MODIS-Based Evaluation of Low Cloud Amount Indices: “Do the Spatiotemporal Scales Matter?”

It has been widely discussed that marine low clouds play an important role in simulating future climate in response to natural and/or anthropogenic forcings. Previously, several studies have suggested indices for low cloud amount (LCA) by representing atmospheric stability. In this study, four LCA indices (lower-tropospheric stability [LTS], estimated inversion strength [EIS], estimated cloud-top entrainment index [ECTEI], and estimated low-level cloud fraction [ELF]) were calculated from reanalysis data from and used to measure the coherence between these indices and MODIS observations of LCA.

These indices show generally good agreement with MODIS LCA observations. For example, all four indices show strong correlations with observation near the coast of Peru in South America. However, their relationships with MODIS observations are not as strong near the coast of California, particularly for the EIS and ECTEI indices. We also found that the relationships varied depending on the spatiotemporal scales, with weaker relationships on smaller horizontal and shorter temporal scales. Based on these findings, we examined regional characteristics to identify factors that may be disrupting the relationship between LCA indices and MODIS observations. We also discussed the effects of these factors on spatiotemporal scales.