Andrew A. Lacis

A parameterization for the absorption of solar radiation in the earth's atmosphere

Abstract A method is described for rapidly computing the amount of solar energy absorbed at the earths surface and in the atmosphere as a function of altitude. The method is a parametric treatment, but the form of the solution and the coefficients involved are based on accurate multiple-scattering computations. In this treatment the absorption varies with the amount and type of clouds, the humidity, the zenith angle of the sun, and the albedo of the earths surface. Within the stratosphere the absorption also depends on the vertical distribution of ozone. This parameterization for solar radiation is being used in current versions of the global atmospheric circulation model developed at the Goddard Institute for Space Studies.

Calculation of radiative fluxes from the surface to top of atmosphere based on ISCCP and other global data sets: Refinements of the radiative transfer model and the input data

[1] We continue reconstructing Earth’s radiation budget from global observations in as much detail as possible to allow diagnosis of the effects of cloud (and surface and other atmospheric constituents) variations on it. This new study was undertaken to reduce the most noticeable systematic errors in our previous results (flux data set calculated mainly using International Satellite Cloud Climatology Project–C1 input data (ISCCP-FC)) by exploiting the availability of a more advanced NASA Goddard Institute for Space Studies (GISS) radiative transfer model and improved ISCCP cloud climatology and ancillary data sets. The most important changes are the introduction of a better treatment of ice clouds, revision of the aerosol climatology, accounting for diurnal variations of surface skin/air temperatures and the cloud-radiative effects on them, revision of the water vapor profiles used, and refinement of the land surface albedos and emissivities. We also extend our previous flux results, limited to the top of atmosphere (TOA) and surface (SRF), to also include three levels within the atmosphere, forming one integrated vertical atmospheric flux profile from SRF to TOA, inclusive, by combining a new climatology of cloud vertical structure with the ISCCP cloud product. Using the new radiative transfer model and new input data sets, we have produced an 18-year at 3-hour time steps, global at 280-km intervals, radiative flux profile data set (called ISCCP-FD) that provides full- and clear-sky, shortwave and longwave, upwelling and downwelling fluxes at five levels (SRF, 680 mbar, 440 mbar, 100 mbar, and TOA). Evaluation is still only possible for TOA and SRF fluxes: Comparisons of monthly, regional mean values from FD with Earth Radiation Budget Experiment, Clouds and the Earth’s Radiant Energy System and Baseline Surface Radiation Network values suggest that we have been able to reduce the overall uncertainties from 10–15 to 5–10 W/m 2 at TOA and from 20–25 to 10– 15 W/m 2 at SRF. Annual mean pressure-latitude cross sections of the cloud effects on atmospheric net radiative fluxes show that clouds shift the longwave cooling downward in the Intertropical Convergence Zone, acting to stabilize the tropical atmosphere while increasing the horizontal heating gradient forcing the Hadley circulation, and shift the longwave cooling upward in the midlatitude storm zones, acting to destabilize the baroclinic zones while decreasing the horizontal heating gradient there. INDEX TERMS: 1620 Global Change: Climate dynamics (3309); 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 3359 Meteorology and Atmospheric Dynamics: Radiative processes; KEYWORDS: Earth radiation budget, surface radiation budget (SRB), cloud vertical structure, ERBE, CERES, BSRN Citation: Zhang, Y., W. B. Rossow, A. A. Lacis, V. Oinas, and M. I. Mishchenko (2004), Calculation of radiative fluxes from the surface to top of atmosphere based on ISCCP and other global data sets: Refinements of the radiative transfer model and the input data, J. Geophys. Res., 109, D19105, doi:10.1029/2003JD004457.

A description of the correlated k distribution method for modeling nongray gaseous absorption, thermal emission, and multiple scattering in vertically inhomogeneous atmospheres

We describe a radiative transfer method for treating nongray gaseous absorption and thermal emission in vertically inhomogeneous multiple scattering atmospheres. We derive probability density distributions of absorption coefficient strength from line-by-line calculations to construct line-by-line and band model based k distributions. The monotonic ordering of absorption coefficient strengths in these k distributions implicitly preserves the monochromatic structure of the atmosphere at different pressure levels, thus simulating monochromatic spectral integration at a fraction of the line-by-line computing cost. The k distribution approach also permits accurate modeling of overlapping absorption by different atmospheric gases and accurate treatment of nongray absorption in multiple scattering media. To help verify the accuracy of the correlated k distribution method, we compare radiative cooling rates by atmospheric water vapor, CO2, and ozone against line-by-line calculations. The results show the correlated k distribution method is capable of achieving numerical accuracy to within 1% of cooling rates obtained with line-by-line calculations throughout the troposphere and most of the stratosphere.

Modeling of particle size distribution and its influence on the radiative properties of mineral dust aerosol

The radiative parameters of mineral aerosols are strongly dependent on particle size. Therefore explicit modeling of particle size distribution is needed to calculate the radiative effects and the climate impact of mineral dust. We describe a parameterization of the global mineral aerosol size distribution in a transport model using eight size classes between 0.1 and 10 μm. The model prescribes the initial size distribution using soil texture data and aerosol size measurements close to the ground. During transport, the size distribution changes as larger particles settle out faster than smaller particles. Results of Mie scattering calculations of radiative parameters (extinction efficiency, single scattering albedo, asymmetry parameter) of mineral dust are shown at wavelengths between 0.3 and 30 μm for effective particle radii between 0.1 and 10 μm. Also included are radiative properties (reflection, absorption, transmission) calculated for a dust optical thickness of 0.1. Preliminary studies with the Goddard Institute for Space Studies (GISS) general circulation model (GCM), using two particle size modes, show regional changes in radiative flux at the top of the atmosphere as large as +15 W m -2 at solar and +5 W m -2 at thermal wavelengths in the annual mean, indicating that dust forcing is an important factor in the global radiation budget.

Potential climate impact of Mount Pinatubo eruption

We use the GISS global climate model to make a preliminary estimate of Mount Pinatubos climate impact. Assuming the aerosol optical depth is nearly twice as great as for the 1982 El Chichon eruption, the model forecasts a dramatic but temporary break in recent global warming trends. The simulations indicate that Pinatubo occurred too late in the year to prevent 1991 from becoming one of the warmest years in instrumental records, but intense aerosol cooling is predicted to begin late in 1991 and to maximize late in 1992. The predicted cooling is sufficiently large that by mid 1992 it should even overwhelm global warming associated with an El Nino that appears to be developing, but the El Nino could shift the time of minimum global temperature into 1993. The model predicts a return to record warm levels in the later 1990s. We estimate the effect of the predicted global cooling on such practical matters as the severity of the coming Soviet winter and the dates of cherry blossoming next spring, and discuss caveats which must accompany these preliminary simulations.

Aerosol retrievals over the ocean by use of channels 1 and 2 AVHRR data: sensitivity analysis and preliminary results

We outline the methodology of interpreting channels 1 and 2 Advanced Very High Resolution Radiometer (AVHRR) radiance data over the oceans and describe a detailed analysis of the sensitivity of monthly averages of retrieved aerosol parameters to the assumptions made in different retrieval algorithms. The analysis is based on using real AVHRR data and exploiting accurate numerical techniques for computing single and multiple scattering and spectral absorption of light in the vertically inhomogeneous atmosphere-ocean system. We show that two-channel algorithms can be expected to provide significantly more accurate and less biased retrievals of the aerosol optical thickness than one-channel algorithms and that imperfect cloud screening and calibration uncertainties are by far the largest sources of errors in the retrieved aerosol parameters. Both underestimating and overestimating aerosol absorption as well as the potentially strong variability of the real part of the aerosol refractive index may lead to regional and/or seasonal biases in optical-thickness retrievals. The Angström exponent appears to be the aerosol size characteristic that is least sensitive to the choice of aerosol model and should be retrieved along with optical thickness as the second aerosol parameter.

Climate forcing by stratospheric aerosols

We illustrate how climate forcing by stratospheric aerosols depends on aerosol properties. The climate forcing is a function of aerosol size distribution, but the size dependence can be described well by a single parameter: the area-weighted mean radius, reff.If reff is greater than about 2 μm, the global average greenhouse effect of the aerosols exceeds the albedo effect, causing a surface heating. The aerosol climate forcing is less sensitive to other characteristics of the size distribution, the aerosol composition, and the altitude of the aerosols. Thus stratospheric aerosol forcing can be defined accurately from measurements of aerosol, extinction over a broad wavelength range.

Global atmospheric black carbon inferred from AERONET

AERONET, a network of well calibrated sunphotometers, provides data on aerosol optical depth and absorption optical depth at >250 sites around the world. The spectral range of AERONET allows discrimination between constituents that absorb most strongly in the UV region, such as soil dust and organic carbon, and the more ubiquitously absorbing black carbon (BC). AERONET locations, primarily continental, are not representative of the global mean, but they can be used to calibrate global aerosol climatologies produced by tracer transport models. We find that the amount of BC in current climatologies must be increased by a factor of 2–4 to yield best agreement with AERONET, in the approximation in which BC is externally mixed with other aerosols. The inferred climate forcing by BC, regardless of whether it is internally or externally mixed, is ≈1 W/m2, most of which is probably anthropogenic. This positive forcing (warming) by BC must substantially counterbalance cooling by anthropogenic reflective aerosols. Thus, especially if reflective aerosols such as sulfates are reduced, it is important to reduce BC to minimize global warming.

Long-term satellite record reveals likely recent aerosol trend.

Analysis of the long-term Global Aerosol Climatology Project data set reveals a likely decrease of the global optical thickness of tropospheric aerosols by as much as 0.03 during the period from 1991 to 2005. This recent trend mirrors the concurrent global increase in solar radiation fluxes at Earths surface and may have contributed to recent changes in surface climate.

Atmospheric CO2: principal control knob governing Earth's temperature.

Turning Up the Heat The physical effect of atmospheric carbon dioxide on Earths energy budget—that is, its “greenhouse effect”—has been understood for more than 100 years, but its role in climate warming is still not universally accepted. Lacis et al. (p. 356) conducted a set of idealized climate model experiments in which various greenhouse gases were added to or subtracted from the atmosphere in order to illustrate their roles in controlling the temperature of the air. The findings clearly show that carbon dioxide exerts the most control on Earths climate, and that its abundance determines how much water vapor the atmosphere contains, even though the radiative effect of the water vapor is greater than that of carbon dioxide itself. Carbon dioxide is the atmospheric greenhouse gas that exerts the most control on Earth’s climate. Ample physical evidence shows that carbon dioxide (CO2) is the single most important climate-relevant greenhouse gas in Earth’s atmosphere. This is because CO2, like ozone, N2O, CH4, and chlorofluorocarbons, does not condense and precipitate from the atmosphere at current climate temperatures, whereas water vapor can and does. Noncondensing greenhouse gases, which account for 25% of the total terrestrial greenhouse effect, thus serve to provide the stable temperature structure that sustains the current levels of atmospheric water vapor and clouds via feedback processes that account for the remaining 75% of the greenhouse effect. Without the radiative forcing supplied by CO2 and the other noncondensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate into an icebound Earth state.