Istvan Laszlo

Intercomparison of models representing direct shortwave radiative forcing by sulfate aerosols

The importance of aerosols as agents of climate change has recently been highlighted. However, the magnitude of aerosol forcing by scattering of shortwave radiation (direct forcing) is still very uncertain even for the relatively well characterized sulfate aerosol. A potential source of uncertainty is in the model representation of aerosol optical properties and aerosol influences on radiative transfer in the atmosphere. Although radiative transfer methods and codes have been compared in the past, these comparisons have not focused on aerosol forcing (change in net radiative flux at the top of the atmosphere). Here we report results of a project involving 12 groups using 15 models to examine radiative forcing by sulfate aerosol for a wide range of values of particle radius, aerosol optical depth, surface albedo, and solar zenith angle. Among the models that were employed were high and low spectral resolution models incorporating a variety of radiative transfer approximations as well as a line-by-line model. The normalized forcings (forcing per sulfate column burden) obtained with the several radiative transfer models were examined, and the discrepancies were characterized. All models simulate forcings of comparable amplitude and exhibit a similar dependence on input parameters. As expected for a non-light-absorbing aerosol, forcings were negative (cooling influence) except at high surface albedo combined with small solar zenith angle. The relative standard deviation of the zenith-angle-averaged normalized broadband forcing for 15 models was 8% for particle radius near the maximum in this forcing (∼0.2 μm) and at low surface albedo. Somewhat greater model-to-model discrepancies were exhibited at specific solar zenith angles. Still greater discrepancies were exhibited at small particle radii, and much greater discrepancies were exhibited at high surface albedos, at which the forcing changes sign; in these situations, however, the normalized forcing is quite small. Discrepancies among the models arise from inaccuracies in Mie calculations, differing treatment of the angular scattering phase function, differing wavelength and angular resolution, and differing treatment of multiple scattering. These results imply the need for standardized radiative transfer methods tailored to the direct aerosol forcing problem. However, the relatively small spread in these results suggests that the uncertainty in forcing arising from the treatment of radiative forcing of a well-characterized aerosol at well-specified surface albedo is smaller than some of the other sources of uncertainty in estimates of direct forcing by anthropogenic sulfate aerosols and anthropogenic aerosols generally.

Global distribution of photosynthetically active radiation as observed from satellites

Abstract Concern about possible effects of a steady increase in CO2 on the earths climate, and the fact that current estimates of sources and sinks of CO2 do not balance, generated interest to improve knowledge of rates at which carbon is cycled between the oceans, land, and atmosphere. The net primary productivity (NPP)—namely, the rate at which inorganic carbon is transformed into organic matter—is strongly controlled by the availability and intensity of photosynthetically active radiation (PAR); the distribution of photoactive pigments; the efficiency with which the light is absorbed; and the efficiency of its conversion into organic matter. In this study the feasibility to derive one of the above parameters is demonstrated—namely, PAR on a global scale. In the past, information on PAR was obtained from local ground measurements in the 0.4−0.7-µm spectral interval. In the absence of such measurements, PAR was estimated from measured total solar irradiance, using empirical “conversion factors.” It is d...

Suomi‐NPP VIIRS aerosol algorithms and data products

[1]xa0The Visible Infrared Imaging Radiometer Suite (VIIRS) instrument on board the Suomi National Polar-orbiting Partnership (S-NPP) spacecraft was launched in October 2011. The instrument has 22 spectral channels with band centers from 412u2009nm to 12,050u2009nm. The VIIRS aerosol data products are derived primarily from the radiometric channels covering the visible through the short-wave infrared spectral regions (412u2009nm to 2250u2009nm). The major components of the VIIRS aerosol retrieval process are data screening, land inversion, ocean inversion, suspended matter typing, and aggregation. The primary data product produced is the aerosol optical thickness (AOT) environmental data record. A higher resolution AOT intermediate product is also produced. These AOT products and their corresponding retrieval algorithms are described in detail, including theoretical basis, retrieval limitations, and data quality flagging. Preliminary evaluation of the data products has been undertaken by the VIIRS aerosol calibration/validation team using Aerosol Robotic Network ground-based observations to show that the performance of AOT retrievals meets the requirements specified in the Joint Polar Satellite System Level 1 requirements.

Preliminary evaluation of S‐NPP VIIRS aerosol optical thickness

The Visible Infrared Imaging Radiometer Suite (VIIRS) is the next-generation polar-orbiting operational environmental sensor with a capability for global aerosol observations. The VIIRS aerosol Environmental Data Record (EDR) is expected to continue the decade-long successful multispectral aerosol retrieval from the NASAs Earth Observing System Moderate Resolution Imaging Spectroradiometer (MODIS) for scientific research and applications. Since the launch of the Suomi National Polar-orbiting Partnership (S-NPP), the VIIRS aerosol calibration/validation team has been continuously monitoring, evaluating, and improving the performance of VIIRS aerosol retrievals. In this study, the VIIRS aerosol optical thickness (AOT) at 550u2009nm EDR at current Provisional maturity level is evaluated by comparing it with MODIS retrievals and measurements from the Aerosol Robotic Network (AERONET) and the Maritime Aerosol Network (MAN). The VIIRS global mean AOT at 550u2009nm differs from that of MODIS by approximately −0.01 over ocean and 0.03 over land (0.00 and −0.01 for the collocated retrievals) but shows larger regional biases. Global validation with AERONET and with MAN measurements shows biases of 0.01 over ocean and −0.01 over land, with about 64% and 71% of retrievals falling within the expected uncertainty range established by MODIS over ocean (±(0.03u2009+u20090.05AOT)) and over land (±(0.05u2009+u20090.15AOT)), respectively. The VIIRS retrievals over land exhibit slight overestimation over vegetated surfaces and underestimation over soil-dominated surfaces. These results show that the VIIRS AOT at 550u2009nm product provides a solid global data set for quantitative scientific investigations and environmental monitoring.

Shortwave cloud-radiative forcing at the top of the atmosphere at the surface and of the atmospheric column as determined from ISCCP C1 data

To understand the role of clouds in the atmospheric circulation and in the modulation of energy available at the surface, their effect on the atmospheric and surface absorption should be determined. The C1 data of the International Satellite Cloud Climatology Project, along with the Satellite Algorithm for Shortwave Radiation Budget, are used to estimate the shortwave cloud effects in terms of the cloud-radiative forcing at the top of the atmosphere (TOA), at the surface and of the atmospheric column on a global scale for the July months of 1983-1985. Global means of TOA cloud forcing range from [minus]43.6 (1983) to [minus]39.1 Wm[sup [minus]2] (1985). The cloud forcing for July 1985 is underestimated, by about 8 Wm[sup [minus]2], compared with that obtained from the Earth Radiation Budget Experiment. The cloud forcing at the surface is almost identical to that at the TOA, indicating that the effect of clouds on the shortwave energy budget of the surface-atmosphere system is such that most of the cooling is at the surface. Regression analysis of the computed fluxes shows a strong linear correlation between the TOA and surface cloud forcing. The monthly averaged regional values of the atmospheric cloud forcing are generally less than themorexa0» estimated uncertainty of 20 Wm[sup [minus]2]. Assuming that the uncertainties cancel, the global mean of the atmospheric cloud forcing is between 1 and 2 Wm[sup [minus]2], suggesting a slight warming owing to the presence of clouds. 35 refs., 9 figs., 3 tabs.«xa0less

Validation and expected error estimation of suomi‐NPP VIIRS aerosol optical thickness and ångström exponent with AERONET

The new-generation polar-orbiting operational environmental sensor, the Visible Infrared Imaging Radiometer Suite (VIIRS) on board the Suomi National Polar-orbiting Partnership (S-NPP) satellite, provides critical daily global aerosol observations. As older satellite sensors age out, the VIIRS aerosol product will become the primary observational source for global assessments of aerosol emission and transport, aerosol meteorological and climatic effects, air quality monitoring, and public health. To prove their validity and to assess their maturity level, the VIIRS aerosol products were compared to the spatiotemporally matched Aerosol Robotic Network (AERONET) measurements. Over land, the VIIRS aerosol optical thickness (AOT) environmental data record (EDR) exhibits an overall global bias against AERONET of −0.0008 with root-mean-square error (RMSE) of the biases as 0.12. Over ocean, the mean bias of VIIRS AOT EDR is 0.02 with RMSE of the biases as 0.06. The mean bias of VIIRS Ocean Angstrom Exponent (AE) EDR is 0.12 with RMSE of the biases as 0.57. The matchups between each product and its AERONET counterpart allow estimates of expected error in each case. Increased uncertainty in the VIIRS AOT and AE products is linked to specific regions, seasons, surface characteristics, and aerosol types, suggesting opportunity for future modifications as understanding of algorithm assumptions improves. Based on the assessment, the VIIRS AOT EDR over land reached Validated maturity beginning 23 January 2013; the AOT EDR and AE EDR over ocean reached Validated maturity beginning 2 May 2012, excluding the processing error period 15 October to 27 November 2012. These findings demonstrate the integrity and usefulness of the VIIRS aerosol products that will transition from S-NPP to future polar-orbiting environmental satellites in the decades to come and become the standard global aerosol data set as the previous generations missions come to an end.

Estimates of surface ultraviolet radiation over north America using Geostationary Operational Environmental Satellites observations

[1]xa0Information on ultraviolet (UV) radiative fluxes is needed for public safety, understanding biodiversity, and for chemical transport modeling. Space-based observations can provide homogeneous and systematic estimates of the UV flux over large regions. In the past, UV flux estimates have been made from polar orbiting satellites; such estimates lack information on diurnal variability that can result in significant errors in UV dose (diurnally integrated UV flux). An algorithm has been developed to estimate diurnally varying spectral UV flux at the surface based on information from geostationary satellites (cloud amount, surface albedo and aerosols) and from polar orbiting satellites (ozone). Algorithm evaluation is done by comparison with ground-based observations made between January 1998 and December 2000 over eighteen stations of the United States Department of Agriculture (USDA)s UV monitoring network. A good agreement between ground-based observations and satellite estimates is found with a mean bias (satellite − ground) of +3.5% for all-sky (cloudy + clear) cases. A negative mean bias of the same magnitude is found for clear-sky cases. Root mean square (RMS) differences are 25% and 14% for all-sky and clear-sky cases, respectively. Using simulations, it is shown that when only one observation near noontime is used to estimate UV dose, errors in the range of −61% to 48% can result, depending on cloud conditions. The RMS difference is 9% and it increases to 13% when off-noon hour (±2 hrs) observations are used to estimate the UV flux over Queenstown, MD.

An enhanced VIIRS aerosol optical thickness (AOT) retrieval algorithm over land using a global surface reflectance ratio database

The Visible/Infrared Imager Radiometer Suite (VIIRS) on board the Suomi National Polar-orbiting Partnership (S-NPP) satellite has been retrieving aerosol optical thickness (AOT), operationally and globally, over ocean and land since shortly after S-NPP launch in 2011. However, the current operational VIIRS AOT retrieval algorithm over land has two limitations in its assumptions for land surfaces: (1) it only retrieves AOT over the dark surfaces and (2) it assumes that the global surface reflectance ratios between VIIRS bands are constants. In this work, we develop a surface reflectance ratio database over land with a spatial resolution 0.1°u2009×u20090.1° using 2u2009years of VIIRS top of atmosphere reflectances. We enhance the current operational VIIRS AOT retrieval algorithm by applying the surface reflectance ratio database in the algorithm. The enhanced algorithm is able to retrieve AOT over both dark and bright surfaces. Over bright surfaces, the VIIRS AOT retrievals from the enhanced algorithm have a correlation of 0.79, mean bias of −0.008, and standard deviation (STD) of error of 0.139 when compared against the ground-based observations at the global AERONET (Aerosol Robotic Network) sites. Over dark surfaces, the VIIRS AOT retrievals using the surface reflectance ratio database improve the root-mean-square error from 0.150 to 0.123. The use of the surface reflectance ratio database also increases the data coverage of more than 20% over dark surfaces. The AOT retrievals over bright surfaces are comparable to MODIS Deep Blue AOT retrievals.

Interannual variability of solar irradiance over the Amazon Basin including the 1982-83 El Nino Year

Surface solar irradiance (SW[down arrow]) was derived over the extended Amazon Basin using AVHRR observations from polar-orbiting satellites during four July months ( 1983-1986). Observations from the geostationary satellite GOES for July 1983 were also used to assess diurnal effects. Both satellite datasets are part of the Satellite Cloud Climatology Project (ISCCP) B3 product. It was demonstrated that it is now possible to derive long-term surface SW[down arrow], which can be useful in climate studies, and that the accuracy of the derived fields is sufficient to detect interannual differences that can exceed at times 70 W M[sup [minus]2]. The variability of the daily totals of SW[down arrow] from the monthly means was similar during three of the four years investigated, yet, during the El Nino year of 1982-83, north of 10[degrees]N such variability increased drastically. This increase could be attributed to a changed pattern of convective activity as a result of higher SST off the coast of Peru. For the first time, the El Nino influence on the large-scale variability of the SW[down arrow] was demonstrated.

Comparison of single‐channel and multichannel aerosol optical depths derived from MAPSS data

[1]xa0Previous comparisons of the single-channel and multichannel aerosol products reported in the Clouds and the Earths Radiant Energy System (CERES) Single Scanner Footprint (SSF) data sets showed systematic differences that were partly attributed to differences in sampling and cloud screening. This study concentrates on quantifying the aerosol optical depth (AOD) differences when the above differences are absent and exactly the same clear radiances are inputted to the aerosol algorithms used to generate the two products. This is accomplished by retrieving AOD with the single-channel algorithm at 22 oceanic locations from the reflectance data in the Moderate Resolution Imaging Spectroradiometer (MODIS) Atmosphere Parameters Subset Statistics (MAPSS) data set for the period of 2000–2007 and then by comparing them to the corresponding MODIS AOD data reported in MAPSS. Comparisons of AODs are performed for two MODIS instruments flown onboard the Terra and Aqua platforms at two wavelengths. On average, the mean differences are wavelength and platform dependent. The single-channel 644-nm AODs are larger by 0.004–0.015 (∼2–9%) than those from the multichannel algorithm. The mean AOD at 1632 nm from both algorithms are very similar from Terra, but the single-channel AOD from Aqua at 2119 nm is lower by 0.02 (∼24%). The mean absolute differences are 0.022–0.025 and do not change much with wavelength or platform. Slight dependence of the mean differences on the scattering angle is observed, which is partially explained by the differences between the retrieved aerosol model in the multichannel retrieval and the fixed aerosol model used in the single-channel algorithm.