37 research outputs found
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INDOEX aerosol optical depths and radiative forcing derived from AVHRR
The Indian Ocean Experiment (INDOEX) had as a primary objective
determining the radiative forcing due to anthropogenic aerosols over
climatologically significant space and time scales: the Indian Ocean during the
winter monsoon, January-March. During the winter monsoon, polluted, low-level
air from the Asian subcontinent blows over the Arabian Sea and Indian
Ocean. As part of INDOEX, aerosol optical depths were derived from Advanced
Very High Resolution Radiometer (AVHRR) data for the cloud-free ocean regions.
The AVHRR radiances were first calibrated using the interior zone of the Antarctic
and Greenland ice sheets, which proved to be radiometrically stable calibration
targets. Optical depths were derived by matching the observed radiances to
radiances calculated for a wide range of optical depths and viewing geometry.
Optical depths derived with the AVHRR were compared with those derived with
NASA's Aerosol Robotic Network (AERONET) CIMEL instrument at the Center
for Clouds, Chemistry, and Climate's Kaashidhoo Observatory, as well as with
other surface and shipboard observations taken in the INDOEX region. The
retrieved and surface-based optical depths agreed best for a new 2-channel, 2-
aerosol model scheme in which the AVHRR observations at O·64 and O·84 microns
were used to determine relative amounts of marine and polluted continental aerosol
and then the resulting aerosol mixture was used to derive the optical depths.
Broadband radiative transfer calculations for the mixture of marine and polluted
continental aerosols were combined with the 0·64 and 0·84-micron AVHRR
radiances to determine the radiative forcing due to aerosols in the INDOEX region.
Monthly composites of aerosol optical depth and top of the atmosphere, surface,
and atmospheric radiative forcing were derived from calibrated AVHRR radiances
for January-March 1996-2000. An inter-annual variability in the magnitude and
spatial extent of high value regions is noted for derived optical depths and radiative
forcing, with highest values reached in 1999, particularly in the Bay of Bengal
which during the IFP was covered by plumes from Indochina. Frequency
distributions of the optical depth for 1⁰ x 1⁰ latitude-longitude regions are well
represented by gamma distribution functions. The day-to-day and year-to-year
variability of the optical depth for such regions is correlated with the long term
average optical depth. Interannual variability of the monthly mean optical depths
for such regions is found to be as large as the day to day
The energy budget of the middle and upper stratosphere.
Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology, 1973.Bibliography: leaves 113-116.M.S
Evaluation of the aerosol indirect effect in marine stratocumulus clouds: Droplet number, size, liquid water path, and radiative impact
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Effects of threshold retrievals on estimates of the aerosol indirect radiative forcing
Empirical estimates of the aerosol indirect radiative forcing often rely on threshold cloud retrievals applied to multispectral satellite imagery data. In such retrievals, pixels having radiances that surpass prescribed thresholds are assumed to be overcast even if they are only partially cloud covered. This assumption leads to cloud visible optical depths that are underestimated and droplet radii that are overestimated. As regional cloud cover increases, overcast pixels become more common and the biases in cloud properties decrease. Because aerosol optical depths derived from cloud-free pixels also increase with regional cloud cover (Loeb and Manalo-Smith, 2005), the biases in threshold-derived cloud properties can be mistakenly interpreted as being evidence for the effects of aerosols on clouds. Because of the biases, threshold retrievals of cloud properties are likely to lead to overestimates of the aerosol indirect forcing. A retrieval scheme that accounts for fractional cloud cover within an imager pixel is used to estimate the enhancement in the indirect radiative forcing that arises from threshold cloud retrievals. The enhancements prove to be relatively small, approximately 20%. If cloud liquid water is held fixed to estimate the forcing, the biases in threshold-derived droplet radii and in the sensitivity of droplet radii to changes in aerosol nearly cancel so that the estimates are almost the same for threshold and partly cloudy pixel retrievals
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Multiyear Advanced Very High Resolution Radiometer observations of summertime stratocumulus collocated with aerosols in the northeastern Atlantic
Advanced Very High Resolution Radiometer (AVHRR) 4-km data were collected over the northeast Atlantic for May–August 1995–1999. Aerosol optical depth at 0.55 μm was retrieved in pixels identified as being cloud-free ocean. In pixels identified as containing clouds from single-layered, low-level cloud systems over oceans, the following cloud properties were retrieved: 0.64-μm cloud optical depth, droplet effective radius, cloud layer altitude, pixel-scale fractional cloud cover, column liquid water amount and column droplet concentration. Aerosol and cloud properties were averaged in 1º x 1º latitude-longitude regions. Regions that contained clouds were limited to those in which all the clouds were part of a single-layered, low-level cloud system. Aerosol and cloud properties were compared only in 1º regions that had sufficient numbers of both cloud-free pixels that yielded aerosol retrievals and cloudy pixels that yielded retrievals of cloud properties within a single overpass. The comparisons were collected in 5º x 5º latitude-longitude regions to determine trends. Within each 5º region the cloud properties were similar from year to year, permitting the data to be composited for all 5 years. Aerosol optical depth decreased systematically with time, probably as a result of the increase in solar zenith angle due to the precession of the satellite orbit. Within the 5º regions, as aerosol optical depth increased, droplet effective radius decreased, cloud optical depth increased, and droplet column number concentration increased, qualitatively consistent with the trends expected for the aerosol indirect effect. In some regions, liquid water path decreased as aerosol optical depth increased, contrary to the trends expected for the suppression of drizzle. Within each 5º region, clouds in clean air, as indicated by their collocation with relatively small aerosol optical depths, had larger droplets and smaller cloud optical depths than clouds in polluted air, as indicated by their collocation with relatively large aerosol optical depths. On average, the aerosol indirect radiative forcing for overcast conditions was about twice as large as the direct radiative forcing for cloud-free conditions. In most of the 5º regions increases in cloud liquid water with increasing aerosol optical depth enhanced the ratios by 20–30% over those calculated from the changes in droplet effective radius and an assumption of constant cloud liquid water. In some 5º regions, however, like those just to the west of the Iberian peninsula, the column amount of cloud liquid water decreased with increasing aerosol optical depth
The Appearance and Disappearance of Ship Tracks on Large Spatial Scales
The 1-km advanced very high resolution radiometer observations from the morning, NOAA-12, and afternoon,
NOAA-11, satellite passes over the coast of California during June 1994 are used to determine the altitudes,
visible optical depths, and cloud droplet effective radii for low-level clouds. Comparisons are made between
the properties of clouds within 50 km of ship tracks and those farther than 200 km from the tracks in order to
deduce the conditions that are conducive to the appearance of ship tracks in satellite images. The results indicate
that the low-level clouds must be sufficiently close to the surface for ship tracks to form. Ship tracks rarely
appear in low-level clouds having altitudes greater than 1 km. The distributions of visible optical depths and
cloud droplet effective radii for ambient clouds in which ship tracks are embedded are the same as those for
clouds without ship tracks. Cloud droplet sizes and liquid water paths for low-level clouds do not constrain the
appearance of ship tracks in the imagery. The sensitivity of ship tracks to cloud altitude appears to explain why
the majority of ship tracks observed from satellites off the coast of California are found south of 358N. A small
rise in the height of low-level clouds appears to explain why numerous ship tracks appeared on one day in a
particular region but disappeared on the next, even though the altitudes of the low-level clouds were generally
less than 1 km and the cloud cover was the same for both days. In addition, ship tracks are frequent when lowlevel
clouds at altitudes below 1 km are extensive and completely cover large areas. The frequency of imagery
pixels overcast by clouds with altitudes below 1 km is greater in the morning than in the afternoon and explains
why more ship tracks are observed in the morning than in the afternoon. If the occurrence of ship tracks in
satellite imagery data depends on the coupling of the clouds to the underlying boundary layer, then cloud-top
altitude and the area of complete cloud cover by low-level clouds may be useful indices for this coupling.This work was supported in part by the Office of Naval Research and by the National Science Foundation through the Center for Clouds, Chemistry and Climate at the Scripps Institution of Oceanography, an NSF Science and Technology Center
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Aerosol and cloud property relationships for summertime stratiform clouds in the northeastern Atlantic from Advanced Very High Resolution Radiometer observations
Advanced Very High Resolution Radiometer (AVHRR) 4-km data collected over the northeastern Atlantic off the coast of the Iberian Peninsula for May to August 1995 were used to investigate the feasibility of empirically deriving estimates of the aerosol indirect radiative forcing. A retrieval scheme was used to derive cloud visible optical depth, droplet effective radius, cloud layer altitude, and pixel-scale fractional cloud cover. A two-channel aerosol retrieval scheme was used to determine aerosol optical depth in cloud-free pixels. Mean aerosol optical depths derived from the cloud-free pixels in 1º x 1º latitude-longitude regions on a given satellite overpass were associated with mean cloud properties derived from the cloudy pixels in the same region for the same satellite overpass. The analysis was restricted to 1º regions that contained only singlelayered, low-level cloud systems. Because aerosol and cloud properties are highly variable, results for the 4-month period were composited into 5º x 5º latitude-longitude regions and averaged to obtain reliable trends in the cloud properties as functions of aerosol burden. Consistent with expectations for the aerosol indirect effect, in some 5º regions, droplet effective radii decreased, and cloud visible optical depths increased as aerosol optical depths increased. The hypothesis that drizzle is suppressed in polluted clouds predicts that liquid water path should increase as aerosol burden increases. In three of the thirteen 5º regions studied, the liquid water path increased as aerosol optical depth increased, but in none of the regions was the increase in cloud liquid water statistically significant. In the remaining regions, cloud liquid water remained constant or even decreased with increasing aerosol optical depth. In many of the 5º regions, the retrieved aerosol optical depth increased as the percentage of cloudy pixels increased. Consistent with expectations from adiabatic cloud parcel models, droplet effective radius, cloud optical depth, and cloud liquid water path also increased as fractional cloud cover increased. The simultaneous increase in retrieved aerosol and cloud optical depths with increasing fractional cloud cover might have been due to the aerosol indirect effect, but it might also have resulted from processes that affect both the cloud and aerosol properties as cloud cover changes. The dependence on fractional cloud cover suggests that some of the trends between aerosol optical depth and the cloud properties cannot be solely attributed to the effects of the aerosols. For comparison with previous studies, the simultaneous changes in aerosol and cloud properties were used to estimate the daily average aerosol indirect forcing for overcast conditions in the summertime northeastern Atlantic. The magnitude of the indirect forcing relative to that of the direct forcing reported here is smaller than estimates reported by others
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Aerosol absorption over the clear-sky oceans deduced from POLDER-1 and AERONET observations
We estimate aerosol absorption over the clear-sky oceans using aerosol geophysical products from POLDER-1 space measurements and absorption properties from ground-based AERONET measurements. Our best estimate is 2.5 Wm-2 averaged over the 8-month lifetime of POLDER-1. Low and high absorption estimates are 2.2 and 3.1 Wm-2 based on the variability in aerosol single scattering albedo observed by AERONET. Main sources of uncertainties are the discrimation of the aerosol type from satellite measurements, and potential clear-sky bias induced
by the cloud-screening procedure
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Morning-to-Afternoon Evolution of Marine Stratus Polluted by Underlying Ships: Implications for the Relative Lifetimes of Polluted and Unpolluted Clouds
Ship tracks appearing in both the morning and afternoon Moderate Resolution Imaging Spectroradiometer (MODIS) imagery for the Pacific Ocean off the west coast of the United States were used to study the morning-to-afternoon evolution of marine stratus polluted by underlying ships and nearby uncontaminated stratus. Analyzed 925-hPa winds were used to predict the afternoon positions of ship tracks found in the morning imagery. Droplet effective radii, visible optical depths, and liquid water amounts were analyzed for morning and afternoon clouds that, based on the low-level winds, were taken to be the same clouds. As found in a previous study by Segrin et al., both morning and afternoon polluted clouds had smaller droplet radii, larger optical depths, and smaller liquid water amounts than the nearby unpolluted clouds. In contrast to the Segrin et al. study, however, the droplet effective radii decreased significantly from morning to afternoon in both the polluted and unpolluted clouds, with the rate of decrease being twice as large for the unpolluted clouds. The larger decrease in the unpolluted clouds is thought to be caused by drizzle, which is probably absent in the polluted clouds. The observations suggest that, with their slower rate of liquid loss, polluted clouds could have longer lifetimes than their unpolluted counterparts. Of interest is that clouds with similar droplet radii but smaller optical depths, and thus smaller droplet number concentrations and liquid water amounts, exhibited higher sensitivities to the effects of elevated particle concentrations and a greater likelihood of appearing in both the morning and afternoon satellite overpasses
The Appearance and Disappearance of Ship Tracks on Large Spatial Scales
The 1-km advanced very high resolution radiometer observations from the morning, NOAA-12, and afternoon,
NOAA-11, satellite passes over the coast of California during June 1994 are used to determine the altitudes,
visible optical depths, and cloud droplet effective radii for low-level clouds. Comparisons are made between
the properties of clouds within 50 km of ship tracks and those farther than 200 km from the tracks in order to
deduce the conditions that are conducive to the appearance of ship tracks in satellite images. The results indicate
that the low-level clouds must be sufficiently close to the surface for ship tracks to form. Ship tracks rarely
appear in low-level clouds having altitudes greater than 1 km. The distributions of visible optical depths and
cloud droplet effective radii for ambient clouds in which ship tracks are embedded are the same as those for
clouds without ship tracks. Cloud droplet sizes and liquid water paths for low-level clouds do not constrain the
appearance of ship tracks in the imagery. The sensitivity of ship tracks to cloud altitude appears to explain why
the majority of ship tracks observed from satellites off the coast of California are found south of 358N. A small
rise in the height of low-level clouds appears to explain why numerous ship tracks appeared on one day in a
particular region but disappeared on the next, even though the altitudes of the low-level clouds were generally
less than 1 km and the cloud cover was the same for both days. In addition, ship tracks are frequent when lowlevel
clouds at altitudes below 1 km are extensive and completely cover large areas. The frequency of imagery
pixels overcast by clouds with altitudes below 1 km is greater in the morning than in the afternoon and explains
why more ship tracks are observed in the morning than in the afternoon. If the occurrence of ship tracks in
satellite imagery data depends on the coupling of the clouds to the underlying boundary layer, then cloud-top
altitude and the area of complete cloud cover by low-level clouds may be useful indices for this coupling.This work was supported in part by the Office of Naval Research and by the National Science Foundation through the Center for Clouds, Chemistry and Climate at the Scripps Institution of Oceanography, an NSF Science and Technology Center