Projection of High Clouds and the Link to Ice Hydrometeors: An Approach Using Long-Term Global Cloud System–Resolving Simulations

Abstract

This study examines projections of high clouds related to sea surface temperature (SST) change using 14-km simulation output from NICAM, a global cloud system–resolving model. This study focuses on the vertical and horizontal structure of high cloud response to the SST pattern and how these cloud responses are linked to ice hydrometeors, such as cloud ice, snow, and graupel, which are not resolved by conventional general circulation models (GCMs). Under the present climate, the vertical and horizontal structure of the simulated increase in tropical high cloud amount for positive tropical mean HadISST SST anomalies has similar behavior to that of the GCM-Oriented CALIPSO Cloud Product (GOCCP) cloud fraction for HadISST SST. We further show that cloud ice is the main contributor to the simulated high cloud amount. Under a warming climate, the composite vertical and horizontal structure of the tropical high cloud response to the SST shows similar behavior to that under the present climate, but the amplitude of the variation is greater by a factor of 1.5 and the variation is more widespread. This amplification contributes to the high cloud increase under the warming climate, which is directly linked to the wider spatial extent of cloud ice in the eastern Pacific region. This study specifically reveals the similarity of the patterns of the responses of the high cloud fraction and cloud ice to global warming, indicating that an appropriate treatment of the complete spectrum of ice hydrometeors in global climate models is key to simulating high clouds and their response to global warming.