Johan F. de Haan

Single scattering of light by circular cylinders

We consider two topics pertaining to light scattering by circular cylinders. (A) Scattering properties of cylinders with increasing aspect ratio are studied. It is shown that the solution for finite cylinders does not converge to the solution for infinitely long cylinders if the aspect ratio increases. This is due to differences in the treatment of diffraction for finite and infinite cylinders. (B) Finite cylinders have sharp edges, so their scattering properties differ from those of spheroids having the same aspect ratio. To illustrate these differences we present scattering matrix elements of cylinders and spheroids for a large set of aspect ratios. To handle the large amount of data, the scattering matrix elements as functions of aspect ratio and scattering angle are presented in so-called three-dimensional figures.

A biwavelength analysis of Pioneer Venus polarization observations

A new method for analyzing Pioneer Venus polarimetry data on a pixel-wise basis is presented. Quasi-simultaneous observations at two wavelengths (550 and 935 nm) are combined and compared with results of multiple scattering calculations. In this manner, hypotheses about particle size distributions in the upper part of the Venus atmosphere are tested. Particles composed of a sulfuric acid solution are considered, and a distinction is made between large and small particles, called cloud and haze particles, respectively. Three model atmospheres have been investigated: (1) a single layer containing cloud particles, (2) a single layer containing a mixture of cloud and haze particles, and (3) a two layer model with an upper layer composed of haze particles and a lower layer containing cloud particles. It is found that all three models agree with the observations at phase angles near 20°, but that the first model cannot be made to agree with the observations for phase angles near 90°. This confirms the presence of haze particles in the Venus atmosphere found earlier by Kawabata et al. [1980]. We find that the haze particles may be situated either above or mixed within the main cloud deck of Venus. We derived effective radii between 0.85 and 1.15 μm for the cloud particles, and effective radii of 0.2 or 0.3 μm for the haze particles. When the haze particles are situated above the cloud layer, the haze optical thickness can take values of up to 0.6 at 550 nm. When the haze particles are mixed with the cloud particles, their contribution to the total atmospheric scattering coefficient at 550 nm can become as large as 70%.

Spatial variations of Venus' cloud properties derived from polarimetry

We study satellite polarization data of the clouds of Venus obtained by the Pioneer Venus Orbiter from 1978 through 1990. We present a new method for comparing these data to results of exact multiple scattering computations. This method has been applied to the analysis of a single disk distribution of the polarization at wavelengths 550 and 935 nm, using a simple model for the atmosphere of Venus. We find little variation in the cloud particle size distribution for the equatorial part of the disk. For this region, the effective particle radius is about 1.0 micrometers and the width of the size distribution decreases when approaching the terminator. However, our analysis of observations at higher latitudes suggests that for these regions a different explanation is needed. Here, an upper haze layer with smaller particles than those of the underlying cloud but having the same composition explains the observations well. The optical thickness of this haze is between 0.1 and 0.5 for an effective haze particle radius of 0.40 micrometers .

Influence of aerosol on the degree of linear polarization of skylight in the O2-A band

We have investigated the influence of atmospheric aerosol on the degree of linear polarization P of diffusely transmitted radiation within the O2-A band, between 755 and 775 nm. In this wavelength interval, the molecular absorption optical thickness of the atmosphere ranges approximately from 10-3 to 2 X 102, whereas the atmospheric molecular scattering optical thickness is an small as 0.025. Radiative transfer calculations have been performed for a terrestrial-like atmosphere to calculate P of the transmitted radiation as a function of the atmospheric molecular absorption optical thickness. Results are presented both for a clear atmosphere and for aerosol loaded atmospheres. It is shown that for small absorption optical thicknesses P is mainly determined by first order scattering by particles in the lower atmospheric layers. For absorption optical thicknesses larger than about 10, the degree of linear polarization of the transmitted radiation is dominated by P of the radiation that has taken the shortest optical path through the atmosphere. For the zenith sky radiation, P is then mainly determined by single scattering in the upper atmospheric layers. For radiation from other directions, P then largely results from two scatterings: the first one by particles in the upper atmospheric layers in the nadir direction, and the second one by particles in the lower layers. It is concluded that information on tropospheric aerosol particles can be derived from polarization measurements of the diffusely transmitted radiation outside the O2-A absorption band. In the strongest parts of the band, zenith sky polarization measurements contain information on the stratospheric aerosol particles.

Derivation of cloud properties from biwavelength polarization measurements

The influence of the top altitude of terrestrial liquid water clouds on the polarization of sunlight reflected by a cloudy atmosphere is theoretically investigated. It is compared to the influences of other cloud properties and of stratospheric aerosols. Using a typical atmosphere model, we show how accurately the cloud top pressure may be derived from nadir measurements performed by satellites of the polarization at 350 nm. The accuracy of the derived pressure is about 5 mb under favorable conditions, when the accuracy of polarization measurements is 0.1%, and it depends mainly on knowledge of the density of the cloud and of the stratospheric aerosol optical thickness. The stratospheric aerosol optical thickness may be estimated with an accuracy of 0.02 using observations of the polarization at 670 nm having an accuracy of 0.1%.