Evaluation of the MODIS Collection 6 multilayer cloud detection algorithm through comparisons with CloudSat Cloud Profiling Radar and CALIPSO CALIOP products

Abstract. Since multilayer cloud scenes are common in the atmosphere and can be an
important source of uncertainty in passive satellite sensor cloud
retrievals, the MODIS MOD06 and MYD06 standard cloud optical property products
include a multilayer cloud detection algorithm to assist with data quality
assessment. This paper presents an evaluation of the Aqua MODIS MYD06
Collection 6 multilayer cloud detection algorithm through comparisons with
active Cloud Profiling Radar (CPR) and Cloud-Aerosol Lidar with
Orthogonal Polarization (CALIOP) products that have the ability to provide cloud
vertical distributions and directly classify multilayer cloud scenes and
layer properties. To compare active sensor products with an imager such as
MODIS, it is first necessary to define multilayer clouds in the context of
their radiative impact on cloud retrievals. Three main parameters have thus
been considered in this evaluation: (1) the maximum separation distance
between two cloud layers, (2) the thermodynamic phase of those layers and
(3) the upper-layer cloud optical thickness. The impact of including the
Pavolonis–Heidinger multilayer cloud detection algorithm, introduced in
Collection 6, to assist with multilayer cloud detection has also been
assessed. For the year 2008, the MYD06 C6 multilayer cloud detection
algorithm identifies roughly 20 % of all cloudy pixels as multilayer
(decreasing to about 13 % if the Pavolonis–Heidinger algorithm output
is not used). Evaluation against the merged CPR and CALIOP 2B-CLDCLASS-lidar
product shows that the MODIS multilayer detection results are quite
sensitive to how multilayer clouds are defined in the radar and lidar product
and that the algorithm performs better when the optical thickness of the
upper cloud layer is greater than about 1.2 with a minimum layer separation
distance of 1 km. Finally, we find that filtering the MYD06 cloud optical
properties retrievals using the multilayer cloud flag improves aggregated
statistics, particularly for ice cloud effective radius.