Viewing Geometry Dependencies in MODIS Cloud Products

Abstract

Characterizing the earth's global cloud field is important for the proper assessment of the global radiation budget and hydrologic cycle. This characterization can only be achieved with satellite measurements. For complete daily coverage across the globe, polar-orbiting satellites must take observations over a wide range of sensor zenith angles. This paper uses Moderate Resolution Imaging Spectroradiometer (MODIS) Level-3 data to determine the effect that sensor zenith angle has on global cloud properties including the cloud fraction, cloud-top pressure, effective radii, and optical thickness. For example, the MODIS cloud amount increases from 57% to 71% between nadir and edge-of-scan (;678) observations, for clouds observed between 358N and 358S latitude. These increases are due to a combination of factors, including larger pixel size and longer observation pathlength at more oblique sensor zenith angles. The differences caused by sensor zenith angle bias in cloud properties are not readily apparent in monthly mean regional or global maps because the averaging of multiple satellite overpasses together ''washes out'' the zenith angle artifact. Furthermore, these differences are not constant globally and are dependent on the cloud type being observed.

Global and regional estimates of warm cloud droplet number concentration based on 13 years of AQUA-MODIS observations

“…Results here are largely consistent with earlier results reported in Maddux et al (2010) and are likely caused by increasing field-ofview size and three-dimensional radiative transfer effects toward the edge of the scan. In addition, dependencies of retrievals on scattering angle are also found in the rainbow and backscattering of the phase function.…”

“…For example, Maddux et al (2010) find significantly larger effective radius retrievals along with lower optical depth retrievals near the scan edge than in the centre of the scan. Grosvenor and Wood (2014) address the dependency of cloud microphysical retrievals on solar zenith angle, demonstrating possible increases in CDNC by 40 to 70 % at solar zenith angles higher than about 70 • .…”

“…A variety of observational case studies and process studies were also published using similar approaches for deriving CDNC (Boers et al, 2006;George and Wood, 2010;Painemal and Zuidema, 2010;Rausch et al, 2010;. Various authors have addressed shortcomings and issues related to CDNC climatologies (Merk et al, 2016;Grosvenor and Wood, 2014) as well as issues related to the cloud retrievals underlying the CDNC climatologies (Zhang and Platnick, 2011;Nakajima et al, 2010;Maddux et al, 2010;Horvath et al, 2014). Painemal and Zuidema (2011) validate MODIS-derived CDNC against in situ observations taken in the South Pacific during VOCALS-Rex (Wood et al, 2011).…”

Low-Cloud Feedbacks from Cloud-Controlling Factors: A Review

“…The larger spread among Q15 estimates might be consistent with the fact that cloud fraction is more difficult to measure. As a result, there may be greater differences between cloud fraction datasets than those describing cloud-radiative effect (Maddux et al 2010;Pincus et al 2012). …”

“…However, the results are less clear when using observations of cloud fraction since cloud feedbacks may also result from changes in other cloud properties, especially the distribution of optical thickness (Zelinka et al 2012). Cloud fraction itself is also relatively sensitive to details of the observing system (Maddux et al 2010;Pincus et al 2012) and how this system changes over time (Norris and Evan 2015). Nonetheless, these concerns are somewhat mitigated by the fact that the observed variability in cloud reflectance in tropical low-cloud regions is primarily driven by changes in low-cloud fraction (Klein and Hartmann 1993;George and Wood 2010).…”