Indirect Radiative Forcing Research Articles

Investigations on direct and indirect effect of nitrate on temperature and precipitation in China using a regional climate chemistry modeling system

The tropospheric atmosphere chemistry model (TACM), including a condensed gaseous chemical mechanism and an inorganic aerosol thermodynamic equilibrium submodel, was coupled with the regional climate model (RegCM3) to build a Regional Climate Chemistry Modeling System (RegCCMS), which was applied to investigate the spatial and temporal distribution of anthropogenic nitrate aerosol, radiative forcing, and also its climatic effect over China. Modeling results show that the annual average surface concentration and column burden of nitrate are 2.19 μg/m3 and 5.05 mg/m2, respectively. The countrywide annual average direct radiative forcing, first indirect radiative forcing, and total radiative forcing at the top of atmosphere induced by nitrate are −0.88 W/m2, −2.47 W/m2, and −2.52 W/m2, respectively. Numerical experiments indicate that surface air temperature decreases and precipitation reduces when nitrate aerosol is included in the coupled modeling system. Changes in annual surface air temperature due to direct effect, first indirect effect, second indirect effect, and combined effect are −0.04°C, −0.11°C, −0.68°C and −0.78°C, respectively. The corresponding precipitation reduction is −0.05 mm/d, −0.10 mm/d, −0.42 mm/d, and −0.52 mm/d. Variations of surface temperature and precipitation due to the combined effect are less than the sum of individual direct, first indirect, and second indirect effect, showing a strong nonlinearity between radiative forcing and climate change. The indirect effect of nitrate is stronger than its direct effect. These preliminary results suggest that nitrate aerosol has unignorable effect on regional climate in China compared to sulfate and carbonaceous aerosols.

Ion‐mediated nucleation in the atmosphere: Key controlling parameters, implications, and look‐up table

Nucleation is an important source of atmospheric particles and ubiquitous ions in the atmosphere have long been known to promote nucleation. An ion‐mediated nucleation (IMN) mechanism based on a kinetic model is supported by recent measurements of the excess charge on freshly nucleated particles and ion cluster evolution during nucleation events. Here we investigate the dependence of steady state IMN rate (JIMN) on key controlling parameters. We find that sulfuric acid vapor concentration, temperature, relative humidity, ionization rate, and surface area of preexisting particles have profound and nonlinear impacts on JIMN. The sensitivities of JIMN to the changes in these key parameters may imply important physical feedback mechanisms involving climate and emission changes, solar variations, nucleation, aerosol number abundance, and aerosol indirect radiative forcing. We also describe a five‐dimensional JIMN look‐up table derived from the most recent version of the IMN model, with the key parameters covering a wide range of atmospheric conditions. With the look‐up table and a multiple‐variable interpolation subroutine, JIMN and the properties of critical clusters can be determined efficiently and accurately under given atmospheric conditions. The look‐up table reduces the computational costs of the IMN rate calculations significantly (by a factor of around 8000) and can be readily incorporated into multidimensional models to study the secondary particle formation via IMN and associated climatic and health effects.

Effect of biomass burning on marine stratocumulus clouds off the California coast

Abstract. Aerosol-cloud interactions are considered to be one of the most important and least known forcings in the climate system. Biomass burning aerosols are of special interest due to their radiative impact (direct and indirect effect) and their potential to increase in the future due to climate change. Combining data from Geostationary Operational Environmental Satellite (GOES) and MODerate-resolution Imaging Spectroradiometer (MODIS) with passive tracers from the FLEXPART Lagrangian Particle Dispersion Model, the impact of biomass burning aerosols on marine stratocumulus clouds has been examined in June and July of 2006–2008 off the California coast. Using a continental tracer, the indirect effect of biomass burning aerosols has been isolated by comparing the average cloud fraction and cloud albedo for different meteorological situations, and for clean versus polluted (in terms of biomass burning) continental air masses at 14:00 local time. Within a 500 km-wide band along the coast of California, biomass burning aerosols, which tend to reside above the marine boundary layer, increased the cloud fraction by 0.143, and the cloud albedo by 0.038. Absorbing aerosols located above the marine boundary layer lead to an increase of the lower tropospheric stability and a reduction in the vertical entrainment of dry air from above, leading to increased cloud formation. The combined effect was an indirect radiative forcing of −7.5% ±1.7% (cooling effect) of the outgoing radiative flux at the top of the atmosphere on average, with a bias due to meteorology of +0.9%. Further away from the coast, the biomass burning aerosols, which were located within the boundary layer, reduced the cloud fraction by 0.023 and the cloud albedo by 0.006, resulting in an indirect radiative forcing of +1.3% ±0.3% (warming effect) with a bias of +0.5%. These results underscore the dual role that absorbing aerosols play in cloud radiative forcing.

Evidence for Asian dust effects from aerosol plume measurements during INTEX-B 2006 near Whistler, BC

Abstract. Several cases of aerosol plumes resulting from trans-Pacific transport were observed between 2 km and 5.3 km at Whistler, BC from 22 April 2006 to 15 May 2006. The fine particle (<1 μm) chemical composition of most of the plumes was dominated by sulphate that ranged from 1–5 μg m−3 as measured with a Quadrapole Aerosol Mass Spectrometer (Q-AMS). Coarse particles (>1 μm) were enhanced in all sulphate plumes. Fine particle organic mass concentrations were relatively low in most plumes and were nominally anti-correlated with the increases in the number concentrations of coarse particles. The ion chemistry of coarse particles sampled at Whistler Peak was dominated by calcium, sodium, nitrate, sulphate and formate. Scanning transmission X-ray microscopy of coarse particles sampled from the NCAR C-130 aircraft relatively close to Whistler indicated carbonate, potassium and organic functional groups, in particular the carboxyl group. Asian plumes reaching Whistler, BC during the INTEX-B study were not only significantly reduced of fine particle organic material, but organic compounds were attached to coarse particles in significant quantities. Suspension of dust with deposited organic material and scavenging of organic materials by dust near anthropogenic sources are suggested, and if any secondary organic aerosol (SOA) was formed during transport from Asian source regions across the Pacific it was principally associated with the coarse particles. An average of profiles indicates that trans-Pacific transport between 2 and 5 km during this period increased ozone by about 10 ppbv and fine particle sulphate by 0.2–0.5 μg m−3. The mean sizes of the fine particles in the sulphate plumes were larger when dust particles were present and smaller when the fine particle organic mass concentration was larger and dust was absent. The coarse particles of dust act to accumulate sulphate, nitrate and organic material in larger particles, diminishing the role of these compounds in indirect radiative forcing, but potentially enhancing their roles in direct radiative forcing.

Indirect radiative forcing and climatic effect of the anthropogenic nitrate aerosol on regional climate of China

The regional climate model (RegCM3) and a tropospheric atmosphere chemistry model (TACM) were coupled, thus a regional climate chemistry modeling system (RegCCMS) was constructed, which was applied to investigate the spatial distribution of anthropogenic nitrate aerosols, indirect radiative forcing, as well as its climatic effect over China. TACM includes the thermodynamic equilibrium model ISORROPIA and a condensed gas-phase chemistry model. Investigations show that the concentration of nitrate aerosols is relatively high over North and East China with a maximum of 29 µg m−3 in January and 8 µg m−3 in July. Due to the influence of air temperature on thermodynamic equilibrium, wet scavenging of precipitation and the monsoon climate, there are obvious seasonal differences in nitrate concentrations. The average indirect radiative forcing at the tropopause due to nitrate aerosols is −1.63 W m−2 in January and −2.65 W m−2 in July, respectively. In some areas, indirect radiative forcing reaches −10 W m−2. Sensitivity tests show that nitrate aerosols make the surface air temperature drop and the precipitation reduce on the national level. The mean changes in surface air temperature and precipitation are −0.13 K and −0.01 mm d−1 in January and −0.09 K and −0.11 mm d−1 in July, respectively, showing significant differences in different regions.

Radiative susceptibility of cloudy atmospheres to droplet number perturbations: 2. Global analysis from MODIS

Global distributions of albedo susceptibility for areas covered by liquid clouds are presented for 4 months in 2005. The susceptibility estimates are based on expanded definitions presented in a companion paper and include relative cloud droplet number concentration (CDNC) changes, perturbations in cloud droplet asymmetry parameter and single‐scattering albedo, atmospheric/surface effects, and incorporation of the full solar spectrum. The cloud properties (optical thickness and effective radius) used as input in the susceptibility calculations come from MODIS Terra and Aqua Collection 5 gridded data. Geographical distributions of susceptibility corresponding to absolute (“absolute cloud susceptibility”) and relative (“relative cloud susceptibility”) CDNC changes are markedly different indicating that the detailed nature of the cloud microphysical perturbation is important for determining the radiative forcing associated with the first indirect aerosol effect. However, both types of susceptibility exhibit common characteristics such as significant reductions when perturbations in single‐scattering properties are omitted, significant increases when atmospheric absorption and surface albedo effects are ignored, and the tendency to decrease with latitude, to be higher over ocean than over land, and to be statistically similar between the morning and afternoon MODIS overpasses. The satellite‐based susceptibility analysis helps elucidate the role of present‐day cloud and land surface properties in indirect aerosol forcing responses. Our realistic yet moderate CDNC perturbations yield forcings on the order of 1–2 W m−2 for cloud optical property distributions and land surface spectral albedos observed by MODIS. Since susceptibilities can potentially be computed from model fields, these results have practical application in assessing the reasonableness of model‐generated estimates of the aerosol indirect radiative forcing.

Estimation of Asian dust aerosol effect on cloud radiation forcing using Fu-Liou radiative model and CERES measurements

Abstract. The impact of Asian dust on cloud radiative forcing during 2003–2006 is studied by using the Clouds and Earth's Radiant Energy Budget Scanner (CERES) data and the Fu-Liou radiative transfer model. Analysis of satellite data shows that the dust aerosol significantly reduced the cloud cooling effect at TOA. In dust contaminated cloudy regions, the 4-year mean values of the instantaneous shortwave, longwave and net cloud radiative forcing are −138.9, 69.1, and −69.7 Wm−2, which are 57.0, 74.2, and 46.3%, respectively, of the corresponding values in pristine cloudy regions. The satellite-retrieved cloud properties are significantly different in the dusty regions and can influence the radiative forcing indirectly. The contributions to the cloud radiation forcing by the dust direct, indirect and semi-direct effects are estimated using combined satellite observations and Fu-Liou model simulation. The 4-year mean value of combination of dust indirect and semi-direct shortwave radiative forcing (SWRF) is 82.2 Wm−2, which is 78.4% of the total dust effect. The dust direct effect is only 22.7 Wm−2, which is 21.6% of the total effect. Because both first and second indirect effects enhance cloud cooling, the aerosol-induced cloud warming is mainly the result of the semi-direct effect of dust.

Modelling sea salt aerosol and its direct and indirect effects on climate

Abstract. A size-dependent sea salt aerosol parameterization was developed based on the piecewise log-normal approximation (PLA) for aerosol size distributions. Results of this parameterization from simulations with a global climate model produce good agreement with observations at the surface and for vertically-integrated volume size distributions. The global and annual mean of the sea salt burden is 10.1 mg m−2. The direct radiative forcing is calculated to be −1.52 and −0.60 W m−2 for clear sky and all sky, respectively. The first indirect radiative forcing is about twice as large as the direct forcing for all-sky (−1.34 W m−2). The results also show that the total indirect forcing of sea salt is −2.9 W m−2 if climatic feedbacks are taken into account. The sensitivity of the forcings to changes in the burdens and sizes of sea salt particles was also investigated based on additional simulations with a different sea salt source function.

Limits on climate sensitivity derived from recent satellite and surface observations

An analysis of satellite and surface measurements of aerosol optical depth suggests that global average of aerosol optical depth has been recently decreasing at the rate of around 0.0014/a. This decrease is nonuniform with the fastest decrease observed over the United States and Europe. The observed rate of decreasing aerosol optical depth produces the top of the atmosphere radiative forcing that is comparable to forcing due to the current rate of increasing atmospheric concentration of carbon dioxide and other greenhouse gases. Consequently, both increasing atmospheric concentration of greenhouse gases and decreasing loading of atmospheric aerosols are major contributors to the top‐of‐atmosphere radiative forcing. We find that the climate sensitivity is reduced by at least a factor of 2 when direct and indirect effects of decreasing aerosols are included, compared to the case where the radiative forcing is ascribed only to increases in atmospheric concentrations of carbon dioxide. We find the empirical climate sensitivity to be between 0.29 and 0.48 K/Wm−2 when aerosol direct and indirect radiative forcing is included.

Integrating biomass, sulphate and sea-salt aerosol responses into a microphysical chemical parcel model: implications for climate studies

Aerosols are known to influence significantly the radiative budget of the Earth. Although the direct effect (whereby aerosols scatter and absorb solar and thermal infrared radiation) has a large perturbing influence on the radiation budget, the indirect effect (whereby aerosols modify the microphysical and hence the radiative properties and amounts of clouds) poses a greater challenge to climate modellers. This is because aerosols undergo chemical and physical changes while in the atmosphere, notably within clouds, and are removed largely by precipitation. The way in which aerosols are processed by clouds depends on the type, abundance and the mixing state of the aerosols concerned. A parametrization with sulphate and sea-salt aerosol has been successfully integrated within the Hadley Centre general circulation model (GCM). The results of this combined parametrization indicate a significantly reduced role, compared with previous estimates, for sulphate aerosol in cloud droplet nucleation and, consequently, in indirect radiative forcing. However, in this bicomponent system, the cloud droplet number concentration, N(d) (a crucial parameter that is used in GCMs for radiative transfer calculations), is a smoothly varying function of the sulphate aerosol loading. Apart from sea-salt and sulphate aerosol particles, biomass aerosol particles are also present widely in the troposphere. We find that biomass smoke can significantly perturb the activation and growth of both sulphate and sea-salt particles. For a fixed salt loading, N(d) increases linearly with modest increases in sulphate and smoke masses, but significant nonlinearities are observed at higher non-sea-salt mass loadings. This non-intuitive N(d) variation poses a fresh challenge to climate modellers.

Indirect radiative forcing of climate change through ozone effects on the land-carbon sink

The evolution of the Earth's climate over the twenty-first century depends on the rate at which anthropogenic carbon dioxide emissions are removed from the atmosphere by the ocean and land carbon cycles. Coupled climate-carbon cycle models suggest that global warming will act to limit the land-carbon sink, but these first generation models neglected the impacts of changing atmospheric chemistry. Emissions associated with fossil fuel and biomass burning have acted to approximately double the global mean tropospheric ozone concentration, and further increases are expected over the twenty-first century. Tropospheric ozone is known to damage plants, reducing plant primary productivity and crop yields, yet increasing atmospheric carbon dioxide concentrations are thought to stimulate plant primary productivity. Increased carbon dioxide and ozone levels can both lead to stomatal closure, which reduces the uptake of either gas, and in turn limits the damaging effect of ozone and the carbon dioxide fertilization of photosynthesis. Here we estimate the impact of projected changes in ozone levels on the land-carbon sink, using a global land carbon cycle model modified to include the effect of ozone deposition on photosynthesis and to account for interactions between ozone and carbon dioxide through stomatal closure. For a range of sensitivity parameters based on manipulative field experiments, we find a significant suppression of the global land-carbon sink as increases in ozone concentrations affect plant productivity. In consequence, more carbon dioxide accumulates in the atmosphere. We suggest that the resulting indirect radiative forcing by ozone effects on plants could contribute more to global warming than the direct radiative forcing due to tropospheric ozone increases.

Accurate Monitoring of Terrestrial Aerosols and Total Solar Irradiance: Introducing the Glory Mission

The NASA Glory mission is intended to facilitate and improve upon long-term monitoring of two key forcings influencing global climate. One of the mission's principal objectives is to determine the global distribution of detailed aerosol and cloud properties with unprecedented accuracy, thereby facilitating the quantification of the aerosol direct and indirect radiative forcings. The other is to continue the 28-yr record of satellite-based measurements of total solar irradiance from which the effect of solar variability on the Earth's climate is quantified. These objectives will be met by flying two state-of-the-art science instruments on an Earth-orbiting platform. Based on a proven technique demonstrated with an aircraft-based prototype, the Aerosol Polarimetry Sensor (APS) will collect accurate multiangle photopolarimetric measurements of the Earth along the satellite ground track within a wide spectral range extending from the visible to the shortwave infrared. The Total Irradiance Monitor (TIM) is an improved version of an instrument currently flying on the Solar Radiation and Climate Experiment (SORCE) and will provide accurate and precise measurements of spectrally integrated sunlight illuminating the Earth. Because Glory is expected to fly as part of the A-Train constellation of Earth-orbiting spacecraft, the APS data will also be used to improve retrievals of aerosol climate forcing parameters and global aerosol assessments with other A-Train instruments. In this paper, we detail the scientific rationale and objectives of the Glory mission, explain how these scientific objectives dictate the specific measurement strategy, describe how the measurement strategy will be implemented by the APS and TIM, and briefly outline the overall structure of the mission. It is expected that the Glory results will be used extensively by members of the climate, solar, atmospheric, oceanic, and environmental research communities as well as in education and outreach activities.

Intensification of Pacific storm track linked to Asian pollution

Indirect radiative forcing of atmospheric aerosols by modification of cloud processes poses the largest uncertainty in climate prediction. We show here a trend of increasing deep convective clouds over the Pacific Ocean in winter from long-term satellite cloud measurements (1984-2005). Simulations with a cloud-resolving weather research and forecast model reveal that the increased deep convective clouds are reproduced when accounting for the aerosol effect from the Asian pollution outflow, which leads to large-scale enhanced convection and precipitation and hence an intensified storm track over the Pacific. We suggest that the wintertime Pacific is highly vulnerable to the aerosol-cloud interaction because of favorable cloud dynamical and microphysical conditions from the coupling between the Pacific storm track and Asian pollution outflow. The intensified Pacific storm track is climatically significant and represents possibly the first detected climate signal of the aerosol-cloud interaction associated with anthropogenic pollution. In addition to radiative forcing on climate, intensification of the Pacific storm track likely impacts the global general circulation due to its fundamental role in meridional heat transport and forcing of stationary waves.

Indirect radiative forcing of the ozone layer during the 21st century

The response of a coupled two‐dimensional radiative‐chemical‐dynamical model to possible 21st century changes of the greenhouse gasses (GHGs) carbon dioxide, nitrous oxide and methane are explored using a range of IPCC marker scenarios of GHG emissions. The changes to the ozone layer caused by these GHGs are found to be relatively large (e.g., up to 5% global mean column ozone changes and 30% local changes for CO2 using the IPCC A2 scenario between 2000 and 2100) and the mechanisms for these changes are discussed. The ozone changes are compared to the recovery of ozone due to expected decreases in chlorine containing compounds. Since carbon dioxide, nitrous oxide, and methane affect ozone they induce an indirect radiative forcing in addition to their direct radiative forcing. These indirect radiative forcings are computed using a combination of accurate line‐by‐line and band radiative transfer models and are compared to the radiative forcing of ozone during the 1979–2000 time period. Although the changes in ozone are large at some altitudes over the 2000–2100 time horizon, the range of associated future indirect radiative forcings from ozone over the range of IPCC scenarios are found to be −0.1 to 0.1 W m−2, which is small compared with the corresponding range of total direct radiative forcing of 2.2 to 6.2 W m−2 for these GHGs over this time horizon.

Multiyear ground‐based and satellite observations of aerosol properties over a tropical urban area in India

AbstractAerosol particle size distributions along with their spatial and temporal variability are important for describing both direct and indirect radiative forcing. In this study, the variation of black carbon (BC) aerosols, total aerosol mass loading and aerosol optical depth (AOD) over an urban region of Hyderabad, south India, was analyzed for 3 consecutive years from 2003 to 2005. The AOD was measured using a handheld multichannel sun‐photometer at six wavelengths centered on 380, 440, 500, 675, 870 and 1020 nm and aerosol mass–size distribution was made using a quartz crystal microbalance (QCM) cascade impactor. In addition, satellite remote‐sensing data from nighttime DMSP‐OLS images were analyzed for inferring ancillary sources of aerosols. Results from temporal analysis (2004–2006) suggest that aerosol mass loading and BC mass concentration increased considerably over the 3‐year time‐period mainly due to increasing vehicular traffic from urban population growth. DMSP‐OLS nighttime images for different years suggested higher forest fire occurrences in the year 2004 compared to other years. The annual mean AOD at 550 nm from moderate resolution imaging spectroradiometer (MODIS) showed relatively high values during 2004. Copyright © 2007 Royal Meteorological Society

Impact of cloud-borne aerosol representation on aerosol direct and indirect effects

Abstract. Aerosol particles attached to cloud droplets are much more likely to be removed from the atmosphere and are much less efficient at scattering sunlight than if unattached. Models used to estimate direct and indirect effects of aerosols employ a variety of representations of such cloud-borne particles. Here we use a global aerosol model with a relatively complete treatment of cloud-borne particles to estimate the sensitivity of simulated aerosol, cloud and radiation fields to various approximations to the representation of cloud-borne particles. We find that neglecting transport of cloud-borne particles introduces little error, but that diagnosing cloud-borne particles produces global mean biases of 20% and local errors of up to 40% for aerosol, droplet number, and direct and indirect radiative forcing. Aerosol number, aerosol optical depth and droplet number are significantly underestimated in regions and seasons where and when wet removal is primarily by stratiform rather than convective clouds (polar regions during winter), but direct and indirect effects are less biased because of the limited sunlight there and then. A treatment that predicts the total mass concentration of cloud-borne particles for each mode yields smaller errors and runs 20% faster than the complete treatment. The errors are much smaller than current estimates of uncertainty in direct and indirect effects of aerosols, which suggests that the treatment of cloud-borne aerosol is not a significant source of uncertainty in estimates of direct and indirect effects.

Sulfate aerosols forcing: An estimate using a three-dimensional interactive chemistry scheme

The tropospheric sulfate radiative forcing has been calculated using an interactive chemistry scheme in LMD-GCM. To estimate the radiative forcing of sulfate aerosol on climate, a consistent interaction between atmospheric circulation and radiation computation has been allowed in LMD-GCM. The model results indicate that the change in the sulfate aerosols number concentration is negatively correlated to the indirect radiative forcing. The model simulated annual mean direct radiative forcing ranges from −0.1 to −1.2 W m −2, and indirect forcing ranges from −0.4 to −1.6 W m −2. The global annual mean direct effect estimated by the model is −0.48 W m −2, and that of indirect is −0.68 W m −2.

Model intercomparison of indirect aerosol effects

Abstract. Modeled differences in predicted effects are increasingly used to help quantify the uncertainty of these effects. Here, we examine modeled differences in the aerosol indirect effect in a series of experiments that help to quantify how and why model-predicted aerosol indirect forcing varies between models. The experiments start with an experiment in which aerosol concentrations, the parameterization of droplet concentrations and the autoconversion scheme are all specified and end with an experiment that examines the predicted aerosol indirect forcing when only aerosol sources are specified. Although there are large differences in the predicted liquid water path among the models, the predicted aerosol first indirect effect for the first experiment is rather similar, about −0.6 Wm−2 to −0.7 Wm−2. Changes to the autoconversion scheme can lead to large changes in the liquid water path of the models and to the response of the liquid water path to changes in aerosols. Adding an autoconversion scheme that depends on the droplet concentration caused a larger (negative) change in net outgoing shortwave radiation compared to the 1st indirect effect, and the increase varied from only 22% to more than a factor of three. The change in net shortwave forcing in the models due to varying the autoconversion scheme depends on the liquid water content of the clouds as well as their predicted droplet concentrations, and both increases and decreases in the net shortwave forcing can occur when autoconversion schemes are changed. The parameterization of cloud fraction within models is not sensitive to the aerosol concentration, and, therefore, the response of the modeled cloud fraction within the present models appears to be smaller than that which would be associated with model "noise". The prediction of aerosol concentrations, given a fixed set of sources, leads to some of the largest differences in the predicted aerosol indirect radiative forcing among the models, with values of cloud forcing ranging from −0.3 Wm−2 to −1.4 Wm−2. Thus, this aspect of modeling requires significant improvement in order to improve the prediction of aerosol indirect effects.

Multiyear Advanced Very High Resolution Radiometer observations of summertime stratocumulus collocated with aerosols in the northeastern Atlantic

Advanced Very High Resolution Radiometer (AVHRR) 4‐km data were collected over the northeast Atlantic for May–August 1995–1999. Aerosol optical depth at 0.55 μm was retrieved in pixels identified as being cloud‐free ocean. In pixels identified as containing clouds from single‐layered, low‐level cloud systems over oceans, the following cloud properties were retrieved: 0.64‐μm cloud optical depth, droplet effective radius, cloud layer altitude, pixel‐scale fractional cloud cover, column liquid water amount and column droplet concentration. Aerosol and cloud properties were averaged in 1° × 1° latitude‐longitude regions. Regions that contained clouds were limited to those in which all the clouds were part of a single‐layered, low‐level cloud system. Aerosol and cloud properties were compared only in 1° regions that had sufficient numbers of both cloud‐free pixels that yielded aerosol retrievals and cloudy pixels that yielded retrievals of cloud properties within a single overpass. The comparisons were collected in 5° × 5° latitude‐longitude regions to determine trends. Within each 5° region the cloud properties were similar from year to year, permitting the data to be composited for all 5 years. Aerosol optical depth decreased systematically with time, probably as a result of the increase in solar zenith angle due to the precession of the satellite orbit. Within the 5° regions, as aerosol optical depth increased, droplet effective radius decreased, cloud optical depth increased, and droplet column number concentration increased, qualitatively consistent with the trends expected for the aerosol indirect effect. In some regions, liquid water path decreased as aerosol optical depth increased, contrary to the trends expected for the suppression of drizzle. Within each 5° region, clouds in clean air, as indicated by their collocation with relatively small aerosol optical depths, had larger droplets and smaller cloud optical depths than clouds in polluted air, as indicated by their collocation with relatively large aerosol optical depths. On average, the aerosol indirect radiative forcing for overcast conditions was about twice as large as the direct radiative forcing for cloud‐free conditions. In most of the 5° regions increases in cloud liquid water with increasing aerosol optical depth enhanced the ratios by 20−30% over those calculated from the changes in droplet effective radius and an assumption of constant cloud liquid water. In some 5° regions, however, like those just to the west of the Iberian peninsula, the column amount of cloud liquid water decreased with increasing aerosol optical depth.

Aircraft measurements of cloud droplet spectral dispersion and implications for indirect aerosol radiative forcing

Using a large amount of aircraft measurements of cloud droplet size distributions, the relationship between cloud spectral relative dispersion (ɛ) and cloud droplet number concentration (Nc) is studied. The results indicate that the value of ɛ varies between 0.2 to 0.8 when the cloud droplet number concentration is low (about 50 cm−3), and converges toward a narrow range of 0.4 to 0.5 when the cloud number concentration is higher. Because the distribution of the cloud droplet size is an important parameter in estimating the first indirect radiative effect of aerosols on the climate system, the uncertainty in the corresponding radiative forcing can be reduced by 10–40% (depending on cloud droplet number density) under high aerosol loading. This finding is important for improving climate change projections, especially for the regions where aerosol loading is high and continues to increase.