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Robert F. Cahalan
and
Joachim H. Joseph

Abstract

Landsat Multispectral Scanner (MSS) and Thematic Mapper (TM) data, with 80 and 30 m spatial resolution, respectively, have been employed to study the spatial structure of boundary-layer and intertropical convergence zone (ITCZ) clouds. The probability distributions of cloud area and cloud perimeters are found to approximately follow a power-law, with a different power (i.e., fractal dimension) for each cloud type. They are better approximated by a double power-law behavior, indicating a change in the fractal dimension at a characteristic size which depends upon cloud type. The fractal dimension also changes with threshold. The more intense cloud areas are found to have a higher perimeter fractal dimension, perhaps indicative of the increased turbulence at cloud top. A detailed picture of the inhomogeneous spatial structure of various cloud types will contribute to a better understanding of basic cloud processes, and also has implications for the remote sensing of clouds, for their effects on remote sensing of other parameters, and for the parameterization of clouds in general circulation models, all of which rely upon plane-parallel radiative transfer algorithms.

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Joachim H. Joseph
and
Robert F. Cahalan

Abstract

Histograms of nearest neighbor spacings of fair weather cumulus at 15 locations Over the world's oceans are presented based on the analysis of high resolution LANDSAT 3 Multispectral Scanner images for amounts of cloud cover ranging from 0.6% to 37.6%. These histograms are found to be essentially the same at all locations analysed, similarly to our previous findings on the size distributions and the fractal dimensions of the perimeters for this cloud type.

The nearest neighbor spacings are linearly dependent on the effective cloud radii, with a proportionality factor ranging from five to twenty. The histograms peak at about 0.5 km. Nearest-neighbor spacings smaller than about a kilometer, associated with cumulus clouds with an effective radius less than a few hundred meters, have a distribution of cloud centers that is almost independently distributed in the horizontal plane and show a tendency for the formation of clumps. Larger spacings of up to thirty kilometers occur and are associated with the larger clouds. These latter spacings are not independent.

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Yuri Mekler
and
Joachim H. Joseph

Abstract

An empirical method to measure the spectral surface albedo of surfaces from LANDSAT imagery is presented and analyzed.

The empiricism in the method is due only to the fact that three parameters of the solution must be determined for each spectral photograph of an image on the basis of independently known albedos at three points. The approach is otherwise based on exact solutions of the radiative transfer equation for upwelling intensity.

Application of the method allows the routine construction of spectral albedo maps from satellite imagery, without requiring detailed knowledge of the atmospheric aerosol content, as long as the optical depth is less than 0.75, and of the calibration of the satellite sensor.

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Joachim H. Joseph
and
Noah Wolfson

Abstract

Representative average values of the absorption to backscatter ratio of the aerosols in two typical synoptic conditions, namely cloudless khamsinic days and normal cloudless days, have been deduced from four years of data at two sites in Israel, one in the coastal plain and one in the Judean hills.

These values (15 for cloudless days and 11 for khamsinic days) have been compared to those, available in, or deducible from, published studies of the aerosol effects in various parts of the world.

It was found that the presence in the atmosphere of most of the aerosols induces a net heating effect, as far as the solar radiation balance is affected, over the regions in which they were measured.

We have further evaluated the complex part of the refractive index, averaged over the solar spectrum between 0.3 and 2.5 μm (0.03 ± 0.02 for khamsinic days and 0.08 ± 0.02 for normal days). This result is important because of the little data available for this parameter so far.

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George Ohring
and
Joachim H. Joseph

Abstract

A simple criterion is presented for determining whether the combined local atmospheric infrared cooling rate at a given height in the presence of two gases, absorbing in the same spectral region, is less than or greater than the cooling rate of either of the gases alone. The criterion is applied to infrared cooling of a tropical atmosphere.

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Joachim H. Joseph
,
Alexander Manes
, and
Dov Ashbel

Abstract

The mass of natural aerosol injected into the atmosphere by desert wind systems of the Khamsinic type has been estimated from long series of normal-incidence spectral solar radiation measurements and from synoptic analysis of the temporal and areal extent of such phenomena.

From these data the regional and global dust input into the atmosphere due to deserts has been inferred and compared to other estimates.

A tentative value for the effect of such desert aerosols on the average global albedo and surface temperature is also presented.

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George Gutman
,
George Ohring
, and
Joachim H. Joseph

Abstract

A method is suggested for introducing long-term interaction between the geobotanic state and climate (a biogeophysical feedback mechanism) into climate models. It is based upon making the geobotanic state, characterized by the snow-free surface albedo and the water availability parameter, dependent upon the ratio of annual radiation balance to annual precipitation (the so-called radiative index of dryness).

This approach is illustrated using a zonally averaged annual steady-state climate model which is based on the hemispheric climate model of Ohring and Adler. Zonal data statistics are employed to obtain simple relationships consistent with the zonality of the system. The heating parameterization of the original model is modified so that precipitation and cloud amount are computed using vertical velocity at 500 mb, which is calculated from the thermodynamic equation.

Experiments with the model indicate that the simulated climate and geobotanic zones are in good agreement with observations. Sensitivity studies suggest that biogeophysical feedback has a negligible effect on the model's response to solar constant variations but may be important in the evaluation of the long-term impact of surface albedo changes.

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Zev Levin
,
Joachim H. Joseph
, and
Yuri Mekler

Abstract

Simultaneous measurements of optical depth and Size distribution in a dust storm are presented. The measured and derived properties of the aerosol are compared with each other and with other results published in the scientific literature. We observe some global commonality in the measured size spectra of desert aerosols especially for post-frontal conditions. On the other hand, during the passage of the front itself, high aerosol concentrations with a sharp peak in radius at −1 μm were observed. Generally, these were not similar to other size distributions reported in the literature.

The imaginary part of the refractive index in the spectral region 0.3-1.7 μm was found to be similar to that found in other deserts. Comparison of the optical measurements with the direct sampling data suggests that the general time trends of the size distributions, as measured in situ, are followed by the optical depth and its variations with wavelength. On the other hand, detailed short-term fluctuations detected by our direct measurements are not followed by the optical method. We have observed that a simple power law for the size distribution, in the range r>0.15μm is a reasonable approximation only during clear and calm conditions with small optical depth. During the dust storm itself, the deviations from a power law are lame as shown by both direct in situ and optical observations.

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Binyamin U. Neeman
,
George Ohring
, and
Joachim H. Joseph

Abstract

A parameterization of quasi-geostrophic eddy transport that takes into account the time variation of the eddy transfer coefficients according to Green's theory is studied. A relation proposed by Green connects the vertical integral of the meridional heat flux at 50°N with the second power of the 500 mb temperature difference between the boundaries of baroclinic activity. It is found that the fourth power in this relation, rather than the original second power, is obtained from analysis of zonal/monthly-mean observational data at 500 mb. For the temperature difference at 1 000 mb, however, the same analysis yields a power of 1.5.

The differences in the seasonal simulation of different powers in the eddy transfer relation are explored in a two-level statistical dynamical zonally averaged model (SDZAM), and it is found that an appropriate choice of power may be of special importance if the model is devised to simulate the seasonal climate cycle, or to test astronomical changes inducing different seasonalities. With the second power in 500 rob, the particular SDZAM being tested simulates an oversensitivity in the high latitude temperature response to the seasonal cycle/astronomical changes, due to its undersensitivity in the simulation of changes in the meridional eddy heat flux. A comparison of the results of a second power at the surface level vs a fourth power at 500 mb is difficult due to the need to retune the model, but a certain advantage to the latter model is detected.

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Binyamin U. Neeman
,
Joachim H. Joseph
, and
George Ohring

Abstract

An efficient longwave scheme for climate models originally suggested by Sasamori is modified to correctly simulate the water vapor-temperature feedback mechanism. It is found that the modified scheme with a fixed cloud-top altitude (FCA) correctly simulates the longwave sensitivity to surface temperature, F / T s , over the clear portion of the sky, but that over the cloud portion of the sky F / T <s remains too high. The fixed cloud-top temperature (FCT) method is similarly reviewed and tested. Comparisons with observational Budyko-type correlations are shown to be indecisive over the question of FCA vs FCT.

A direct observational correlation between the effective cloud-top and surface temperatures, based on annually averaged cloud statistics data, suggests a variable cloud-top temperature (VCT) model. In such a model, the temperature of the elective cloud-top layer is varied according to changes in the surface temperature at a rate which is intermediate between that of the FCA and FCT models. This model results in a reasonable F / T s over the cloud portion of the sky.

The modified longwave scheme is implemented into a zonally averaged dynamic climate model. It is shown that when the VCT mechanism is invoked, climate sensitivity is doubled compared to that simulated with the FCA model. The importance of simulating not only the correct longwave flux, but also the correct longwave sensitivity to temperature changes is therefore stressed for radiation schemes in studies involved with climate change.

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