Detection of Cloud-Base Height Using Jenoptik CHM15K and Vaisala CL31 Ceilometers

Twelve case studies of multilayer cloud-base height (CBH) retrievals from two collocated ceilometers (Vaisala CL31 and Jenoptik CHM15K) have been analyzed. The studies were performed during the period from September to December 2008 at the Mace Head Atmospheric Research Station in Ireland. During the period of measurement, the two instruments provided vertical profiles of backscattered laser signal as well as the manufacturer's operational cloud-base product. The cases selected covered a diverse range of cloud-cover conditions, ranging from single to multiple cloud layers and from cloud-base heights varying from only a few hundreds meters per day up to 3-5 km in a few hours. The results show significant offsets between the two manufacturer-derived CBHs along with a considerable degree of scatter. Using a newly developed temporal height-tracking (THT) algorithm applied to both ceilometers, significant improvement in the correlation between CBH derived from both instruments results in a correlation coefficient increasing to R(2) = 0.997 (with a slope of 0.998) from R(2) = 0.788 (with an associated slope of 0.925). Also, the regression intercept (offset) is reduced from 160 m to effectively zero (-3 m). For the worst individual case study, using the THT algorithm resulted in the correlation coefficient improving from R(2) = 0.52, using the manufacturer's output, to R(2) = 0.97 with a reduction in the offset reducing from 569 to 32 m. Applying the THT algorithm to the backscatter profiles of both instruments led to retrieved cloud bases that are statistically consistent with each other and ensured reliable detection of CBH, particularly when inhomogeneous cloud fields were present and changing rapidly in time. The THT algorithm also overcomes multiple false cloud-base detections associated with the manufacturer's output of the two instruments.