During the past several years, the sensitivity of the 8-year old Raman lidar at the ACRF Southern Great Plains (SGP) site has decreased between a factor of 3-4, resulting in a lower maximum altitude and increased random error in the geophysical properties (such as water vapor mixing ration and aerosol extinction profiles) derived from lidar. In September 2004, a ten-month effort to refurbish and upgrade the aging lidar was completed, as ARM scientists visited the site to align the instrument’s newly-refurbished receiving telescope, install new interference filters, and optimize signal levels. Replacing the telescope and interference filters returned the lidar’s sensitivity back to its original levels, while upgrades to the detection electronics - and subsequent removal of the neutral density filters - boosted the signals in the nitrogen and aerosol channels an order of magnitude, resulting in a substantial improvement in the capability of the lidar for water vapor, cloud, and aerosol studies.

A unique capability of the SGP Raman lidar is the ability to automatically switch between daytime and nighttime operations. This provides continuous, uninterrupted multiple-day measurements, as shown by this chart with uptime data in monthly increments from February 1998 through April 2004.

Following several years of deteriorating performance by the Lidar, a committee was formed in December 2003 to recommend ways to restore the instrument’s performance and recommend improvements and future upgrades. Based on the committee’s recommendations, many optical elements were replaced, including the external window, the input lens to the beam-expanding (transmit) telescope, and beam-turning mirrors. However, these changes had minimal impact on signal levels, so the decision was made to refurbish the telescope and replace the interference filters. In May, ARM scientists installed new signal processing and data acquisition electronics from Licel, combing both analog and photon counting into a single package. Then, in June, the 24-inch, 200-pound receiving telescope was sent to the vendor to have all three mirrors resurfaced. The refurbished telescope was reinstalled by SGP Operations staff in August in preparation for the final alignment, filter installation, and signal optimization activities in September.

Data from the SGP’s Raman lidar—the first operational turn-key Raman lidar in the world—are critical for various projects in the ARM Program’s Aerosol Working Group, and provide unique and valuable data for cirrus cloud property studies. Future improvements to the lidar system include refinements to the automatic alignment capability, and possibly additional channels for measuring temperature profiles and liquid water in thin clouds.