Tilting Ceilometers To Improve Cloud Base Height Detection in Precipitation

Precipitation adversely affects ceilometer cloud base height (CBH) detection. A strong increase in signal return induced by falling rain drops or snow flakes causes the ceilometer to report a CBH much lower than the actual CBH. Some international meteorological agencies tilt the ceilometer from vertical in an effort to report a more representative CBH during periods of precipitation. The National Weather Service (NWS) Automated Surface Observing System (ASOS) program conducted a seven-month evaluation of this technique. The study included two 25,000-foot (25K) ceilometers tilted at 8° from vertical and two vertically-pointed 25K ceilometers. Reference CBH measurements were obtained using pilot balloons from 0 to 2,000 feet. Results have shown that the tilted 25K ceilometers do not significantly improve CBH detection in liquid precipitation from 0 to 2,000 feet.