Abstract. The Cloud-Aerosol
Lidar and Infrared Pathfinder Satellite Operations (CALIPSO) mission released
version 4.1 (V4) of their lidar level 2 cloud and aerosol data products in
November 2016. These new products were derived from the CALIPSO V4 lidar
level 1 data, in which the calibration of the measured backscatter data at
both 532 and 1064 nm was significantly improved. This paper describes
updates to the V4 level 2 cloud–aerosol discrimination (CAD) algorithm that
more accurately differentiate between clouds and aerosols throughout the
Earth's atmosphere. The level 2 data products are improved with new CAD
probability density functions (PDFs) that were developed to accommodate
extensive calibration changes in the level 1 data. To enable more reliable
identification of aerosol layers lofted into the upper troposphere and lower
stratosphere, the CAD training dataset used in the earlier data releases was
expanded to include stratospheric layers and representative examples of
volcanic aerosol layers. The generic “stratospheric layer” classification
reported in previous versions has been eliminated in V4, and cloud–aerosol
classification is now performed on all layers detected everywhere from the
surface to 30 km. Cloud–aerosol classification has been further extended to
layers detected at single-shot resolution, which were previously classified
by default as clouds. In this paper, we describe the underlying rationale
used in constructing the V4 PDFs and assess the performance of the V4 CAD
algorithm in the troposphere and stratosphere. Previous misclassifications of
lofted dust and smoke in the troposphere have been largely improved, and
volcanic aerosol layers and aerosol layers in the stratosphere are now being
properly classified. CAD performance for single-shot layer detections is also
evaluated. Most of the single-shot layers classified as aerosol occur within
the dust belt, as may be expected. Due to changes in the 532 nm calibration
coefficients, the V4 feature finder detects ∼9.0 % more features at
night and ∼2.5 % more during the day. These features are typically
weakly scattering and classified about equally as clouds and aerosols. For
those tropospheric layers detected in both V3 and V4, the CAD classifications
of more than 95 % of all cloud and daytime aerosol layers remain
unchanged, as do the classifications of ∼89 % of nighttime aerosol
layers. Overall, the nighttime net cloud and aerosol fractions remain
unchanged from V3 to V4, but the daytime net aerosol fraction is increased by
about 2 % and the daytime net cloud fraction is decreased by about
2 %.