Variability of the Daily-Mean Shortwave Cloud Radiative Forcing at the Surface at a Midlatitude Site in Southwestern Europe

Abstract The shortwave cloud radiative forcing is calculated from surface measurements taken in Évora from 2003 to 2010 with a multifilter rotating shadowband radiometer (MFRSR) and with an Eppley black and white pyranometer. A new approach to estimate the clear-sky irradiance based on radiative transfer calculations is also proposed. The daily-mean values of the cloud radiative forcing (absolute and normalized) as well as their monthly and seasonal variabilities are analyzed. The study shows greater variability of radiative forcing during springtime with respect to the other seasons. The mean daily cloudy periods have seasonal variation proportional to the seasonal variation of the cloud radiative forcing, with maximum values also occurring during springtime. The minimum values found for the daily-mean cloud radiative forcing are −139.5 and −198.4 W m−2 for MFRSR and Eppley data, respectively; the normalized values present about 40% of sample amplitude, both for MFRSR and Eppley. In addition, a quantitative relationship between the MFRSR and Eppley cloud radiative forcings applicable to other locations is proposed.

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Regional and monthly and clear-sky aerosol direct radiative effect (and forcing) derived from the GlobAEROSOL-AATSR satellite aerosol product

Atmospheric Chemistry and Physics ◽

10.5194/acp-13-393-2013 ◽

2013 ◽

Vol 13 (1) ◽

pp. 393-410 ◽

Cited By ~ 9

Author(s):

G. E. Thomas ◽

N. Chalmers ◽

B. Harris ◽

R. G. Grainger ◽

E. J. Highwood

Keyword(s):

Optical Properties ◽

Radiative Forcing ◽

Radiative Effect ◽

Mean Values ◽

Clear Sky ◽

Aerosol Loading ◽

Uncertainty Estimates ◽

Wide Range ◽

Aerosol Radiative ◽

Radiation Scheme

Abstract. Using the GlobAEROSOL-AATSR dataset, estimates of the instantaneous, clear-sky, direct aerosol radiative effect and radiative forcing have been produced for the year 2006. Aerosol Robotic Network sun-photometer measurements have been used to characterise the random and systematic error in the GlobAEROSOL product for 22 regions covering the globe. Representative aerosol properties for each region were derived from the results of a wide range of literature sources and, along with the de-biased GlobAEROSOL AODs, were used to drive an offline version of the Met Office unified model radiation scheme. In addition to the mean AOD, best-estimate run of the radiation scheme, a range of additional calculations were done to propagate uncertainty estimates in the AOD, optical properties, surface albedo and errors due to the temporal and spatial averaging of the AOD fields. This analysis produced monthly, regional estimates of the clear-sky aerosol radiative effect and its uncertainty, which were combined to produce annual, global mean values of (−6.7 ± 3.9) W m−2 at the top of atmosphere (TOA) and (−12 ± 6) W m−2 at the surface. These results were then used to give estimates of regional, clear-sky aerosol direct radiative forcing, using modelled pre-industrial AOD fields for the year 1750 calculated for the AEROCOM PRE experiment. However, as it was not possible to quantify the uncertainty in the pre-industrial aerosol loading, these figures can only be taken as indicative and their uncertainties as lower bounds on the likely errors. Although the uncertainty on aerosol radiative effect presented here is considerably larger than most previous estimates, the explicit inclusion of the major sources of error in the calculations suggest that they are closer to the true constraint on this figure from similar methodologies, and point to the need for more, improved estimates of both global aerosol loading and aerosol optical properties.

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Spectral and Broadband Longwave Downwelling Radiative Fluxes, Cloud Radiative Forcing, and Fractional Cloud Cover over the South Pole

Journal of Climate ◽

10.1175/jcli3525.1 ◽

2005 ◽

Vol 18 (20) ◽

pp. 4235-4252 ◽

Cited By ~ 32

Author(s):

Michael S. Town ◽

Von P. Walden ◽

Stephen G. Warren

Keyword(s):

Water Vapor ◽

Radiative Forcing ◽

Cloud Cover ◽

South Pole ◽

The South ◽

Near Surface ◽

Cloud Radiative Forcing ◽

Clear Sky ◽

Fractional Cloud ◽

Visual Observations

Abstract Annual cycles of downwelling broadband infrared radiative flux and spectral downwelling infrared flux were determined using data collected at the South Pole during 2001. Clear-sky conditions are identified by comparing radiance ratios of observed and simulated spectra. Clear-sky fluxes are in the range of 110–125 W m−2 during summer (December–January) and 60–80 W m−2 during winter (April–September). The variability is due to day-to-day variations in temperature, strength of the surface-based temperature inversion, atmospheric humidity, and the presence of “diamond dust” (near-surface ice crystals). The persistent presence of diamond dust under clear skies during the winter is evident in monthly averages of clear-sky radiance. About two-thirds of the clear-sky flux is due to water vapor, and one-third is due to CO2, both in summer and winter. The seasonal constancy of this approximately 2:1 ratio is investigated through radiative transfer modeling. Precipitable water vapor (PWV) amounts were calculated to investigate the H2O/CO2 flux ratio. Monthly mean PWV during 2001 varied from 1.6 mm during summer to 0.4 mm during winter. Earlier published estimates of PWV at the South Pole are similar for winter, but are 50% lower for summer. Possible reasons for low earlier estimates of summertime PWV are that they are based either on inaccurate hygristor technology or on an invalid assumption that the humidity was limited by saturation with respect to ice. The average fractional cloud cover derived from the spectral infrared data is consistent with visual observations in summer. However, the wintertime average is 0.3–0.5 greater than that obtained from visual observations. The annual mean of longwave downwelling cloud radiative forcing (LDCRF) for 2001 is about 23 W m−2 with no apparent seasonal cycle. This is about half that of the global mean LDCRF; the low value is attributed to the small optical depths and low temperatures of Antarctic clouds.

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Technical Note: Estimating aerosol effects on cloud radiative forcing

Atmospheric Chemistry and Physics ◽

10.5194/acp-13-9971-2013 ◽

2013 ◽

Vol 13 (19) ◽

pp. 9971-9974 ◽

Cited By ~ 105

Author(s):

S. J. Ghan

Keyword(s):

Radiative Forcing ◽

Technical Note ◽

Anthropogenic Aerosol ◽

Cloud Radiative Forcing ◽

Aerosol Radiative Forcing ◽

Clear Sky ◽

Aerosol Scattering ◽

The Difference ◽

Aerosol Effects ◽

The Impact

Abstract. Estimating anthropogenic aerosol effects on the planetary energy balance through the aerosol influence on clouds using the difference in cloud radiative forcing from simulations with and without anthropogenic emissions produces estimates that are positively biased. A more representative method is suggested using the difference in cloud radiative forcing calculated as a diagnostic with aerosol scattering and absorption neglected. The method also yields an aerosol radiative forcing decomposition that includes a term quantifying the impact of changes in surface albedo. The method requires only two additional diagnostic calculations: the whole-sky and clear-sky top-of-atmosphere radiative flux with aerosol scattering and absorption neglected.

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Regional and monthly and clear-sky aerosol direct radiative effect (and forcing) derived from the GlobAEROSOL-AATSR satellite aerosol product

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-12-18459-2012 ◽

2012 ◽

Vol 12 (7) ◽

pp. 18459-18497 ◽

Cited By ~ 3

Author(s):

G. E. Thomas ◽

N. Chalmers ◽

B. Harris ◽

R. G. Grainger ◽

E. J. Highwood

Keyword(s):

Optical Properties ◽

Radiative Forcing ◽

Radiative Effect ◽

Mean Values ◽

Clear Sky ◽

Aerosol Loading ◽

Uncertainty Estimates ◽

Wide Range ◽

Aerosol Radiative ◽

Radiation Scheme

Abstract. Using the GlobAEROSOL-AATSR dataset, estimates of the instantaneous, clear-sky, direct aerosol radiative effect and radiative forcing have been produced for the year 2006. Aerosol Robotic Network sun-photometer measurements have been used to characterise the random and systematic error in the GlobAEROSOL product for 22 regions covering the globe. Representative aerosol properties for each region have been derived from the results of a wide range of literature sources and, along with the de-biased GlobAEROSOL AODs, were used to drive an offline version of the Met Office unified model radiation scheme. In addition to the mean AOD, best-estimate run of the radiation scheme, a range of additional calculations were done to propagate uncertainty estimates in the AOD, optical properties, surface albedo and errors due to the temporal and spatial averaging of the AOD fields. This analysis produced monthly, regional estimates of the clear-sky aerosol radiative effect and its uncertainty, which produce annual, global mean values of (−6.7 ± 3.9) W m−2 at the top of atmosphere (TOA) and (−12 ± 6) W m−2 at the surface. These results were then used to produce estimates of regional, clear-sky aerosol direct radiative forcing, using modelled pre-industrial AOD fields for 1750 calculated for the AEROCOM PRE experiment. However, as it was not possible to quantify the uncertainty in the pre-industrial aerosol loading, these figures can only be taken as indicative and their uncertainties as lower bounds on the likely errors. Although the uncertainty on aerosol radiative effect presented here is considerably larger than most previous estimates, the explicit inclusion of the major sources of error in the calculations suggest that they are closer to the true constraint on this figure from similar methodologies, and point to the need for more, improved estimates of both global aerosol loading and aerosol optical properties.

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Impact of different definitions of clear-sky flux on the determination of longwave cloud radiative forcing: NICAM simulation results

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-10-22093-2010 ◽

2010 ◽

Vol 10 (9) ◽

pp. 22093-22107

Author(s):

B. J. Sohn ◽

T. Nakajima ◽

M. Satoh ◽

H.-S. Jang

Keyword(s):

Radiative Forcing ◽

Atmospheric Model ◽

Satellite Measurements ◽

Cloud Radiative Forcing ◽

Cloud Forcing ◽

Clear Sky ◽

Atmospheric State ◽

The Impact ◽

Mean State

Abstract. Using one month of the cloud-resolving Nonhydrostatic Icosahedral Atmospheric Model (NICAM) simulations, we examined the impact of different definitions of clear-sky flux on the determination of longwave cloud radiative forcing (CRF). Because the satellite-like cloud-free composite preferentially samples drier conditions relative to the all-sky mean state, the conventional clear-sky flux calculation using the all-sky mean state in the model may represent a more humid atmospheric state in comparison to the cloud-free state. The drier bias is evident for the cloud-free composite in the NICAM simulations, causing an overestimation of the longwave CRF by about 10% compared to the NICAM simulated longwave CRF. Overall, water vapor contributions of up to 10% of the total longwave CRF should be added to make the NICAM-generated cloud forcing comparable to the satellite measurements.

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Correlation of cloud radiative forcing with clear-sky flux: Seasonal variations of longwave components

International Journal of Climatology ◽

10.1002/(sici)1097-0088(199710)17:12<1369::aid-joc199>3.0.co;2-u ◽

1997 ◽

Vol 17 (12) ◽

pp. 1369-1379

Author(s):

Yongseung Kim

Keyword(s):

Seasonal Variations ◽

Radiative Forcing ◽

Cloud Radiative Forcing ◽

Clear Sky

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Impact of different definitions of clear-sky flux on the determination of longwave cloud radiative forcing: NICAM simulation results

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-11641-2010 ◽

2010 ◽

Vol 10 (23) ◽

pp. 11641-11646 ◽

Cited By ~ 25

Author(s):

B. J. Sohn ◽

T. Nakajima ◽

M. Satoh ◽

H.-S. Jang

Keyword(s):

Radiative Forcing ◽

Atmospheric Model ◽

Satellite Measurements ◽

Cloud Radiative Forcing ◽

Cloud Forcing ◽

Clear Sky ◽

Atmospheric State ◽

The Impact ◽

Mean State

Abstract. Using one month of the cloud-resolving Nonhydrostatic Icosahedral Atmospheric Model (NICAM) simulations, we examined the impact of different definitions of clear-sky flux on the determination of longwave cloud radiative forcing (CRF). Because the satellite-like cloud-free composite preferentially samples drier conditions relative to the all-sky mean state, the conventional clear-sky flux calculation using the all-sky mean state in the model may represent a more humid atmospheric state in comparison to the cloud-free state. The drier bias is evident for the cloud-free composite in the NICAM simulations, causing an overestimation of the longwave CRF by about 10% compared to the NICAM simulated longwave CRF. Overall, water vapor contributions of up to 10% of the total longwave CRF should be taken account for making model-generated cloud forcing comparable to the satellite measurements.

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Dry Bias in Satellite-Derived Clear-Sky Water Vapor and Its Contribution to Longwave Cloud Radiative Forcing

Journal of Climate ◽

10.1175/jcli3948.1 ◽

2006 ◽

Vol 19 (21) ◽

pp. 5570-5580 ◽

Cited By ~ 28

Author(s):

Byung-Ju Sohn ◽

Johannes Schmetz ◽

Rolf Stuhlmann ◽

Joo-Young Lee

Keyword(s):

Water Vapor ◽

Radiative Forcing ◽

Active Regions ◽

Tropical Regions ◽

Cloud Radiative Forcing ◽

Cloud Data ◽

Clear Sky ◽

Cloud Development ◽

Sampling Problem ◽

Sky Conditions

Abstract In this paper, the amount of satellite-derived longwave cloud radiative forcing (CRF) that is due to an increase in upper-tropospheric water vapor associated with the evolution from clear-sky to the observed all-sky conditions is assessed. This is important because the satellite-derived clear-sky outgoing radiative fluxes needed for the CRF determination are from cloud-free areas away from the cloudy regions in order to avoid cloud contamination of the clear-sky fluxes. However, avoidance of cloud contamination implies a sampling problem as the clear-sky fluxes represent an area drier than the hypothetical clear-sky humidity in cloudy regions. While this issue has been recognized in earlier works this study makes an attempt to quantitatively estimate the bias in the clear-sky longwave CRF. Water vapor amounts in the 200–500-mb layer corresponding to all-sky condition are derived from microwave measurements with the Special Sensor Microwave Temperature-2 Profiler and are used in combination with cloud data for determining the clear-sky water vapor distribution of that layer. The obtained water vapor information is then used to constrain the humidity profiles for calculating clear-sky longwave fluxes at the top of the atmosphere. It is shown that the clear-sky moisture bias in the upper troposphere can be up to 40%–50% drier over convectively active regions. Results indicate that up to 12 W m−2 corresponding to about 15% of the satellite-derived longwave CRF in tropical regions can be attributed to the water vapor changes associated with cloud development.

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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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