Optical Thickness Distribution Research Articles

Average aerosol extinction and water vapor profiles over the Southern Great Plains

An operational Raman lidar deployed at the DOE Atmospheric Radiation Measurement site in Oklahoma has collected more than 7500 h of aerosol and water vapor data between April 1998 and January 2000. These data, which span a wide variety of atmospheric conditions, have been analyzed as a function of season, integrated amount, and time of day. The scale height of the mean aerosol profiles varies considerably as both a function of season and aerosol optical thickness, with the mean scale height increasing from less than 1 km in the winter to over 2 km during turbid summer days. The mean scale height of the water vapor remained very close to 2 km, regardless of season or precipitable water vapor. Furthermore, the distribution of aerosol optical thickness shows a slight shift to smaller values at night compared to day, and the mean aerosol profiles show little diurnal dependence except at the top of the boundary layer.

Retrieval of columnar water vapour over land from backscattered solar radiation using the Medium Resolution Imaging Spectrometer

We describe a new algorithm to derive columnar water vapour under cloud-free conditions over land from backscattered solar radiation in the near-infrared. The algorithm will be used in ESA's Medium Resolution Imaging Spectrometer (MERIS) ground processor. It is based on radiative transfer simulations, where the radiance ratio between the MERIS channels 15 (900 nm) and 14 (885 nm) is used in an inversion procedure based on regressions. The theoretical accuracy of the algorithm is about 1.7 kg/m 2. We discuss and quantify possible error sources using radiative transfer simulations. These error sources are variable aerosol optical thickness, type, and vertical distribution, spectral variations in land surface reflectance, deviations between the actual and nominal bandsetting of the MERIS, sensor noise, variations in surface pressure; and temperature variations. For validation, we re-calibrate the algorithm to the bandsettings of the Modular Optoelectronical Scanner (MOS), which has been flown on the Indian IRS platform since 1996. Comparisons of the retrieved water vapour path (WVP) with colocated radiosoundings for 239 cases in the period 1996–1999 show a RMSE of 2.49 kg/m 2 with a BIAS component of 0.04 kg/m 2.

Sources and distributions of dust aerosols simulated with the GOCART model

The global distribution of dust aerosol is simulated with the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model. In this model all topographic lows with bare ground surface are assumed to have accumulated sediments which are potential dust sources. The uplifting of dust particles is expressed as a function of surface wind speed and wetness. The GOCART model is driven by the assimilated meteorological fields from the Goddard Earth Observing System Data Assimilation System (GEOS DAS) which facilitates direct comparison with observations. The model includes seven size classes of mineral dust ranging from 0.1–6 μm radius. The total annual emission is estimated to be between 1604 and 1960 Tg yr−1 in a 5‐year simulation. The model has been evaluated by comparing simulation results with ground‐based measurements and satellite data. The evaluation has been performed by comparing surface concentrations, vertical distributions, deposition rates, optical thickness, and size distributions. The comparisons show that the model results generally agree with the observations without the necessity of invoking any contribution from anthropogenic disturbances to soils. However, the model overpredicts the transport of dust from the Asian sources to the North Pacific. This discrepancy is attributed to an overestimate of small particle emission from the Asian sources.

Microphysical and radiative properties of stratocumulus clouds: the EUCREX mission 206 case study

In this conclusion paper, remote sensing retrievals of cloud optical thickness performed during the EUCREX mission 206 are analyzed. The comparison with estimates derived from in situ measurements demonstrates that the adiabatic model of cloud microphysics is more realistic than the vertically uniform plane parallel model (VUPPM) for parameterization of optical thickness. The analysis of the frequency distributions of optical thickness in the cloud layer then shows that the adiabatic model provides a good prediction when the cloud layer is thick and homogeneous, while it overestimates significantly the optical thickness when the layer is thin and broken. Finally, it is shown that the effective optical thickness over the whole sampled cloud is smaller than the adiabatic prediction based on the mean geometrical thickness of the cloud layer. The high sensitivity of the optical thickness on cloud geometrical thickness suggests that the effect of aerosol and droplet concentration on precipitation efficiency, and therefore on cloud extent and lifetime, is likely to be more significant than the Twomey effect.

Global three‐dimensional simulation of aerosol optical thickness distribution of various origins

A global three‐dimensional model that can treat transportation of various species of aerosols in the atmosphere is developed using a framework of an atmospheric general circulation model (AGCM). Main aerosols in the troposphere, i.e., soil dust, carbonaceous (organic and black carbon), sulfate, and sea‐salt aerosols, are introduced into this model. Prior to the model calculations the meteorological parameters are calculated by the AGCM with the nudging technique using reanalysis data. To evaluate aerosol effects on the climate system and to compare simulated results with observations, the optical thickness and Ångström exponent are also calculated taking into account the size distribution and composition. The model results are validated by both measured surface aerosol concentrations and retrieved aerosol optical parameters from National Oceanic and Atmospheric Administration/Advanced Very High Resolution Radiometer. A general agreement is found between the simulated result and the observation globally and seasonally. One of the significant results is that the simulated relative contribution of anthropogenic carbonaceous aerosols to the total optical thickness is comparable to that of sulfate aerosols at midlatitudes of the Northern Hemisphere, which agrees with recent observations. This result leads to a conclusion that the radiative effect evaluation of aerosols on the climate system is necessary to be modified because optical properties of carbonaceous aerosols are different from those of sulfate aerosols. The other finding is that the seasonal shift off the west coast of North Africa observed by satellites, i.e., the latitude of the maximum optical thickness moves seasonally, is also reproduced in consideration of a mixed state of soil dust and carbonaceous aerosols.

Temporal and spatial distribution of the stratospheric aerosol optical thickness at 1013 nm derived from the ORA Experiment after the Pinatubo eruption

In this article, we present aerosol optical thickness data, computed from the aerosol extinctions measured by the ORA experiment at 1013 nm. Temporal and latitudinal dependences are discussed for the period Aug. 1992 ­ May 1993. A comparison with SAGE II optical thickness data is presented, and shows good agreement.

Satellite and ground‐based study of optical properties of 1997 Indonesian Forest Fire aerosols

Optical properties of biomass burning aerosols in the event of Indonesian forest fires in 1997 were studied by groundbased sky radiometry and satellite remote sensing with AVHRR and TOMS radiometers. The AVHRR‐derived optical thickness distribution agreed with the distribution of TOMS‐derived UV‐absorbing aerosol index and with the optical thickness measured by sky radiometry and sunphotometry. The single scattering albedo of aerosols was fairly constant as 0.9 in the September–October period. Relationship between Ångström turbidity factor and exponent supported the polydispersion consisted of aged small particles. This observation was consistent with the fact that the retrieved volume size distribution by sky radiometry has a distinct accumulation mode with a peak radius of 0.25 µm. Those optical properties of smoke aerosols seem to reflect the specific chemical structure of Indonesian forest fire aerosols, i.e., a mixture of carbonaceous and sulfate particles.

Albedo bias and the horizontal variability of clouds in subtropical marine boundary layers: Observations from ships and satellites

Cloud optical properties vary dramatically at spatial scales smaller than typical grid cells in large‐scale models, which can cause a significant overestimate of cloud albedo by the model. This plane parallel homogeneous (PPH) albedo bias exist may be reduced if the mean cloud optical thickness and the amount of variability are available, but little is known about how much variability exists in nature and to what factors it is sensitive. The authors combine 1331 observations made by volunteer surface observers with satellite imagery to assess the relationships between cloud fraction, cloud optical properties, and cloud type in marine boundary layer clouds off the coast of California during summer. Estimates of cloud fraction from the two datasets are in best agreement when a reflectance threshold between 0.09 and 0.10 is used. Satellite‐derived cloud fraction increases slowly with sensor resolution at spatial scales from 1 to 32 km. Cloud fraction in scenes dominated by cumulus is much more sensitive to the reflectance threshold used for cloud detection than are scenes containing stratiform clouds. The mean magnitude of the PPH bias found here, 0.025, is considerably smaller than those found in other recent studies. When fit to the observed distributions of optical thickness both log‐normal and gamma distributions substantially reduce the PPH bias. The mean and dispersion of log optical thickness are related to cloud type: optical thickness increases as cloud type changes from cumuliform to stratiform, while the relative amount of variability decreases. The authors suggest a basis for the parameterization of unresolved variability in large scale models.

Similarity and diversity of OCTS and POLDER from an ocean color analysis point of view

Comparison and combination of two ocean color sensors, ADEOS/OCTS and POLDER, are treated in this work. These new sensors possess high radiometric sensitivity and multi-spectral bands in the visible and near-infrared wavelengths. It is of interest to mention that the POLDER provides the data at the multiviewing angles. These directional measurements include significant information of atmospheric aerosols. In addition POLDER measures polarization as well as radiance. It is shown here that aerosol retrieval can be efficiently pursued by a combination of radiance and polarization. As a result, it is found that the retrieved distribution of aerosol optical thickness and the phytoplankton pigment concentration near the sea surface obtained from the OCTS data coincide with those from the POLDER data within the tolerances required at the present time.

Retrieval of aerosol optical thickness and size distribution over ocean from the MODIS airborne simulator during TARFOX

Radiation and in‐situ measurements collected during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) are used to test the method for remote sensing of aerosol properties and loading from the MODIS instrument. MODIS, a Moderate Resolution Imaging Spectroradiometer, will be launched in 1999 aboard the first EOS (Earth Observing System). Following the MODIS procedure [Tanré et al, 1997], the spectral radiance at the top of the atmosphere (TOA) measured over the ocean in a wide spectral range (0.55–2.13 μm) is used to derive the aerosol optical thickness (proportional to the aerosol total loading) and the aerosol size distribution (integrated over the vertical column) of the ambient (undisturbed) aerosol by comparing measured radiances with values in look‐up table (LUT). The LUT includes the gas‐phase oxidation accumulation mode, cloud‐phase accumulation mode, and a coarse mode that represents maritime particles (salt) and dust. In each inversion, one accumulation and one coarse mode can be retrieved. The inversion retrieves the ratio of the contribution to the optical thicknesses of the two particle modes and the mean particle size that best fits the measurements. This algorithm is successfully applied to the data sets acquired during TARFOX. The MODIS airborne simulator (MAS) aboard the NASA ER‐2 aircraft flew several times during the experiment above the University of Washington C‐131A research aircraft on which the six‐channel Ames Airborne Tracking Sun Photometer (AATS‐6) was mounted. It flew also above surface‐based Sun photometers. Optical thicknesses (at λ = 550 nm) as well as the spectral dependence from the various data sets compare very well.

Influence of the Pinatubo eruption on the aerosol optical depth in the Arctic in the summer of 1993

In the summer of 1993, measurements of the spatial distribution of atmospheric optical aerosol thickness were conducted in the region of the Laptev Sea, the Kara Sea and the Taimyr peninsula, using an aircraft visible and UV band spectrophotometer. The Arctic atmosphere's aerosol optical depth was measured using I1-18 ‘Cyclone’ aircraft-meteolab as a platform at an altitude ranges of 100–8500 m. It was observed that the troposphere aerosol was concentrated in the altitude range 100–4000 m. The light extinction of the troposphere column was approximately 0.05 for a wavelength of 400 nm. A comparison with the results of measurements made in the same region in March-April 1990 shows a decrease of more than 3–5 times in troposphere aerosol optical depth in the Arctic during summer months. It was also found that there was a relatively clean area above 4000 m in this season in the Arctic troposphere. In addition, an increase in aerosol optical thickness in the stratosphere is observed. The value of aerosol optical depth measured from the level 8200 m was 0.1 for a wavelength of 500 nm. The spectral dependence of the stratospheric optical thickness has a complicated form with at least two local maxima. Estimates of the size and concentration of stratospheric aerosol particles are in agreement with in-situ measurements after the Mount Pinatubo eruption.

Beam size and collimation effects in spectroscopic ellipsometry of transparent films with optical thickness inhomogeneity

In spectroscopic ellipsometry (SE) measurements of thin films with inhomogeneous optical thickness, the spot size of the probe beam on the sample surface is a highly critical parameter in the treatment of the experimental data. SE measurements were performed on transparent samples prepared by various techniques: plasma enhanced chemical vapor deposition of diamond film, spin-coated PMMA films and thermally grown SiO 2 film. The first two deposition techniques can produce films with inhomogeneous optical thickness. By comparing SE measurements using different set-up configurations, we demonstrate that a combination of lenses and small diaphragms is most suitable for minimizing the effects related to the optical thickness distribution and light beam divergence. In this way mistakes are avoided in extracting physical parameters from SE optical modeling of these films.

Contribution to the atmospheric mineral aerosol load from land surface modification

An estimation of the contribution of mineral dust from disturbed soils (i.e., soils affected by human activity and/or climate variability) to the total atmospheric mineral aerosol load is presented. A three‐dimensional atmospheric dust transport model was used to simulate the distribution of dust optical thickness in response to individual dust sources, which include natural soils known to have been affected by the Saharan/Sahelian boundary shift, cultivation, deforestation, and wind erosion. The distributions extracted from advanced very high resolution radiometer (AVHRR) optical thickness retrievals were used to constrain likely source combinations. The results indicate that observed features like the seasonal shift of maximum optical thickness caused by Saharan dust over the Atlantic ocean are best reproduced if disturbed sources contribute 30–50% of the total atmospheric dust loading.

Precipitation in Stratocumulus Clouds: Observational and Modeling Results

The spatial and temporal variability of precipitating stratocumulus layers is examined using aircraft observations, satellite retrievals of cloud optical depth, and one-dimensional models that include coalescence and a simple representation of layer turbulence. The aircraft observations show large horizontal variations in cloud thickness and precipitation, with local rain rates 4–5 times larger than the replacement moisture flux, and evidence for precipitation scavenging of small cloud droplets. The satellite observations show that, despite this local water loss, the distribution of cloud optical thickness remains nearly constant over the course of a day, indicating that on larger scales precipitation removal and cloud-top entrainment are in approximate balance with the vapor flux. The authors apply analytic and numerical models of steady-state precipitation to the observed microphysical conditions, and find that the models can match the drop size distributions observed during both heavy and light stratocumulus rainfall, but are especially sensitive to the processes governing the growth rate of the smallest drizzle drops.

Impact of Kuwait's oil-fire smoke cloud on the sky of Bahrain

The effects of the Kuwaiti oil well fires of 1991 on the atmospheric parameters of Bahrain (∼600 km southeast of Kuwait) were observed. Solar radiation, optical thickness, ultraviolet radiation, horizontal visibility, temperature, and solar spectral distribution were measured for 1991 and compared to the long-term values of 1985–1990. The relative monthly solar radiation in Bahrain was reduced by 8% (February) when 50 oil wells were burning and reduced further to 20% when 470 oil wells were on fire (April–July). In November 1991, when there were 12 oil wells burning, the recorded solar radiation became nearly equal to the long-term average. The monthly average daily optical thickness, τ, for the direct or beam solar radiation was calculated. The values of τ were found to be larger in 1991 than the average for the years 1985–1990 by nearly 58% during June and returned to normal in October (after nearly all the oil well fires were extinguished). The clear and smoked sky solar spectra distribution were detected before and during the burning of the Kuwait oil wells. Large absorption of the solar radiation was noticed on the 2nd and 3rd of March, 1991. The daily average infrared radiation during 1990 was found to be 6700.4 Whm −2 and shifted to 9182.1 Whm −2 in 1991. Comparison was also made between 1990 and 1991 data of the global solar radiation and the temperature. The effect of the burning oil wells in Kuwait on the horizontal visibility in Bahrain have also been studied. The changes in the above parameters were mainly attributed to the plumes driven by the wind from the burning oil wells to the Island of Bahrain.

Detection of the SO2 Atmosphere on Io with the Hubble Space Telescope

Observations of the trailing hemisphere of Io made with the Faint Object Spectrograph of the Hubble Space Telescope in March 1992 have resulted in the first detection of atmospheric SO2 absorption bands in the ultraviolet. These observations represent only the third positive means of detection of what is widely believed to be Io's primary atmospheric constituent. Below ∼2130 Å the geometric albedo of the satellite is dominated by SO2 gas absorption band signatures, which have been analyzed using models that include the effects of optical thickness, temperature, and spatial distribution. The disk-integrated HST data cannot resolve the spatial distribution, but it is possible to define basic properties and set constraints on the atmosphere at the time of the observations, Hemispheric atmospheres with average column density N = 6-10 × 1015 cm-2 and Tgas = 110-500 K fit the data, with preference for temperatures of ∼200-250 K. Better fits are found as the atmosphere is spatially confined, with a limit of ∼8% hemispheric areal coverage and N ≈ 3 × 1017 cm-2 with colder 110-250 K temperatures. A dense (N ≥1016 cm-2), localized component of SO2 gas, such ns that possibly associated with active volcanoes, can generate the observed spectral contrast only when the atmosphere is cold (110 K) and an extended component such as Pele is included. The combination of a dense, localized atmosphere with a tenuous component (N < 1016 cm-2 either patchy or extended) also fits the data. In all cases the best fit models imply a disk-averaged column density larger than exospheric but ∼10-30 times less than the previous upper limit from near-UV observations.

Temporal and spatial variability of aerosol optical depth in the Sahel region in relation to vegetation remote sensing

In order to monitor the aerosol characteristics needed for atmospheric correction of remotely sensed data, a network of sun photometers was established in the Sahel region of Senegal, Mali, and Niger. Data analysis suggests that there is a high spatial variability of the aerosol optical thickness tau(a) in the western Sahel region. At a 67 percent confidence level the instantaneous values of tau(a) can be extrapolated approximately 270-400 km with an error tolerance of 50 percent. Spatial variability in the dry season is found to be of a similar magnitude. The ranges of variations in the NDVI in the Sahel region are shown to be approximately 0.02 and 0.01, respectively, due to commonly observed fluctuations in the aerosol optical thickness and aerosol size distribution.

Fringe pattern analysis using hybrid image processing

Hybrid image processing using optical defocusing and digital computing techniques is investigated for use in the analysis of interferograms or moire fringes. The optical thickness distribution of a glass plate is measured from the interferogram, and the power distribution of an eyeglass lens is tested from the moire pattern. The optical defocusing technique is also used for preprocessing of the input image to suppress high frequency noises. The optimum defocus amount relating to the processed result is discussed.

The effects of aerosols on the response of a two-dimensional zonally-averaged climate model

The response of climate processes to changes in aerosols is investigated using a two-dimensional zonally-averaged climate model. To account for the spatial and temporal heterogeneities of aerosol distributions, a strongly scattering maritime aerosol over ocean and a strongly absorbing rural aerosol over land are used in the layer between the surface and the 700 mb level. The effects of aerosols, gases and clouds on the radiative transfer are computed concurrently using a parameterized form of a two-stream approximation. Aerosols induce a reduction of the solar absorption at the surface and an enhancement of atmospheric absorption. The resulting changes in the climate model parameters are not only a function of the radiatively-active atmospheric constituents, but also of the amplifying or dampening effects of feedback processes employed in the model calculations. For the specified aerosol types and the optical thickness distribution given by Potter and Cess, the annual temperature change at the surface is −2.1 °C with ice-albedo feedback. Without ice-albedo feedback the corresponding value is −1.4 °C. In the present study aerosols produce a cooling effect in the atmosphere at all latitudes.

Retrieval of Cloud Cover Parameters from Multispectral Satellite Images

A technique is described for extracting cloud cover parameters from multispectral satellite radiometric measurements. Utilizing three channels from the AVHRR (Advanced Very High Resolution Radiometer) on NOAA polar orbiting satellites, it is shown that one can retrieve four parameters for each pixel: cloud fraction within the FOV, optical thickness, cloud-top temperature and a microphysical model parameter. The last parameter is an index representing the properties of the cloud particle and is determined primarily by the radiance at 3.7 microns. The other three parameters are extracted from the visible and 11 micron infrared radiances, utilizing the information contained in the two-dimensional scatter plot of the measured radiances. The solution is essentially one in which the distributions of optical thickness and cloud-top temperature are maximally clustered for each region, with cloud fraction for each pixel adjusted to achieve maximal clustering.