Top Of The Atmosphere Forcing Research Articles

Long range transport of South and East Asian anthropogenic aerosols counteracting Arctic warming

The large-scale convection during the Asian summer monsoon plays an important role in the rapid transport of boundary layer aerosols into the Asian summer monsoon anticyclone. Here, using the state-of-the-art ECHAM6–HAMMOZ aerosol-chemistry-climate model, we show that these aerosols are further transported to the Arctic along isentropic surfaces by the Brewer-Dobson-Circulation (BDC) during the monsoon season. Our model simulations show that East and South Asian anthropogenic emissions contribute significantly to the aerosol transported to the Arctic, which causes a higher negative net aerosol radiative forcing at the surface (dimming) of −0.09 ± 0.02 Wm−2 and −0.07 ± 0.02 Wm−2, respectively. Over the Arctic, the East Asian anthropogenic aerosols that include large amounts of sulfate cause a seasonal mean net radiative forcing at the top of the atmosphere (TOA) of −0.003 ± 0.001Wm−2 and a surface cooling of −0.56 K while the black carbon dominated aerosol from South Asia shows a positive TOA forcing of +0.004 ± 0.001Wm−2 with an only minor surface cooling of −0.043 K. Overall, the long-range transport of South Asian aerosols results in a notably warming throughout the atmospheric column but minimal temperature response at the Arctic surface. Conversely, East Asian aerosols cool the troposphere and heat the lower stratosphere in the Arctic. The Asian aerosol thus plays an ambivalent role, with the East Asian sources in particular having the potential to counteract the rapid rise in Arctic temperatures and the associated melting of snow and ice.

The Global Patterns of Instantaneous CO2 Forcing at the Top of the Atmosphere and the Surface

Abstract The radiative forcing of carbon dioxide (CO2) at the top of the atmosphere (TOA) has a rich spatial structure and has implications for large-scale climate changes, such as poleward energy transport and tropical circulation change. Beyond the TOA, additional CO2 increases downwelling longwave at the surface, and this change in flux is the surface CO2 forcing. Here we thoroughly evaluate the spatiotemporal variation of the instantaneous, longwave CO2 radiative forcing at both the TOA and surface. The instantaneous forcing is calculated with a radiative transfer model using ERA5 reanalysis fields. Multivariate regression models show that the broadband forcing at the TOA and surface are well predicted by local temperatures, humidity, and cloud radiative effects. The difference between the TOA and surface forcing, the atmospheric forcing, can be either positive or negative and is mostly controlled by the column water vapor, with little explicit dependence on the surface temperature. The role of local variables on the TOA forcing is also assessed by partitioning the change in radiative flux to the component emitted by the surface versus that emitted by the atmosphere. In cold, dry regions, the surface and atmospheric contribution partially cancel out, leading to locally weak or even negative TOA forcing. In contrast, in the warm, moist regions, the surface and atmospheric components strengthen each other, resulting in overall larger TOA forcing. The relative contributions of surface and atmosphere to the TOA forcing depend on the optical thickness in the current climate, which in turn is controlled by the column water vapor.

Assessment of regional aerosol radiative effects under the SWAAMI campaign – Part 2: Clear-sky direct shortwave radiative forcing using multi-year assimilated data over the Indian subcontinent

Abstract. Clear-sky, direct shortwave aerosol radiative forcing (ARF) has been estimated over the Indian region, for the first time employing multi-year (2009–2013) gridded, assimilated aerosol products, as an important part of the South West Asian Aerosol Monsoon Interactions (SWAAMI) which is a joint Indo-UK research field campaign focused at understanding the variabilities in atmospheric aerosols and their interactions with the Indian summer monsoon. The aerosol datasets have been constructed following statistical assimilation of concurrent data from a dense network of ground-based observatories and multi-satellite products, as described in Part 1 of this two-part paper. The ARF, thus estimated, is assessed for its superiority or otherwise over other ARF estimates based on satellite-retrieved aerosol products, over the Indian region, by comparing the radiative fluxes (upward) at the top of the atmosphere (TOA) estimated using assimilated and satellite products with spatiotemporally matched radiative flux values provided by CERES (Clouds and Earth's Radiant Energy System) single-scan footprint (SSF) product. This clearly demonstrated improved accuracy of the forcing estimates using the assimilated vis-à-vis satellite-based aerosol datasets at regional, subregional and seasonal scales. The regional distribution of diurnally averaged ARF estimates has revealed (a) significant differences from similar estimates made using currently available satellite data, not only in terms of magnitude but also the sign of TOA forcing; (b) the largest magnitudes of surface cooling and atmospheric warming over the Indo-Gangetic Plain (IGP) and arid regions from north-western India; and (c) negative TOA forcing over most parts of the Indian region, except for three subregions – the IGP, north-western India and eastern parts of peninsular India where the TOA forcing changes to positive during pre-monsoon season. Aerosol-induced atmospheric warming rates, estimated using the assimilated data, demonstrate substantial spatial heterogeneities (∼0.2 to 2.0 K d−1) over the study domain with the IGP demonstrating relatively stronger atmospheric heating rates (∼0.6 to 2.0 K d−1). There exists a strong seasonality as well, with atmospheric warming being highest during pre-monsoon and lowest during winter seasons. It is to be noted that the present ARF estimates demonstrate substantially smaller uncertainties than their satellite counterparts, which is a natural consequence of reduced uncertainties in assimilated vis-à-vis satellite aerosol properties. The results demonstrate the potential application of the assimilated datasets and ARF estimates for improving accuracies of climate impact assessments at regional and subregional scales.

Aerosol radiative effects with MACv2

Abstract. Monthly global maps for aerosol properties of the Max Planck Aerosol Climatology version 2 (MACv2) are applied in an offline radiative transfer model to determine aerosol radiative effects. This model setup cannot address rapid adjustments by clouds, but current evidence suggests their contribution to be small when compared to the instantaneous radiative forcing. Global maps are presented to detail the regional and seasonal variability associated with (annual) global averages. Radiative effects caused by the aerosol presence (direct effects) and by aerosol modified clouds (indirect effects) are examined. Direct effects are determined for total aerosol, anthropogenic aerosol and extracted individual aerosol components. Indirect effects cover the impact of reduced cloud drop sizes by anthropogenic aerosol. Present-day global annual radiative effects for anthropogenic aerosol yield (1) a climate cooling of −1.0 W m−2 at the top of the atmosphere (TOA); (2) a surface net-flux reduction of −2.1 W m−2; and, by difference; (3) an atmospheric effect of +1.1 W m−2. This atmospheric solar heating is almost entirely a direct effect. On a global basis, indirect effects (−0.65 W m−2) dominate direct effects (−0.35 W m−2) for the present-day climate response at the TOA, whereas the present-day surface radiative budget is more strongly reduced by direct effects (−1.45 W m−2) than by indirect effects (−0.65 W m−2). Natural aerosols are on average less absorbing and larger in size. However, their stronger solar TOA cooling efficiency is offset by a non-negligible infrared (IR) greenhouse warming efficiency. In the sum the global average annual direct forcing efficiencies (per unit AOD) for natural and anthropogenic aerosol are similar: −12 W m−2 per unit AOD for all-sky conditions and −24 W m−2 per unit AOD for clear-sky conditions. The present-day direct TOA impact by all soot (BC) is +0.55 W m−2, when globally and annually averaged. Between +0.25 and +0.45 W m−2 of that can be attributed to anthropogenic sources, depending on assumptions for the preindustrial BC reference state. Similarly, the preindustrial fine-mode reference uncertainty has a strong influence not just on the direct effect but even more on the indirect effect. Present-day aerosol TOA forcing is estimated to stay within the −0.7 to −1.6 W m−2 range (with the best estimate at −1.0 W m−2). Calculations with scaled temporal changes to anthropogenic AOD from global modeling indicate that the global annual aerosol forcing has not changed much over the last decades, despite strong shifts in regional maxima for anthropogenic AOD. These regional shifts explain most solar insolation (brightening or dimming) trends that have been observed by ground-based radiation data.

Contrasting variation in aerosol optical properties during dust episodes in the Middle East and Southwest Asia: Model results and ground measurement

Dust storms deteriorated air quality over the Gulf Region, Iraq, Iran, and Pakistan during the last decade. The purpose of this study is to investigate the changes in aerosol optical and radiative properties during a dust episode over the various locations in the Middle East and Southwest Asia using data from the MODerate resolution Imaging Spectroradiometer (MODIS) and the Aerosol Robotic Network (AERONET) during March, 2012. Maximum aerosol optical depth (AOD) values were found to be 2.18, 1.30, 4.33 and 1.80 over Lahore, Kanpur, Kaust, and Mezaira, respectively. The Volume Size Distributions, Single Scattering Albedo, Refractive Index, and Asymmetry parameter indicated that coarse mode aerosols were predominant relative to fine mode aerosols during the dust event. The average shortwave aerosol radiative forcing (ARF) values at the earth’s surface were found to be -96±45 W m-2, -86±22 W m-2, -77±51 W m-2, and -75±40 W m-2, over Lahore, Kanpur, Kaust and Mezaira, respectively. Likewise, the averaged ARF values over Lahore, Kanpur, Kaust and Mezaira at the top of the atmosphere (TOA) were found to be -45±25 W m-2, -27±9 W m-2, -41±29 W m-2, and -75±40 W m-2, respectively. The large differences between surface and TOA forcing produced significant heating within the atmosphere.

Radiative Forcing Estimation of Aerosols at an Urban Site near the Thar Desert Using Ground-Based Remote Sensing Measurements

ABSTRACT The focus of present study is to quantify the radiation budget of aerosols over Jaipur (Northwestern, India) from 2011 to 2015. The Aerosol radiative forcing (ARF) has been determined for shortwave spectrum (0.3–3.0 µm) individually for the top of the atmosphere (TOA), bottom of the atmosphere (BOA) and within the atmosphere (ATM) over study region. Santa Barbara DISORT Atmospheric Radiative Transfer model (SBDART) is used to simulate the aerosols radiative effect. The inter-annual monthly average of ARF at TOA during 2011–2015 is found between –11.40 to –5.60 W m–2, while the ARF at BOA is found to be between –32.2 to –22.49 W m–2. Likewise, the ARF within the atmosphere (ATM) comes between 14.04 to 22.47 W m–2 over Jaipur. The SBDART model is run discretely for Dust period (DSP) and non-Dust Period (NDP) during the year 2012 to inspect the change in ARF during extreme events over the Jaipur site. During DSP, the net TOA and BOA forcing are found in the range –20.71 to –16.81 W m–2 and –45.15 to –39.6 W m–2, respectively, and net ATM forcing varies in the range 22.7 to 24.4 W m–2. For the NDP, the corresponding value varies in the range –10.1 to –6.6 W m–2 and –23.6 to –22.3 W m–2. The net ATM forcing during NDP is between 12.2 to 17.05 W m–2. The value of BOA increases more than ~67% during DSP than NDP. The more increase (–ve) in surface forcing represents the cooling of the surface during DSP. The results depict that dust over Jaipur in the vicinity of the Thar Desert is scattering in nature with high value (> 0.95) of SSA. The scattering is mostly high during summer and low in winter.

A new method for evaluating the impact of vertical distribution on aerosol radiative forcing in general circulation models

Abstract. The quantification and understanding of direct aerosol forcing is essential in the study of climate. One of the main issues that makes its quantification difficult is the lack of a complete understanding of the role of the vertical distribution of aerosols and clouds. This work aims at reducing the uncertainty of aerosol top-of-the-atmosphere (TOA) forcing due to the vertical superposition of several short-lived atmospheric components, in particular different aerosol species and clouds. We propose a method to quantify the contribution of different parts of the atmospheric column to the TOA forcing as well as to evaluate the contribution to model differences that is exclusively due to different spatial distributions of aerosols and clouds. We investigate the contribution of aerosol above, below and in clouds by using added diagnostics in the aerosol–climate model LMDz. We also compute the difference between the TOA forcing of the ensemble of the aerosols and the sum of the forcings from individual species in clear sky. This difference is found to be moderate for the global average (14%) but can reach high values regionally (up to 100%). Nonlinear effects are even more important when superposing aerosols and clouds. Four forcing computations are performed: one where the full aerosol 3-D distribution is used, and then three where aerosols are confined to regions above, inside and below clouds, respectively. We find that the TOA forcing of aerosols depends crucially on the presence of clouds and on their position relative to that of the aerosol, in particular for black carbon (BC). We observe a strong enhancement of the TOA forcing of BC above clouds, attenuation for BC below clouds, and a moderate enhancement when BC is found within clouds. BC above clouds accounts for only about 30% of the total BC optical depth but for 55% of the forcing, while forcing efficiency increases by a factor of 7.5 when passing from below to above clouds. The different behaviour of forcing nonlinearities for these three components of the atmospheric column encouraged us to develop the method for application to inter-model variability studies by reading 3-D aerosol and cloud fields from different general circulation models (GCMs) into the same model. We apply the method to the comparison of forcing due to the aerosols and clouds distributions of the general circulation models LMDz and SPRINTARS. The different amount of BC above but also within clouds is revealed to play a major role on the differences of cloudy-sky forcings between the two models, which can exceed 100% regionally.

Adjustment to Radiative Forcing in a Simple Coupled Ocean–Atmosphere Model

Abstract This study calculates the adjustment to radiative forcing in a simple model of a mixed layer ocean coupled to the overlying atmosphere. One application of the model is to calculate how dust aerosols perturb the temperature of the atmosphere and ocean, which in turn influence tropical cyclone development. Forcing at the top of the atmosphere (TOA) is the primary control upon both the atmospheric and ocean temperature anomalies, both at equilibrium and during most of the adjustment to the forcing. Ocean temperature is directly influenced by forcing only at the surface, but is indirectly related to forcing at TOA due to heat exchange with the atmosphere. Within a few days of the forcing onset, the atmospheric temperature adjusts to heating within the aerosol layer, reducing the net transfer of heat from the ocean to the atmosphere. For realistic levels of aerosol radiative forcing, the perturbed net surface heating strongly opposes forcing at the surface. This means that surface forcing dominates the ocean response only within the first few days following a dust outbreak, before the atmosphere has responded. This suggests that, to calculate the effect of dust upon the ocean temperature, the atmospheric adjustment must be taken into account explicitly and forcing at TOA must be considered in addition to the surface forcing. The importance of TOA forcing should be investigated in a model where vertical and lateral mixing of heat are calculated with fewer assumptions than in the simple model presented here. Nonetheless, the fundamental influence of TOA forcing appears to be only weakly sensitive to the model assumptions.

Radiative effects of aerosols over Indo-Gangetic plain: environmental (urban vs. rural) and seasonal variations

Aerosol radiative effects over two environmentally distinct locations, Kanpur (urban site) and Gandhi College (rural location) in the Indo-Gangetic plain (IGP), a regional aerosol hot spot, utilizing the measured optical and physical characteristics of aerosols, an aerosol optical properties model and a radiative transfer model, are examined. Shortwave aerosol radiative forcing (ARF) at the top of the atmosphere (TOA) is < -12 W m( - 2) over Kanpur and Gandhi College. ARF at the surface is ≥ -30 W m( - 2). Atmospheric warming is maximum during premonsoon (>30 W m( - 2)). Shortwave atmospheric heating due to aerosols is >0.4 K/day over IGP and peaks during premonsoon at >0.6 K/day due to lower single scattering albedo (SSA) and higher surface albedo. TOA forcing is always less negative over Kanpur when compared to Gandhi College due to lower surface albedo except in postmonsoon owing to higher SSA. This happens as TOA forcing depends on SSA and surface albedo in addition to aerosol optical depth. The magnitude of longwave forcing and atmospheric cooling in an absolute sense is significantly small and contributes only about 20% or less to the net (shortwave + longwave) forcing. Aerosol radiative effects over these two locations, despite differences in aerosol characteristics, are similar, thus confirming that aerosols and their radiative influence get transported due to circulation. ARF over Kanpur and Gandhi College is an order of magnitude higher when compared to greenhouse gas forcing. A large reduction in surface reaching solar irradiance accompanied by large atmospheric warming can have implications on precipitation and hydrological cycle, and these aerosol radiative effects should be included while performing regional-scale aerosol climate assessments.

A long-term study of aerosol modulation of atmospheric and surface solar heating over Pune, India

ABSTRACTImplications of aerosol characteristics, observed during a five-year (2004–2009) period over Pune (a tropical urban location), to short-wave radiation budget are reported. A discrete ordinate radiative transfer (DISORT) model with a code, namely, Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART), has been used to carry out the radiative transfer computations. The validity of the method is demonstrated using independent ground-based remote sensing observations. Uncertainties in the estimates are also quantified. Clear-sky forcing reveals the points that include (1) Large negative bottom-of-the-atmosphere (BOA) forcing (more than−30 Wm−2) in all the months with peaks during October, December and March when the surface forcing exceeds~−40 Wm−2, and (2) Surface forcing values are higher for pre-monsoon months, while they are comparable for winter and post-monsoon months. The top-of-the-atmosphere (TOA) forcing is found to be negative during all the seasons. Large differences between TOA and BOA forcing during pre-monsoon, winter and post-monsoon indicate large absorption of radiant energy (~30 Wm−2) within the atmosphere during these seasons, thus increasing atmospheric heating by~1 K/d. These values imply that aerosols have considerable impact on the atmosphere–surface system by causing substantial warming/cooling at the atmosphere/surface. This persistent trend in strong atmospheric absorption is likely to alter atmospheric thermodynamic conditions and thus affects circulation considerably.

A 1-year remote sensing study of radiative effects of aerosol and clouds over the NE Mediterranean

Direct aerosol radiative forcing (ARF) and cloud radiative forcing (CRF) over the Aegean Sea are investigated using 1 year of almost continuous observations of aerosol optical properties, cloud data and Clouds and the Earth's Radiant Energy System (CERES) satellite short-wave (SW) and long-wave (LW) fluxes during 2005–2006. Aerosol optical depth (AOD) values from moderate resolution imaging spectroradiometer (MODIS) over the Aegean Sea are found to be high during summer and spring months. This period of the year, the air masses come predominantly from the eastern and northeastern part of Europe. The monthly mean ARF at the top of the atmosphere (TOA) over the Aegean Sea resulted in net cooling and ranged from −6.0 to −15.8 W m−2, within the range of results from previous studies. The presence of fine particles greatly influenced the ARF. The CRF was also characterized by net cooling and ranged from −23 to −84 W m−2 and appeared to be driven primarily by changes in cloud fraction. An apparent aerosol weekly cycle with lower values during the weekend as well as a cloud fraction weekly cycle with lower values during the weekend is observed over the Aegean Sea, which, if confirmed, could result in net TOA forcing of around −20 W m−2.

Aerosol radiative effects over an urban location and a remote site in western India: Seasonal variability

Seasonal variability in radiative effects due to aerosols in the shortwave and longwave regions is studied utilizing measured aerosol optical properties and models over two distinct environments, namely, Ahmedabad (urban, industrialised location) and Gurushikhar (high altitude remote site) in western India. Top of the atmosphere (TOA) forcing in shortwave varies from positive to negative over Ahmedabad, while over Gurushikhar it is negative throughout the year because of lower surface reflectance and higher single scattering albedo (SSA). Aerosol radiative forcing at the surface (SFC) is >−30 Wm −2 over Ahmedabad. In Gurushikhar SFC forcing increases from winter (−7 Wm −2 ) to premonsoon (−14 Wm −2 ), which is attributed to the increase in the amount of dust aerosol loading. Aerosol radiative forcing in the atmosphere (ATM) peaks during postmonsoon with a value of 54 Wm −2 over Ahmedabad, while ATM warming over Gurushikhar peaks during premonsoon (9 Wm −2 ). Shortwave heating rate is >0.9 K d −1 over Ahmedabad while it is an order of magnitude lower over Gurushikhar. Aerosols produce a positive TOA and SFC forcing in the longwave and cool the atmosphere. Anthropogenic influence is evident from the aerosol radiative effects obtained over Ahmedabad and Gurushikhar. The abundance of fine (coarse) mode aerosols over Ahmedabad (Gurushikhar) results in higher (lower) aerosol optical depth, lower (higher) SSA, and higher (lower) aerosol radiative forcing. Owing to the dissimilar nature of shortwave and longwave radiative effects of aerosols, when coarse mode aerosols are abundant over a region, longwave radiative effects can balance shortwave atmospheric warming produced by aerosols. ► Top of the atmosphere forcing varies from positive to negative over Ahmedabad while it is negative over Gurushikhar. ► Abundance of fine mode absorbing aerosols give rise to higher atmospheric warming in Ahmedabad. ► Shortwave heating rate is an order of magnitude lower over Gurushikhar than Ahmedabad. ► Longwave cooling balances the shortwave atmospheric warming when coarse mode aerosols are abundant.

On realistic size equivalence and shape of spheroidal Saharan mineral dust particles applied in solar and thermal radiative transfer calculations

Abstract. Realistic size equivalence and shape of Saharan mineral dust particles are derived from in-situ particle, lidar and sun photometer measurements during SAMUM-1 in Morocco (19 May 2006), dealing with measured size- and altitude-resolved axis ratio distributions of assumed spheroidal model particles. The data were applied in optical property, radiative effect, forcing and heating effect simulations to quantify the realistic impact of particle non-sphericity. It turned out that volume-to-surface equivalent spheroids with prolate shape are most realistic: particle non-sphericity only slightly affects single scattering albedo and asymmetry parameter but may enhance extinction coefficient by up to 10 %. At the bottom of the atmosphere (BOA) the Saharan mineral dust always leads to a loss of solar radiation, while the sign of the forcing at the top of the atmosphere (TOA) depends on surface albedo: solar cooling/warming over a mean ocean/land surface. In the thermal spectral range the dust inhibits the emission of radiation to space and warms the BOA. The most realistic case of particle non-sphericity causes changes of total (solar plus thermal) forcing by 55/5 % at the TOA over ocean/land and 15 % at the BOA over both land and ocean and enhances total radiative heating within the dust plume by up to 20 %. Large dust particles significantly contribute to all the radiative effects reported. They strongly enhance the absorbing properties and forward scattering in the solar and increase predominantly, e.g., the total TOA forcing of the dust over land.

Direct climatic effect of dust aerosol in the NCAR Community Atmosphere Model Version 3 (CAM3)

Direct climate responses to dust shortwave and longwave radiative forcing (RF) are studied using the NCAR Community Atmosphere Model Version 3 (CAM3). The simulated RF at the top of the atmosphere (TOA) is −0.45 W m−2 in the solar spectrum and +0.09 W m−2 in the thermal spectrum on a global average. The magnitude of surface RF is larger than the TOA forcing, with global mean shortwave forcing of −1.76 W m−2 and longwave forcing of +0.31 W m−2. As a result, dust aerosol causes the absorption of 1.1 W m−2 in the atmosphere. The RF of dust aerosol is predicted to lead to a surface cooling of 0.5 K over the Sahara Desert and Arabian Peninsula. In the meantime, the upper troposphere is predicted to become warmer because of the absorption by dust. These changes in temperature lead to a more stable atmosphere, which results in increases in surface humidity. The upward sensible and latent heat fluxes at the surface are reduced, largely balancing the surface energy loss caused by the backscattering and absorption of dust aerosol. Precipitation is predicted to decrease moderately on a global scale.

Distribution and direct radiative forcing of carbonaceous and sulfate aerosols in an interactive size‐resolving aerosol–climate model

A multimode, two‐moment aerosol model has been incorporated in the NCAR CAM3 to develop an interactive aerosol–climate model and to study the impact of anthropogenic aerosols on the global climate system. Currently, seven aerosol modes, namely three for external sulfate and one each for external black carbon (BC), external organic carbon (OC), sulfate/BC mixture (MBS; with BC core coated by sulfate shell), and sulfate/OC mixture (MOS; a uniform mixture of OC and sulfate) are included in the model. Both mass and number concentrations of each aerosol mode, as well as the mass of carbonaceous species in the mixed modes, are predicted by the model so that the chemical, physical, and radiative processes of various aerosols can be formulated depending on aerosol's size, chemical composition, and mixing state. Comparisons of modeled surface and vertical aerosol concentrations, as well as the optical depth of aerosols with available observations and previous model estimates, are in general agreement. However, some discrepancies do exist, likely caused by the coarse model resolution or the constant rates of anthropogenic emissions used to test the model. Comparing to the widely used mass‐only method with prescribed geometric size of particles (one‐moment scheme), the use of prognostic size distributions of aerosols based on a two‐moment scheme in our model leads to a significant reduction in optical depth and thus the radiative forcing at the top of the atmosphere (TOA) of particularly external sulfate aerosols. The inclusion of two types of mixed aerosols alters the mass partitioning of carbonaceous and sulfate aerosol constituents: about 35.5%, 48.5%, and 32.2% of BC, OC, and sulfate mass, respectively, are found in the mixed aerosols. This also brings in competing effects in aerosol radiative forcing including a reduction in atmospheric abundance of BC and OC due to the shorter lifetime of internal mixtures (cooling), a mass loss of external sulfate to mixtures (warming), and an enhancement in atmospheric heating per BC mass due to the stronger absorption extinction of the MBS than external BC (warming). The combined result of including a prognostic size distribution and the mixed aerosols in the model is a much smaller total negative TOA forcing (−0.12 W m−2) of all carbonaceous and sulfate aerosol compounds compared to the cases using one‐moment scheme either excluding or including internal mixtures (−0.42 and −0.71 W m−2, respectively). In addition, the global mean all‐sky TOA direct forcing of aerosols is significantly more positive than the clear‐sky value due to the existence of low clouds beneath the absorbing (external BC and MBS) aerosol layer, particularly over a dark surface. An emission reduction of about 44% for BC and 38% of primary OC is found to effectively change the TOA radiative forcing of the entire aerosol family by −0.14 W m−2 for clear‐sky and −0.29 W m−2 for all‐sky.

Fog-induced variations in aerosol optical and physical properties over the Indo-Gangetic Basin and impact to aerosol radiative forcing

Abstract. A detailed study on the changes in aerosol physical and optical properties during fog events were made in December 2004 at Hissar (29.13° N, 75.70° E), a city located in the Indo-Gangetic basin. The visible aerosol optical depth was relatively low (0.3) during the initial days, which, however, increased (0.86) as the month progressed. The increasing aerosol amount, the decreasing surface temperature and a higher relative humidity condition were found favoring the formation of fog. The fog event is also found to alter the aerosol size distribution. An increase in the number concentration of the nucleation mode (radius&lt;0.1 μm) particles, along with a decrease in the mode radius showed the formation of freshly nucleated aerosols. In the case of accumulation mode (0.1 μm&lt;radius&lt;1.0 μm) an increase in the mode radius was observed showing the hygroscopic and coagulation growth of particles. The observed aerosol optical depth spectra are model fitted to infer the aerosol components which are further used to compute the aerosol radiative forcing. The top of the atmosphere forcing is found to increase during foggy days due to large backscattering of radiation back to space. It is also shown that during foggy days, as the day progresses the RH value decreases, which reduces the forcing value while the increasing solar elevation increases the forcing value. Thus the fog event which prolongs longer into the daytime has a stronger effect on the diurnally averaged aerosol radiative forcing than those events which are confined only to the early morning hours.

Sensitivity of climate forcing and response to dust optical properties in an idealized model

An idealized global climate model is used to explore the response of the climate to a wide range of dust radiative properties and dust layer heights. The top‐of‐the‐atmosphere (TOA) shortwave forcing becomes more negative as the broadband shortwave single scattering albedo increases and the broadband shortwave asymmetry parameter decreases, but the sensitivity is highly dependent on the location of the dust layer with respect to clouds. The longwave TOA forcing is most affected by the height of the dust layer. The net TOA forcing is most sensitive to the shortwave single scattering albedo and shortwave asymmetry parameter. The surface and atmospheric temperature responses are approximately linear with respect to the TOA forcing, as opposed to the surface or atmospheric forcings. Thus the TOA forcing can be used to estimate both the surface and atmospheric temperature responses to dust. The corresponding changes in latent and sensible heat fluxes are essential for the close relationship of the surface temperature response to the TOA forcing. Estimating the hydrological cycle response requires knowledge of the vertical distribution of dust with respect to clouds or other reflective particles. The sensitivity of the latent heat flux to variations in the shortwave single scattering albedo changes sign with dust height. The latent heat flux change becomes less negative as the shortwave single scattering albedo increases if the dust layer is below clouds. However, when the dust is above clouds, the latent heat response becomes more negative as the single scattering albedo increases.

Direct radiative effect of mineral dust and volcanic aerosols in a simple aerosol climate model

Airborne mineral dust can influence the climate by altering the radiative properties of the atmosphere, but the magnitude of the effect is uncertain. An idealized global model is developed to study the dust‐climate system. The model determines the dust longwave and shortwave direct radiative forcing, as well as the resulting temperature changes, based on the specified dust distribution, height, and optical properties. Comparisons with observations and general circulation results indicate that the model produces realistic results for the present‐day dust distribution as well as for volcanic aerosols. Although the model includes many simplifications, it can still provide insight into dust‐climate system behavior. Recent observations suggest that dust may absorb less solar radiation than previously thought. Experiments with the model suggest that previous studies which used more absorbing dust may be underestimating the effect of dust. Increasing the solar single scattering albedo value from 0.85 to 0.97, corresponding to recent measurements, more than doubles the modeled global average top‐of‐the‐atmosphere (TOA) shortwave direct forcing for the present‐day dust distribution, while the surface shortwave forcing is halved. The corresponding temperature decreases are larger for the larger single scattering albedo, and the latent and sensible heat fluxes decreases are smaller. The dust forcing and climate response are approximately linear with respect to optical depth. However, the relationship depends on the relative magnitudes of shortwave versus longwave TOA forcing. Thus the net TOA forcing alone does not determine the steady state climate response.

Estimation of aerosol optical properties and radiative effects in the Ganga basin, northern India, during the wintertime

An aerosol model has been developed using mass size distributions of various chemical components measured at Kanpur (an urban location in the Ganga basin, GB, in northern India) and applied to estimate the radiative effects of the aerosols over the entire GB during the winter season. The number size distribution of various species was derived from the measured mass concentration, and the optical properties were calculated using Mie theory. The maximum anthropogenic contribution to the total extinction was estimated to be ∼83%. The relative contributions of various species to the aerosol optical depth (AOD) at 0.5 μm are in the following order: (NH4)2SO4 (nss‐SO4, 30%), nitrate (NO3−, 24%), salt (mainly NaCl and KCl, 18%), dust (17%) and black carbon (BC, 11%). Relative contribution of nss‐SO4, NO3− and salt to the calculated AOD decreases with wavelength, and that of dust increases with wavelength, whereas BC contribution is spectrally insensitive. The extinction coefficient strongly depends on the RH, as the scattering by fine mode fraction, which contributes 88% to the total extinction, is enhanced at high ambient RH. The spectral variation of absorption coefficient indicates that the most likely source of BC in this region is fossil fuel. The spectral variation of single scattering albedo (SSA) in the coarse mode fraction suggests mixing of BC and dust particles. During the observational period, the mean shortwave (SW) clear sky top of the atmosphere (TOA) and surface forcing over Kanpur are estimated to be −13 ± 3 and −43 ± 8 W m−2, respectively. The corresponding longwave forcings are 3.6 ± 0.7 and 2.9 ± 0.6 W m−2, respectively. Mean AOD at 0.55 μm over the GB as derived from MODIS data is 0.36 ± 0.14. Extending our model over the entire GB, the net mean TOA and surface forcing become −6.4 and −30.2 W m−2 (with overall ∼15% uncertainty). This results in high atmospheric absorption (+23.8 W m−2), translating into a heating rate of 0.67 K day−1. The SW surface to TOA forcing ratio (∼3.7) over the GB is 23% higher than the corresponding value for Indian Ocean. The aerosols reduce the incoming solar radiation reaching the surface by ∼19%, which has significant effect on the regional climate.

Global anthropogenic aerosol direct forcing derived from satellite and ground‐based observations

A global estimate of the direct effects of anthropogenic aerosols on solar radiation in cloudy skies is obtained by integrating satellite and ground‐based observations with models of aerosol chemistry, transport, and radiative transfer. The models adopt global distribution of aerosol optical depths (from MODIS), clouds, water vapor, ozone, and surface albedo from various satellite climatology. Gaps and errors in satellite derived aerosol optical depths are filled and corrected by surface network (AERONET), and an aerosol chemical‐transport model (GOCART) by using statistical techniques. Using these derived aerosol properties and other related variables, we generate climatological monthly mean anthropogenic aerosol forcing for both clear and average cloudy skies. Unless otherwise stated, our estimates are for average cloudy skies, also referred to as all sky conditions. The global annual mean direct forcing is −0.35 Wm−2 (range of −0.6 to −0.1 Wm−2) at the top‐of‐the atmosphere (TOA), +3.0 Wm−2 (range of +2.7 to +3.3Wm−2) in the atmosphere, and −3.4 Wm−2 (range of −3.5 to −3.3 Wm−2) at the surface. The uncertainty of about 10–20% in the surface and atmosphere forcing translates into a six fold uncertainty in the TOA forcing because the TOA forcing is a small sum of two large terms (surface and atmosphere) of opposing signs. Given the current state of observations and modeling, it is very difficult to further reduce the uncertainty in the estimated TOA forcing. The major contributors to the uncertainty in atmospheric absorption are from the uncertainty in the vertical distribution of aerosols and the single scattering albedo of aerosols. The TOA forcing in clear skies is a factor of two different, while the surface and atmosphere forcing terms differ by only about 10–25%. Another major finding of this study is that the reduction in the surface solar radiation is a factor of 10 larger than the reduction in net solar (down minus up) radiation at TOA. The TOA forcing changes sign regionally, whereas the surface forcing is always negative. Thus caution must be exercised against relying too strongly on assessing the aerosol impacts based solely on global mean forcing. Aerosols over the NH contribute about 64% to the global surface forcing. Regionally the populated tropical regions contribute the most to the global surface forcing, with Asia the largest contributor. Roughly 49% of the total surface forcing is over the oceanic regions. Most of the previous global aerosol forcing estimate studies were conducted with a chemical transport model coupled to a general circulation model with model generated aerosols and cloudiness. Thus the present study, which adopts observed aerosol properties and observed three dimensional cloudiness, provides an independent approach for estimating the aerosol forcing.