An Intercomparison of Cloud Droplet Number Concentration Estimates from the MODIS Collection 5.1 and 6 Cloud Products

Abstract:

Cloud droplet number concentration (CDNC) is a property of particular interest to the cloud/aerosol modeling community. Estimates from MODIS are often used as an observational reference. With changes to the cloud screening and optical property retrieval algorithms between Collections 5.1 and 6, there is additional uncertainty with regard to which product provides the more representative estimate.

In order to quantify and better understand the differences in estimates between both products, CDNC is calculated from one year of Level 2 MODIS optical property retrievals and aggregated to Level 3 (1x1 degree) for the 1.6, 2.1 and 3.7 μm channels over global ocean scenes. Several comparisons between Collections 5.1 and 6 are made. 1) Investigating how MODIS Cloud Product quality flag selections affect CDNC estimates. 2) The dependence of the 3.7 and 1.6 μm values on the retrieval success of the 2.1 μm value in Collection 5.1 relative to the independent retrievals provided in Collection 6. 3) The inclusion of partly cloudy retrievals in Collection 6, which are excluded from the Collection 5.1 dataset via the Clear Sky Restoral Algorithm. Differences in estimates are presented in both a regional and temporal context.