Cloud optical depth

Conditions Cloud optical depth Above cloud Below cloud... 

Stamnes, K., J. Slusser, and M. Bowen, Biologically Effective Ultraviolet Radiation, Total Ozone Abundance, and Cloud Optical Depth at McMurdo Station, Antarctica, September 15, 1988 through April 15, 1989, Geophys. Res. Lett., 17, 2181-2184 (1990). 

The seasonal cycle of CCN has also been shown to be correlated with that of cloud optical depth in one remote marine area (Boers et al., 1994), and the isotope composition of non-sea salt sulfate over remote regions of the southern Pacific Ocean has been shown to be consistent with a DMS source (Calhoun et al., 1991). 

Boers, R., G. P. Ayers, and J. L. Gras, Coherence between Seasonal Variation in Satellite-Derived Cloud Optical Depth and Boundary Layer CCN Concentrations at a Mid-Latitude Southern Hemisphere Station, Tellus, 46B, 123-131 (1994). 

Loeb, N. G., and R. Davies, Observational Evidence of Plane Parallel Model Biases Apparent Dependence of Cloud Optical Depth on Solar Zenith Angle, J. Geophys. Res., 101, 1621-1634 (1996). 

The meteorological fields are supplied by the limited area model ALADIN-Austria (http //www.cnrm.meteo.fr/aladin/). It is run twice a day at the ZAMG and renders forecasts for 48 h. The meteorological fields have a temporal resolution of 1 h. The data is provided on 45 levels, and model has a horizontal resolution of 9.6 km. Fields of wind, temperatiue, pressure, convective and large scale precipitation, snow cover, solar radiation and specific humidity are extracted directly from the ALADIN dataset. The other fields, cloud optical depth, cloud water- and precipitation water content have to be parameterised (Seinfeld and Pandis 1998) from the ALADIN output. 

Key variables interstitial/activated fraction, CCN size/composition, cloud drop size/number/liquid water content (LWC), cloud optical depth (COD), and updraft velocity... 

Schwartz S. E., Harshvardhan, and Benkovitz C. (2002) Influence of anthropogenic aerosol on cloud optical depth and albedo shown by satellite measurements and chemical transport modeling. Proc. Natl Acad. Sci. USA 99, 1784-1789. 

DMS reactions in the troposphere are believed to lead to enhanced reflectivity of marine clouds [171] and thus DMS emissions may have a cooling influence on the atmosphere. One of the best demonstrations of the link between the natural atmospheric sulfur cycle and the physical climate system are the observations that link the satellite derived stratus cloud optical depth and observed DMS derived cloud condensation nuclei (CCN) concentrations at Cape Grim, Australia [175]. Statistical evidence indicates that the optical depth of the clouds is correlated with the number of CCN in the atmosphere. Thus, any UV-related changes at the surface of the ocean that result in the alteration in DMS flux to the atmosphere and the subsequent formation of CCN would also alter the atmospheric radiation budget for the affected region. 

R. Boers, P. Ayers, J.L. Gras (1994). Coherence between seasonal variation in satellite-derived cloud optical depth and boundary layer CCN concentrations at a mid-latitude Southern Hemisphere station. Tellus Ser. B. Chem. Phys. Meteorol, 46, 123-131. 

TABLE 3.14 Calculated Enhancements or Depressions of Actinic Fluxes above and below Perfectly Light-Diffusing Clouds of Different Optical Depths"... 

The cloudless case shown first for a small solar zenith angle and typical summertime conditions shows an enhancement due to reflections from the surface. The cloud with an optical depth of 8 corresponds to a total of 67% transmission of the light through the cloud, but essentially all of it is diffused by the cloud and is therefore not directly transmitted light. The cloud with an optical depth of 128 only transmits a total of 9% of the light, essentially all of which is again diffuse. 

From these and other data it follows that accuracy in the estimate of radiation balance as a function of space coordinates depends on cloud distribution, their state, and atmospheric pollution, as well as on the chosen size of pixels for spatial averaging. In this connection, Henderson and Chylek (2005) used image data from the Multispectral Thermal Imager to evaluate the effect of spatial resolution on aerosol optical depth retrieval from satellite imagery. It was shown that aerosol optical depth (AOD) depends weakly on pixel size in the range 40 x 80 m2 to 2,040 x 4,080 m2 in the absence of clouds and changes monotonically with the growing size of pixels in clouds. 

It is generally very difficult to measure total gas column densities through a cloud, while it is relatively straightforward to measure die extinction (essentially optical depth) due to dust at some wavelength. The extinction is related to the total dust column density via a dust model, including a particle size distribution. These dust models are typically not unique, hence the additional constraint from the elemental abundances of dust constituents, in particular Si and Fe. Therefore, the conversion factor between the two is an important number, and significant efforts have been directed toward its measurement. 

The daughter nuclide may be stable or unstable (radioactive), debris disk a circumstellar disk in which the majority of the dust is not derived from the collapsing molecular cloud, but from the collisions of minor bodies in the disk. The typical masses and optical depths of these disks are several orders of magnitudes lower than those typical to accretion disks, desorption changing from an adsorbed state on a surface to a gaseous or liquid state. 

Fig. 9. Estimated change of kinetic gas temperature and equilibrium temperature of dust grains as a function of the distance r to the surface of the cloud. The calculation assumes a linear change of density n y- r, a constant ionization rate by subcosmic particles and penetration of UV photons X > 912 A into the cloud only from the outside. Absorption of subcosmic particles is neglected. The upper scale indicates the optical depth of the dust grains in the visual...

In Section B we have discussed how the basic quantities of line emission and absorption, the excitation temperature Tex and optical depth r can be determined from observations. Energies required for rotational excitation are generally low enough (< 200 cm-1) so that the rotational levels are expected to be populated even at the very low kinetic temperatures of the interstellar molecular clouds. On the other hand, with a few exceptions such as H20 and NH3, one may assume that only the lowest energy levels of interstellar molecules are populated. Under these conditions the observable fractional column density Nx may not deviate appreciably from the total column density N of a molecule, which can be computed by means of Eq. (17) on the assumption of LTE. 

The radiative transfer model in Madronich (1987) permits the proper treatment of several cloud layers, each with height-dependent liquid water contents. The extinction coefficient of cloud water is parameterized as a function of the cloud water computed by the three-dimensional model based on a parametrization given by Slingo (1989). For the Madronich scheme used in WRF/Chem, the effective radius of the cloud droplets follows Jones et al. (1994). For aerosol particles, a constant extinction profile with an optical depth of 0.2 is applied. 

First indirect effect - affect cloud drop size, number, reflectivity, and optical depth via cloud condensation nuclei (CCN) ... 

All of the clouds are low density, because the visibility inside the densest region of the clouds is a few kilometers. The average and maximum optical depths (t) in visible light of all cloud layers are 29 and 40, respectively, versus average and maximum t values of 6 and 350 for terrestrial clouds. Average mass densities for Venus clouds are 0.01-0.02 g m versus an average mass density of 0.1-0.5 g m for fog clouds on Earth. Venus cloud layers are typically divided into the subcloud haze (32-48 km), the lower cloud (48-51 km), middle cloud (51-57 km), upper cloud (57-70 km), and upper haze (70-90 km). 

Consider a uniform cold cloud through which starlight passes to the observer. The cloud is seeded with atoms absorbing at the wavelength Ao with an absorption coefficient given by the transition s /-value and a profile /(AA). Assume the cloud to be transparent at all other wavelengths. Around Ao, the transmitted intensity of starlight is /(AA) = /o exp(—r(AA)), where Iq is the intensity in the local continuum, and r(AA) is the cloud s optical depth at AA from Ao. 

Fq = incident solar flux, W m-2 Ac = fraction of the surface covered by clouds Ta = fractional transmittance of the atmosphere Rs = albedo of the underlying Earth surface tt) = single-scattering albedo of the aerosol (3 = upscatter fraction of the aerosol x = aerosol optical depth... 

If aerosol number concentrations are substantially increased as a result of anthropogenic emissions over that in the absence of such emissions, the number concentration of cloud droplets, which is governed by the number concentration of aerosol particles below cloud, may also be increased. An increased number concentration of cloud droplets leads, in turn, to an enhanced multiple scattering of light within clouds and to an increase in the optical depth and albedo of the cloud. The areal extent of the cloud and its lifetime may also increase. This is the essence of the indirect effect of aerosols on climate. A key measure of aerosol influences on cloud droplet number concentrations is the number concentration of cloud condensation nuclei (CCN). 

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