Shortwave Radiative Forcing Research Articles

Solar geoengineering using solid aerosol in the stratosphere

Abstract. Solid aerosol particles have long been proposed as an alternative to sulfate aerosols for solar geoengineering. Any solid aerosol introduced into the stratosphere would be subject to coagulation with itself, producing fractal aggregates, and with the natural sulfate aerosol, producing liquid-coated solids. Solid aerosols that are coated with sulfate and/or have formed aggregates may have very different scattering properties and chemical behavior than uncoated non-aggregated monomers do. We use a two-dimensional (2-D) chemistry–transport–aerosol model to capture the dynamics of interacting solid and liquid aerosols in the stratosphere. As an example, we apply the model to the possible use of alumina and diamond particles for solar geoengineering. For 240 nm radius alumina particles, for example, an injection rate of 4 Tg yr−1 produces a global-average shortwave radiative forcing of −1.2 W m−2 and minimal self-coagulation of alumina although almost all alumina outside the tropics is coated with sulfate. For the same radiative forcing, these solid aerosols can produce less ozone loss, less stratospheric heating, and less forward scattering than sulfate aerosols do. Our results suggest that appropriately sized alumina, diamond or similar high-index particles may have less severe technology-specific risks than sulfate aerosols do. These results, particularly the ozone response, are subject to large uncertainties due to the limited data on the rate constants of reactions on the dry surfaces.

Predicting the mineral composition of dust aerosols – Part 1: Representing key processes

Abstract. Soil dust aerosols created by wind erosion are typically assigned globally uniform physical and chemical properties within Earth system models, despite known regional variations in the mineral content of the parent soil. Mineral composition of the aerosol particles is important to their interaction with climate, including shortwave absorption and radiative forcing, nucleation of cloud droplets and ice crystals, heterogeneous formation of sulfates and nitrates, and atmospheric processing of iron into bioavailable forms that increase the productivity of marine phytoplankton. Here, aerosol mineral composition is derived by extending a method that provides the composition of a wet-sieved soil. The extension accounts for measurements showing significant differences between the mineral fractions of the wet-sieved soil and the emitted aerosol concentration. For example, some phyllosilicate aerosols are more prevalent at silt sizes, even though they are nearly absent at these diameters in a soil whose aggregates are dispersed by wet sieving. We calculate the emitted mass of each mineral with respect to size by accounting for the disintegration of soil aggregates during wet sieving. These aggregates are emitted during mobilization and fragmentation of the original undispersed soil that is subject to wind erosion. The emitted aggregates are carried far downwind from their parent soil. The soil mineral fractions used to calculate the aggregates also include larger particles that are suspended only in the vicinity of the source. We calculate the emitted size distribution of these particles using a normalized distribution derived from aerosol measurements. In addition, a method is proposed for mixing minerals with small impurities composed of iron oxides. These mixtures are important for transporting iron far from the dust source, because pure iron oxides are more dense and vulnerable to gravitational removal than most minerals comprising dust aerosols. A limited comparison to measurements from North Africa shows that the model extensions result in better agreement, consistent with a more extensive comparison to global observations as well as measurements of elemental composition downwind of the Sahara, as described in companion articles.

Impacts of cloud overlap assumptions on radiative budgets and heating fields in convective regions

Impacts of cloud overlap assumptions on radiative budgets and heating fields are explored with the aid of a cloud-resolving model (CRM), which provided cloud geometry as well as cloud micro and macro properties. Large-scale forcing data to drive the CRM are from TRMM Kwajalein Experiment and the Global Atmospheric Research Program's Atlantic Tropical Experiment field campaigns during which abundant convective systems were observed. The investigated overlap assumptions include those that were traditional and widely used in the past and the one that was recently addressed by Hogan and Illingworth (2000), in which the vertically projected cloud fraction is expressed by a linear combination of maximum and random overlap, with the weighting coefficient depending on the so-called decorrelation length Lcf. Results show that both shortwave and longwave cloud radiative forcings (SWCF/LWCF) are significantly underestimated under maximum (MO) and maximum-random (MRO) overlap assumptions, whereas remarkably overestimated under the random overlap (RO) assumption in comparison with that using CRM inherent cloud geometry. These biases can reach as high as 100 Wm−2 for SWCF and 60 Wm−2 for LWCF. By its very nature, the general overlap (GenO) assumption exhibits an encouraging performance on both SWCF and LWCF simulations, with the biases almost reduced by 3-fold compared with traditional overlap assumptions. The superiority of GenO assumption is also manifested in the simulation of shortwave and longwave radiative heating fields, which are either significantly overestimated or underestimated under traditional overlap assumptions. The study also pointed out the deficiency of constant assumption on Lcf in GenO assumption. Further examinations indicate that the CRM diagnostic Lcf varies among different cloud types and tends to be stratified in the vertical. The new parameterization that takes into account variation of Lcf in the vertical well reproduces such a relationship and leads to better simulations on radiative heating fields. It is therefore desirable to specify or parameterize Lcf in terms of cloud categories rather than constantly specified if to further improve the model performance.

Effect of particle shape on dust shortwave direct radiative forcing calculations based on MODIS observations for a case study

Assuming spheroidal and spherical particle shapes for mineral dust aerosols, the effect of particle shape on dust aerosol optical depth retrievals, and subsequently on instantaneous shortwave direct radiative forcing (SWDRF) at the top of the atmosphere (TOA), is assessed based on Moderate Resolution Imaging Spectroradiometer (MODIS) data for a case study. Specifically, a simplified aerosol retrieval algorithm based on the principle of the Deep Blue aerosol retrieval method is employed to retrieve dust aerosol optical depths, and the Fu-Liou radiative transfer model is used to derive the instantaneous SWDRF of dust at the TOA for cloud-free conditions. Without considering the effect of particle shape on dust aerosol optical depth retrievals, the effect of particle shape on the scattering properties of dust aerosols (e.g., extinction efficiency, single scattering albedo and asymmetry factor) is negligible, which can lead to a relative difference of at most 5% for the SWDRF at the TOA. However, the effect of particle shape on the SWDRF cannot be neglected provided that the effect of particle shape on dust aerosol optical depth retrievals is also taken into account for SWDRF calculations. The corresponding results in an instantaneous case study show that the relative differences of the SWDRF at the TOA between spheroids and spheres depend critically on the scattering angles at which dust aerosol optical depths are retrieved, and can be up to 40% for low dust-loading conditions.

Intercomparison of Surface Radiative Fluxes in the Arctic Ocean

AbstractRecent satellite data analysis has provided improved data sets relevant to the surface energy budget in the Arctic Ocean. In this paper, surface radiation properties in the Arctic Ocean obtained from the Surface Radiation Budget (SRB3.0) and the International Satellite Cloud Climatology Project (ISCCP-FD) during 1984–2007 are analyzed and compared. Our analysis suggests that these datasets show encouraging agreement in basin-wide averaged seasonal cycle and spatial distribution of surface albedo; net surface shortwave and all-wave radiative fluxes; and shortwave, longwave, and all-wave cloud radiative forcings. However, a systematic large discrepancy is detected for the net surface longwave radiative flux between the two data sets at a magnitude of∴23 W m∴2, which is primarily attributed to significant differences in surface temperature, particularly from April to June. Moreover, the largest difference in surface short-wave and all-wave cloud radiative forcings between the two data sets is apparen...

Potential climate effect of mineral aerosols over West Africa. Part I: model validation and contemporary climate evaluation

Mineral dusts present in the atmosphere can play an important role in climate over West Africa and surrounding regions. However, current understanding regarding how dust aerosols influence climate of West Africa is very limited. In this study, a regional climate model is used to investigate the potential climatic impacts of dust aerosols. Two sets of simulations driven by reanalysis and Earth System Model boundary conditions are performed with and without the representation of dust processes. The model, regardless of the boundary forcing, captures the spatial and temporal variability of the aerosol optical depth and surface concentration. The shortwave radiative forcing of dust is negative at the surface and positive in the atmosphere, with greater changes in the spring and summer. The presence of mineral dusts causes surface cooling and lower troposphere heating, resulting in a stabilization effect and reduction in precipitation in the northern portion of the monsoon close to the dust emissions region. This results in an enhancement of precipitation to the south. While dusts cause the lower troposphere to stabilize, upper tropospheric cooling makes the region more prone to intense deep convection as is evident by a simulated increase in extreme precipitation. In a companion paper, the impacts of dust emissions on future West African climate are investigated.

Dust aerosol radiative effects during summer 2012 simulated with a coupled regional aerosol–atmosphere–ocean model over the Mediterranean

Abstract. The present study investigates the radiative effects of dust aerosols in the Mediterranean region during summer 2012 using a coupled regional aerosol–atmosphere–ocean model (CNRM-RCSM5). A prognostic aerosol scheme, including desert dust, sea salt, organic, black-carbon and sulphate particles, has been integrated to CNRM-RCSM5 in addition to the atmosphere, land surface and ocean components. An evaluation of this aerosol scheme of CNRM-RCSM5, and especially of the dust aerosols, has been performed against in situ and satellite measurements, showing its ability to reproduce the spatial and temporal variability of aerosol optical depth (AOD) over the Mediterranean region in summer 2012. The dust vertical and size distributions have also been evaluated against observations from the TRAQA/ChArMEx campaign. Three simulations have been carried out for summer 2012 with CNRM-RCSM5, including the full prognostic aerosol scheme, only monthly-averaged AOD means from the aerosol scheme or no aerosols at all, in order to focus on the radiative effects of dust particles and the role of the prognostic scheme. Surface short-wave aerosol radiative forcing variability is found to be more than twice as high over regions affected by dust aerosols, when using a prognostic aerosol scheme instead of monthly AOD means. In this case downward surface solar radiation is also found to be better reproduced according to a comparison with several stations across the Mediterranean. A composite study over 14 stations across the Mediterranean, designed to identify days with high dust AOD, also reveals the improvement of the representation of surface temperature brought by the use of the prognostic aerosol scheme. Indeed the surface receives less radiation during dusty days, but only the simulation using the prognostic aerosol scheme is found to reproduce the observed intensity of the dimming and warming on dusty days. Moreover, the radiation and temperature averages over summer 2012 are also modified by the use of prognostic aerosols, mainly because of the differences brought in short-wave aerosol radiative forcing variability. Therefore this first comparison over summer 2012 highlights the importance of the choice of the representation of aerosols in climate models.

Role of vertical structure of cloud microphysical properties on cloud radiative forcing over the Asian monsoon region

Five years (2006–2010) of clouds and earth’s radiant energy system (CERES) and CloudSat data have been analyzed to examine the role of vertical structure of cloud microphysical properties on cloud radiative forcing (CRF) parameters at the top-of-the atmosphere over the Asian monsoon region during the summer monsoon season (June–September) and the Pacific warm pool region during April. Vertical profile of cloud properties (optical depth, cloud liquid water content and cloud ice water content) derived from CloudSat data has been used for the present analysis. Shortwave, longwave and net CRF derived from the CERES data have been used. The results suggest an imbalance between shortwave cloud radiative forcing and longwave cloud radiative forcing over the Asian monsoon region consistent with the results reported earlier. The present analysis suggests that over the Bay-of-Bengal (BoB), vertical profile of cloud microphysical properties determine more than 50 % of variance in CRF. However, over the Pacific warm pool region, cloud microphysical property profiles does not contribute significantly to variance in net CRF (<10 %). Over the BoB, large asymmetry between shortwave and longwave CRF is caused by large amounts of cloud liquid water content in the layer between the surface and 9 km. The present study highlights the importance of accurate representation of cloud microphysical properties in determining the influence of clouds on the radiative balance over the top-of-the atmosphere.

Rapid Adjustments of Cloud and Hydrological Cycle to Increasing CO2: a Review

Rapid cloud response to instantaneous radiative perturbation in the troposphere due to change in CO2 concentration is called cloud adjustment. Cloud adjustment develops on a short timescale because it is separated from surface temperature-mediated changes in cloud. Adjustments in cloud and tropospheric properties including the hydrological cycle have attracted considerable attention because of their importance in the interpretation of mechanisms of climate change and the identification of sources of uncertainty in climate sensitivity. Modeling studies have clearly revealed that major aspects of the tropospheric adjustment including the warming and drying of the troposphere, associated reduction of low cloud and increasing shortwave cloud radiative forcing, downward shift of the low-cloud layer, and slowdown of the global hydrological cycle, are common among many climate model simulations. Combinations of model simulations with realistic and idealized aqua-planet settings have helped demonstrate the roles of land and robust aspects of the tropospheric adjustment.

Simulation of carbonaceous aerosols over the Third Pole and adjacent regions: distribution, transportation, deposition, and climatic effects

Carbonaceous aerosols including black carbon and organic carbon over the Third Pole regions are simulated using a regional climate model (RegCM4.3) coupled with a chemistry–aerosol module. Results show that the model can simulate well the climatology of the Third Pole region in monsoon and non-monsoon seasons, but the model shows a cold bias and an overestimation of precipitation over the Himalayas and the northern Tibetan Plateau. The model also performs reasonably well in terms of aerosol optical depth and near surface aerosol concentration when compared with satellite datasets and in situ observations. BC wet deposition in monsoon seasons is more (less) than that in non-monsoon seasons in the southern (northwestern) parts of the Third Pole region. Westerly winds prevail throughout the year and transport carbonaceous particles from central Asia to the northern Tibetan Plateau. In the monsoon period, aerosols can cross the Himalayas and can be transported to high altitudes by the southwesterly winds over South Asia. Dry deposition shows a topography-controlled distribution, with low fluxes within and high fluxes outside of the Tibetan Plateau. Mixed carbonaceous aerosols produce positive shortwave radiative forcing in the atmosphere and negative forcing at the surface. Shortwave forcing is with less magnitude over the Third Pole region. Longwave radiation forcing is negative over the Pamir Plateau and positive over the Tibetan Plateau during monsoon season. In non-monsoon season, longwave radiative forcing is negative in the Himalayas and southern parts of the Tibetan Plateau. Aerosols increase surface air temperatures by 0.1–0.5 °C over the Tibetan Plateau and decrease temperatures in South Asia during the monsoon season. In the non-monsoon period, temperatures decrease by 0.1–0.5 °C over the southern Tibetan Plateau. Spatial changes in temperature are consistent with the distribution of longwave radiative forcing, which indicates that aerosols’ longwave radiative forcing probably plays an important role in the climatic impact of aerosols over the Third Pole region.

Mineral dust aerosol distributions, its direct and semi-direct effects over South Africa based on regional climate model simulation

The present contribution investigates the seasonal mean mass distributions, direct and semi-direct climatic effects of desert dust aerosols over South Africa, using the 12 year runs of Regional Climate Model (RegCM4). The results have shown that the desert dust particles which burden the western and southern regions of South Africa are mainly produced from the Kalahari and Namib Desert areas. At the surface and within the atmosphere, the short- and long-wave radiative forcing (RF) of dust showed contrasting effects. However, due to the dust short-wave RF dominant influence, the Net-RF of dust causes reduction on net radiation absorbed by the surface via enhancing radiative heating in the atmosphere. The radiative feedbacks of desert dust particles predominantly result in a positive response on net atmospheric radiative heating rate, Cloud Cover (CC) and cloud liquid water path. The CC enhancement and Net-RF of dust, cooperatively, induce reduction in surface temperature (up to −1.1 K) and surface sensible heat flux (up to −24 W/m2). The presence of desert dust aerosol also causes boundary layer height reduction, surface pressure enhancement and dynamical changes. Overall, the present contribution underscores the importance of including the effects of wind-eroded dust particles in climate change studies over South Africa.

Improving subtropical boundary layer cloudiness in the 2011 NCEP GFS

Abstract. The current operational version of National Centers for Environmental Prediction (NCEP) Global Forecasting System (GFS) shows significant low cloud bias. These biases also appear in the Coupled Forecast System (CFS), which is developed from the GFS. These low cloud biases degrade seasonal and longer climate forecasts, particularly of short-wave cloud radiative forcing, and affect predicted sea surface temperature. Reducing this bias in the GFS will aid the development of future CFS versions and contributes to NCEP's goal of unified weather and climate modelling. Changes are made to the shallow convection and planetary boundary layer parameterisations to make them more consistent with current knowledge of these processes and to reduce the low cloud bias. These changes are tested in a single-column version of GFS and in global simulations with GFS coupled to a dynamical ocean model. In the single-column model, we focus on changing parameters that set the following: the strength of shallow cumulus lateral entrainment, the conversion of updraught liquid water to precipitation and grid-scale condensate, shallow cumulus cloud top, and the effect of shallow convection in stratocumulus environments. Results show that these changes improve the single-column simulations when compared to large eddy simulations, in particular through decreasing the precipitation efficiency of boundary layer clouds. These changes, combined with a few other model improvements, also reduce boundary layer cloud and albedo biases in global coupled simulations.

Restricting 32–128 km horizontal scales hardly affects the MJO in the Superparameterized Community Atmosphere Model v.3.0 but the number of cloud‐resolving grid columns constrains vertical mixing

AbstractThe effects of artificially restricting the 32–128 km horizontal scale regime on MJO dynamics in the Superparameterized Community Atmosphere Model v.3.0 have been explored through reducing the extent of its embedded cloud resolving model (CRM) arrays. Two and four‐fold reductions in CRM extent (from 128 to 64 km and 32 km) produce statistical composite MJO signatures with spatial scale, zonal phase speed, and intrinsic wind‐convection anomaly structure that are all remarkably similar to the standard SPCAM's MJO. This suggests that the physics of mesoscale convective organization on 32–128 km scales are not critical to MJO dynamics in SPCAM and that reducing CRM extent may be a viable strategy for 400% more computationally efficient analysis of superparameterized MJO dynamics. However several unexpected basic state responses caution that extreme CRM domain reduction can lead to systematic mean state issues in superparameterized models. We hypothesize that an artificial limit on the efficiency of vertical updraft mixing is set by the number of grid columns available for compensating subsidence in the embedded CRM arrays. This can lead to reduced moisture ventilation supporting too much liquid cloud and thus an overly strong cloud shortwave radiative forcing, particularly in regions of deep convection.

Sensitivity simulations with direct shortwave radiative forcing by aeolian dust during glacial cycles

Abstract. Possible feedback effects between aeolian dust, climate and ice sheets are studied for the first time with an Earth system model of intermediate complexity over the late Pleistocene period. Correlations between climate and dust deposition records suggest that aeolian dust potentially plays an important role for the evolution of glacial cycles. Here climatic effects from the dust direct radiative forcing (DRF) caused by absorption and scattering of solar radiation are investigated. Key elements controlling the dust DRF are the atmospheric dust distribution and the absorption-scattering efficiency of dust aerosols. Effective physical parameters in the description of these elements are varied within uncertainty ranges known from available data and detailed model studies. Although the parameters can be reasonably constrained, the simulated dust DRF spans a~wide uncertainty range related to the strong nonlinearity of the Earth system. In our simulations, the dust DRF is highly localized. Medium-range parameters result in negative DRF of several watts per square metre in regions close to major dust sources and negligible values elsewhere. In the case of high absorption efficiency, the local dust DRF can reach positive values and the global mean DRF can be insignificantly small. In the case of low absorption efficiency, the dust DRF can produce a significant global cooling in glacial periods, which leads to a doubling of the maximum glacial ice volume relative to the case with small dust DRF. DRF-induced temperature and precipitation changes can either be attenuated or amplified through a feedback loop involving the dust cycle. The sensitivity experiments suggest that depending on dust optical parameters, dust DRF has the potential to either damp or reinforce glacial–interglacial climate changes.

Biophysical forcings of land‐use changes from potential forestry activities in North America

Land‐use changes through forestry and other activities alter not just carbon storage, but biophysical properties, including albedo, surface roughness, and canopy conductance, all of which affect temperature. This study assessed the biophysical forcings and climatic impact of vegetation replacement across North America by comparing satellite‐derived albedo, land surface temperature (LST), and evapotranspiration (ET) between adjacent vegetation types. We calculated radiative forcings (RF) for potential local conversions from croplands (CRO) or grasslands (GRA) to evergreen needleleaf (ENF) or deciduous broadleaf (DBF) forests. Forests generally had lower albedo than adjacent grasslands or croplands, particularly in locations with snow. They also had warmer nighttime LST, cooler daily and daytime LST in warm seasons, and smaller daily LST ranges. Darker forest surfaces induced positive RFs, dampening the cooling effect of carbon sequestration. The mean (±SD) albedo‐induced RFs for each land conversion were equivalent to carbon emissions of 2.2 ± 0.7 kg C/m2 (GRA–ENF), 2.0 ± 0.6 kg C/m2 (CRO–ENF), 0.90 ± 0.50 kg C/m2 (CRO–DBF), and 0.73 ± 0.22 kg C/m2 (GRA–DBF), suggesting that, given the same carbon sequestration potential, a larger net cooling (integrated globally) is expected for planting DBF than ENF. Both changes in LST and ET induce longwave RFs that sometimes had values comparable to or even larger than albedo‐induced shortwave RFs. Sensible heat flux, on average, increased when replacing CRO with ENF, but decreased for conversions to DBF, suggesting that DBF tends to cool near‐surface air locally while ENF tends to warm it. This local temperature effect showed some seasonal variation and spatial dependence, but did not differ strongly by latitude. Overall, our results show that a carbon‐centric accounting is, in many cases, insufficient for climate mitigation policies. Where afforestation or reforestation occurs, however, deciduous broadleaf trees are likely to produce stronger cooling benefits than evergreen needleleaf trees provide.

Assessing the effects of anthropogenic aerosols on Pacific storm track using a multiscale global climate model.

Atmospheric aerosols affect weather and global general circulation by modifying cloud and precipitation processes, but the magnitude of cloud adjustment by aerosols remains poorly quantified and represents the largest uncertainty in estimated forcing of climate change. Here we assess the effects of anthropogenic aerosols on the Pacific storm track, using a multiscale global aerosol-climate model (GCM). Simulations of two aerosol scenarios corresponding to the present day and preindustrial conditions reveal long-range transport of anthropogenic aerosols across the north Pacific and large resulting changes in the aerosol optical depth, cloud droplet number concentration, and cloud and ice water paths. Shortwave and longwave cloud radiative forcing at the top of atmosphere are changed by -2.5 and +1.3 W m(-2), respectively, by emission changes from preindustrial to present day, and an increased cloud top height indicates invigorated midlatitude cyclones. The overall increased precipitation and poleward heat transport reflect intensification of the Pacific storm track by anthropogenic aerosols. Hence, this work provides, for the first time to the authors' knowledge, a global perspective of the effects of Asian pollution outflows from GCMs. Furthermore, our results suggest that the multiscale modeling framework is essential in producing the aerosol invigoration effect of deep convective clouds on a global scale.

Evaluation of cloud vertical structure simulated by recent BCC_AGCM versions through comparison with CALIPSO-GOCCP data

The abilities of BCC_AGCM2.1 and BCC_AGCM2.2 to simulate the annual-mean cloud vertical structure (CVS) were evaluated through comparison with GCM-Oriented CALIPSO Cloud Product (CALIPSO-GOCCP) data. BCC_AGCM2.2 has a dynamical core and physical processes that are consistent with BCC_AGCM2.1, but has a higher horizontal resolution. Results showed that both BCC_AGCM versions underestimated the global-mean total cloud cover (TCC), middle cloud cover (MCC) and low cloud cover (LCC), and that BCC_AGCM2.2 underestimated the global-mean high cloud cover (HCC). The global-mean cloud cover shows a systematic decrease from BCC_AGCM2.1 to BCC_AGCM2.2, especially for HCC. Geographically, HCC is significantly overestimated in the tropics, particularly by BCC_AGCM2.1, while LCC is generally overestimated over extra-tropical lands, but significantly underestimated over most of the oceans, especially for subtropical marine stratocumulus clouds. The leading EOF modes of CVS were extracted. The BCC_AGCMs perform well in reproducing EOF1, but with a larger variance explained. The two models also capture the basic features of EOF3, except an obvious deficiency in eigenvector peaks. EOF2 has the largest simulation biases in both position and strength of eigenvector peaks. Furthermore, we investigated the effects of CVS on relative shortwave and longwave cloud radiative forcing (RSCRF and RLCRF). Both BCC_AGCM versions successfully reproduce the sign of regression coefficients, except for RLCRF in PC1. However, the RSCRF relative contributions from PC1 and PC2 are overestimated, while the relative contribution from PC3 is underestimated in both BCC_AGCM versions. The RLCRF relative contribution is underestimated for PC2 and overestimated for PC3.

Impact of Biomass Burning on Aerosol Size Distribution, Aerosol Optical Properties and Associated Radiative Forcing

ABSTRACTThe influence of biomass burning on aerosol size distributions, particle number and radiative forcing has been studied at a rural site in Spain. It has been found that air contaminated by aerosols from biomass burning presents four times the total number of particles registered in non-contaminated air. In the case of the smallest fraction of the fine mode, between 0.1 and 0.2 µm, the increase soars to over seven times the total number of particles. An analysis of the evolution of the count mean diameter in the fine mode (CMDf) in the 8 daily measurements has revealed a decrease of over 25% in this parameter in the modified measurements when compared with measurements that were not contaminated by aerosols from biomass burning. In contrast, when the aerosol transport time is long, the increases detected in CMDf range between 15% and 100% when compared with measurements of air by non-aged aerosol from biomass burning. Shortwave radiative forcings have been calculated for these high loads of fine aerosols with GAME (Global Atmospheric Model) software. For the August event, the daytime average of surface radiative forcing is –66 (±30) W/m2, and at the top of the atmosphere the forcing is –32 (±12) W/m2. Induced daytime average of atmospheric radiative forcing reaches 34 (±20) W/m2. The study demonstrates that wildfires affect not only the number of particles and the size distribution, causing a clear increase in the number of aerosols in the atmosphere, but they are also responsible for altering the local radiative balance.

Impact of aerosol on surface reaching solar irradiance over Mohal in the northwestern Himalaya, India

The present study, for the first time during 2007, is focused to examine the impact of aerosols on surface reaching solar irradiance over Mohal (31.9°N, 77.12°E, 1154m amsl) in the northwestern part of the Indian Himalaya. The study also aims to estimate shortwave aerosol radiative forcing (SWARF) and its effect on regional climate. The multi-wavelength solar radiometer (MWR) is used to measure aerosol optical depth (AOD) over a wider spectrum, i.e. ultraviolet, visible and near-infrared. The AOD is obtained by analyzing the data from MWR following the Langley technique. The radiative transfer model is used along with Optical Properties of Aerosols and Clouds model to estimate the SWARF. Aerosol shows a great efficiency to reduce substantial fraction of energy from the surface reaching direct solar beam, i.e. 154Wm−2μm−1 per unit AOD at 0.5μm. The SWARF at the surface, top of the atmosphere and the atmosphere is estimated to be −18.5±1.7, +0.6±3.7 and +19.1±3.1Wm−2, respectively. The large SWARF at the surface stood during the summer (April–July), while small during the monsoon (August–September). Moderate SWARF is obtained in the autumn (October–November) and winter (December–March). The study estimates a notable extinction in incoming solar radiation relatively with lower atmospheric heating from 0.41 to 0.73Kday−1. The potential effect of aerosol is found relatively higher on high aerosol loading days. On these days, the lower atmospheric heating increases by a factor 1.8 (during dust events) and 1.7 (during biomass burning). This study concludes that aerosols produce significant reduction in incoming solar radiation with substantial increase in lower atmospheric heating, leading to a remarkable effect on the atmospheric stability. In addition, as a subject of future interest, the present study has also important implications on the atmospheric circulation and regional climate.

Technical Note: Evaluating a simple parameterization of radiative shortwave forcing from surface albedo change

Abstract. Land-use activities affect Earth's energy balance not only via biogeochemical emissions, but also through perturbations in surface albedo, the latter of which is often excluded in impact assessment studies. In this short technical note, we present and compare a simple model for estimating shortwave radiative forcings at the top of Earth's atmosphere to a more sophisticated 8-stream radiative transfer model based on a discrete ordinate method. Outcomes from monthly albedo change simulations for ten globally distributed regions and a single year revealed that the simple model – based on a single exogenously supplied meteorological variable – performed quite well, having a sample correlation coefficient of 0.93 and a normalized root mean square error of 7.2%. Simple models like the one presented here can offer an attractive and efficient means for non-experts to begin including albedo change considerations in climate impact assessment studies enveloping land use activities.