Conclusions

The work presented in this thesis has shown that the installation of the iodine absorption filter has substantially improved the performance of the University of Wisconsin-Madison High Spectral Resolution lidar (HSRL). The new system design that includes the iodine absorption filter, polarization, and multiple scattering measurement capabilities is shown. The high resolution etalon, that was used in the earlier system to separate between aerosol and molecular scattering, provided only 1:2 separation for the aerosol backscatter signal between channels. The iodine absorption filter has been shown to suppress the aerosol backscatter signal on molecular channel down to 0.08% and therefore the aerosol cross talk signal on the molecular signal can be easily removed even for optically thick clouds. In the etalon based system, the determination of the system calibration terms was sufficient to provide accurate inversions only for the clear air aerosols and thin cirrus clouds. The use of the iodine absorption filter has also improved the robustness of the HSRL, reduced the complexity of the system, and increased the optical transmission of the system, so that accurate measurements of the optical depth, backscatter cross section and phase function can be made. Also the simultaneous measurements of depolarization and multiple scattering are performed. The HSRL implementation shows a depolarization measurement technique that uses the one transmitter laser and one detector to measure both polarization components. Therefore, no calibration of the receiver is required. The multiple scattering measurements are realized with a separate channel that allows measurements of signal strength variations as function of field of view simultaneously with the measurements of the narrow field of view channels.

The iodine absorption filter provides an absolute wavelength reference for the HSRL measurements. The iodine absorption line observed through a cell with 50% transmission on the line center is used for the wavelength locking of the HSRL transmitter laser. This provides stable operation over a long period of time without a need for frequent calibrations. Measurements have shown that the laser wavelength is maintained within 0.052 pm.

The stability and reliability of the system have been tested by operating the iodine absorption based HSRL at the University of Wisconsin-Madison campus. Starting from July 1993, the HSRL has been routinely operated and data from different atmospheric conditions have been recorded. This dataset contains 30 cirrus cloud cases. The measurements show that accurate measurements of optical properties of the atmosphere can be performed. The improved measurement accuracy has made possible to measure optical depth profiles inside the clouds. The current HSRL can be used to probe clouds that have optical depths up to 3. This means that most of the cirrus cloud cases can be fully observed and the bases of thick water clouds can be measured up to 300--500 m inside the cloud. This has been achieved by using the iodine absorption filter, high laser pulse repetition rate, small pulse energy per laser pulse, and very fast photon counting data system. The measurement accuracy of the HSRL is high enough to provide accurate measurements of optical parameters of clouds and strong clear air aerosol layers within 3 min averaging time, but the accurate measurements of clear air optical parameters require longer averaging times. The clear air optical parameters can be measured up to 35 km. The error analysis shows that accuracy of the HSRL measurements is mostly limited by the photon counting statistics. The system performance can be increased by increasing the system detection efficiency or/and increasing the transmitted laser power. The greatest improvement would be a photodetector with higher quantum efficiency and faster count rate capability.

The depolarization data obtained by the HSRL shows the ability of the HSRL to distinguish between water and ice clouds. It is shown, that the 160rad field of view of the spectrometer channels effectively suppresses the multiple scattering effects on the measured depolarization ratio. Therefore, a reliable separation between water and ice clouds is possible. The HSRL measurements have shown, that traditional systems with 1--5 mrad field of views cannot reliably separate between water and ice, because the depolarization observed with a 1 mrad field of view is comparable to the ice depolarization within a small penetration depth from the water cloud base due to the multiple scattering. The error analysis shows, that the depolarization of the clear air aerosol layers can be observed with better than 10 % accuracy. The accuracy of the cloud depolarization measurements is better than 1 %. The measurements of depolarizations of weak aerosol layers and molecular backscatterer are limited by the photon counting statistics.

The study of the cloud depolarizations between August 2 and November 11, 1993 shows that in 45 % of cirrus cloud cases simultaneous observations of supercooled water at cirrus cloud altitudes were made. The average cirrus cloud depolarization is shown to increase from 33% at -5C temperature to 41% at -60C. The observed behavior is different than observed by Platt et al. [43]. The depolarization observed by Platt et al. ranged from 15% to 40% between temperatures from -10 to -60 C. The largest difference between the HSRL measurements and Platt's measurements is observed for the temperature range from -30 to -10 C, where Platt observed low cirrus cloud depolarizations. The low values of depolarization in Platt's measurement are most probably caused by multiple scattering from supercooled water droplets, because the system used for this measurement had a 2.5 mrad field of view. The HSRL measurements show that supercooled water clouds have been found at temperatures as low as -40C and pure water clouds have been found at temperatures above 0C. No water has been found at temperatures below -40C and the presence of cirrus disappears at temperatures above 0C. In the HSRL measurements, the cirrus cloud depolarization ratios for all temperatures are close to the values observed for the temperatures without supercooled water clouds. The small difference in the observed depolarization as a function of temperature may be a result of different shapes, sizes, and orientations of the ice crystals at different temperatures.

The depolarization of the molecular backscatter is 0.7--0.8%, when measured without the low resolution etalons. When the low resolution etalons are used, a 0.55--0.6% depolarization is measured. The depolarization measured for the signal from the Cabannes line and the rotational Raman lines is 1.5 % without any spectral filters. The measured molecular depolarization value is larger than the expected 0.4 % depolarization of Cabannes line. The system filter bandpass admits a small fraction of the rotational Raman lines and blocks part of the Cabannes line and therefore an increase on the depolarization ratio is observed due to the presence of the highly depolarized rotational Raman lines. The model calculation for the depolarization transmission of the system show, that a 0.56 --0.62 % depolarization is expected for the case where no low resolution etalons were used. A 0.402 --0.425 % depolarization is expected, when one or two etalon are used. The depolarization observed with the HSRL are larger than the expected values. The cause of the small difference between expected and measured depolarization values is currently unknown. The further analysis of the depolarization measurement accuracy of the HSRL requires an advanced study of effects of the iodine spectrum and rotational Raman lines to the depolarization.

The HSRL measurements require a knowledge of the atmospheric temperature profile. The atmospheric temperature profile measured with the HSRL shows that the HSRL can be used to measure temperature with a high enough accuracy so that the measured temperature profiles can be used for the analysis of the HSRL data. Therefore, the requirement for radiosonde measurements of atmospheric temperature could be eliminated. Before the temperature measurements with the HSRL can be routinely performed, the effects of aerosol backscatter signal to the molecular transmission of the iodine absorption filter have to be removed.

This study has provided an instrument basis for a design of a simple and robust lidar for the measurements of the optical properties of the atmosphere. The University of Wisconsin HSRL provides a unique instrument for the measurements of the cloud optical properties and the data measured with the HSRL provides useful information that can be used for the climate models that study the effects of clouds to the earths atmosphere.