Edward M. Patterson

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.

Initial measurements using a 1.54-μm 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.

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.

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.

Boundary layer height measurements with an eyesafe Lidar

We have developed and operated an eyesafe lidar in support of an intensive set of air chemistry measurements in Atlanta, Georgia, which were part of the Southern Oxidants Research Program (SORP) during the summer of 1992. The lidar was used to monitor the thickness of the mixed layer by measuring the vertical distribution of boundary layer aerosols. The lidar system is based on a Raman-shifted Nd:YAG laser source at 1.54 microns wavelength with a pulse energy of 40 mJ and a pulse repetition frequency of 4 Hz. The receiver aperture was 46 mm in diameter and an InGaAs PIN diode was used as the detector. The lidar data was typically averaged over 1000 laser pulses, which required about 4 minutes. The lidar returns were range corrected to yield profiles of signal versus altitude in which the signal is proportional to the atmospheric backscatter coefficient. The profiles showed the vertical extent of boundary layer aerosols, and this was interpreted to find the mixed layer thickness. Data was acquired on nine days in July and August 1992. Measurements were typically made at 15-minute intervals from early morning until midafternoon. Mixed layer thicknesses provided by the lidar have been shown to be consistent with balloon sonde results, and they have proved to be useful in interpreting atmospheric chemistry results.

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%.

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.

Eye-safe lidar as an undergraduate research experience

Agnes Scott College and the Georgia Institute of Technology are jointly developing an eye safe atmospheric lidar as a unique hands-on research experience for undergraduates, primarily undergraduate women. Students from both institutions will construct the lidar under the supervision of Agnes Scott and Georgia Tech faculty members. The engineering challenges of making lidar accessible and appropriate for undergraduates are described. The project is intended to serve as a model for other schools.

Design and performance of a 100 inch lidar facility

A lidar system based on the 100 in. optical collimator at Wright- Patterson Air Force Base has been developed for middle atmosphere studies. The system has been demonstrated by recording Rayleigh backscatter returns from mesospheric air molecules at altitudes up to 90 km. These returns were then used to develop atmospheric density profiles. The design of the system provided several unique engineering challenges due to the long focal length and size of the collimator used as the receiver telescope. Careful optical engineering in the receiver and an innovative, modular approach led to a design that eliminates potential problems due to defocus, detector nonuniformity, and detector saturation.