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- Books in the Atmospheric and Oceanographic Sciences Library series - Wheelers Books
- Cloud Optics
Despite absence of discernible dependence of optical depth or albedo on modeled sulfate loading, examination of the dependence of these quantities on LWP readily permits detection and quantification of increases correlated with sulfate loading, which are otherwise masked by variability of LWP, demonstrating brightening of clouds because of the Twomey effect on a synoptic scale. Median cloud-top spherical albedo was enhanced over these episodes, relative to the unperturbed base case for the same LWP distribution, by 0.
Aerosol particles serve as nuclei for formation of cloud droplets in the earth's atmosphere. As enhanced concentrations of aerosol particles from industrial activities lead to increased concentrations of cloud droplets, it was pointed out some time ago by Twomey 1 , 2 that such enhanced cloud droplet concentrations would be expected to increase cloud reflectivity for a given cloud liquid water content. This enhancement of cloud albedo would be expected to have resulted in reduction in absorption of solar radiation by the Earth-atmosphere system over the industrial period and a cooling influence on climate.
Estimates of the global average magnitude of this effect suggest that it is substantial in the context of climate forcing by increased concentrations of greenhouse gases over the industrial period 3 — 7. Enhancement of albedo of marine stratus clouds globally by 0. Such a global mean forcing would more than offset the warming influence caused by increased CO 2 8. Current estimates of this indirect aerosol forcing are mainly model-based and highly uncertain 5 — The uncertainty arises from uncertainties in both the global distribution and loading of anthropogenic aerosols and the relation between aerosol loading and cloud microphysics and reflectivity.
Because of such uncertainties in model-based estimates, it is imperative to obtain measurement-based estimates of the aerosol indirect forcing. As aerosols are short-lived in the troposphere about 1 week , their distribution is quite inhomogeneous spatially and temporally, indicative of the need for rather detailed global characterization, that is, by satellite.
Satellite surveys alone might be expected to permit characterizing and quantifying indirect forcing on large scales but have thus far been of limited utility. Large-scale surveys have shown cloud-drop effective radius of warm clouds to be systematically lower 19 , 20 and number concentration systematically greater 21 in the anthropogenically influenced Northern Hemisphere than in the relatively unperturbed Southern Hemisphere, consistent with the Twomey mechanism of indirect aerosol forcing.
Likewise, fairly strong spatial correlations have been reported of monthly mean aerosol optical depth and number concentration with effective radius negative correlation and optical depth positive correlation of low clouds, all of which would be consistent with the Twomey effect 22 , However, the expected enhancement of cloud albedo caused by the Twomey effect has not been identified in interhemispheric comparisons 20 , Likewise, examination for aerosol enhancement of monthly mean cloud albedo as a function of distance off of continents has been negative 25 or shown only marginal indication of enhancement 26 ; evidence has been presented of enhanced reflectivity of marine stratocumulus clouds locally in the vicinity of copper smelters in Peru under conditions of offshore flow Recently we demonstrated the ability to use satellite measurements to detect influences of anthropogenic aerosols on cloud properties in a situation where enhanced aerosol loading was indicated by a chemical transport model This approach uses a chemical transport model driven by archived output of a numerical weather prediction model to identify locations and times of higher or lower aerosol loading.
This approach also takes advantage of the high dynamic range of aerosol loading that results from synoptic-scale variation in meteorology responsible for transport, conversion, and removal of tropospheric aerosols. Examination of cloud-drop effective radius and number concentration in conjunction with modeled sulfate concentration allowed identification of the influence of sulfate.
Here we extend this approach to examine the aerosol influence on cloud optical depth and albedo taking advantage of the ability to simultaneously determine these quantities and cloud liquid water path LWP from the satellite data, thereby allowing the strong influence of LWP on cloud albedo to be accounted for.
The chemical transport and transformation model 29 , which represents mixing ratios of sulfur dioxide and sulfate as functions of location, altitude, and time, is driven by the 6-hour forecast data of the European Centre for Medium Range Weather Forecasts ECMWF, ref.
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Briefly, the model represents transport of geographically distributed anthropogenic emissions 31 of SO 2 and sulfate and biogenic emissions of dimethyl sulfide DMS by the controlling three-dimensional wind fields. The model, which has horizontal grid spacing of 1. The model, which has been extensively evaluated by comparison with in situ measurements 32 , provides a fairly accurate, albeit not exact, indicator of the sulfate loading as a function of location and time. This study focuses on two episodes during April, , during which modeled concentration of sulfate aerosol at locations over the North Atlantic well removed from local sources exhibited substantial increase and subsequent decrease resulting from transport from anthropogenic continental sources.
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The meteorological conditions giving rise to these episodes are presented and analyzed in ref. During the first episode April 2—8 , sulfate from western Europe was transported westward over the North Atlantic under the influence of a persistent intense cut-off low-pressure system. The study area is situated toward the edge of the region most strongly influenced by the transported continental sulfate. Modeled sulfate in the study area, which was distributed vertically mainly between the surface and 3 km, reached a maximum on April 5 Fig.
During the second episode April 24—29 , the model indicates advection of air containing high sulfate concentrations from the northeastern United States, in part under the influence of a cut-off low-centered northwest of the Iberian peninsula Cloud microphysical properties were retrieved for a 3. Cloud-top temperatures were indicative mainly of liquid-water clouds with average cloud-top heights ranging from 1. Multiple cloud layers were indicated on April 24 and 29, but cloud properties inferred from satellite measurements show no appreciable differences for the several layers.
Modeled sulfate was present mainly between the surface and 4 km; sulfate column burden peaked on April 27 Fig.
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Although aerosol from other sources would certainly have been present in both episodes, sulfate from continental sources is expected to have been a major aerosol species at these remote marine locations under these flow conditions. Sulfate column burden vertical integral of concentration in mid North Atlantic at Universal Time Coordinated on April 2—8, , and April 24—29, , as evaluated with a chemical transport and transformation model. Boxes denote areas chosen for analysis of satellite retrievals of cloud properties.
Two sets of data are shown for April 8, 27, and 28, for which the study area was within range of the satellite on two successive overpasses. We present the algorithm architecture, inversion procedure, retrieval quality flags, initial validation results, and results from a year long OMI record — including global climatology of the frequency of occurrence, ACAOD, and aerosol-corrected cloud optical depth. The long-term OMACA record reveals several important regions of the world, including Southeastern Atlantic Ocean, southern Indian Ocean, South-East Asia, tropical Atlantic Ocean off the coast of western Africa, and northern Arabian sea where the carbonaceous aerosols from the seasonal biomass burning and mineral dust originated over the continents are found to overlie low-level cloud decks with moderate 0.
No significant long-term trend in the frequency of occurrence of aerosols above the clouds and ACAOD is noticed when OMI observations that are free from the row anomaly throughout the operation are considered. If not accounted, the effects of aerosol absorption above the clouds introduce low bias in the retrieval of cloud optical depth with a profound impact at increasing ACAOD and cloud brightness.
Books in the Atmospheric and Oceanographic Sciences Library series - Wheelers Books
The OMACA aerosol product from OMI presented in this paper offers a crucial missing piece of information of the aerosol loading above cloud that will help us to quantify the radiative effects of clouds when overlaid with aerosols and its resultant impact on cloud properties and climate. Journal metrics Journal metrics IF 3. Discussion papers Copyright https: This work is distributed under the Creative Commons Attribution 4.
This book summarizes current knowledge on cloud optical properties, for example their ability to absorb, transmit, and reflect light, which depends on the clouds geometrical and microphysical characteristics such as sizes of droplets and crystals, their shapes, and structures. In addition, problems related to the image transfer through clouds and cloud remote sensing are addressed in this book in great detail. This book can serve as a major introductory text in cloud optics for students; it can also be an important source of information on theoretical cloud optics for cloud physicists, meteorologists and optical engineers.
All basic ideas of optics as related to scattering of light in clouds e.
Cloud Optics
Mie theory and radiative transfer are considered in a self consistent way. Consequently, the book can also be a useful textbook to newcomers to the field. Additional Details Series Volume Number.