Vaisala Laser Ceilometer CT12K Cloud Fraction Retrieval Analysis

A set of three recently acquired Vaisala Laser Ceilometers (model CT12K) were configured and tested during an undergraduate research experience in Summer 2012. After adapting wireless communications, an instrument was mounted on the Oceanography and Meteorology Building at Texas A&M University in September 2012. The CT12K measures cloud base heights to approximately 12,000 feet for up to two cloud layers, as well as providing a raw backscatter profile. However, the instrument does not directly supply cloud fraction, presenting the challenge of developing our own cloud fraction approaches. Atmospheric Infrared Sounder (AIRS) cloud fraction retrievals and a nearby ceilometer operated by the National Weather Service are used for comparison and method development.

Two different methods are used to calculate cloud fraction. The first method uses the ratio of cloud base observations to total number of observations over a specified short time interval. This method allows for an approximation of instantaneous cloud cover. The second method uses the ratio of the number of fifteen-minute samples when clouds were detected to the total number of fifteen-minute samples over a specified time interval. Limitations of these methods stem from the ceilometer's difficulty in detecting high clouds and the potential for second-layer cloud bases to be obscured by thick first-layer clouds. Here we present an analysis of the two cloud fraction methods and discuss the impact of the hardware limitations. Additionally, we will discuss the process of repurposing ceilometers to educational usage and our efforts to add a companion instrument to Texas A&M's REU site in the Costa Rican cloud forest.