Introduction

Cirrus clouds have a direct impact on the radiation balance of the Earth-Atmosphere system. These clouds reflect a portion of the incoming solar radiation and partially absorb the outgoing infrared radiation. The reflection of the incoming solar radiation reduces the energy added to the system. The partial absorption of the outgoing infrared radiation reduces the energy lost to space. This occurs because the cirrus clouds absorb the upwelling infrared radiation and radiate energy at longer wavelengths (associated with the lower temperatures at the cirrus cloud heights) in all directions. The energy radiated downward by the cloud is put back into the system, warming the levels beneath the cloud, while a reduced amount of upwelling infrared radiation leaves the system. Since the effective temperature of the planet is dependent upon the balance between the incoming and outgoing radiation, the change in the radiation balance caused by cirrus clouds has to be understood to predict future climate change.

The cirrus cloud morphological and optical properties alter the Earths radiation balance. The cloud structural properties which affect the radiation balance are: the cloud height, latitude, and the frequency of cloud occurrence. The height of the cirrus cloud governs its radiative temperature. Since the cirrus cloud height, the insolation, and the Earths surface temperature are functions of latitude, the effect of cirrus clouds on the radiative balance also changes with latitude (Platt (1981), p. 674-676). The frequency of occurrence of the cirrus clouds will control the overall impact of these clouds on the radiative balance. The more often cirrus clouds occur, the greater their effect will be on the global energy balance. The optical properties of the cirrus clouds moderate both the incoming and the outgoing radiation. The scattering properties of ice crystals at visible wavelengths control the amount of downwelling solar radiation reaching the lower atmosphere while the absorptive and scattering properties of the ice crystals at infrared wavelengths governs the amount of infrared radiation escaping to space.

Climatologies have been compiled to determine the cirrus cloud frequency around the planet. Although there have been many cloud climatologies over the years, few have dealt with cirrus cloud coverage. One of the first extensive cloud climatologies to include cirrus clouds was compiled by London (1957). He assembled a large number of surface cloud observations from the Northern Hemisphere recorded in the 1930's and the 1940's and separated them according to cloud types, one of which was cirrus clouds. Recent climatologies have been compiled by Barton (1983), Woodbury and McCormick (1986), Prabhakara et al. (1988), Wylie and Menzel (1989), and Warren (1985). While the instruments and techniques used in these studies differ, each of these climatologies is limited by a lack of detailed global coverage. One cirrus climatology which was not regionally limited was compiled by Wylie et al. (1993). This four year cloud climatology used the NOAA polar orbiting HIRS (High resolution Infrared Radiation Sounder) multispectral infrared data. The cirrus clouds, detected using the CO slicing technique (Smith and Platt (1978), p. 1797--1798), were present in 42% of the satellite observations on the average.

Cirrus cloud climatologies have been compiled from ground based point measurements and area averaged satellite data. The satellite and ground based instruments measure different atmospheric scales. The satellite climatology compiled by Wylie et al. (1993) averages 20 km by 20 km pixels containing cirrus clouds to produce 2 latitude by 3 longitude grids. The ground based point measurements observed only a small portion of the atmospheric structure (which may or may not contain cirrus clouds) which is advected over the instrument position. The cirrus cloud structure on a scale between the point measurements and the area averaged satellite measurements, the mesoscale, is unknown. In some cases, the mesoscale cirrus cloud structure has been inferred from point measurements. To ascertain the variability of the cirrus clouds on this intermediate scale, and thereby the accuracy of cirrus cloud point measurements in determining the overall cirrus cloud structure, cirrus clouds have to be measured throughout a mesoscale volume. One instrument capable of making these measurements is a volume imaging lidar.

The cirrus cloud optical properties, which depend directly upon the particle composition, shape, size, and number density, also affect the Earths radiation budget. In situ measurements have been used to determine the cirrus cloud particle composition, size, shape, and number density. Cirrus clouds consist of ice crystals with maximum lengths typically in the range of 20-2000 m (Liou (1986), p. 1172). These crystals are large compared to visible wavelength radiation and are approximately equal to or greater than the wavelengths of infrared radiation.

A relationship has been used by climate modelers to parameterize the cirrus cloud visible and infrared optical properties. From Mie theory, as a spherical particle becomes large compared to the wavelength of the incident light, the scattering efficiency of the particle converges to two while the absorption efficiency of the particle converges to one (Liou (1980), p. 139). Since the cirrus cloud ice crystals are large compared to the wavelength of visible radiation and since the complex part of the index of refraction (which is associated with absorption by the particle) at visible wavelengths is small, scattering will dominate the interaction of solar radiation with the ice crystals. At infrared wavelengths, absorption will dominate the interaction between the ice particles and the radiation since the cirrus cloud ice crystals are highly absorbing at 10 m (Dorsey (1940), p. 491). Due to the differing radiative properties of the ice crystals at the two wavelengths, the cirrus cloud optical properties at visible and infrared wavelengths has be related through Mie theory by a ratio of efficiencies, the scattering efficiency divided by the absorption efficiency. This ratio is approximately two when the particle absorption equals the particle emission at infrared wavelengths. This efficiency ratio can also be stated in terms of the cirrus cloud optical depth at the two different wavelengths: the visible scattering optical depth divided by the infrared absorption optical depth multiplied by a ratio of visible extinction efficiency to the infrared extinction efficiency. Both models and cirrus cloud measurements have been used to test this optical depth relationship for nonspherical ice crystals. A model Minnis (1991) used three size distributions of hexagonal ice crystals and calculated a ratio for the optical depths ranging from 2.06 to 2.22. For these calculations, it was assumed that the extinction efficiencies at the two wavelengths were equal. Measurements from a FIRE (First ISCCP Cloud Climatology Project) IFO (Intensive Field Operation) which used ground based and satellite based instruments suggested a 2.13 ratio between the visible and infrared optical depths for the cirrus clouds (Minnis et al. (1990)). To calculate the visible optical depths for the FIRE data from the measured visible radiances, the ground albedo and the cloud cover within each pixel had to be known. Measurements by Platt et al. (1980) from a ground based lidar and satellite radiometers suggested a ratio less than 2.0. A method is described in this thesis which allows for the calculation of the visible optical depths for cirrus clouds in a mesoscale volume using a ground based volume imaging lidar. Knowledge of the cirrus cloud visible optical depths over a mesoscale region allows for a direct comparison with the cirrus cloud infrared optical depths measured by satellite radiometers for the same region. This method of comparing the cirrus clouds on the mesoscale can also be used to validate the cirrus cloud detection techniques from satellite radiometers.

This thesis quantitatively describes the variability of the cirrus cloud optical and morphological properties within a mesoscale volume measured by the University of Wisconsin Volume Imaging Lidar (VIL). The cirrus cloud cover within the measured volume is calculated and compared to cirrus cloud point measurements made with the University of Wisconsin High Spectral Resolution Lidar (HSRL). The difference between the point and area cloud covers is used to illustrate the importance of sampling errors in single point measurements when they are used to describe cirrus clouds throughout a mesoscale volume. This is accomplished by estimating the change in solar flux at the surface of the Earth using the difference in the average cloud cover detected by the two lidars and the measured cirrus cloud visible optical depths. The visible scattering properties of the cirrus clouds are also calculated throughout the mesoscale volume. This is achieved by directly calibrating the VIL backscattered signal to the cirrus cloud aerosol backscatter cross sections measured by the HSRL. This is possible since both instruments were aligned to simultaneously view the same cirrus clouds. The variability of the cirrus cloud aerosol backscatter cross sections will be determined using the calibrated VIL signal within the mesoscale volume. The cirrus cloud visible scattering properties can then be used to calculate the visible optical depth of the cirrus clouds in the volume. These visible optical depths will then be directly compared to the infrared optical depths for the cirrus clouds calculated from VAS (VISSR Atmospheric Sounder) radiance measurements. The ratio of the visible and infrared optical depths is compared to Mie theory and results from previous experiments.