Top Of The Atmosphere Research Articles

Interannual variation of global net radiation flux as measured from space

AbstractThe global net radiation flux (NRF) in and out of the climate system at the top of the atmosphere (TOA) varies at interannual time scales, reflecting the complexity of the processes responsible for attaining global energy equilibrium. These processes are investigated in this study using the previously unexplored data acquired by a bolometric type sensor installed in the PICARD microsatellite. The obtained anomalies in the NRF (PICARD‐NRF) are compared to the global NRF changes at the TOA measured by the Clouds and Earth's Radiant Energy System mission (CERES‐NRF). The interanual PICARD‐NRF is strongly correlated with the matching period CERES‐NRF; the bootstrapped correlation at the 95%(+0.85 and +0.97) confidence intervals is +0.93. Consistency in the interannual variability in the NRF derived by two completely independent measurement systems enhances confidence in the estimated magnitude of these variations. To reveal the possible drivers of the NRF interannual variability, the NRF values were compared with the multivariate El Niño–Southern Oscillation index.

A case study of the radiative effect of aerosols over Europe: EUCAARI-LONGREX

Abstract. The radiative effect of anthropogenic aerosols over Europe during the 2008 European Integrated Project on Aerosol Cloud Climate and Air Quality Interactions Long Range Experiment (EUCAARI-LONGREX) campaign has been calculated using measurements collected by the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft and radiative transfer modelling. The aircraft sampled anthropogenically perturbed air masses across north-western Europe under anticyclonic conditions with aerosol optical depths ranging from 0.047 to 0.357. For one specially designed “radiative closure” flight, simulated irradiances have been compared to radiation measurements for a case of aged European aerosol in order to explore the validity of model assumptions and the degree of radiative closure that can be attained given the spatial and temporal variability of the observations and their measurement uncertainties. Secondly, the diurnally averaged aerosol radiative effect throughout EUCAARI-LONGREX has been calculated. The surface radiative effect ranged between −3.9 and −22.8 W m−2 (mean −11 ± 5 W m−2), whilst top-of-the-atmosphere (TOA) values were between −2.1 and −12.0 W m−2 (mean −5 ± 3 W m−2). We have quantified the uncertainties in our calculations due to the way in which aerosols and other parameters are represented in a radiative transfer model. The largest uncertainty in the aerosol radiative effect at both the surface and the TOA comes from the spectral resolution of the information used in the radiative transfer model (∼ 17 %) and the aerosol description (composition and size distribution) used in the Mie calculations of the aerosol optical properties included in the radiative transfer model (∼ 7 %). The aerosol radiative effect at the TOA is also highly sensitive to the surface albedo (∼ 12 %).

Rayleigh radiance computations for satellite remote sensing: accounting for the effect of sensor spectral response function.

To understand and assess the effect of the sensor spectral response function (SRF) on the accuracy of the top of the atmosphere (TOA) Rayleigh-scattering radiance computation, new TOA Rayleigh radiance lookup tables (LUTs) over global oceans and inland waters have been generated. The new Rayleigh LUTs include spectral coverage of 335-2555 nm, all possible solar-sensor geometries, and surface wind speeds of 0-30 m/s. Using the new Rayleigh LUTs, the sensor SRF effect on the accuracy of the TOA Rayleigh radiance computation has been evaluated for spectral bands of the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi National Polar-orbiting Partnership (SNPP) satellite and the Joint Polar Satellite System (JPSS)-1, showing some important uncertainties for VIIRS-SNPP particularly for large solar- and/or sensor-zenith angles as well as for large Rayleigh optical thicknesses (i.e., short wavelengths) and bands with broad spectral bandwidths. To accurately account for the sensor SRF effect, a new correction algorithm has been developed for VIIRS spectral bands, which improves the TOA Rayleigh radiance accuracy to ~0.01% even for the large solar-zenith angles of 70°-80°, compared with the error of ~0.7% without applying the correction for the VIIRS-SNPP 410 nm band. The same methodology that accounts for the sensor SRF effect on the Rayleigh radiance computation can be used for other satellite sensors. In addition, with the new Rayleigh LUTs, the effect of surface atmospheric pressure variation on the TOA Rayleigh radiance computation can be calculated precisely, and no specific atmospheric pressure correction algorithm is needed. There are some other important applications and advantages to using the new Rayleigh LUTs for satellite remote sensing, including an efficient and accurate TOA Rayleigh radiance computation for hyperspectral satellite remote sensing, detector-based TOA Rayleigh radiance computation, Rayleigh radiance calculations for high altitude lakes, and the same Rayleigh LUTs are applicable for all satellite sensors over the global ocean and inland waters. The new Rayleigh LUTs have been implemented in the VIIRS-SNPP ocean color data processing for routine production of global ocean color and inland water products.

Martian Top of the Atmosphere 10–420 nm spectral irradiance database and forecast for solar cycle 24

Ultraviolet radiation from 10 to 420nm reaching Mars Top of the Atmosphere (TOA) and surface is important in a wide variety of fields such as space exploration, climate modeling, and spacecraft design, as it has impact in the physics and chemistry of the atmosphere and soil. Despite the existence of databases for UV radiation reaching Earth TOA, based in space-borne instrumentation orbiting our planet, there is no similar information for Mars. Here we present a Mars TOA UV spectral irradiance database for solar cycle 24 (years 2008–2019), containing daily values from 10 to 420nm. The values in this database have been computed using a model that is fed by the Earth-orbiting Solar Radiation and Climate Experiment (SORCE) data. As the radiation coming from the Sun is not completely isotropic, in order to eliminate the geometrically related features but being able to capture the general characteristics of the solar cycle stage, we provide 3-, 7- and 15-days averaged values at each wavelength. Our database is of interest for atmospheric modeling and spectrally dependent experiments on Mars, the analysis of current and upcoming surface missions (rovers and landers) and orbiters in Mars. Daily values for the TOA UV conditions at the rover Curiosity location, as well as for the NASA Insight mission in 2016, and ESA/Russia ExoMars mission in 2018 are provided.

Influence of the vertical absorption profile of mixed Asian dust plumes on aerosol direct radiative forcing over East Asia

We estimate the aerosol direct radiative forcing (ADRF) and heating rate profiles of mixed East Asian dust plumes in the solar wavelength region ranging from 0.25 to 4.0 μm using the Santa Barbara Discrete Ordinate Atmospheric Radiative Transfer (SBDART) code. Vertical profiles of aerosol extinction coefficients and single-scattering albedos (SSA) were derived from measurements with a multi-wavelength Raman lidar system. The data are used as input parameters for our radiative transfer calculations. We considered four cases of radiative forcing in SBDART: 1. dust, 2. pollution, 3. mixed dust plume and the use of vertical profiles of SSA, and 4. mixed dust plumes and the use of column-averaged values of SSA. In our sensitivity study we examined the influence of SSA and aerosol layer height on our results. The ADRF at the surface and in the atmosphere shows a small dependence on the specific shape of the aerosol extinction vertical profile and its light-absorption property for all four cases. In contrast, at the top of the atmosphere (TOA), the ADRF is largely affected by the vertical distribution of the aerosols extinction. This effect increases if the light-absorption capacity (decrease of SSA) of the aerosols increases. We find different radiative effects in situations in which two layers of aerosols had different light-absorption properties. The largest difference was observed at the TOA for an absorbing aerosol layer at high altitude in which we considered in one case the vertical profile of SSA and in another case the column-averaged SSA only. The ADRF at the TOA increases when the light-absorbing aerosol layer is located above 3 km altitude. The differences between height-resolved SSA, which can be obtained from lidar data, and total layer-mean SSA indicates that the use of a layer-mean SSA can be rather misleading as it can induce a large error in the calculation of the ADRF at the TOA, which in turn may cause errors in the vertical profiles of heating rates.

Sensitivity of Amazonian TOA flux diurnal cycle composite monthly variability to choice of reanalysis.

Amazonian deep convection experiences a strong diurnal cycle driven by the cycle in surface sensible heat flux, which contributes to a significant diurnal cycle in the top of the atmosphere (TOA) radiative flux. Even when accounting for seasonal variability, the TOA flux diurnal cycle varies significantly on the monthly timescale. Previous work shows evidence supporting a connection between variability in the convective and radiative cycles, likely modulated by variability in monthly atmospheric state (e.g., convective instability). The hypothesized relationships are further investigated with regression analysis of the radiative diurnal cycle and atmospheric state using additional meteorological variables representing convective instability and upper tropospheric humidity. The results are recalculated with three different reanalyses to test the reliability of the results. The radiative diurnal cycle sensitivity to upper tropospheric humidity is about equal in magnitude to that of convective instability. In addition, the results are recalculated with the data subdivided into the wet and dry seasons. Overall, clear‐sky radiative effects have a dominant role in radiative diurnal cycle variability during the dry season. Because of this, even in a convectively active region, the clear‐sky radiative effects must be accounted for in order to fully explain the monthly variability in diurnal cycle. Finally, while there is general agreement between the different reanalysis‐based results when examining the full data time domain (without regard to time of year), there are significant disagreements when the data are divided into wet and dry seasons. The questionable reliability of reanalysis data is a major limitation.

Significant cooling effect on the surface due to soot particles over Brahmaputra River Valley region, India: An impact on regional climate

Black carbon (BC) is an important atmospheric aerosol constituent that affects the climate by absorbing (directly) the sunlight and modifying cloud characteristics (indirectly). Here, we present first time yearlong measurements of BC and carbon monoxide (CO) from an urban location of Guwahati located in the Brahmaputra River valley (BRV) in the northeast region of India from 1st July 2013 to 30th June 2014. Daily BC concentrations varied within the range of 2.86 to 11.56μgm−3 with an annual average of 7.17±1.89μgm−3, while, CO varied from 0.19 to 1.20ppm with a mean value of 0.51±0.19ppm during the study period. The concentrations of BC (8.37μgm−3) and CO (0.67ppm) were ~39% and ~55% higher during the dry months (October to March) than the wet months (April to September) suggesting that seasonal changes in meteorology and emission sources play an important role in controlling these species. The seasonal ΔBC/ΔCO ratios were highest (lowest) in the pre-monsoon (winter) 18.1±1.4μgm−3ppmv−1 (12.6±2.2μgm−3ppmv−1) which indicate the combustion of biofuel/biomass as well as direct emissions from fossil fuel during the pre-monsoon season. The annual BC emission was estimated to be 2.72Gg in and around Guwahati which is about 44% lower than the mega city ‘Delhi’ (4.86Gg). During the study period, the annual mean radiative forcing (RF) at the top of the atmosphere (TOA) for clear skies of BC was +9.5Wm−2, however, the RF value at the surface (SFC) was −21.1Wm−2 which indicates the net warming and cooling effects, respectively. The highest RF at SFC was in the month of April (−30Wm−2) which is coincident with the highest BC mass level. The BC atmospheric radiative forcing (ARF) was +30.16 (annual mean) Wm−2 varying from +23.1 to +43.8Wm−2. The annual mean atmospheric heating rate (AHR) due to the BC aerosols was 0.86Kday−1 indicates the enhancement in radiation effect over the study region. The Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) captured the seasonal cycle of observed BC fairly well but underestimated the observed BC during the month of May–August. Model results show that BC at Guwahati is controlled mainly by anthropogenic emissions except during the pre-monsoon season when open biomass burning also makes a similar contribution.

Unforced Surface Air Temperature Variability and Its Contrasting Relationship with the Anomalous TOA Energy Flux at Local and Global Spatial Scales*

Abstract Unforced global mean surface air temperature () is stable in the long term primarily because warm anomalies are associated with enhanced outgoing longwave radiation () to space and thus a negative net radiative energy flux (, positive downward) at the top of the atmosphere (TOA). However, it is shown here that, with the exception of high latitudinal and specific continental regions, warm unforced surface air temperature anomalies at the local spatial scale [T(θ, ϕ), where (θ, ϕ) = (latitude, longitude)] tend to be associated with anomalously positive N(θ, ϕ). It is revealed that this occurs mainly because warm T(θ, ϕ) anomalies are accompanied by anomalously low surface albedo near sea ice margins and over high altitudes, low cloud albedo over much of the middle and low latitudes, and a large water vapor greenhouse effect over the deep Indo-Pacific. It is shown here that the negative versus relationship arises because warm anomalies are associated with large divergence of atmospheric energy transport over the tropical Pacific [where the N(θ, ϕ) versus T(θ, ϕ) relationship tends to be positive] and convergence of atmospheric energy transport at high latitudes [where the N(θ, ϕ) versus T(θ, ϕ) relationship tends to be negative]. Additionally, the characteristic surface temperature pattern contains anomalously cool regions where a positive local N(θ, ϕ) versus T(θ, ϕ) relationship helps induce negative . Finally, large-scale atmospheric circulation changes play a critical role in the production of the negative versus relationship as they drive cloud reduction and atmospheric drying over large portions of the tropics and subtropics, which allows for greatly enhanced .

Overview of the Chemistry-Aerosol Mediterranean Experiment/Aerosol Direct Radiative Forcing on the Mediterranean Climate (ChArMEx/ADRIMED) summer 2013 campaign

The Chemistry-Aerosol Mediterranean Experiment (ChArMEx; http://charmex.lsce.ipsl.fr) is a collaborative research program federating international activities to investigate Mediterranean regional chemistry-climate interactions. A special observing period (SOP-1a) including intensive airborne measurements was performed in the framework of the Aerosol Direct Radiative Forcing on the Mediterranean Climate (ADRIMED) project during the Mediterranean dry season over the western and central Mediterranean basins, with a focus on aerosol-radiation measurements and their modeling. The SOP-1a took place from 11 June to 5 July 2013. Airborne measurements were made by both the ATR-42 and F-20 French research aircraft operated from Sardinia (Italy) and instrumented for in situ and remote-sensing measurements, respectively, and by sounding and drifting balloons, launched in Minorca. The experimental set-up also involved several ground-based measurement sites on islands including two ground-based reference stations in Corsica and Lampedusa and secondary monitoring sites in Minorca and Sicily. Additional measurements including lidar profiling were also performed on alert during aircraft operations at EARLINET/ACTRIS stations at Granada and Barcelona in Spain, and in southern Italy. Remote sensing aerosol products from satellites (MSG/SEVIRI, MODIS) and from the AERONET/PHOTONS network were also used. Dedicated meso-scale and regional modelling experiments were performed in relation to this observational effort. We provide here an overview of the different surface and aircraft observations deployed during the ChArMEx/ADRIMED period and of associated modeling studies together with an analysis of the synoptic conditions that determined the aerosol emission and transport. Meteorological conditions observed during this campaign (moderate temperatures and southern flows) were not favorable to produce high level of atmospheric pollutants nor intense biomass burning events in the region. However, numerous mineral dust plumes were observed during the campaign with main sources located in Morocco, Algeria and Tunisia, leading to aerosol optical depth (AOD) values ranging between 0.2 to 0.6 (at 440 nm) over the western and central Mediterranean basins. Associated aerosol extinction values measured on-board the ATR-42 within the dust plume show local maxima reaching up to 150 Mm−1. Non negligible aerosol extinction (about 50 Mm−1) was also been observed within the Marine Boundary Layer (MBL). By combining ATR-42 extinction, absorption and scattering measurements, a complete optical closure has been made revealing excellent agreement with estimated optical properties. Associated calculations of the dust single scattering albedo (SSA) have been conducted, which show a moderate variability (from 0.90 to 1.00 at 530 nm). In parallel, active remote-sensing observations from the surface and onboard the F-20 aircraft suggest a complex vertical structure of particles and distinct aerosol layers with sea-salt and pollution located within the MBL, and mineral dust and/or aged north American smoke particles located above (up to 6–7 km in altitude). Aircraft and balloon-borne observations show particle size distributions characterized by large aerosols (> 10 μm in diameter) within dust plumes. In terms of shortwave (SW) direct forcing, in-situ surface and aircraft observations have been merged and used as inputs in 1-D radiative transfer codes for calculating the direct radiative forcing (DRF). Results show significant surface SW instantaneous forcing (up to −90 W m−2 at noon). Associated 3-D modeling studies from regional climate (RCM) and chemistry transport (CTM) models indicate a relatively good agreement for simulated AOD compared with measurements/observations from the AERONET/PHOTONS network and satellite data, especially for long-range dust transport. Calculations of the 3-D SW (clear-sky) surface DRF indicate an average of about −10 to −20 W m−2 (for the whole period) over the Mediterranean Sea together with maxima (−50 W m−2) over northern Africa. The top of the atmosphere (TOA) DRF is shown to be highly variable within the domain, due to moderate absorbing properties of dust and changes in the surface albedo. Indeed, 3-D simulations indicate negative forcing over the Mediterranean Sea and Europe and positive forcing over northern Africa.

Modulation in Direct Radiative Forcing Caused by Wind Generated Sea-Salt Aerosols

Sea-salt aerosols, prominent natural aerosols over the ocean, play a vital role in direct and indirect radiative forcing. Since surface winds are the prime cause of sea-salt generation, we have developed an empirical relationship between aerosol optical depth (AOD) and sea-surface winds over the study region (60-70 degrees E; 40 degrees S-20 degrees N). The latitudinal variation of background aerosol optical depth (tau(0)) and the wind index (b) are estimated as they are essential inputs for the estimation of the spatial variation of sea-salt aerosols and are used over the Arabian Sea (AS) to generate spatial heterogeneity of sea-salt aerosols. The latitudinal variation of tau(0) and b show a nearly exponential and linear increase, respectively, as we moved towards the north. We used an empirical-cum-model approach to construct an aerosol system to reproduce the observed AOD and aerosol optical properties. Utilizing this information as input to a radiative transfer model, we worked out direct radiative forcing (DRF) over the study region in the short wave (0.2-4 mu m) and long wave (8-14 mu m) region at the surface, top of the atmosphere (TOA) and in the atmosphere. Short wave cooling at the surface, TOA and heating in the atmosphere are estimated as 40 W m(-2), 32 W m(-2) and 8 W m(-2), respectively. Long wave heating due to the sea-salt aerosols estimated at the surface, TOA and in the atmosphere is about 9 W m-2, 6 W m-2 and 15 W m-2 respectively. Long wave forcing (LWF) partly counterbalances the effect of short wave forcing (SWF) and the cooling at the surface and at TOA. The highest value of such an offset at the surface was observed over the AS (similar to 23%) and that at TOA was similar to 19%, obviously at high wind conditions. This implies that over the AS sea-salt aerosols (in coarse mode) contribute significantly to LWF compared to other adjacent oceanic regions.

Comparison of Radiative Energy Flows in Observational Datasets and Climate Modeling

AbstractThis study examines radiative flux distributions and local spread of values from three major observational datasets (CERES, ISCCP, and SRB) and compares them with results from climate modeling (CMIP3). Examinations of the spread and differences also differentiate among contributions from cloudy and clear-sky conditions. The spread among observational datasets is in large part caused by noncloud ancillary data. Average differences of at least 10 W m−2 each for clear-sky downward solar, upward solar, and upward infrared fluxes at the surface demonstrate via spatial difference patterns major differences in assumptions for atmospheric aerosol, solar surface albedo and surface temperature, and/or emittance in observational datasets. At the top of the atmosphere (TOA), observational datasets are less influenced by the ancillary data errors than at the surface. Comparisons of spatial radiative flux distributions at the TOA between observations and climate modeling indicate large deficiencies in the strength and distribution of model-simulated cloud radiative effects. Differences are largest for lower-altitude clouds over low-latitude oceans. Global modeling simulates stronger cloud radiative effects (CRE) by +30 W m−2 over trade wind cumulus regions, yet smaller CRE by about −30 W m−2 over (smaller in area) stratocumulus regions. At the surface, climate modeling simulates on average about 15 W m−2 smaller radiative net flux imbalances, as if climate modeling underestimates latent heat release (and precipitation). Relative to observational datasets, simulated surface net fluxes are particularly lower over oceanic trade wind regions (where global modeling tends to overestimate the radiative impact of clouds). Still, with the uncertainty in noncloud ancillary data, observational data do not establish a reliable reference.

Projected response of East Asian summer monsoon system to future reductions in emissions of anthropogenic aerosols and their precursors

The response of the East Asian summer monsoon (EASM) system to reductions in emissions of anthropogenic aerosols and their precursors at the end of the twenty-first century projected by Representative Concentration Pathway 4.5 is studied using an aerosol-climate model with aerosol direct, semi-direct, and indirect effects included. Our results show that the global annual mean aerosol effective radiative forcing at the top of the atmosphere (TOA) is +1.45 W m−2 from 2000 to 2100. The summer mean net all-sky shortwave fluxes averaged over the East Asian monsoon region (EAMR) at the TOA and surface increased by +3.9 and +4.0 W m−2, respectively, due to the reductions of aerosols in 2100 relative to 2000. Changes in radiations affect local thermodynamic and dynamic processes and the hydrological cycle. The summer mean surface temperature and pressure averaged over the EAMR are shown to increase by 1.7 K and decreased by 0.3 hPa, respectively, due to the reduced aerosols. The magnitudes of these changes are larger over land than ocean, causing a marked increase in the contrast of land-sea surface temperature and pressure in the EAMR, thus strengthening the EASM. The summer mean southwest and south winds at 850 hPa are enhanced over eastern and southern China and the surrounding oceans, and the East Asian subtropical jet shifted northward due to the decreases of aerosols. These factors also indicate enhanced EASM circulation, which in turn causes a 10 % increase in summer mean precipitation averaged over the EAMR.

Aerosol direct radiative forcing in desert and semi-desert regions of northwestern China

The optical properties of dust aerosols were measured using narrow-band data from a portable sun photometer at four desert and semi-desert stations in northwestern China from 2004 to 2007. Ground-based and satellite observations indicated absorbing dust aerosol loading over the region surrounded by eight large-scale deserts. Radiation forcing was identified by using the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model. The ranges of annual mean aerosol optical depth (AOD), Angström exponents, and single-scattering albedo (SSA) were from 0.25 to 0.35, from −0.73 to 1.18, and from 0.77 to 0.86, respectively. The ranges of annual mean aerosol direct radiative forcing values at the top of the atmosphere (TOA), mid-atmosphere, and on the surface were from 3.9 to 12.0, from 50.0 to 53.1, and from −39.1 to −48.1W/m2, respectively. The aerosols' optical properties and radiative characteristics showed strong seasonal variations in both the desert and semi-desert regions. Strong winds and relatively low humidity will lead dust aerosols in the atmosphere to an increase, which played greatly affected these optical properties during spring and winter in northwestern China. Based on long-term observations and retrieved data, aerosol direct radiative forcing was confirmed to heat the atmosphere (50–53W/m2) and cool the surface (−39 to −48W/m2) above the analyzed desert. Radiative forcing in the atmosphere in spring and winter was 18 to 21W/m2 higher than other two seasons. Based on the dust sources around the sites, the greater the AOD, the more negative the forcing. The annual averaged heating rates for aerosols close to the ground (1km) were approximately 0.80–0.85K/day.

Absorbing and scattering aerosols over the source region of biomass burning emissions: Implications in the assessment of optical and radiative properties

Abstract The current study focuses on the assessment of model simulated optical and radiative properties of aerosols incorporating the measured chemical composition of aerosol samples collected at Patiala during October, 2011–February, 2012. Monthly average mass concentration of PM2.5, elemental carbon (EC), primary organic carbon (POC), water-soluble (WS) and insoluble (INS) aerosols ranged from 120 to 192, 6.2 to 7.2, 20 to 39, 59 to 111 and 35 to 90 μg m−3, respectively. Mass concentration of different components of aerosols was further used for the assessment of optical properties derived from Optical Properties of Aerosols and Clouds (OPAC) model simulations. Microtops based measured aerosol optical depth (AOD500) ranged from 0.47 to 0.62 showing maximum value during November and December, and minimum during February. Angstrom exponent (α380-870) remained high (>0.90) throughout the study period except in February (0.74), suggesting predominance of fine mode particles over the study region. The observed ratio of scattering to absorbing aerosols was incorporated in OPAC model simulations and single scattering albedo (SSA at 500 nm) so obtained ranged between 0.80 and 0.92 with relatively low values during the period of extensive biomass burning. In the present study, SBDART based estimated values of aerosol radiative forcing (ARF) at the surface (SRF) and top of the atmosphere (TOA) ranged from −31 to −66 Wm-2 and -2 to −18 W m−2 respectively. The atmospheric ARF, ranged between + 18 and + 58 Wm-2 resulting in the atmospheric heating rate between 0.5 and 1.6 K day−1. These results signify the role of scattering and absorbing aerosols in affecting the magnitude of aerosol forcing.

Interhemispheric Variability of Earth’s Radiation

Abstract The variability of interhemispheric symmetry of Earth’s energy serves as an independent indicator of climate change. The analysis of updated data obtained from satellite measurements at the top of the atmosphere (TOA) shows that in accord with Earth’s orbital requirements the annually averaged incident solar radiation is the same in the Northern and Southern Hemispheres, the annual mean of the reflected shortwave radiation is almost north–south symmetric, and the annual mean of the outgoing longwave radiation is larger in the Northern Hemisphere by 1.4 W m−2. These mean radiations systematically differ from the mean radiations found from the numerical atmospheric models that participated in the Coupled Model Intercomparison Project phase 5 (CMIP5). The hemispheric differences of the TOA radiations vary on the annual and interannual time scales. The multidecadal variability in Earth’s north–south temperature difference reveals a similarity of trends in both hemispheres. The Atlantic meridional transport (in contrast to the Pacific meridional transport) is found to be coherent with the interhemispheric ocean heat content (OHC) difference on decadal and multidecadal time scales, indicating a critical role of the Atlantic in the interhemispheric energy balance change.

The ADV/ASV AATSR aerosol retrieval algorithm: current status and presentation of a full-mission AOD dataset

ABSTRACTAn advanced along-track scanning radiometer (AATSR) global multi-year aerosol retrieval algorithm is described. Over land, the AATSR dual-view (ADV) algorithm utilizes the measured top of the atmosphere (TOA) reflectance in both the nadir and forward views to decouple the contributions of the atmosphere and the surface to retrieve aerosol properties. Over ocean, the AATSR single-view (ASV) algorithm minimizes the discrepancy between the measured and modelled TOA reflectances in one of the views to retrieve the aerosol information using an ocean reflectance model. Necessary steps to process global, multi-year aerosol information are presented. These include cloud screening, the averaging of measured reflectance, and post-processing. Limitations of the algorithms are also discussed. The main product of the aerosol retrieval is the aerosol optical depth (AOD) at visible/near-infrared wavelengths. The retrieved AOD is validated using data from the surface-based AERONET and maritime aerosol network (MAN) sun photometer networks as references. The validation shows good agreement with the reference (r = 0.85, RMSE = 0.09 over land; r = 0.83, RMSE = 0.09 at coasts and r = 0.96, RMSE = 0.06 over open ocean). The results of the aerosol retrievals are presented for the full AATSR mission years 2002–2012 with seasonally averaged time series for selected regions.

Ice water content vertical profiles of high-level clouds: classification and impact on radiative fluxes

Abstract. In this article, we discuss the shape of ice water content (IWC) vertical profiles in high ice clouds and its effect on their radiative properties, both in short- and in long-wave bands (SW and LW). Based on the analysis of collocated satellite data, we propose a minimal set of primitive shapes (rectangular, isosceles trapezoid, lower and upper triangle), which represents the IWC profiles sufficiently well. About 75 % of all high-level ice clouds (P < 440 hPa) have an ice water path (IWP) smaller than 100 g m−2, with a 10 % smaller contribution from single layer clouds. Most IWC profiles (80 %) can be represented by a rectangular or isosceles trapezoid shape. However, with increasing IWP, the number of lower triangle profiles (IWC rises towards cloud base) increases, reaching up to 40 % for IWP values greater than 300 g m−2. The number of upper triangle profiles (IWC rises towards cloud top) is in general small and decreases with IWP, with the maximum occurrence of 15 % in cases of IWP less than 10 g m−2. We propose a statistical classification of the IWC shapes using IWP as a single parameter. We have estimated the radiative effects of clouds with the same IWP and with different IWC profile shapes for five typical atmospheric scenarios and over a broad range of IWP, cloud height, cloud vertical extent, and effective ice crystal diameter (De). We explain changes in outgoing LW fluxes at the top of the atmosphere (TOA) by the cloud thermal radiance while differences in TOA SW fluxes relate to the De vertical profile within the cloud. Absolute differences in net TOA and surface fluxes associated with these parameterized IWC profiles instead of assuming constant IWC profiles are in general of the order of 1–2 W m−2: they are negligible for clouds with IWP < 30 g m−2, but may reach 2 W m−2 for clouds with IWP > 300 W m−2.

The Response of the Tropical Atlantic and West African Climate to Saharan Dust in a Fully Coupled GCM

Abstract This study examines the climate response in West Africa and the tropical Atlantic to an idealized aerosol radiative forcing from Saharan mineral dust, comparable to the observed changes between the 1960s and 1990s, using simulations with the fully coupled GFDL Climate Model, version 2.1 (CM2.1), for two optical property regimes: more absorbing (ABS) and more scattering (SCT) dust. For both regimes dust induces significant regional reductions in radiative flux at the surface (approximately −30 W m−2). At the top of the atmosphere (TOA) dust in the two simulations produces a radiative flux anomaly of opposite sign (+30 W m−2 in the ABS case and −20 W m−2 in the SCT case). These differences result in opposing regional hydrologic and thermodynamic effects of dust. The ABS-forced simulations show an increase in the West African monsoon resulting from dust, whereas in the SCT-forced simulations dust causes a decrease in the monsoon. This is due to moist enthalpy changes throughout the atmospheric column over West Africa creating either horizontal divergence or convergence near the surface, respectively. In the tropical North Atlantic, dust acts to cool the ocean surface. However, in the subsurface the ABS-forced simulations show a decrease in upper-ocean heat content, while the SCT-forced simulations show an increase in upper-ocean heat content. The peak differences primarily arise from the wind stress curl response to a shift in the Atlantic ITCZ and associated mixed layer depth anomalies. Changes to upper-ocean currents are also found to be important in transporting energy across the equator.

Next-generation angular distribution models for top-of-atmosphere radiative flux calculation from CERES instruments: validation

Abstract. Radiative fluxes at the top of the atmosphere (TOA) from the Clouds and the Earth's Radiant Energy System (CERES) instrument are fundamental variables for understanding the Earth's energy balance and how it changes with time. TOA radiative fluxes are derived from the CERES radiance measurements using empirical angular distribution models (ADMs). This paper evaluates the accuracy of CERES TOA fluxes using direct integration and flux consistency tests. Direct integration tests show that the overall bias in regional monthly mean TOA shortwave (SW) flux is less than 0.2 Wm−2 and the RMSE is less than 1.1 Wm−2. The bias and RMSE are very similar between Terra and Aqua. The bias in regional monthly mean TOA LW fluxes is less than 0.5 Wm−2 and the RMSE is less than 0.8 Wm−2 for both Terra and Aqua. The accuracy of the TOA instantaneous flux is assessed by performing tests using fluxes inverted from nadir- and oblique-viewing angles using CERES along-track observations and temporally and spatially matched MODIS observations, and using fluxes inverted from multi-angle MISR observations. The averaged TOA instantaneous SW flux uncertainties from these two tests are about 2.3 % (1.9 Wm−2) over clear ocean, 1.6 % (4.5 Wm−2) over clear land, and 2.0 % (6.0 Wm−2) over clear snow/ice; and are about 3.3 % (9.0 Wm−2), 2.7 % (8.4 Wm−2), and 3.7 % (9.9 Wm−2) over ocean, land, and snow/ice under all-sky conditions. The TOA SW flux uncertainties are generally larger for thin broken clouds than for moderate and thick overcast clouds. The TOA instantaneous daytime LW flux uncertainties derived from the CERES-MODIS test are 0.5 % (1.5 Wm−2), 0.8 % (2.4 Wm−2), and 0.7 % (1.3 Wm−2) over clear ocean, land, and snow/ice; and are about 1.5 % (3.5 Wm−2), 1.0 % (2.9 Wm−2), and 1.1 % (2.1 Wm−2) over ocean, land, and snow/ice under all-sky conditions. The TOA instantaneous nighttime LW flux uncertainties are about 0.5–1 % (< 2.0 Wm−2) for all surface types. Flux uncertainties caused by errors in scene identification are also assessed by using the collocated CALIPSO, CloudSat, CERES and MODIS data product. Errors in scene identification tend to underestimate TOA SW flux by about 0.6 Wm−2 and overestimate TOA daytime (nighttime) LW flux by 0.4 (0.2) Wm−2 when all CERES viewing angles are considered.

Three‐dimensional constrained variational analysis: Approach and application to analysis of atmospheric diabatic heating and derivative fields during an ARM SGP intensive observational period

AbstractAtmospheric vertical velocities and advective tendencies are essential large‐scale forcing data to drive single‐column models (SCMs), cloud‐resolving models (CRMs), and large‐eddy simulations (LESs). However, they cannot be directly measured from field measurements or easily calculated with great accuracy. In the Atmospheric Radiation Measurement Program (ARM), a constrained variational algorithm (1‐D constrained variational analysis (1DCVA)) has been used to derive large‐scale forcing data over a sounding network domain with the aid of flux measurements at the surface and top of the atmosphere (TOA). The 1DCVA algorithm is now extended into three dimensions (3DCVA) along with other improvements to calculate gridded large‐scale forcing data, diabatic heating sources (Q1), and moisture sinks (Q2). Results are presented for a midlatitude cyclone case study on 3 March 2000 at the ARM Southern Great Plains site. These results are used to evaluate the diabatic heating fields in the available products such as Rapid Update Cycle, ERA‐Interim, National Centers for Environmental Prediction Climate Forecast System Reanalysis, Modern‐Era Retrospective Analysis for Research and Applications, Japanese 55‐year Reanalysis, and North American Regional Reanalysis. We show that although the analysis/reanalysis generally captures the atmospheric state of the cyclone, their biases in the derivative terms (Q1 and Q2) at regional scale of a few hundred kilometers are large and all analyses/reanalyses tend to underestimate the subgrid‐scale upward transport of moist static energy in the lower troposphere. The 3DCVA‐gridded large‐scale forcing data are physically consistent with the spatial distribution of surface and TOA measurements of radiation, precipitation, latent and sensible heat fluxes, and clouds that are better suited to force SCMs, CRMs, and LESs. Possible applications of the 3DCVA are discussed.