NASA - National Aeronautics and Space Administration

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 and 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 uncertainties 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 thin 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 intersensor 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 (−60 to −260 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. Plain Language Summary Cloud-top height (CTH) is an essential climate variable that impacts the Earth’s energy budget and hydrological cycle. We are greatly interested in CTHs for their possible application in detecting signatures of forced climate change in the more than two-decade long (2000–present) CTH record from NASA’s enduring mission, Terra. Since Terra has offered longevity and orbital stability, the remaining criterion for a successful climate dataset is an in-depth understanding and quantification of uncertainty in the data. To ascertain the uncertainty of CTH retrievals from two Terra instruments, namely MISR and MODIS, we compare a subset of their observations against a lidar called CATS that operated from the International Space Station from 2015 to 2017. We determined that both MISR and MODIS have provided us with robust CTHs, with MISR being about twice as accurate and precise as MODIS. Each instrument demonstrates strengths and weaknesses depending on the types of clouds being observed. We note that the MISR error budget is self-contained and that we were able to close the error budget. This study has also provided needed CTH error characteristics that can help inform future satellite architecture for observing CTH.