A radiative climatology of midlatitude cirrus clouds: results from the analysis of 7 years of lidar and IR radiometer observations using the LIRAD method

Cirrus cloud radiative properties have been modeled extensively using in situ and ground based measurements of cloud microphysics. This ongoing research has been performed to study the interaction of cirrus clouds with the Earth's climate system. An important aspect of cirrus cloud research is to determine their effect on the Earth's radiation budget. This can not be done without first determining the relationship between the microphysical, macrophysical, and radiative properties of cirrus clouds.

Past studies have used multiple remote sensor retrieval techniques to derive cloud properties from measurements obtained at various field campaigns. One such technique combines lidar and infrared radiometer measurements (LIRAD) to retrieve macrophysical and radiative properties of cirrus clouds. The LIRAD technique also uses a radiative transfer model to remove the contribution of atmospheric gases from the measured IR radiance. In recent years, microwave radiometer measurements have also been added to correct radiosonde derived water vapor mixing ratios to more accurately model the water vapor amounts in the atmosphere. The macrophysical properties determined by the LIRAD method are cloud height, thickness, and temperature (from radiosonde measurements), while the radiative properties include visible optical depth, cloud emittance, and extinction and absorption coefficients.

The Facility for Atmospheric Remote Sensing (FARS) at the University of Utah has collected ruby lidar and IR radiometer measurements of cirrus clouds since 1992 in support of the Project First ISCCP (International Satellite Cloud Climatology Program) Regional Experiment (FIRE) Extended Time Observations. The LIRAD technique has been applied to the dataset to retrieve the above mentioned cloud properties and each cloud has been identified by its generating mechanism (i.e. midlatitude synoptic, orographic, or anvil). This study will present results for data obtained between 1992 and March 1999 in the form of a radiative climatology of midlatitude cirrus clouds. We will also present relationships that have been observed between the macrophysical and radiative properties of cirrus clouds. As observed in previous studies, our results show that cloud emittance varies between 0.0 and 1.0 and increases with increasing visible optical depth. We have also observed that cloud emittance approaches but does not reach 1.0 until the visible optical depth exceeds 3.0 and the cloud is appropriately classified as altostratus.