Upper tropospheric ice sensitivity to sulfate geoengineering

Abstract. Aside from the direct surface cooling sulfate geoengineering (SG) would produce, the investigation on possible side-effects of this method is still ongoing, as for instance on upper tropospheric cirrus cloudiness. Goal of the present study is to better understand the SG thermo-dynamical effects on the homogeneous freezing ice formation process. This is done by comparing SG model simulations against a RCP4.5 reference case: in one case the aerosol-driven surface cooling is included and coupled to the stratospheric warming resulting from aerosol absorption of longwave radiation. In a second SG perturbed case, surface temperatures are kept unchanged with respect to the reference RCP4.5 case. Surface cooling and lower stratospheric warming, together, tend to stabilize the atmosphere, thus decreasing turbulence and water vapor updraft velocities (−10 % in our modeling study). The net effect is an induced cirrus thinning, which may then produce a significant indirect negative radiative forcing (RF). This would go in the same direction as the direct effect of solar radiation scattering by the aerosols, thus influencing the amount of sulfur needed to counteract the positive RF due to greenhouse gases. In our study, given a 8 Tg-SO2 equatorial injection in the lower stratosphere, an all-sky net tropopause RF of −2.13 W/m2 is calculated, of which −0.96 W/m2 (45 %) from the indirect effect on cirrus thinning (7.5 % reduction in ice optical depth). When the surface cooling is ignored, the ice optical depth reduction is lowered to 5 %, with an all-sky net tropopause RF of −1.45 W/m2, of which −0.21 W/m2 (14 %) from cirrus thinning. Relatively to the clear-sky net tropopause RF due to SG aerosols (−2.06 W/m2), the cumulative effect of background clouds and cirrus thinning accounts for −0.07 W/m2, due to close compensation of large positive shortwave (+1.85 W/m2) and negative longwave adjustments (−1.92 W/m2). When the surface cooling is ignored, the net cloud adjustment becomes +0.71 W/m2, with the shortwave contribution (+1.97 W/m2) significantly larger in magnitude than the longwave one (−1.26 W/m2). This highlights the importance of including all dynamical feedbacks of SG aerosols.

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Upper tropospheric ice sensitivity to sulfate geoengineering

Atmospheric Chemistry and Physics ◽

10.5194/acp-18-14867-2018 ◽

2018 ◽

Vol 18 (20) ◽

pp. 14867-14887 ◽

Cited By ~ 10

Author(s):

Daniele Visioni ◽

Giovanni Pitari ◽

Glauco di Genova ◽

Simone Tilmes ◽

Irene Cionni

Keyword(s):

Optical Depth ◽

Radiative Forcing ◽

Near Infrared ◽

Cumulative Effect ◽

Stratospheric Warming ◽

Surface Cooling ◽

Reference Case ◽

Clear Sky ◽

Aerosol Absorption ◽

First Case

Abstract. Aside from the direct surface cooling that sulfate geoengineering (SG) would produce, investigations of the possible side effects of this method are still ongoing, such as the exploration of the effect that SG may have on upper tropospheric cirrus cloudiness. The goal of the present study is to better understand the SG thermodynamical effects on the freezing mechanisms leading to ice particle formation. This is undertaken by comparing SG model simulations against a Representative Concentration Pathway 4.5 (RCP4.5) reference case. In the first case, the aerosol-driven surface cooling is included and coupled to the stratospheric warming resulting from the aerosol absorption of terrestrial and solar near-infrared radiation. In a second SG perturbed case, the surface temperatures are kept unchanged with respect to the reference RCP4.5 case. When combined, surface cooling and lower stratospheric warming tend to stabilize the atmosphere, which decreases the turbulence and updraft velocities (−10 % in our modeling study). The net effect is an induced cirrus thinning, which may then produce a significant indirect negative radiative forcing (RF). This RF would go in the same direction as the direct effect of solar radiation scattering by aerosols, and would consequently influence the amount of sulfur needed to counteract the positive RF due to greenhouse gases. In our study, given an 8 Tg-SO2 yr−1 equatorial injection into the lower stratosphere, an all-sky net tropopause RF of −1.46 W m−2 is calculated, of which −0.3 W m−2 (20 %) is from the indirect effect on cirrus thinning (6 % reduction in ice optical depth). When surface cooling is ignored, the ice optical depth reduction is lowered to 3 %, with an all-sky net tropopause RF of −1.4 W m−2, of which −0.14 W m−2 (10 %) is from cirrus thinning. Relative to the clear-sky net tropopause RF due to SG aerosols (−2.1 W m−2), the cumulative effect of the background clouds and cirrus thinning accounts for +0.6 W m−2, due to the partial compensation of large positive shortwave (+1.6 W m−2) and negative longwave adjustments (−1.0 W m−2). When surface cooling is ignored, the net cloud adjustment becomes +0.8 W m−2, with the shortwave contribution (+1.5 W m−2) almost twice as much as that of the longwave (−0.7 W m−2). This highlights the importance of including all of the dynamical feedbacks of SG aerosols.

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Aerosol absorption and radiative forcing

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-7-7171-2007 ◽

2007 ◽

Vol 7 (3) ◽

pp. 7171-7233 ◽

Cited By ~ 7

Author(s):

P. Stier ◽

J. H. Seinfeld ◽

S. Kinne ◽

O. Boucher

Keyword(s):

Optical Depth ◽

Radiative Forcing ◽

Radiative Properties ◽

Refractive Indices ◽

Anthropogenic Aerosol ◽

Clear Sky ◽

Aerosol Absorption ◽

Long Wave ◽

Aerosol Radiative ◽

Cloud Radiative Properties

Abstract. We present a comprehensive examination of aerosol absorption with a focus on evaluating the sensitivity of the global distribution of aerosol absorption to key uncertainties in the process representation. For this purpose we extended the comprehensive aerosol-climate model ECHAM5-HAM by effective medium approximations for the calculation of aerosol effective refractive indices, updated black carbon refractive indices, new cloud radiative properties considering the effect of aerosol inclusions, as well as by modules for the calculation of long-wave aerosol radiative properties and instantaneous aerosol forcing. The evaluation of the simulated aerosol absorption optical depth with the AERONET sun-photometer network shows a good agreement in the large scale global patterns. On a regional basis it becomes evident that the update of the BC refractive indices to Bond and Bergstrom (2006) significantly improves the previous underestimation of the aerosol absorption optical depth. In the global annual-mean, absorption acts to reduce the short-wave anthropogenic aerosol top-of-atmosphere (TOA) radiative forcing clear-sky from –0.79 to –0.53 W m−2 (33%) and all-sky from –0.47 to –0.13 W m−2 (72%). Our results confirm that basic assumptions about the BC refractive index play a key role for aerosol absorption and radiative forcing. The effect of the usage of more accurate effective medium approximations is comparably small. We demonstrate that the diversity in the AeroCom land-surface albedo fields contributes to the uncertainty in the simulated anthropogenic aerosol radiative forcings: the usage of an upper versus lower bound of the AeroCom land albedos introduces a global annual-mean TOA forcing range of 0.19 W m−2 (36%) clear-sky and of 0.12 W m−2 (92%) all-sky. The consideration of black carbon inclusions on cloud radiative properties results in a small global annual-mean all-sky absorption of 0.05 W m−2 and a positive TOA forcing perturbation of 0.02 W m−2. The long-wave aerosol radiative effects are small for anthropogenic aerosols but become of relevance for the larger natural dust and sea-salt aerosols.

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Aerosol absorption and radiative forcing

Atmospheric Chemistry and Physics ◽

10.5194/acp-7-5237-2007 ◽

2007 ◽

Vol 7 (19) ◽

pp. 5237-5261 ◽

Cited By ~ 197

Author(s):

P. Stier ◽

J. H. Seinfeld ◽

S. Kinne ◽

O. Boucher

Keyword(s):

Optical Depth ◽

Radiative Forcing ◽

Radiative Properties ◽

Refractive Indices ◽

Anthropogenic Aerosol ◽

Clear Sky ◽

Aerosol Absorption ◽

Long Wave ◽

Aerosol Radiative ◽

Cloud Radiative Properties

Abstract. We present a comprehensive examination of aerosol absorption with a focus on evaluating the sensitivity of the global distribution of aerosol absorption to key uncertainties in the process representation. For this purpose we extended the comprehensive aerosol-climate model ECHAM5-HAM by effective medium approximations for the calculation of aerosol effective refractive indices, updated black carbon refractive indices, new cloud radiative properties considering the effect of aerosol inclusions, as well as by modules for the calculation of long-wave aerosol radiative properties and instantaneous aerosol forcing. The evaluation of the simulated aerosol absorption optical depth with the AERONET sun-photometer network shows a good agreement in the large scale global patterns. On a regional basis it becomes evident that the update of the BC refractive indices to Bond and Bergstrom (2006) significantly improves the previous underestimation of the aerosol absorption optical depth. In the global annual-mean, absorption acts to reduce the short-wave anthropogenic aerosol top-of-atmosphere (TOA) radiative forcing clear-sky from −0.79 to −0.53 W m−2 (33%) and all-sky from −0.47 to −0.13 W m−2 (72%). Our results confirm that basic assumptions about the BC refractive index play a key role for aerosol absorption and radiative forcing. The effect of the usage of more accurate effective medium approximations is comparably small. We demonstrate that the diversity in the AeroCom land-surface albedo fields contributes to the uncertainty in the simulated anthropogenic aerosol radiative forcings: the usage of an upper versus lower bound of the AeroCom land albedos introduces a global annual-mean TOA forcing range of 0.19 W m−2 (36%) clear-sky and of 0.12 W m−2 (92%) all-sky. The consideration of black carbon inclusions on cloud radiative properties results in a small global annual-mean all-sky absorption of 0.05 W m−2 and a positive TOA forcing perturbation of 0.02 W m−2. The long-wave aerosol radiative effects are small for anthropogenic aerosols but become of relevance for the larger natural dust and sea-salt aerosols.

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Influence of heterogeneous freezing on the microphysical and radiative properties of orographic cirrus clouds

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-18069-2013 ◽

2013 ◽

Vol 13 (7) ◽

pp. 18069-18112

Author(s):

H. Joos ◽

P. Spichtinger ◽

P. Reutter ◽

F. Fusina

Keyword(s):

Optical Depth ◽

Radiative Forcing ◽

Short Wave ◽

Time Of Day ◽

Cirrus Clouds ◽

Radiative Properties ◽

Critical Threshold ◽

Cloud Forcing ◽

Long Wave ◽

Homogeneous Freezing

Abstract. The influence of heterogeneous freezing on the microphysical and optical properties of orographic cirrus clouds has been simulated with the cloud resolving model EULAG. Idealized simulations with different concentrations of ice nuclei (IN) in a dynamically dominated regime with high vertical velocities have been performed. Furthermore the temperature under which the cloud forms as well as the critical supersaturation which is needed for the initiation of heterogenoues freezing have been varied. The short wave, long wave and net cloud forcing has been calculated under the assumption that the clouds form between 06:00 and 12:00 LT or between 12:00 and 18:00 LT, respectively. In general it can be seen that the onset of homogeneous freezing is shifted in time depending on the IN concentration as part of the available water vapor is depleted before the critical threshold for homogeneous freezing is reached. Although the high vertical velocities in an orographic gravity wave lead to a strong adiabatic cooling followed by high ice supersaturations, a small number concentration of IN in the order of 5 L−1 is already able to strongly decrease the simulated ice crystal number burden (ICNB), ice water path (IWP) and optical depth of the cloud. In general, the ICNB, IWP and optical depth strongly decrease when the IN concentrations are increased from 0 to 50 L−1. The absolute values of the short wave, long wave and net cloud forcing are also reduced with increasing IN concentrations. If a cloud produces a net warming or cooling depends on the IN concentration, the temperature and the time of day at which the cloud forms. The clouds that form between 06:00 and 12:00 LT are mainly cooling whereas the clouds with the same microphysical properties can lead to a warming when they form between 12:00 and 18:00 LT. In order to predict the radiative forcing of cirrus clouds it is therefore necessary to take the correct dynamical and thermodynamical processes as well as the possible existence and freezing threshold of heterogeneous INs into account not only for low vertical velocities but also for dynamically dominated regimes like orographic cirrus.

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Earth’s Global Mean Energy Flow System as the Solution of Four Radiative Transfer Constraint Equations

10.21203/rs.3.rs-818026/v1 ◽

2021 ◽

Author(s):

Miklos Zagoni

Keyword(s):

Optical Depth ◽

Energy Flow ◽

Flow System ◽

Lower Boundary ◽

Longwave Radiation ◽

Net Radiation ◽

Data Set ◽

Clear Sky ◽

Cloud Radiative Effect ◽

Global Mean

Abstract Schwarzschild’s equation in two-stream approximation provides a constraint on the net radiation at the lower boundary, as unequivocally connects it to fluxes at the upper boundary (top-of-atmosphere): surface net radiation, balanced by the non-radiative fluxes in radiative-convective equilibrium, is equal to half of the outgoing longwave radiation (OLR), independent of the optical depth. This paper controls this equation on satellite observations and proved to be valid within -2.19 W/m2 for clear-sky, in the annual global mean. The all-sky version of the relationship can easily be constructed, separating atmospheric radiation transfer from the longwave cloud radiative effect (LWCRE); it is valid with a difference of 2.76 W/m2 on the same data set. The equation provides a total (greenhouse) relationship as well, where the surface energy income is given as a function of OLR and the optical depth. This equation is controlled at a particular optical depth of two, and found to be verified with a difference of -2.72 W/m2 for clear-sky in the global mean. An all-sky form is created by including LWCRE, justified within 2.42 W/m2 on the same data set. The four equations together have a mean bias of 0.07 W/m2. This set of equations, using evident definitions between the all-sky and clear-sky fluxes has a solution for the involved components of the global mean energy flow system. Implications on the global mean clear-sky and cloudy energy flow systems are assessed, and the all-sky energy budget is constructed as a weighted sum of the clear and cloudy fluxes.

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Constraining direct aerosol radiative forcing using remote sensing and in-situ constraints

10.5194/egusphere-egu2020-17044 ◽

2020 ◽

Author(s):

Lucia Timea Deaconu ◽

Duncan Watson-Parris ◽

Philip Stier ◽

Lindsay Lee

Keyword(s):

Remote Sensing ◽

Black Carbon ◽

Optical Depth ◽

Radiative Forcing ◽

Removal Rates ◽

Aerosol Radiative Forcing ◽

Aerosol Absorption ◽

Direct Radiative Forcing ◽

Aerosol Radiative

Absorbing aerosols affect the climate system (radiative forcing, cloud formation, precipitation and more) by strongly absorbing solar radiation, particularly at ultraviolet and visible wavelengths. The environmental impacts of an absorbing aerosol layer are influenced by its single scattering albedo (SSA), the albedo of the underlying surface, and also by the atmospheric residence time and column concentration of the aerosols.Black-carbon (BC), the collective term used for strongly absorbing, carbonaceous aerosols, emitted by incomplete combustion of fossil fuel, biofuel and biomass, is a significant contributor to atmospheric absorption and probably a main-driver in inter-model differences and large uncertainties in estimating the aerosol radiative forcing due to aerosol-radiation interaction (RFari). Estimates of BC direct radiative forcing suggest a positive effect of +0.71 Wm-2 (Bond and Bergstrom (2006)) with large uncertainties [+0.08, +1.27] Wm-2. These uncertainties result from poor estimates of BC atmospheric burden (emissions and removal rates) and its radiative properties. The uncertainty in the burden is due to the uncertainty in emissions (7.5 [2, 29] Tg yr-1) and lifetime (removal rates). In comparison with the available observations, global climate models (GCMs) tend to under-predict absorption near source (e.g. at AERONET stations), and over-predict concentrations in remote regions (e.g. as measured by aircraft campaigns). This may be due to GCM&#8217;s weak emissions at the source, but longer lifetime of aerosols in the atmosphere.This study aims to address the parametric uncertainty of GCMs and constrain the direct radiative forcing using a perturbed parameter ensemble (PPE) and a collection of observations, from remote sensing to in-situ measurements. Total atmospheric aerosol extinction is quantified using satellite observations that provide aerosol optical depth (AOD), while the SSA is constrained by the use of high-temporal resolution aerosol absorption optical depth (AAOD) measured with AERONET sun-photometers (for near-source columnar information of aerosol absorption) and airborne black-carbon in-situ measurements collected and synthesised in the Global Aerosol Synthesis and Science Project (GASSP) (for properties of long-range transported aerosols). Measurements from the airborne campaigns ATOM and HIPPO are valuable for constraining aerosol absorption in remote areas, while CLARIFY and ORACLES, that were employed over Southeast Atlantic, are considered in our study for near source observations of biomass burning aerosols transported over the bright surface of stratocumulus clouds.Using the PPE to explore the uncertainties in the aerosol absorption as well as the dominant emission and removal processes, and by comparing with a variety of observations we have confidence to better constrain the aerosol direct radiative forcing.

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Contribution of ammonium nitrate to aerosol optical depth and direct radiative forcing by aerosols over East Asia

Atmospheric Chemistry and Physics ◽

10.5194/acp-14-2185-2014 ◽

2014 ◽

Vol 14 (4) ◽

pp. 2185-2201 ◽

Cited By ~ 30

Author(s):

R. S. Park ◽

S. Lee ◽

S.-K. Shin ◽

C. H. Song

Keyword(s):

East Asia ◽

Ammonium Nitrate ◽

Aerosol Optical Depth ◽

Optical Depth ◽

Radiative Forcing ◽

East Asian ◽

Annual Average ◽

Clear Sky ◽

Direct Radiative Forcing ◽

Sky Conditions

Abstract. This study focused on the contribution of ammonium nitrate (NH4NO3) to aerosol optical depth (AOD) and direct radiative forcing (DRF) by aerosols over an East Asian domain. In order to evaluate the contribution, chemistry-transport model (CTM)-estimated AOD was combined with satellite-retrieved AOD, utilizing a data assimilation technique, over East Asia for the entire year of 2006. Using the assimilated AOD and CTM-estimated aerosol optical properties, the DRF by aerosols was estimated over East Asia via a radiative transfer model (RTM). Both assimilated AOD and estimated DRF values showed relatively good agreements with AOD and DRF by aerosols from AERONET. Based on these results, the contributions of NH4NO3 to AOD and DRF by aerosols (ΦAOD and ΦDRF) were estimated for the four seasons of 2006 over East Asia. Both ΦAOD and ΦDRF showed seasonal variations over East Asia within the ranges between 4.7% (summer) and 31.3% (winter) and between 4.7% (summer) and 30.7% (winter), respectively, under clear-sky conditions, showing annual average contributions of 15.6% and 15.3%. Under all-sky conditions, ΦDRF varied between 3.6% (summer) and 24.5% (winter), showing annual average contribution of 12.1% over East Asia. These annual average contributions of NH4NO3 to AOD and DRF are almost comparable to the annual average mass fractions of NH4NO3 in PM2.5 and PM10 (17.0% and 14.0%, respectively). ΦAOD and ΦDRF were even larger in the locations where NH3 and NOx emission rates are strong, such as the central East China (CEC) region and Sichuan Basin. For example, under clear-sky conditions, both ΦAOD and ΦDRF over the CEC region range between 6.9% (summer) and 47.9% (winter) and between 6.7% (summer) and 47.5% (winter), respectively. Based on this analysis, it was concluded that both ΦAOD and ΦDRF cannot be ignored in East Asian air quality and radiative forcing studies, particularly during winter.

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Contribution of ammonium nitrate to aerosol optical depth and direct radiative forcing by aerosols over East Asia

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-19193-2013 ◽

2013 ◽

Vol 13 (7) ◽

pp. 19193-19235

Author(s):

R. S. Park ◽

S. J. Lee ◽

S.-K. Shin ◽

C. H. Song

Keyword(s):

East Asia ◽

Ammonium Nitrate ◽

Aerosol Optical Depth ◽

Optical Depth ◽

Radiative Forcing ◽

East Asian ◽

Annual Average ◽

Clear Sky ◽

Direct Radiative Forcing ◽

Sky Conditions

Abstract. This study focused on the contribution of ammonium nitrate (NH4NO3) to aerosol optical depth (AOD) and direct radiative forcing (DRF) by aerosols over an East Asian domain. In order to evaluate the contribution, CTM-estimated AOD was combined with satellite-retrieved AOD, utilizing a data assimilation technique, over East Asia for the entire year of 2006. Using the assimilated AOD and CTM-estimated aerosol optical properties, the DRF by aerosols was estimated over East Asia via a radiative transfer model (RTM). Both assimilated AOD and estimated DRF values showed relatively good agreements with AOD and DRF by aerosols from AERONET. Based on these results, the contributions of NH4NO3 to AOD and DRF by aerosols (ΦAOD and ΦDRF) were estimated for four seasons of 2006 over East Asia. Both ΦAOD and ΦDRF showed seasonal variations over East Asia within the ranges between 4.7% (summer) and 31.3% (winter) and between 4.7% (summer) and 30.7% (winter), respectively, under clear-sky conditions, showing annual average contributions of 15.6% and 15.3%. Under all-sky conditions, ΦDRF varied between 3.6% (summer) and 24.5% (winter), showing annual average contribution of 12.1% over East Asia. These annual average contributions of NH4NO3 to AOD and DRF are almost comparable to the annual average mass fractions of NH4NO3 to PM2.5 and PM10 (17.0% and 14.0%, respectively). ΦAOD and ΦDRF were even larger in the locations where NH3 and NOx emission rates are strong like the Central East China (CEC) region and Sichuan basin. For example, under clear-sky conditions, both ΦAOD and ΦDRF over the CEC region range between 6.9% (summer) and 47.9% (winter) and between 6.7% (summer) and 47.5% (winter), respectively. Based on this analysis, it was concluded that both ΦDRF and ΦDRF cannot be ignored in East Asian air quality and radiative forcing studies, particularly during winter.

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Radiative forcing by aerosols as derived from the AeroCom present-day and pre-industrial simulations

Atmospheric Chemistry and Physics ◽

10.5194/acp-6-5225-2006 ◽

2006 ◽

Vol 6 (12) ◽

pp. 5225-5246 ◽

Cited By ~ 515

Author(s):

M. Schulz ◽

C. Textor ◽

S. Kinne ◽

Y. Balkanski ◽

S. Bauer ◽

...

Keyword(s):

Optical Depth ◽

Mass Extinction ◽

Radiative Forcing ◽

Residence Times ◽

Atmospheric Forcing ◽

Annual Average ◽

Extinction Coefficients ◽

Clear Sky ◽

Direct Radiative Forcing ◽

The Difference

Abstract. Nine different global models with detailed aerosol modules have independently produced instantaneous direct radiative forcing due to anthropogenic aerosols. The anthropogenic impact is derived from the difference of two model simulations with prescribed aerosol emissions, one for present-day and one for pre-industrial conditions. The difference in the solar energy budget at the top of the atmosphere (ToA) yields a new harmonized estimate for the aerosol direct radiative forcing (RF) under all-sky conditions. On a global annual basis RF is −0.22 Wm−2, ranging from +0.04 to −0.41 Wm−2, with a standard deviation of ±0.16 Wm−2. Anthropogenic nitrate and dust are not included in this estimate. No model shows a significant positive all-sky RF. The corresponding clear-sky RF is −0.68 Wm−2. The cloud-sky RF was derived based on all-sky and clear-sky RF and modelled cloud cover. It was significantly different from zero and ranged between −0.16 and +0.34 Wm−2. A sensitivity analysis shows that the total aerosol RF is influenced by considerable diversity in simulated residence times, mass extinction coefficients and most importantly forcing efficiencies (forcing per unit optical depth). The clear-sky forcing efficiency (forcing per unit optical depth) has diversity comparable to that for the all-sky/ clear-sky forcing ratio. While the diversity in clear-sky forcing efficiency is impacted by factors such as aerosol absorption, size, and surface albedo, we can show that the all-sky/clear-sky forcing ratio is important because all-sky forcing estimates require proper representation of cloud fields and the correct relative altitude placement between absorbing aerosol and clouds. The analysis of the sulphate RF shows that long sulphate residence times are compensated by low mass extinction coefficients and vice versa. This is explained by more sulphate particle humidity growth and thus higher extinction in those models where short-lived sulphate is present at lower altitude and vice versa. Solar atmospheric forcing within the atmospheric column is estimated at +0.82±0.17 Wm−2. The local annual average maxima of atmospheric forcing exceed +5 Wm−2 confirming the regional character of aerosol impacts on climate. The annual average surface forcing is −1.02±0.23 Wm−2. With the current uncertainties in the modelling of the radiative forcing due to the direct aerosol effect we show here that an estimate from one model is not sufficient but a combination of several model estimates is necessary to provide a mean and to explore the uncertainty.

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Sir John Houghton (30 December 1931 — 15 April 2020) and radiation transfer

10.5194/egusphere-egu21-1 ◽

2021 ◽

Author(s):

Miklos Zagoni

Keyword(s):

Optical Depth ◽

Greenhouse Effect ◽

Radiative Forcing ◽

Radiation Transfer ◽

Lapse Rate ◽

Cloud Radiative Forcing ◽

Outgoing Radiation ◽

Clear Sky ◽

Ipcc Report ◽

Net Flux

IPCC announced that the WGI contribution to AR6 will be dedicated to the memory of leading climate scientist Sir John Houghton. Sir John died of complications from COVID-19 one year ago. He helped creating the IPCC in 1988, and served as Chair and Co-Chair of WGI from 1988 to 2002. In this presentation we focus on two aspects of his work: radiation transfer and cloud radiative forcing. &#8212; His book &#8220;The Physics of Atmospheres&#8221; (third edition, 2002) says: &#8220;The equation of radiative transfer through the slab, which includes both absorption and emission, is sometimes known as Schwarzschild&#8217;s equation&#8221; (Eq. 2.3, p.11). Introducing a constant &#1060; net flux (Eq. 2.5) being equal to the outgoing radiation, the black-body function B of the atmosphere is given as a function of &#1060; and the optical depth as B = &#1060;(&#967;* + 1)/2&#960; (Eq. 2.12). He says, &#8220;it is easy to show that there must be a temperature discontinuity at the lower boundary&#8221;: Bg &#8211; B0 = &#1060;/2&#960; (Eq. 2.13). Fig. 2.4 displays the net flux at the boundary as half of the outgoing radiation, independently of the optical depth. He notes: &#8220;Such a steep lapse rate will soon be destroyed by the process of convection&#8221;, and continues: &#8220;Combining (2.12) and (2.13) we find Bg = &#1060;(&#967;* + 2)/2&#960; &#8221; (Eq. 2.15, section 2.5 The greenhouse effect). We controlled Eq. (2.13) on 20 years of clear-sky CERES EBAF Ed4.1 global mean data and found it satisfied with a difference of -2.28 Wm-2. The validity of this equation casts constraint on the surface net radiation and on the corresponding non-radiative fluxes in the hydrological cycle by connecting them unequivocally to half of the outgoing longwave radiation. We constructed the all-sky version of the equation by separating atmospheric radiation transfer from longwave cloud effect, and found it valid within 2.84 Wm-2. We computed Eq. (2.15) with a special optical depth of &#967;* = 2 for clear-sky; it is justified with a difference of -2.88 Wm-2. We also created its all-sky version; the difference is 2.46 Wm-2. Altogether, the four equations are satisfied on 20-yr of CERES data with a mean bias of 0.035 Wm-2. We show that the four equations together determine a clear-sky and an all-sky greenhouse factor as 1/3 and 0.4. Data from Wild et al. (2018) and IPCC AR5 (2013) show g(clear) = (398 &#8211; 267)/398 = 0.33 and g(all) = (398 &#8211; 239)/398 = 0.3995. The IPCC reports predict an enhanced greenhouse effect from human emissions. According to the above arithmetic solutions, Earth&#8217;s observed greenhouse factors are equal to the theoretical ones without any deviation or enhancement. &#8212; The first IPCC report states that cloud radiative forcing is governed by cloud properties as cloud amount, reflectivity, vertical distribution and optical depth. Here we show that the TOA net CRF (= SWCRF + LWCRF) in equilibrium is equivalent to TOA net clear-sky imbalance, hence to determine its magnitude only clear-sky fluxes are needed.

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