Influence of high altitude clouds on upper tropospheric radiance measurements.

Altitude profiles of atmospheric window radiance measured with upward-looking sensors frequently show a rapid decrease in radiance with increasing height over a narrow altitude region in the upper troposphere. This region of rapid decrease is termed a radiometric knee in the altitude profile. The top of this knee defines a radiometric tropopause with a latitudinal height dependence similar to that of the usually defined barometric tropopause. Atmospheric window (10-12-microm) radiance at these altitudes can be associated with the presence of ice particulates. Comparison of the measurements with predicted altitude profiles of atmospheric radiance from the LOWTRAN 7 atmospheric model code shows that a well-defined knee occurs when there is a cloud layer (liquid or ice) such as a subvisual cirrus cloud present. The rate and magnitude of the radiance decrease depend on the optical depth and, therefore, the water content of the layer. Atmospheric background radiance values for near horizontal (large zenith angle) viewing with upward-looking sensors can be as much as a factor of 100 lower above the knee than below it. Comparisons between calculated and observed radiance profiles were used to estimate the vertical extent, total optical depth, and water content of the clouds.

Simplified ultraviolet and visible wavelength atmospheric propagation model.

We have developed a program to model atmospheric propagation and lidar return at visible and UV wavelengths. This model combines a transmission code suitable for use in the visible and UV regions with a backscatter code for Mie and fluorescence lidar return calculations and a sky background radiance code into a modular menu-driven user friendly FORTRAN program for an IBM PC or PC compatible system. This propagation model includes attenuation due to molecular scattering, molecular absorption, and particulate attenuation. The wavelength dependence of our aerosol attenuation is parametrized in terms of the visual range to provide an approximate match for UV and visible horizontal attenuation data. This aerosol model is compared with the AFGL standard aerosol models and experimental data on atmospheric attenuation as a function of the visual range.

Initial measurements using a 1.54-microm eyesafe Raman shifted lidar.

We have demonstrated an eyesafe lidar system for cloud and aerosol studies using 45-mJ/pulse 1.54-microm radiation generated by wavelength shifting the output from a pulsed Q-switched Nd:YAG laser using a CH(4)Raman cell.

Diffuse reflectance and diffuse transmission measurements of aerosol absorption at the First International Workshop on light absorption by aerosol particles.

Aerosol absorption measurements using diffuse reflectance and transmission techniques were made as part of the First International Workshop on light absorption by aerosol particles during July and August 1980. Our diffuse reflectance measurements were used to determine the imaginary component of the refractive index and the mass absorption coefficient for the aerosols and for the bulk material from which the aerosols were generated. Mass absorption coefficients measured by these techniques appear to be lower than those determined by diffuse transmission techniques. Our diffuse transmission measurements of sigma(A), the volume absorption coefficient, show that the measured response for this technique will be dependent on filter orientation and on the type of filter. Our data suggest that there may also be a dependence on the physical properties of the aerosol. Average omega values calculated on the basis of these diffuse transmission measurements range from 0.18 for soots to 0.98 for ammonium sulfate.

Volume scattering ratios determined by the polar and the integrating nephelometer: a comparison.

Polar nephelometer and integrating nephelometer measurements of the volume scattering coefficient for well-documented aerosols were made as a part of the First International Workshop on light absorption by aerosol partioles. These measurements showed a good overall agreement between the two methods, with an average difference between the polar nephelometer and integrating nephelometer data of approximately 10%.

Orbiting lidar simulations. 1: Aerosol and cloud measurements by an independent-wavelength technique.

Aerosol and cloud measurements are simulated for a space shuttle lidar. Expected errors (in signal, transmission, density, and calibration) are calculated algebraically and checked by simulating measurements and retrievals using random number generators. Vertical resolution is 0.1-0.5 km in the troposphere, 0.5-2.0 km above, except 0.25-1.0 km in mesospheric cloud and aerosol layers. Horizontal resolution is 100-2000 km. By day vertical structure is retrieved for tenuous clouds, Saharan aerosols, and boundary layer aerosols (at 0.53 and 1.06 microm) as well as strong volcanic stratospheric aerosols (at 0.53 microm). Quantitative backscatter is retrieved provided that particulate optical depth does not exceed approximately 0.3. By night all these constituents are retrieved plus upper tropospheric and stratospheric aerosols (at 1.06 microm), mesospheric aerosols (at 0.53 microm), and noctilucent clouds (at 1.06 and 0.53 microm). Molecular density is a leading source of error in measuring nonvolcanic stratospheric and upper tropospheric aerosols.

Measurements of the imaginary part of the refractive index between 300 and 700 nanometers for mount st. Helens ash.

The absorption properties, expressed as a wavelength-dependent imaginary index of refraction, of the Mount St. Helens ash from the 18 May 1980 eruption were measured between 300 and 700 nanometers by diffuse reflectance techniques. The measurements were made for both surface and stratospheric samples. The stratospheric samples show imaginary index values that decrease from approximately 0.01 to 0.02 at 300 nanometers to about 0.0015 at 700 nanometers. The surface samples show less wavelength variation in imaginary refractive index over this spectral range.

Kubelka-Munk optical properties of a barium sulfate white reflectance standard.

We have measured the Kubelka-Munk scattering and absorption coefficients for a barium sulfate white reflectance standard. These measurements have been based on measurements of the absolute reflectance for the particular barium sulfate samples whose scattering and absorption coefficients were measured. This method gives results that are different from earlier measurements; the differences are significant for measurements of the optical properties of atmospheric aerosols.

Atmospheric extinction between 0.55 microm and 10.6 microm due to soil-derived aerosols.

We have calculated the extinction expected for soil-derived aerosols for several laser wavelengths and have shown that these aerosols result in approximately neutral extinction between 0.55 microm and 10.6 microm. This result is valid whenever the extinction is due primarily to soil-derived aerosols, and it appears to be independent of whether the aerosols are of local origin. Comparison with visibility data shows that visibility reductions due to these aerosols are of relatively greater importance in arid or semiarid areas. Lower bounds on the frequency of occurrence of such visibility reductions may also be determined from analysis of visibility data.