Assessment and Error Analysis of Terra-MODIS and MISR Cloud-top Heights
through Comparison with ISS-CATS lidar
Abstract
Cloud-top heights (CTH) from the Multiangle Imaging Spectroradiometer
(MISR) and the Moderate Resolution Imaging Spectroradiometer (MODIS) on
Terra constitute our longest-running single-platform CTH record from a
stable orbit. Here, we provide the first evaluation of the Terra Level 2
CTH record against collocated International Space Station Cloud-Aerosol
Transport System (CATS) lidar observations between 50ºN - 50ºS. Bias and
precision of Terra CTH relative to CATS is shown to be strongly tied to
cloud horizontal and vertical heterogeneity and altitude. For
single-layered, unbroken, optically thick clouds observed over all
altitudes, the uncertainty in MODIS and MISR CTH are -540±690 m and
-280±370 m, respectively. The uncertainties are generally smaller for
lower altitude clouds and larger for optically thinner clouds. For
multi-layered clouds, errors are summarized herein using both absolute
CTH and CATS-layer-altitude proximity to Terra CTH. We show that MISR
detects the lower cloud in a two-layered system, provided top-layer
optical depth < ~0.3, but MISR low-cloud CTH
errors are unaltered by the presence of thin cirrus. Systematic and
random errors are propagated to explain inter-sensor disagreements, as
well as to provide the first estimate of the MISR stereo-opacity bias.
For MISR, altitude-dependent wind-retrieval bias (-90 to -110 m) and
stereo-opacity bias (-110 to -150 m) and for MODIS, CO2-slicing bias due
to geometrically thick cirrus leads to overall negative CTH bias. MISR’s
precision is largely driven by precision in retrieved wind-speed (3.7 m
s-1), whereas MODIS precision is driven by forward-modeling uncertainty.