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NASA NTRS1995

Remote sensing of smoke, clouds, and radiation using AVIRIS during SCAR experiments

During the past two years, researchers from several institutes joined together to take part in two SCAR experiments. The SCAR-A (Sulfates, Clouds And Radiation - Atlantic) took place in the mid-Atlantic region of the United States in July, 1993. remote sensing data were acquired with the Airborne Visible Infrared Imaging Spectrometer (AVIRIS), the MODIS Airborne Simulator (MAS), and a RC-10 mapping camera from an ER-2 aircraft at 20 km. In situ measurements of aerosol and cloud microphysical properties were made with a variety of instruments equipped on the University of Washington's C-131A research aircraft. Ground based measurements of aerosol optical depths and particle size distributions were made using a network of sunphotometers. The main purpose of SCAR-A experiment was to study the optical, physical and chemical properties of sulfate aerosols and their interaction with clouds and radiation. Sulfate particles are believed to affect the energy balance of the earth by directly reflecting solar radiation back to space and by increasing the cloud albedo. The SCAR-C (Smoke, Clouds And Radiation - California) took place on the west coast areas during September - October of 1994. Sets of aircraft and ground-based instruments, similar to those used during SCAR-A, were used during SCAR-C. Remote sensing of fires and smoke from AVIRIS and MAS imagers on the ER-2 aircraft was combined with a complete in situ characterization of the aerosol and trace gases from the C-131A aircraft of the University of Washington and the Cesna aircraft from the U.S. Forest Service. The comprehensive data base acquired during SCAR-A and SCAR-C will contribute to a better understanding of the role of clouds and aerosols in global change studies. The data will also be used to develop satellite remote sensing algorithms from MODIS on the Earth Observing System.

Gao, Bo-Cai↗
NASA NTRS1995

Unified description of accretion flows around black holes

We provide a unified description of thermal equilibria of black hole accretion disks, including the newly discovered advection-dominated solutions. We classify the solutions on the basis of optical depth and impotance of advection cooling. We demonstrate that only four physically distinct topological types of equilibria exist. Two of the types correspond to optically thin and optically thick equilibria, while the other two types are distinguished by whether advection is negligible or dominant. A stable Shakura-Sunyaev disk exists only for accretion rates dot-M below a certain maximum. However, there is a critical viscosity parameter alpha(sub crit), which is a function of radius, such that for alpha greater than alpha(sub crit) advection-dominated solutions exist for all dot-M. Even when alpha less than alpha(sub crit), the advection-dominated solutions are available for a wide range of dot-M except for a gap around the Eddington rate. We therefore suggest that advection-dominated flows may be more common than standard thin disks in black hole systems. For certain ranges of radii and dot-M, no stable steady state solution is possible. In these cases, we suggest that limit cycle behavior may occur, leading to variability.

Chen, Xingming↗
NASA NTRS1999

Realistic NLTE Radiative Transfer for Modeling Stellar Winds

This NASA grant supported the development of codes to solve the non-LTE multi-level spherical radiative transfer problem in the presence of velocity fields. Much of this work was done in collaboration with Graham Harper (CASA, University of Colorado). These codes were developed for application to the cool, low-velocity winds of evolved late-type stars. Particular emphasis was placed on modeling the wind of lambda Velorum (K4 lb), the brightest K supergiant in the sky, based on extensive observations of the ultraviolet spectrum with the HST/GHRS from GO program 5307. Several solution techniques were examined, including the Eddington factor Approach described in detail by Bennett & Harper (1997). An Eddington factor variant of Harper's S-MULTI code (Harper 1994) for stationary atmospheres was developed and implemented, although full convergence was not realized. The ratio of wind terminal velocity to turbulent velocity is large (approx. 0.3-0.5) in these cool star winds so this assumption of stationarity provides reasonable starting models. Final models, incorporating specified wind laws, were converged using the comoving CRD S-MULTI code. Details of the solution procedure were published by Bennett & Harper (1997). Our analysis of the wind of lambda Vel, based on wind absorption superimposed on chromospheric emission lines in the ultraviolet, can be found in Carpenter et al. (1999). In this paper, we compare observed wind absorption features to an exact CRD calculation in the comoving frame, and also to a much quicker, but approximate, method using the SEI (Sobolev with Exact Integration) code of Lamers, Cerruti-Sola, & Perinotto (1987). Carpenter et al. (1999) provide detailed comparisons of the exact CRD and approximate SEI results and discuss when SEI is adequate to use for computing wind line profiles. Unfortunately, the observational material is insufficient to unambiguously determine the wind acceleration law for lambda Vel. Relatively few unblended Fe II lines of optical depth sensitive to the wind acceleration region are present in the existing HST/GHRS data set. Most of the Fe II lines are either too optically thick (resulting in a board, black wind absorption profile) or too optically thin (resulting in no wind absorption feature present). Also, most of the ultraviolet spectra obtained from HST GO-5307 was at medium resolution (R approx. 40,000, corresponding to a velocity resolution of 7.5 km/s). This is simply inadequate to resolve the turbulence in the outer wind; a key parameter in theoretical wind models. We can now say that an unambiguous determination of the wind velocity law in lambda Vel will require complete coverage of the ultraviolet spectrum at high dispersion (R approx. 10(exp 5), or 3 km/s). This is now feasible usin, the STIS echelle spectrograph on-board HST.

Bennett, Philip D.↗
NASA NTRS2008

Sensitivity of Photolysis Frequencies and Key Tropospheric Oxidants in a Global Model to Cloud Vertical Distributions and Optical Properties

As a follow-up study to our recent assessment of the radiative effects of clouds on tropospheric chemistry, this paper presents an analysis of the sensitivity of such effects to cloud vertical distributions and optical properties in a global 3-D chemical transport model (GEOS4-Chem CTM). GEOS-Chem was driven with a series of meteorological archives (GEOS1-STRAT, GEOS-3 and GEOS-4) generated by the NASA Goddard Earth Observing System data assimilation system, which have significantly different cloud optical depths (CODs) and vertical distributions. Clouds in GEOS1-STRAT and GEOS-3 have more similar vertical distributions while those in GEOS-4 are optically much thinner in the tropical upper troposphere. We find that the radiative impact of clouds on global photolysis frequencies and hydroxyl radical (OH) is more sensitive to the vertical distribution of clouds than to the magnitude of column CODs. Model simulations with each of the three cloud distributions all show that the change in the global burden of O3 due to clouds is less than 5%. Model perturbation experiments with GEOS-3, where the magnitude of 3-D CODs are progressively varied by -100% to 100%, predict only modest changes (<5%) in global mean OH concentrations. J(O1D), J(NO2) and OH concentrations show the strongest sensitivity for small CODs and become insensitive at large CODs due to saturation effects. Caution should be exercised not to use in photochemical models a value for cloud single scattering albedo lower than about 0.999 in order to be consistent with the current knowledge of cloud absorption at the UV wavelength. Our results have important implications for model intercomparisons and climate feedback on tropospheric photochemistry.

Liu, Hongyu↗
NASA NTRS2024

Long-Term Trends in Aerosols, Low Clouds, and Large-scale Meteorology over the Western North Atlantic from 2003 to 2020

A continuous decrease of aerosol over the western North Atlantic Ocean (WNAO) on decadal timescales provides a long-term experiment to evaluate how other natural and anthropogenic processes affect the manifestation of aerosol-cloud interactions in this region. Furthermore, the WNAO is a natural laboratory with diverse aerosol sources, marine boundary layer clouds that are more variable than marine stratocumulus deck regions, and unique flow regimes set up by the Gulf Stream and semi-permanent Bermuda High. We investigate how satellite-retrieved macrophysical and microphysical properties of low clouds and the surface shortwave irradiance changed from 2003 to 2020, in tandem with this aerosol decrease. The decadal changes in large-scale meteorology relating to the North Atlantic Oscillation (NAO) are also examined. We find no significant changes in low-cloud fraction but a widespread reduction in low-cloud optical depth attributed to fewer and larger cloud droplets with almost no change in cloud liquid water path. Despite robust signals in low-cloud optical properties together with aerosol decrease, a corresponding increase in the surface shortwave irradiance trends, also called surface brightening, is lacking. This absence of brightening is potentially due to concomitant changes found in large-scale meteorology associated with NAO— a Bermuda High strengthening, sea surface warming, and atmospheric moistening— as well as an increase in high cloud fraction that can counteract the surface brightening. Ultimately, our findings suggest that spatial patterns in the decadal meteorological variability, likely set up by NAO, contribute more to the surface cloud radiative effect over the WNAO than aerosol-cloud interactions.

J Minnie Park↗
NASA NTRS1996

A High Spectral Resolution Lidar Based on Absorption Filter

A High Spectral Resolution Lidar (HSRL) that uses an iodine absorption filter and a tunable, narrow bandwidth Nd:YAG laser is demonstrated. The iodine absorption filter provides better performance than the Fabry-Perot etalon that it replaces. This study presents an instrument design that can be used a the basis for a design of a simple and robust lidar for the measurement of the optical properties of the atmosphere. The HSRL provides calibrated measurements of the optical properties of the atmospheric aerosols. These observations include measurements of aerosol backscatter cross sections, optical depth, backscatter phase function depolarization, and multiple scattering. The errors in the HSRL data are discussed and the effects of different errors on the measured optical parameters are shown.

Piironen, Paivi↗
NASA NTRS2014

The Soft-X-Ray Emission of Ark 120. XMM-Newton, NuSTAR, and the Importance of Taking the Broad View

We present simultaneous XMM-Newton and NuSTAR observations of the 'bare' Seyfert 1 galaxy, Ark 120, a system in which ionized absorption is absent. The NuSTAR hard-X-ray spectral coverage allows us to constrain different models for the excess soft-X-ray emission. Among phenomenological models, a cutoff power law best explains the soft-X-ray emission. This model likely corresponds to Comptonization of the accretion disc seed UV photons by a population of warm electrons: using Comptonization models, a temperature of approximately 0.3 kiloelectronvolts and an optical depth of approximately 13 are found. If the UV-to-X-ray OPTXAGNF model is applied, the UV fluxes from the XMM-Newton Optical Monitor suggest an intermediate black hole spin. Contrary to several other sources observed by NuSTAR, no high-energy cutoff is detected with a lower limit of 190 kiloelectronvolts.

accretion↗
NASA NTRS2021

Variabilities in Pm2.5 and Black Carbon Surface Concentrations Reproduced by Aerosol Optical Properties Estimated by In-Situ Data, Ground Based Remote Sensing and Modeling

Because of the increased temporal and spatial resolutions of the sensors onboard recently launched satellites, satellite-based surface aerosol concentration, which is usually estimated from the aerosol optical depth (AOD), is expected to become a strategic tool for air quality studies in the future. By further exploring the relationships of aerosol concentrations and their optical properties using ground observations, the accuracies of these products can be improved. Here, we analyzed collocated observations of surface mass concentrations of fine particulate matter (PM2.5) and black carbon (BC), as well as columnar aerosol optical properties from a sky radiometer and aerosol extinction profiles obtained by multi-axis differential optical absorption spectroscopy (MAX-DOAS), during the 2019–2020 period. We focused the analyses on a daily scale, emphasizing the role of the ultraviolet (UV) spectral region. Generally, the correlation between the AOD of the fine fraction (i.e., fAOD) and the PM2.5 surface concentration was moderately strong, regardless of considerations of boundary layer humidity and altitude. In contrast, the fAOD of the partial column below 1 km, which was obtained by combining sky radiometer and MAX-DOAS retrievals, better reproduced the variability of the PM2.5 and resulted in a linear relationship. In the same manner, we demonstrated that the absorption AOD of the fine fraction (fAAOD) of the partial column was related to the variability of the BC concentration. Analogous analyses based on aerosol products from the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) confirmed these findings and highlighted the importance of the shape of the aerosol profile. Overall, our results indicated a remarkable consistency among the retrieved datasets, and between the datasets and MERRA-2 products. These results confirmed the well-known sensitivity to aerosol absorption in the UV spectral region; they also highlighted the efficacy of combined MAX-DOAS and sky radiometer observations.

Alessandro Damiani↗
NASA NTRS1977

The far-infrared spectrum of the core of Sagittarius B2

A Michelson interferometer aboard NASA's Kuiper Airborne Observatory has been used to measure the spectrum of Sgr B2 from 40 to 200 kaysers with 5-kayser resolution in a 1.4-arcmin beam. The measured spectrum is smooth and featureless with a broad maximum at about 85 kaysers. The data can be fitted analytically with a model corresponding to thermal emission by a uniform sla of dust filling the beam, with an average temperature of approximately 32 K, an optical depth at 100 microns of about 1.6, and a spectral index of the dust emissivity about 1.5. The absence of features implies either that the source is optically thick or that the emission spectrum of the individual grains is smooth in the passband. The possible physical significance of this model is discussed.

Erickson, E. F.↗
NASA NTRS1985

Generation and transfer of polarized radiation in the solar atmosphere: Physical mechanisms and magnetic-field diagnostics

The main physical mechanisms responsible for the generation and transfer of polarized radiation in the solar atmosphere can be classified in a suitable bidimensional diagram with an indicator of the magnetic field strength on its vertical axis and an indicator of the radiation field anisotropy on its horizontal axis. The various polarimetric observations performed on solar spectral lines are interpreted with different theoretical schemes according to their classification in the diagram and to the optical depths involved. These theoretical schemes, and the associated diagnostic tools for inferring the magnetic field vector from observations are reviewed. In particular, the role of magneto-optical effects in determining the direction of the observed linear polarization in active regions is discussed in some detail.

Deglinnocenti, E. L.↗
NASA NTRS1988

Some constraints on a greenhouse atmosphere for Triton

The possibility that a thick atmosphere exists around Neptune's satellite Triton is examined. The IR optical depth in the gray atmosphere approximation is computed for a range of possible surface compositions, albedos, and gravities. It is found that a self-sustaining optically-thick atmosphere is possible if molecular nitrogen and/or hydrogen are present. It is suggested that bimodal behavior of Triton's atmosphere is possible as seasonal effects and volatile distribution alter the distribution of thermal emission.

Nolan, Michael C.↗
NASA NTRS2000

Ground-Based Lidar Measurements of Aerosols During ACE-2 Instrument Description, Results, and Comparisons with Other Ground-Based and Airborne Measurements

A micro-pulse lidar system (MPL) was used to measure the vertical and horizontal distribution or aerosols during the Aerosol Characterization Experiment 2 (ACE-2) in June and July of 1997. The MPL measurements were made at the Izana observatory (IZO), a weather station located on a mountain ridge (28 deg 18'N, 16 deg 30'W, 2367 m asl) near the center of the island of Tenerife, Canary Islands. The MPL was used to acquire aerosol backscatter, extinction, and optical depth profiles for normal background periods and periods influenced by Saharan dust from North Africa. System tests and calibration procedures are discussed, and in analysis of aerosol optical profiles acquired during ACE-2 is presented. MPL data taken during normal IZO conditions (no dust) showed that upslope aerosols appeared during the day and dissipated at night and that the layers were mostly confined to altitudes a few hundred meters above IZO. MPL data taken during a Saharan dust episode on 17 July showed that peak aerosol extinction values were an order of magnitude greater than molecular scattering over IZO. and that the dust layers extended to 5 km asl. The value of the dust backscatter-extinction ratio was determined to be 0.027 + 0.007 per sr. Comparisons of the MPL data with data from other co-located instruments showed good agreement during the dust episode.

Welton, Ellsworth J.↗
NASA NTRS2000

Ground-Based Lidar Measurements of Aerosols During ACE-2: Instrument Description, Results, and Comparisons with Other Ground-Based and Airborne Measurements

A micro-pulse lidar system (MPL) was used to measure the vertical and horizontal distribution of aerosols during the Aerosol Characterization Experiment 2 (ACE-2) in June and July of 1997. The MPL measurements were made at the Izana observatory (IZO), a weather station located on a mountain ridge (28 deg 18 min N, 16 deg 30 min W, 2367 m asl) near the center of the island of Tenerife, Canary Islands. The MPL was used to acquire aerosol backscatter, extinction, and optical depth profiles for normal background periods and periods influenced by Saharan dust from North Africa. System tests and calibration procedures are discussed, and an analysis of aerosol optical profiles acquired during ACE-2 is presented. MPL data taken during normal IZO conditions (no dust) showed that upslope aerosols appeared during the day and dissipated at night and that the layers were mostly confined to altitudes a few hundred meters above IZO. MPL data taken during a Saharan dust episode on 17 July showed that peak aerosol extinction values were an order of magnitude greater than molecular scattering over IZO, and that the dust layers extended to 5 km asl. The value of the dust backscatter-extinction ratio was determined to be 0.027 +/- 0.007 sr(exp -1). Comparisons of the MPL data with data from other collocated instruments showed good agreement during the dust episode.

Welton, Ellsworth J.↗
NASA NTRS2002

Lidar Observations of Tropospheric Aerosols Over Northeastern South Africa During the ARREX and SAFARI-2000 Dry Season Experiments

During the ARREX-1999 and SAFARI-2000 Dry Season experiments a micropulse lidar (523 nm) instrument was operated at the Skukuza Airport in northeastern South Africa. The Mar was collocated with a diverse array of passive radiometric equipment. For SAFARI-2000 the processed Mar data yields a daytime time-series of layer mean/derived aerosol optical properties, including extinction-to-backscatter ratios and vertical extinction cross-section profile. Combined with 523 run aerosol optical depth and spectral Angstrom exponent calculations from available CIMEL sun-photometer data and normalized broadband flux measurements the temporal evolution of the near surface aerosol layer optical properties is analyzed for climatological trends. For the densest smoke/haze events the extinction-to-backscatter ratio is found to be between 60-80/sr, and corresponding Angstrom exponent calculations near and above 1.75. The optical characteristics of an evolving smoke event from SAFARI-2000 are extensively detailed. The advecting smoke was embedded within two distinct stratified thermodynamic layers, causing the particulate mass to advect over the instrument array in an incoherent manner on the afternoon of its occurrence. Surface broadband flux forcing due to the smoke is calculated, as is the evolution in the vertical aerosol extinction profile as measured by the Han Finally, observations of persistent elevated aerosol during ARREX-1999 are presented and discussed. The lack of corroborating observations the following year makes these observation; both unique and noteworthy in the scope of regional aerosol transport over southern Africa.

Campbell, James R.↗
NASA NTRS1986

Satellite measurements of physical properties of Saharan dust

The physical properties of Saharan dust obtained from AVHRR and VISSR images are studied. The techniques of Fraser (1976) and Kaufman and Fraser (1985) are used to derive the aerosol optical depth, mass, and single scattering albedo for the region extending from the west coast of Africa to the Barbados Island for the period of June 21-25, 1984. Optical properties measured by satellite are compared to aircraft measurements taken near Barbados Island during the same period. Remote measurement of thermal properties is also discussed.

Lee, Tae Young↗
NASA NTRS1988

Models of the quasar population. II - The effects of dust obscuration

A model for dust obscuration is proposed to account for the apparent lack of high-redshift quasars which are assumed to be obscured by dust in intervening galaxies. The distribution function for the optical depth as a function of redshift is constructed and used to determine the expected properties of quasars. The apparent magnitude distribution at three points is used to fit the model. Optical results are given for the comoving number density, redshift distribution, and mean color excess. If the theory is correct, evidence for dust may be seen in quasar spectra and a correlation between quasar continuum slopes and strength of metal line absorption systems should be observed.

Heisler, Julia↗
NASA NTRS1990

Remote sensing over oceans of optically thin cirrus and its significance

The extinction characteristics of thin ice-crystal clouds over the oceanic convective rain belts in the tropics and midlatitudes, obtained from the infrared interferometer spectrometer flown on Nimbus 4 satellite in 1970 are examined. It is observed that the optical depth of these clouds in the RF window region is less than 3 and their spectral features are consistent with the cloud particle size of about 10 micron. These optically-thin ice-crystal clouds are extensions of the anvil clouds generated by deep convective systems. The existence of such clouds extending from the upper troposphere into the lower stratosphere in the polar regions during winter and early spring is indicated, and the result of a theoretical simulation are analyzed.

Cuddapah, Prabhakara↗
NASA NTRS2004

Initial Validation and Results of Geoscience Laser Altimeter System Optical Properties Retrievals

Verification of Geoscience Laser Altimeter System (GLAS) optical retrievals is . problematic in that passage over ground sites is both instantaneous and sparse plus space-borne passive sensors such as MODIS are too frequently out of sync with the GLAS position. In October 2003, the GLAS Validation Experiment was executed from NASA Dryden Research Center, California to greatly increase validation possibilities. The high-altitude NASA ER-2 aircraft and onboard instrumentation of Cloud Physics Lidar (CPL), MODIS Airborne Simulator (MAS), and/or MODIS/ASTER Airborne Simulator (MASTER) under-flew seven orbit tracks of GLAS for cirrus, smoke, and urban pollution optical properties inter-comparisons. These highly calibrated suite of instruments are the best data set yet to validate GLAS atmospheric parameters. In this presentation, we will focus on the inter-comparison with GLAS and CPL and draw preliminary conclusions about the accuracies of the GLAS 532nm retrievals of optical depth, extinction, backscatter cross section, and calculated extinction-to-backscatter ratio. Comparisons to an AERONET/MPL ground-based site at Monterey, California will be attempted. Examples of GLAS operational optical data products will be shown.

Hlavka, Dennis L.↗