Observations suggest that Arctic mean temperatures are increasing. Climate models attribute the warming to the radiative effects of increased concentrations of greenhouse gases. However, predictions of future Arctic climate show wide variations from one model to another. Studies of these differences show that the greatest source of uncertainty relates to the modeling of clouds. Different models use different parameterizations to describe clouds and as a result they generate different cloud predictions. Very little cloud data is available for validation. Modelers require information on cloud optical depth, cloud altitudes and cloud phase.

Arctic cloud observations have been limited by several factors including: 1) the lack of observers, 2) the difficulty of seeing clouds during the long Arctic night, 3) a lack of visual and temperature contrast between clouds and the snow surface, which impedes satellite cloud retrievals, and 4) the inconsistent reporting of diamond dust precipitation.

The University of Wisconsin has constructed a High Spectral Resolution Lidar (HSRL) for long-term Arctic cloud and aerosol studies. It provides calibrated profiles of backscatter cross section, optical depth and depolarization. Unlike traditional backscatter lidars, which are unable to rigorously correct profiles for attenuation, the HSRL provides absolutely calibration measurements. It is designed to operate as an Internet appliance that requires only electrical power, an Internet connection, and a zenith facing window for operation. The output beam of the lidar is safe for direct viewing and presents no safety hazard. All control and data transfer is accomplished via the Internet. It requires no routine on site attention other than cleaning the window.

The new HSRL is currently installed under a zenith-facing window on the top of our laboratory. The system operates 24-hours/day and more than one year of nearly continuous data has been acquired as part of testing prior to Arctic deployment. Data is automatically transfered in real time from the lidar to our archive computer via a fault-tolerant client-server application where it is stored on raid disk system. All data can be accessed through a publicly accessible web site: 'lidar.ssec.wisc.edu'. Real time access is provided by a web tools which process data on demand and return images, profile plots and netCDF files of processed data.

This paper will present examples illustrating HSRL measurement capabilities. HSRL measurements showing the variation of the observed cloud fraction with changes in the threshold optical depth needed to detect a cloud will also be presented.