2886 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 45, NO. 9, SEPTEMBER 2007 Differences Between Collection 4 and 5 MODIS Ice Cloud Optical/Microphysical Products and Their Impact on Radiative Forcing Simulations Ping Yang, Lei Zhang, Gang Hong, Shaima L. Nasiri, Bryan A. Baum, Hung-Lung Huang, Michael D. King, Senior Member, IEEE, and Steven Platnick Abstract—This paper reports on the comparison of two latest versions (collections 4 and 5) of ice cloud products derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) measurements. The differences between the bulk optical proper- ties of ice clouds used in collections 4 and 5 and the relevant impact on simulating the correlation of the bidirectional reflection functions at two MODIS bands centered at 0.65 (or 0.86) and 2.13 μm are investigated. The level-3 MODIS ice cloud properties (specifically, ice cloud fraction, optical thickness, and effective particle size in this paper) from the collection 4 and 5 datasets are compared for a tropical belt of 30 ◦ S–30 ◦ N. Furthermore, the impact of the differences between the MODIS collection 4 and 5 ice cloud products on the simulation of the radiative forcing of these clouds is investigated. Over the tropics, the averaged ice cloud fraction from collection 5 is 1.1% more than the collection 4 counterpart, the averaged optical thickness from collection 5 is 1.2 larger than the collection 4 counterpart, and the averaged effective particle radius from collection 5 is 1.8 μm smaller than the collection 4 counterpart. Moreover, the magnitude of the differences between collection 5 and 4 ice cloud properties also depends on the surface characteristics, i.e., over land or over ocean. The differences of these two datasets (collections 4 and 5) of cloud properties can have a significant impact on the simulation of the radiative forcing of ice clouds. In terms of total (longwave plus shortwave) cloud radiative forcing, the differences between the collection 5 and 4 results are distributed primarily between -60 and 20 W · m -2 but peak at 0 W · m -2 . Index Terms—Aqua, clouds, Moderate Resolution Imaging Spectroradiometer (MODIS), radiative forcing, remote sensing, static libraries. Manuscript received December 15, 2006; revised February 6, 2007. This work was supported by a NASA research Grant (NNG04GL24G) from the NASA Radiation Sciences Program managed by Dr. H. Maring (previously by Dr. D. Anderson) and the MODIS Program managed by Dr. P. Bontempi. Regarding the development of the radiative properties (particularly the single- scattering properties) of ice clouds from theoretical perspective, P. Yang also acknowledges support from the National Science Foundation Physical Meteorology Program (ATM-0239605) managed by Dr. A. Detwiler. P. Yang, G. Hong, and S. L. Nasiri are with the Department of Atmospheric Sciences, Texas A&M University, College Station, TX 77843 USA (e-mail: pyang@ariel.met.tamu.edu). L. Zhang was with the Department of Atmospheric Sciences, Texas A&M University, College Station, TX 77843 USA. He is now with the College of Atmospheric Sciences, Lanzhou University, Lanzhou 730000, China. B. A. Baum and H.-L. Huang are with Space Science and Engineering Center, University of Wisconsin, Madison, WI 53706 USA. M. D. King and S. Platnick are with the Earth Sciences Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771 USA. Digital Object Identifier 10.1109/TGRS.2007.898276 I. I NTRODUCTION W ITH THE increasing awareness of the importance of ice clouds [25], [29] in the terrestrial atmosphere, re- search has addressed numerous issues relevant to ice clouds from various perspectives, including in situ measurements of the microphysical properties of ice clouds [13], theoretical investigations of the single and multiple scattering and absorp- tion properties of ice clouds [1], [30], [36], [37], [48], [49], [56]–[58], efforts to parameterize the bulk radiative properties of these clouds [7], [8], [10], [33] for applications to climate models, the impacts of ice clouds on the radiation spectrum [53] and climate feedback [46], and the retrieval of ice cloud optical and microphysical properties from airborne and satel- lite measurements [1], [2], [6], [15], [20], [22], [34], [35], [52]. However, the representation of these clouds in general circulation models (GCMs) is rather primitive in the sense that substantial uncertainties exist in the basic cloud climatologies derived from the GCM simulations, and the cloud distributions simulated from many GCMs are quite different from those inferred from satellite observations [61]. To improve the repre- sentation of ice clouds in GCMs, it is critical to understand the global ice cloud climatology to provide crucial constraints on the parameterization of various cloud microphysical processes and cloud-radiation interactions in GCMs. To this end, reliable satellite-based retrievals of ice cloud properties on a global scale are necessary. The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments [17], [18], [44], [55] on the NASA Earth Observing System Terra and Aqua platforms include 36 spec- tral channels covering essentially all the key atmospheric bands located between 0.415 and 14.235 µm [41] and provide advanced capabilities to study ice clouds, although MODIS does not have far-infrared-radiation (far-IR) spectral bands. Note that the far-IR is important to the energetics of the Earth’s atmosphere. The MODIS measurements have been extensively used in cloud property retrievals and for the cloud clearing (e.g., [23] and [24]) that is necessary for inferring aerosol, surface, and atmospheric profile properties. With precomputed static libraries of ice cloud radiances, a bispectral technique [39] can be used to simultaneously infer the optical thickness and effective particle size of an ice cloud from the MODIS measurements [18], [21], [41] during daytime conditions. The MODIS cloud (level-2) pixel-level products are available for individual granules, known as MOD06 and MYD06 for Terra 0196-2892/$25.00 © 2007 IEEE