Evaluate and improve of low-cloud simulations in climate models

Climate model Evaluations

Low clouds play a critical role in our climate system. Therefore, a faithful simulation of low clouds in the global climate model (GCM) is critical for the projection of future climate and understanding of aerosol-cloud interactions. Unfortunately, it turns out to be an extremely challenging task. Among others, an important reason is that many physical processes in low clouds occur at the spatial scales much smaller than the typical resolution of GCMs, making the simulation of these processes in GCMs highly challenging. One of our recent research directions aims to identify and understand the problems associated with low-cloud simulations in the GCMs and explore pathways for improvements. As the first one of a series of papers, Song et al. [2018b] (led by my postdoc) compared the low-cloud simulations in widely used GCM with satellite observations. We found that the GCM significantly underestimates the low-cloud fraction over the tropical region. More importantly we are able to identify one important reason that has partly caused the low-cloud deficiency in the model, that is, the warm-rain process in the GCM is too frequent that causes low clouds in the model to rain out their water and dissipate too quickly. Later, my postdoc Song led another study that elucidated the importance of considering subgrid cloud variation in the comparison between low-resolution GCM cloud simulations and high-resolution satellite observation [Song et al., 2018a]. Inspired by these two studies, we derived the subgrid variations of cloud microphysical properties over the tropical oceans using satellite observations and use the results to investigate the implications of subgrid cloud variation on the warm-rain parameterization in GCMs [Zhang et al., 2019]. In short, our study elucidated that not only the subgrid variation of cloud water, but the subgrid variation of cloud droplet number concentration and its covariation with cloud water should also be considered in the warm-rain parameterization but are overlooked in most previous studies. Currently, supported by a grant from the DOE, we are further investigating the impacts of subgrid cloud variations on the parametrization of warm rain and thereby the lifetime of low clouds in GCMs.