. Furthermore, a graphical user interface is now available, which greatly simplifies the usage of the model, especially for new users. This paper gives an overview of libRadtran version 2.0.1 with a focus on new features. Applications including these new features are provided as examples of use. A complete description of libRadtran and all its input options is given in the user manual included in the libRadtran software package, which is freely available at http://www.libradtran.org.

Representative wavelengths absorption parameterization applied to satellite channels and spectral bands

Accurate modeling of wavelength-integrated radiative quantities, e.g. integrated over a spectral band or an instrument channel response function, requires computations for a large number of wavelengths if the radiation is affected by gas absorption which typically comprises a complex line structure. In order to increase computational speed of modeling radiation in the Earth׳s atmosphere, we parameterized wavelength-integrals as weighted means over representative wavelengths. We parameterized spectral bands of different widths (1cm−1, 5cm−1, and 15cm−1) in the solar and thermal spectral range, as well as a number of instrument channels on the ADEOS, ALOS, EarthCARE, Envisat, ERS, Landsat, MSG, PARASOL, Proba, Sentinel, Seosat, and SPOT satellites. A root mean square relative deviation lower than 1% from a “training data set” was selected as the accuracy threshold for the parameterization of each band and channel. The training data set included high spectral resolution calculations of radiances at the top of atmosphere for a set of highly variable atmospheric states including clouds and aerosols. The gas absorption was calculated from the HITRAN 2004 spectroscopic data set and state-of-the-art continuum models using the ARTS radiative transfer model. Three representative wavelengths were required on average to fulfill the accuracy threshold. We implemented the parameterized spectral bands and satellite channels in the uvspec radiative transfer model which is part of the libRadtran software package. The parameterization data files, including the representative wavelengths and weights as well as lookup tables of absorption cross sections of various gases, are provided at the libRadtran webpage.In the paper we describe the parameterization approach and its application. We validate the approach by comparing modeling results of parameterized bands and channels with results from high spectral resolution calculations for atmospheric states that were not part of the training data set. Irradiances are not only compared at the top of atmosphere but also at the surface for which this parameterization approach was not optimized. It is found that the parameterized bands and channels provide a good compromise between computation time requirements and uncertainty for typical radiative transfer problems. In particular for satellite radiometer simulations the computation time requirement and the parameterization uncertainty is low. Band-integrated irradiances at any level as well as heating and cooling rates below 20km can also be modeled with low uncertainty.

Stochastic parameterization for light absorption by internally mixed BC/dust in snow grains for application to climate models

A stochastic approach has been developed to model the positions of BC (black carbon)/dust internally mixed with two snow grain types: hexagonal plate/column (convex) and Koch snowflake (concave). Subsequently, light absorption and scattering analysis can be followed by means of an improved geometric-optics approach coupled with Monte Carlo photon tracing to determine BC/dust single-scattering properties. For a given shape (plate, Koch snowflake, spheroid, or sphere), the action of internal mixing absorbs substantially more light than external mixing. The snow grain shape effect on absorption is relatively small, but its effect on asymmetry factor is substantial. Due to a greater probability of intercepting photons, multiple inclusions of BC/dust exhibit a larger absorption than an equal-volume single inclusion. The spectral absorption (0.2–5 µm) for snow grains internally mixed with BC/dust is confined to wavelengths shorter than about 1.4 µm, beyond which ice absorption predominates. Based on the single-scattering properties determined from stochastic and light absorption parameterizations and using the adding/doubling method for spectral radiative transfer, we find that internal mixing reduces snow albedo substantially more than external mixing and that the snow grain shape plays a critical role in snow albedo calculations through its forward scattering strength. Also, multiple inclusion of BC/dust significantly reduces snow albedo as compared to an equal-volume single sphere. For application to land/snow models, we propose a two-layer spectral snow parameterization involving contaminated fresh snow on top of old snow for investigating and understanding the climatic impact of multiple BC/dust internal mixing associated with snow grain metamorphism, particularly over mountain/snow topography.

Entire thermal infrared spectrum observed for first time

The driving mechanism of the greenhouse effect and the underpinning of modern anthropogenic warming is the absorption, emission, and transmission of infrared radiation by atmospheric gases. The heat‐trapping ability of a gas depends on its chemical composition, and each type of gas absorbs infrared radiation of different energies. The amount of infrared radiation that escapes into space depends on the net effect of the myriad gases in the atmosphere, with water vapor being the primary gaseous absorber of infrared radiation. Water vapor absorbs a wide range of infrared radiation, masking the effects of other gases. In fact, in many spectral regions (or infrared radiation energy bands), water vapor is so strongly absorbing that it makes testing the accuracy of infrared radiation absorption parameterizations used in general circulation models difficult.

Parameterization of light absorption by components of seawater in optically complex coastal waters of the Crimea Peninsula (Black Sea)

The absorption of sunlight by oceanic constituents significantly contributes to the spectral distribution of the water-leaving radiance. Here it is shown that current parameterizations of absorption coefficients do not apply to the optically complex waters of the Crimea Peninsula. Based on in situ measurements, parameterizations of phytoplankton, nonalgal, and total particulate absorption coefficients are proposed. Their performance is evaluated using a log-log regression combined with a low-pass filter and the nonlinear least-square method. Statistical significance of the estimated parameters is verified using the bootstrap method. The parameterizations are relevant for chlorophyll a concentrations ranging from 0.45 up to 2 mg/m(3).

Influence of the prescribed solar spectrum on calculations of atmospheric temperature

Significant differences in heating rates are found when two solar irradiance spectra are used in a line‐by‐line radiative transfer code. Compared with a spectrum of recent satellite data an older theoretical spectrum gives 20–40% more heating in the ozone Hartley band, important in the upper stratosphere. The spectra are implemented in a broad‐band radiation code to which some improvements are also made to the ozone absorption parameterization. A widely‐used spectrum of ground‐based data from 1960s gives somewhat lower heating rates. The effects of the changes in the spectrum, and the broad‐band scheme, on the temperatures simulated by a middle atmosphere GCM are investigated. The model has previously shown a warm bias, compared with climatology, around the stratopause but this is significantly reduced when the former spectrum is substituted for the latter, and the new ozone parameterization incorporated. The change in spectrum accounts for two‐thirds of the improvement.

The Use of New Parameterizations for Gaseous Absorption in the CLIRAD-SW Solar Radiation Code for Models

The new gaseous absorption parameterizations are incorporated in the CLIRAD-SW solar radiation code for models, openly distributed for the scientific community. In the new parameterizations, the magnitude of absorption coefficients in each homogeneous layer depends on both species concentrations in the layer and species amounts accumulated along the direct solar radiation path from the top of the atmosphere (TOA). The number of the k-distribution terms varies from 1 to 4 in each of the eight bands. The total number of pseudomonochromatic intervals in the new version of the code is 15, compared with 38 currently in CLIRAD-SW. This reduces computational time of the code by approximately 2 times and facilitates its using in the numerical models. The error of the new version of CLIRAD-SW is determined here as the difference between the flux or heating rate values calculated with the code and line-by-line model. The surface and top-of-the-atmosphere flux difference is less than 1.5 W m−2 in calculations for the standard gaseous atmospheres and less than 7 W m−2 in calculations for the standard gaseous atmosphere with aerosol or cloud scattering and absorption incorporated. The relative flux error is less than 1% for all cases of gaseous atmosphere with and without molecular scattering and aerosol extinction. This error is less than 1.5% for the cases with cloudiness. The errors of heating rate calculations in the clear-sky atmospheres are less than 6% up to the height of 70 km, while these errors in cloudy layers reach values of about 30%, which is typical for current broadband parameterizations. The method to reduce these errors is suggested.

Validation of visible/near‐IR atmospheric absorption and solar emission spectroscopic models at 1 cm−1 resolution

A Fourier transform infrared (FTIR) spectrometer, operating at 1 cm−1 resolution between 9000 and 24,669 cm−1 (0.405–1.111 μm) has been used to check the spectral composition of databases that form the basis for most atmospheric absorption parameterizations used in climate models, remote sensing, and other radiative transfer simulations. The spectrometer, operating near sea level under clear skies, obtained relative atmospheric transmission measurements of the direct solar beam by means of a heliostat. The spectroscopic data were compared with a line‐by‐line radiative transfer model (LBLRTM) calculation of direct solar beam flux, which used a input data a monochromatic model extraterrestrial solar flux spectrum currently in common use. This intercomparison revealed that the extraterrestrial solar flux spectrum contains 266 solar absorption features that do not appear in the data, resulting in an excess of approximately 1.92 W m−2 in the model's solar constant. The intercomparison also revealed 97 absorption features in the data that do not appear in the HITRAN‐96 database as used by LBLRTM, resulting in a model underestimate of shortwave absorption of ∼0.23 W m−2 for a solar zenith angle of 42°. These small discrepancies revealed by the intercomparison indicate that current extraterrestrial solar irradiance models and spectroscopic databases used by shortwave atmospheric radiative transfer models are nearly entirely complete for purposes of atmospheric energy budget calculation. Thus clear or cloudy sky ‘excess absorption’ is unlikely to be related to an incomplete identification of atmospheric absorbing gases and their spectroscopic features, at 1 cm−1 resolution, for a clean troposphere of normal composition.

Solar radiation absorption by CO2, overlap with H2O, and a parameterization for general circulation models

Line‐by‐line (LBL) solar radiative transfer solutions are obtained for CO2‐only, H2O‐only, and CO2 + H2O atmospheres, and the contributions by the major CO2 and H2O absorption bands to the heating rates in the stratosphere and troposphere are analyzed. The LBL results are also used to investigate the inaccuracies in the absorption by a CO2 + H2O atmosphere, arising due to a multiplication of the individual gas transmissions averaged over specific spectral widths (Δν). Errors in absorption generally increase with the value of Δν chosen. However, even when the interval chosen for averaging the individual gas transmissions is the entire solar spectrum, there is no serious degradation in the accuracy of the atmospheric absorbed flux (error < 3%) and the heating rates (errors < 10%). A broadband parameterization for CO2 absorption, employed in several weather prediction and climate models, is found to substantially underestimate the LBL heating rates throughout the atmosphere, most notably in the stratosphere (errors > 40%). This parameterization is modified such that the resulting errors are less than 20%. When this modified CO2 parameterization is combined with a recently modified formulation for H2O vapor absorption, the resulting errors in the heating rates are also less than 20%. The application of the modified solar absorption parameterizations in a general circulation model (GCM) causes an increase in the simulated clear sky diabatic heating rates, ranging from nonnegligible (middle stratosphere and lower troposphere) to significant (lower stratosphere and upper troposphere) additions. The results here should enable a continued use of the older broadband parameterizations in GCMs, albeit in modified forms.

Delta‐Eddington approximation for solar radiation in the NCAR community climate model

Motivated by the desire for a more flexible and general solar radiation calculation in the NCAR community climate model (CCM), the δ‐Eddington approximation has been employed in the CCM version 2 (CCM2). Eighteen spectral intervals span the solar spectrum from 0.2 to 5.0 μm. Absorption parameterizations for H2O, O3, CO2, and O2 were developed by making use of the latest theoretical calculations. Water droplet scattering and absorption are parameterized as shown by Slingo (1989). An accurate and efficient convolution of the H2O vapor spectrum with water droplet clouds is presented that yields good agreement with available line‐by‐line (LBL) calculations for single‐layer clouds. A simple and efficient method to simulate partial cloud cover and cloud overlap is included. The simulated albedo‐solar zenith angle dependence agrees very well with adding/doubling scattering calculations. The CCM2 δ‐Eddington method will make possible many interesting applications of CCM2 in the years to come.

A parameterization for absorption due to the A, B, and γ oxygen bands

A new parameterization for the absorption of visible solar radiation due to 0 2 is presented. This broad band parameterization is based on comparison with detailed line-by-line calculations and is shown to be in excellent agreement with laboratory measurements. The importance of solar 0 2 absorption to atmospheric heating rates and temperatures is also discussed. Although the heating caused by the solar 0 2 bands is small (<0.03 K day -1 ), its presence warms the tropical tropopause region by as much as 1.1 K. Finally, the importance of correct asymptotic limits for modeling the absorptance of 0, 2 is illustrated. DOI: 10.1111/j.1600-0889.1985.tb00040.x

A parameterization for absorption due to the A, B, and &amp;#x3b3; oxygen bands

A parameterization for absorption due to the A, B, and &amp;#x3b3; oxygen bands

A paramaterization for the absorption due to the near infrared bands of CO&lt;sub&gt;2&lt;/sub&gt;

A broad band absorption parameterization for the near-infrared bands of CO 2 is described. This parameterization is in excellent agreement with more detailed line-by-line calculations for a large range of pressures, temperatures and amounts of CO 2 . The role of near IR absorption due to CO 2 is investigated for the earth’s atmosphere. It is shown that the presence of these near-lR absorption bands contributes to a heating of nearly 0.2 K/day in the upper stratosphere and 0.05 K/day in the lower stratosphere. Although the magnitude of heating in the lower stratosphere is small, it contributes more heating than ozone near the tropical tropopause region. DOI: 10.1111/j.1600-0889.1985.tb00068.x

A Parameterization of the Absorption in the 15 μm CO2Spectral Region with Application to Climate Sensitivity Studies

A parameterization of the absorption in the 15 μm CO2 spectral region has been developed based upon the wing scaling approximation of Chou and Arking (1980, 1981). The spectrum is divided into a band-wing region and a band-center region, and the CO2 amount in an inhomogeneous atmosphere is scaled separately for the two regions. The spectrally averaged transmittance over each region is then expressed as a simple function of the scaled amount of CO2. Compared to fine-by-line calculations, the error of the parameterization is <0.025 in the transmittance and <0.04°C day−1 in the tropospheric and lower stratospheric cooling rates. The cooling rate error in the upper stratosphere is generally ten than a few tenths of a degree per day except for the region above the 3 mb level where the error is too large to be acceptable for some studies on the phenomena in that region. The effect of the parameterization of absorption due to CO2 on climate studies has been investigated with the Multi-Layer Energy Balance Model (MLEBM) developed at GLAS (Peng et al, 1982). It is found that, compared to the accurate perturbation method, the parameterization introduces very small differences in the model temperatures and radiation budgets for both the normal and doubled CO2 concentrations. In addition, we have investigated the effect of including the CO2 absorption in the margins of the 15 μm spectral band on the CO2 climate sensitivity. It is found that the surface temperature sensitivity is enhanced by 20% for a doubled CO2 concentration and by 30% for a quadrupled CO2 concentration when the spectral range of CO2 absorption is extended from 580–760 to 540–800 cm−1.