Freidenreich, Stuart, David J Paynter, Pu Lin, V Ramaswamy, Alexandra L Jones, Daniel Feldman, and William D Collins, June 2021: An investigation into biases in instantaneous aerosol radiative effects calculated by shortwave parameterizations in two Earth system models. JGR Atmospheres, 126(11), DOI:10.1029/2019JD032323. Abstract
Because the forcings to which Coupled Model Intercomparison Project - Phase 5 (CMIP5) models were subjected were poorly quantified, recent efforts from the Radiative Forcing Model Intercomparison Project (RFMIP) have focused on developing and testing models with exacting benchmarks. Here, we focus on aerosol forcing to understand if for a given distribution of aerosols, participating models are producing a radiometrically-accurate forcing. We apply the RFMIP experimental protocol for assessing flux biases in aerosol instantaneous radiative effect (IRE) on two participating models, GFDL AM4 and CESM 1.2.2. The latter model contains the RRTMG radiation code which is widely used among CMIP6 GCM's. We conduct a series of calculations that test different potential sources of error in these models relative to line-by-line benchmarks. We find two primary sources of error: two-stream solution methods and the techniques to resolve spectral dependencies of absorption and scattering across the solar spectrum. The former is the dominant source of error for both models but the latter is more significant as a contributing factor for CESM 1.2.2. Either source of error can be addressed in future model development, and these results both demonstrate how the RFMIP protocol can help determine the origins of parameterized errors relative to their equivalent benchmark calculations for participating models, as well as highlight a viable path towards a more rigorous quantification and control of forcings for future CMIP exercises.
In this two-part paper, a description is provided of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). This version, with roughly 100km horizontal resolution and 33 levels in the vertical, contains an aerosol model that generates aerosol fields from emissions and a “light” chemistry mechanism designed to support the aerosol model but with prescribed ozone. In Part I, the quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode – with prescribed sea surface temperatures (SSTs) and sea ice distribution – is described and compared with previous GFDL models and with the CMIP5 archive of AMIP simulations. The model's Cess sensitivity (response in the top-of-atmosphere radiative flux to uniform warming of SSTs) and effective radiative forcing are also presented. In Part II, the model formulation is described more fully and key sensitivities to aspects of the model formulation are discussed, along with the approach to model tuning.
In Part II of this two-part paper, documentation is provided of key aspects of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode has been provided in Part I. Part II provides documentation of key components and some sensitivities to choices of model formulation and values of parameters, highlighting the convection parameterization and orographic gravity wave drag. The approach taken to tune the model's clouds to observations is a particular focal point. Care is taken to describe the extent to which aerosol effective forcing and Cess sensitivity have been tuned through the model development process, both of which are relevant to the ability of the model to simulate the evolution of temperatures over the last century when coupled to an ocean model.
A new paradigm in benchmark absorption-scattering radiative transfer is presented that enables both the globally-averaged and spatially-resolved testing of climate model radiation parameterizations in order to uncover persistent sources of biases in the aerosol Instantaneous Radiative Effect (IRE). A proof-of-concept is demonstrated with the GFDL AM4 and CESM 1.2.2 climate models. Instead of prescribing atmospheric conditions and aerosols, as in prior intercomparisons, native snapshots of the atmospheric state and aerosol optical properties from the participating models are used as inputs to an accurate radiation solver to uncover model-relevant biases. These diagnostic results show that the models’ aerosol IRE bias is of the same magnitude as the persistent range cited (~1 W/m2), and also varies spatially and with intrinsic aerosol optical properties. The findings underscore the significance of native model error analysis and its dispositive ability to diagnose global biases, confirming its fundamental value for the Radiative Forcing Model Intercomparison Project.
The Geophysical Fluid Dynamics Laboratory (GFDL) has developed a coupled general circulation model (CM3) for atmosphere, oceans, land, and sea ice. The goal of CM3 is to address emerging issues in climate change, including aerosol-cloud interactions, chemistry-climate interactions, and coupling between the troposphere and stratosphere. The model is also designed to serve as the physical-system component of earth-system models and models for decadal prediction in the near-term future, for example, through improved simulations in tropical land precipitation relative to earlier-generation GFDL models. This paper describes the dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component (AM3) of this model.
Relative to GFDL AM2, AM3 includes new treatments of deep and shallow cumulus convection, cloud-droplet activation by aerosols, sub-grid variability of stratiform vertical velocities for droplet activation, and atmospheric chemistry driven by emissions with advective, convective, and turbulent transport. AM3 employs a cubed-sphere implementation of a finite-volume dynamical core and is coupled to LM3, a new land model with eco-system dynamics and hydrology.
Most basic circulation features in AM3 are simulated as realistically, or more so, than in AM2. In particular, dry biases have been reduced over South America. In coupled mode, the simulation of Arctic sea ice concentration has improved. AM3 aerosol optical depths, scattering properties, and surface clear-sky downward shortwave radiation are more realistic than in AM2. The simulation of marine stratocumulus decks and the intensity distributions of precipitation remain problematic, as in AM2.
The last two decades of the 20th century warm in CM3 by .32°C relative to 1881-1920. The Climate Research Unit (CRU) and Goddard Institute for Space Studies analyses of observations show warming of .56°C and .52°C, respectively, over this period. CM3 includes anthropogenic cooling by aerosol cloud interactions, and its warming by late 20th century is somewhat less realistic than in CM2.1, which warmed .66°C but did not include aerosol cloud interactions. The improved simulation of the direct aerosol effect (apparent in surface clear-sky downward radiation) in CM3 evidently acts in concert with its simulation of cloud-aerosol interactions to limit greenhouse gas warming in a way that is consistent with observed global temperature changes.
Freidenreich, Stuart, and V Ramaswamy, April 2011: Analysis of the biases in the downward shortwave surface flux in the GFDL CM2.1 General Circulation Model. Journal of Geophysical Research: Atmospheres, 116, D08208, DOI:10.1029/2010JD014930. Abstract
Simulations of downward shortwave surface fluxes by the coupled GFDL CM2.1
GCM are compared against climatology derived from the BSRN, GEBA and ISCCP-FD
datasets. The spatial pattern of the model’s biases is evaluated. An investigation is made
of how these relate to accompanying biases in total cloud amount and aerosol optical
depth, and how they affect the surface temperature simulation.
Comparing CM2.1’s clear-sky fluxes against BSRN site values, for European, Asian
and North American locations, there are underestimates in the direct and overestimates in
the diffuse, resulting in underestimates in the total flux. These are related to
overestimates of sulfate aerosol optical depth, arising due to the behavior of the
parameterization function for hygroscopic growth of these aerosols at very high relative
humidity. Contrastingly, flux overestimate biases at lower latitude locations are
associated with underestimates in sea-salt and carbonaceous aerosol amounts. All-sky
flux biases consist of underestimates for North America, Eurasia, southern Africa, and
northern oceans, and overestimates for the Amazon region, equatorial Africa, off the west
coast of the Americas, and southern oceans. These biases show strong correlations with
cloud amount biases. There are modest correlations with cool surface temperature biases
for North America and Eurasia, and warm biases for the Amazon region, and cool (warm)
biases for the northern (southern) oceans. Analyses assuming non-hygroscopicity
illustrate that there’s a reduction of surface temperature biases accompanying a reduction
of sulfate aerosol optical depth biases; whereas, a more significant improvement in the
temperature simulation requires refining the model’s simulation of cloudiness.
Freidenreich, Stuart, and V Ramaswamy, 2005: Refinement of the Geophysical Fluid Dynamics Laboratory solar benchmark computations and an improved parameterization for climate models. Journal of Geophysical Research, 110, D17105, DOI:10.1029/2004JD005471. Abstract PDF
A recent intercomparison study of solar radiative transfer models has revealed a notable difference (5%) in the total spectrum column absorptance, for a specified clear-sky atmospheric profile, between two principal line-by-line benchmark results (namely, the Geophysical Fluid Dynamics Laboratory (GFDL) and the Atmospheric and Environmental Research, Inc. models). We resolve this discrepancy by performing a series of “benchmark” computations which show that the water vapor continuum formulation, spectral line information, and spectral distribution of the solar irradiance at the top of the atmosphere are key factors. Accounting for these considerations reduces the difference between the two benchmarks to less than 1%. The analysis establishes a high level of confidence in the use of benchmark calculations for developing and testing solar radiation parameterizations in weather and climate models. The magnitude of the change in absorption in the newer GFDL benchmark computations, associated with the use of a more recent spectral line catalog and inclusion of the water vapor continuum, has also necessitated revising the solar parameterization used in the operational GFDL general circulation model (GCM). When compared with the newer reference computation, the older parameterization shows an underestimate of the clear-sky heating rate throughout the atmosphere, with the error in the atmospheric solar absorbed flux being about 20 W m-2 for a midlatitude summer atmosphere and overhead Sun. In contrast, the new parameterization improves the representation of the solar absorption and reduces the bias to about 5 W m-2. Another important feature of the new parameterization is a nearly 50% reduction in the number of pseudomonochromatic columnar calculations compared to the older formulation, with only relatively small increases in the biases in absorption for cloudy layers. This yields a reduction of about 10% in the GCM computational time. The effect of the new parameterization on the simulated temperature in the new operational GFDL climate GCM is also examined. There is an increased solar heating; this yields temperature increases exceeding 1 K in the lower stratosphere.
Halthore, R N., D Crisp, S E Schwartz, G P Anderson, A Berk, B Bonnel, Olivier Boucher, F-L Chang, M-D Chou, E E Clothiaux, P Dubuisson, B Fomin, Y Fouquart, Stuart Freidenreich, C Gautier, S Kato, I Laszlo, Z Li, J H Mather, A Plana-Fattori, V Ramaswamy, P Ricchiazzi, Y Shiren, A Trishchenko, and W Wiscombe, 2005: Intercomparison of shortwave radiative transfer codes and measurements. Journal of Geophysical Research: Atmospheres, 110, D11206, DOI:10.1029/2004JD005293. Abstract
Computation of components of shortwave (SW) or solar irradiance in the surface-atmospheric system forms the basis of intercomparison between 16 radiative transfer models of varying spectral resolution ranging from line-by-line models to broadband and general circulation models. In order of increasing complexity the components are: direct solar irradiance at the surface, diffuse irradiance at the surface, diffuse upward flux at the surface, and diffuse upward flux at the top of the atmosphere. These components allow computation of the atmospheric absorptance. Four cases are considered from pure molecular atmospheres to atmospheres with aerosols and atmosphere with a simple uniform cloud. The molecular and aerosol cases allow comparison of aerosol forcing calculation among models. A cloud-free case with measured atmospheric and aerosol properties and measured shortwave radiation components provides an absolute basis for evaluating the models. For the aerosol-free and cloud-free dry atmospheres, models agree to within 1% (root mean square deviation as a percentage of mean) in broadband direct solar irradiance at surface; the agreement is relatively poor at 5% for a humid atmosphere. A comparison of atmospheric absorptance, computed from components of SW radiation, shows that agreement among models is understandably much worse at 3% and 10% for dry and humid atmospheres, respectively. Inclusion of aerosols generally makes the agreement among models worse than when no aerosols are present, with some exceptions. Modeled diffuse surface irradiance is higher than measurements for all models for the same model inputs. Inclusion of an optically thick low-cloud in a tropical atmosphere, a stringent test for multiple scattering calculations, produces, in general, better agreement among models for a low solar zenith angle (SZA = 30°) than for a high SZA (75°). All models show about a 30% increase in broadband absorptance for 30° SZA relative to the clear-sky case and almost no enhancement in absorptance for a higher SZA of 75°, possibly due to water vapor line saturation in the atmosphere above the cloud.
for climate research developed at the Geophysical Fluid Dynamics Laboratory (GFDL) are presented. The atmosphere model, known as AM2, includes a new gridpoint dynamical core, a prognostic cloud scheme, and a multispecies aerosol climatology, as well as components from previous models used at GFDL. The land model, known as LM2, includes soil sensible and latent heat storage, groundwater storage, and stomatal resistance. The performance of the coupled model AM2–LM2 is evaluated with a series of prescribed sea surface temperature (SST) simulations. Particular focus is given to the model's climatology and the characteristics of interannual variability related to E1 Niño– Southern Oscillation (ENSO).
One AM2–LM2 integration was performed according to the prescriptions of the second Atmospheric Model Intercomparison Project (AMIP II) and data were submitted to the Program for Climate Model Diagnosis and Intercomparison (PCMDI). Particular strengths of AM2–LM2, as judged by comparison to other models participating in AMIP II, include its circulation and distributions of precipitation. Prominent problems of AM2– LM2 include a cold bias to surface and tropospheric temperatures, weak tropical cyclone activity, and weak tropical intraseasonal activity associated with the Madden–Julian oscillation.
An ensemble of 10 AM2–LM2 integrations with observed SSTs for the second half of the twentieth century permits a statistically reliable assessment of the model's response to ENSO. In general, AM2–LM2 produces a realistic simulation of the anomalies in tropical precipitation and extratropical circulation that are associated with ENSO.
Barker, H W., Graeme L Stephens, P T Partain, J W Bergman, B Bonnel, K Campana, E E Clothiaux, S Clough, S Cusack, J Delamere, J Edwards, K Franklin Evans, Y Fouquart, Stuart Freidenreich, V Y Galin, Y Hou, S Kato, J-L Li, Eli J Mlawer, J-J Morcrette, W O'Hirok, P Räisänen, V Ramaswamy, B Ritter, Eugene Rozanov, Michael E Schlesinger, K Shibata, P Sporyshev, Z Sun, M Wendisch, N Wood, and S Yang, 2003: Assessing 1D atmospheric solar radiative transfer models: Interpretation and handling of unresolved clouds. Journal of Climate, 16(16), 2676-2699. Abstract PDF
The primary purpose of this study is to assess the performance of 1D solar radiative transfer codes that are used currently both for research and in weather and climate models. Emphasis is on interpretation and handling of unresolved clouds. Answers are sought to the following questions: (i) How well do 1D solar codes interpret and handle columns of information pertaining to partly cloudy atmospheres? (ii) Regardless of the adequacy of their assumptions about unresolved clouds, do 1D solar codes perform as intended?
One clear-sky and two plane-parallel, homogeneous (PPH) overcast cloud cases serve to elucidate 1D model differences due to varying treatments of gaseous transmittances, cloud optical properties, and basic radiative transfer. The remaining four cases involve 3D distributions of cloud water and water vapor as simulated by cloud-resolving models. Results for 25 1D codes, which included two line-by-line (LBL) models (clear and overcast only) and four 3D Monte Carlo (MC) photon transport algorithms, were submitted by 22 groups. Benchmark, domain-averaged irradiance profiles were computed by the MC codes. For the clear and overcast cases, all MC estimates of top-of-atmosphere albedo, atmospheric absorptance, and surface absorptance agree with one of the LBL codes to within ±2%. Most 1D codes underestimate atmospheric absorptance by typically 15-25 W m-2 at overhead sun for the standard tropical atmosphere regardless of clouds.
Depending on assumptions about unresolved clouds, the 1D codes were partitioned into four genres: (i) horizontal variability, (ii) exact overlap of PPH clouds, (iii) maximum/random overlap of PPH clouds, and (iv) random overlap of PPH clouds. A single MC code was used to establish conditional benchmarks applicable to each genre, and all MC codes were used to establish the full 3D benchmarks. There is a tendency for 1D codes to cluster near their respective conditional benchmarks, though intragenre variances typically exceed those for the clear and overcast cases. The majority of 1D codes fall into the extreme category of maximum/random overlap of PPH clouds and thus generally disagree with full 3D benchmark values. Given the fairly limited scope of these tests and the inability of any one code to perform extremely well for all cases begs the question that a paradigm shift is due for modeling 1D solar fluxes for cloudy atmospheres.
Freidenreich, Stuart, and V Ramaswamy, 1999: A new multiple-band solar radiative parameterization for general circulation models. Journal of Geophysical Research, 104(D24), 31,389-31,409. Abstract PDF
An extensive set of line-by-line plus doubling-adding reference computations for both clear and overcast skies has been utilized to develop, calibrate, and verify the accuracy of a new multiple-band solar parameterization, suitable for use in atmospheric general circulation models. In developing this parameterization the emphasis is placed on reproducing accurately the reference absorbed flux in clear and overcast atmospheres. In addition, a significantly improved representation of the reference stratospheric heating profile, in comparison with that derived from older, broadband solar parameterizations, has been attained primarily because of an improved parameterization of CO2 heating. The exponential-sum-fit technique is used to develop the paramterization of water vapor transmission in the main absorbing bands. An absorptivity approach is used to represent the heating contributions by CO2 and O2, and a spectral averaging of the continuum-like properties is used to represent the O3 heating. There are a total of 72 pseudomonochromatic intervals needed to do the radiative transfer problem in the vertically inhomogeneous atmosphere. The delta-Eddington method is used to solve for the reflection and transmission of the homogeneous layers, while the "adding" method is used to combine the layers. The single-scattering properties of the homogeneous layers can account for all types of scattering and absorbing constituents (molecules, drops, ice particles, and aerosols), given their respective single-scattering properties and mass concentrations. With respect to the reference computational results the clear-sky heating rates are generally accurate to within 10%, and the atmospheric absorbed flux is accurate to within 2%. An analysis is made of the factors contributing to the error in the parameterized cloud absorption in the near infrared. Derivation of the representative drop coalbedo for a band using the mean reflection for an infinitely thick cloud (thick-averaging technique) generally results in a better agreement with the reference cloud absorbed flux than that derived using the mean drop coalbedo (thin-averaging technique), except for high, optically thin water clouds. Further, partitioning the 2500 < v < 8200 cm-1 spectral region into several more bands than two (the minimum required) results in an improved representation of the cloud absorbed flux, with a modest increase in the shortwave radiation computational time. The cloud absorbed flux is accurate to within 10%, and the cloudy layer heating rates are accurate to within 15%, for water clouds, while larger errors can occur for ice clouds. The atmospheric absorbed, downward surface, and upward top-of-the-atmosphere fluxes are generally accurate to within 10%.
Corrections appear in: Journal of Geophysical Research, 105(D6), 7371.
Harshvardhan, W R., V Ramaswamy, Stuart Freidenreich, and M Batey, 1998: Spectral characteristics of solar near-infrared absorption in cloudy atmospheres. Journal of Geophysical Research, 103(D22), 28,793-28,799. Abstract PDF
Theoretical and experimentally derived estimates of the atmospheric absorption of solar energy in the presence of clouds have been reported to be at variance for quite a long time. A detailed set of near-monochromatic computations of the reflectance, transmittance, and absorptance of a standard midlatitude atmosphere with embedded water clouds is used to identify spectral features in the solar near-infrared that can be utilized to study this discrepancy. The results are framed in terms of the cloud radiative forcing both at the surface and at the top of the atmosphere, and it is shown that water vapor windows are the most sensitive to variations in cloud optical properties and cloud placement in the vertical. The ratio of the cloud radiative forcing at the surface to that at the top of the atmosphere, R, varies from near zero in the band centers at small wavenumbers for high clouds to ~1 in the band centers at larger wavenumbers for low clouds and to values in excess of 2 in the water vapor windows at small wavenumbers. The possibility of using measurements from space with the future Moderate Resolution Imaging Spectroradiometer (MODIS) and simultaneous surface measurements is discussed. It is also shown that horizontal inhomogeneities in the cloud layers do not alter appreciably the estimates of the R factor, but areal mean cloud absorption is lower for an inhomogeneous cloud having the same mean liquid water as the corresponding homogeneous cloud.
Ramaswamy, V, and Stuart Freidenreich, 1998: A high-spectral resolution study of the near-infrared solar flux disposition in clear and overcast atmospheres. Journal of Geophysical Research, 103(D18), 23,255-23,273. Abstract PDF
The sensitivity of the near-infrared spectral atmospheric and surface fluxes to the vertical location of clouds is investigated, including a study of factors (drop-size distribution, drop optical depth, solar zenith angle, cloud geometrical thickness, atmospheric profiles) which govern this dependence. Because of the effects of the above-cloud, in-cloud and below-cloud water vapor the atmospheric absorbed flux in each spectral band depends critically on the cloud location, with a high cloud resulting in lesser absorption and greater reflection than a low one having the same drop optical depth. The difference between a high and a low cloud forcing of atmospheric absorption increases with drop optical depth. For any optical depth, clouds with larger drops cause a greater forcing of the spectral atmospheric absorption than those with smaller ones, so high clouds can even cause an increase rather than a decrease of the atmospheric absorption relative to clear skies. In contrast, the spectral and total surface fluxes are relatively insensitive to cloud vertical location. Instead, they are determined by the drop characteristics, notably drop optical depth. This near-invariance characteristic is attributable to the fact that most of the insolation reaching the surface is in the weak water vapor spectral absorption regions; here drops dominate the radiative interactions and thus there is little dependence on cloud height. In addition, the overlap of the drop spectral features with the moderate-to-strong vapor absorption bands ensures that insolation in these regimes fails to reach the surface no matter where the cloud is located; instead, these bands contribute the most to atmospheric absorption. The near-invariant behavior of the spectral and total surface flux holds separately for a wide variety of conditions studied. As a consequence, the difference in reflection, between two columns containing clouds with the same optical depth but located at different altitudes, is approximately balanced in magnitude by the difference in the atmospheric absorption; this holds for every spectral interval whether it be a weak, moderate, or strong vapor/drop absorption band. It also follows that the net fluxes at the top and surface of overcast atmospheres do not have a general, unambiguous relationship; this is in sharp contrast to a linear relation between them in clear skies. However, under certain overcast conditions (e.g., specific vertical location of clouds and solar zenith angle), a simple linear relationship is plausible.
The spectral distribution of the incoming solar irradiance varies substantially from the top of the atmosphere to the surface. This occurs because of the selective spectral attenuation by the various atmospheric constituents. Using a line-by-line and doubling-adding solar radiative transfer model, we formulate a prescription that accounts for this variation in the spectral solar irradiance and thereby determine the appropriate spectral weights for low clouds. The results are sufficiently general with respect to cloud top heights ranging from 680 to 860 mbar, while the range of applicability in terms of the solar zenith position extends to sun angles less than 75 degrees. On the basis of the results here we suggest a reference cloud top height of 760 mbar and a reference zenith angle of 53 degrees. The error in the radiative quantities relative to the "benchmark" calculations is generally less than 5% in most of the spectral bands. As a simple application, it is found that the enhancement of cloud absorption in a two band cloud optical properties parameterization can be largely avoided by using this simple modification of the solar spectral irradiance incident at the top of low-lying clouds.
Ramaswamy, V, and Stuart Freidenreich, 1997: Absorption of solar radiation in overcast atmospheres In IRS '96: Current Problems in Atmospheric Radiation, Proceedings of the International Radiation Symposium, Fairbanks, Alaska, 19-24 August 1996. Hampton, Deepak Publishing, 125-127. Abstract
investigate the absorption of solar radiation in the near-infrared spectrum in overcast atmospheres containing water vapor and water drops. We compare the results with that for water vapor absorption only, and examine the quantitative dependence of the absorption on the drop optical depth and vertical location of the cloud.
Freidenreich, Stuart, and V Ramaswamy, 1995: Stratospheric temperature response to improved solar CO2 and H2O parameterizations. Journal of Geophysical Research, 100(D8), 16,721-16,725. Abstract PDF
A fixed-dynamical heating model is used to investigate the temperature changes in the stratosphere due to improved CO2 and H2O shortwave heating parameterizations. Besides being governed by the magnitude of the local heating, the temperature change in any layer due to the improved parameterizations is also dependent on the distribution of the solar heating in other stratospheric layers. This is a consequence of the longwave radiative exchange process, in which the temperature change in other layers, due to the imposed heating perturbations, leads to an exchange of longwave radiative energy with the layer in question, thus affecting its response. Thus the vertical profile of the heating rate becomes a significant factor in determining the stratospheric thermal profile. This investigation also confirms the sensitivity of the temperature response in the lower stratosphere to perturbations in the shortwave CO2 and H2O heating.
Freidenreich, Stuart, and V Ramaswamy, 1993: Solar radiation absorption by CO2, overlap with H2O, and a parameterization for general circulation models. Journal of Geophysical Research, 98(D4), 7255-7264. Abstract
Line-by-line (LBL) solar radiative 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 (delta v). Errors in absorption generally increase with the value of delta v 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.
Ramaswamy, V, and Stuart Freidenreich, 1992: A study of broadband parameterizations of the solar radiative interactions with water vapor and water drops. Journal of Geophysical Research, 97(D11), 11,487-11,512. Abstract PDF
Reference radiative transfer solutions in the near-infrared spectrum, which account for the spectral absorption characteristics of the water vapor molecule and the absorbing-scattering features of water drops, are employed to investigate and develop broadband treatments of solar water vapor absorption and cloud radiative effects. The conceptually simple and widely used Lacis-Hansen parameterization for solar water vapor absorption is modified so as to yield excellent agreement in the clear sky heating rates. The problem of single cloud decks over a nonreflecting surface is used to highlight the factors involved in the development of broadband overcast sky parameterizations. Three factors warrant considerable attention: (1) the manner in which the spectrally dependent drop single-scattering values are used to obtain the broadband cloud radiative properties, (2) the effect of the spectral attenuation by the vapor above the cloud on the determination of the broadband drop reflection and transmission, and (3) the broadband treatment of the spectrally dependent absorption due to drops and vapor inside the cloud. The solar flux convergence in clouds is very sensitive to all these considerations. Ignoring effect 2 tends to overestimate the cloud heating, particularly for low clouds, while a poor treatment of effect 3 tends to an underestimate. A new parameterization that accounts for the aforementioned considerations is accurate to within ~ 30% over a wide range of overcast sky conditions, including solar zenith angles and cloud characteristics (altitudes, drop models, optical depths, and geometrical thicknesses), with the largest inaccuracies occurring for geometrically thick, extended cloud systems containing large amounts of vapor. Broadband methods that treat improperly one or more of the above considerations can yield substantially higher errors (>35%) for some overcast sky conditions while having better agreements over limited portions of the parameter range. For example, a technique that considers effect 3 but ignores effect 2 yields a partial compensation of errors of opposite sign, such that the resulting inaccuracy for geometrically thick clouds can be less than 20%. In contrast to the marked sensitivity of the cloud heating rates, the maximum relative errors in the reflected flux at the top of the overcast atmosphere and the transmitted flux at the surface do not vary appreciably under the various broadband treatments; with the new parameterization, the relative errors are less than 15%. In applying the broadband concept to overcast atmospheres and multiple cloud decks, there are cases when the errors can be larger than stated above. Hence a general use of broadband methods in weather prediction and climate models (e.g., general circulation models) should be accompanied by a realization of the potential inaccuracies that can occur for specific overcast sky cases.
Ramaswamy, V, and Stuart Freidenreich, 1991: Solar radiative line-by-line determination of water vapor absorption and water cloud extinction in inhomogeneous atmospheres. Journal of Geophysical Research, 96(D5), 9133-9157. Abstract PDF
The complete available spectral features (line-by-line, or LBL) of the water vapor molecule in the solar spectrum and a precise treatment of particulate scattering are employed to obtain and analyze the solar radiative fluxes and heating rates in plane-parallel, vertically inhomogeneous model atmospheres containing vapor only, water cloud only, and vapor-plus-cloud present simultaneously. These studies are part of the Intercomparison of Radiation Codes in Climate Models (ICRCCM) project and constitute useful benchmark computations against which results from simpler radiation algorithms can be compared. The "exact" solution of the radiative transfer equation for cloudy atmospheres with the cloud in a single model layer consumes an exorbitant amount of computational resources (~ 100 hours on a Cyber 205). Two other techniques that are considerably more economical are also investigated. These techniques, too, are based on the LBL spectral features of the H2O molecule but consist of an approximation in either the vapor optical depth or in the multiple-scattering process. The technique involving the "binning" of the vapor optical depths yields extremely accurate fluxes and heating rates for both the vapor and vapor-plus-cloud cases; in particular, it is a practical alternative for obtaining benchmark solutions to the solar radiative transfer in overcast atmospheres (3.8 hours). In contrast, the multiple-scattering approximation technique does not yield precise results; however, considering its computational efficiency (0.5 hours), it offers a rapid means to obtain a first-order approximation of the spectrally integrated quantities. The analyses of the alternate techniques suggest their potential use for high spectral resolution sensitivity studies of the radiative effects due to various types of clouds.
