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This review paper aims to provide readers with a broad range of meteorological backgrounds with basic information on marine low clouds and the concept of their parameterizations used in global climate models. The first part of the paper presents basic information on marine low clouds and their importance in climate simulations in a comprehensible way. It covers the global distribution and important physical processes related to the clouds, typical examples of observational and modeling studies of such clouds, and the considerable importance of changes in low clouds for climate simulations. In the latter half of the paper, the concept of cloud parameterizations that determine cloud fraction and cloud water content in global climate models, which is sometimes called cloud "macrophysics", is introduced. In the parameterizations, the key element is how to assume or determine the inhomogeneity of water vapor and cloud water content in model grid boxes whose size is several tens to several hundreds of kilometers. Challenges related to cloud representation in such models that must be tackled in the next couple of decades are discussed. [ABSTRACT FROM AUTHOR]

The new Meteorological Research Institute Earth System Model version 2.0 (MRI-ESM2.0) has been developed based on previous models, MRI-CGCM3 and MRI-ESM1, which participated in the fifth phase of the Coupled Model Intercomparison Project (CMIP5). These models underwent numerous improvements meant for highly accurate climate reproducibility. This paper describes model formulation updates and evaluates basic performance of its physical components. The new model has nominal horizontal resolutions of 100 km for atmosphere and ocean components, similar to the previous models. The atmospheric vertical resolution is 80 layers, which is enhanced from the 48 layers of its predecessor. Accumulation of various improvements concerning clouds, such as a new stratocumulus cloud scheme, led to remarkable reduction in errors in shortwave, longwave, and net radiation at the top of the atmosphere. The resulting errors are sufficiently small compared with those in the CMIP5 models. The improved radiation distribution brings the accurate meridional heat transport required for the ocean and contributes to a reduced surface air temperature (SAT) bias. MRI-ESM2.0 displays realistic reproduction of both mean climate and interannual variability. For instance, the stratospheric quasi-biennial oscillation can now be realistically expressed through the enhanced vertical resolution and introduction of non-orographic gravity wave drag parameterization. For the historical experiment, MRI-ESM2.0 reasonably reproduces global SAT change for recent decades; however, cooling in the 1950s through the 1960s and warming afterward are overestimated compared with observations. MRI-ESM2.0 has been improved in many aspects over the previous models, MRICGCM3 and MRI-ESM1, and is expected to demonstrate superior performance in many experiments planned for CMIP6. [ABSTRACT FROM AUTHOR]

The cloud top height of marine boundary layer clouds (MBLCs) in the mid-latitudes has received less attention than that of subtropical MBLCs and is investigated here using cloud mask data, which were based on observations from the cloud--aerosol lidar and infrared pathfinder satellite observation (CALIPSO) satellite. This study provides a comprehensive overview of the observational characteristics of variations in cloud top height of MBLCs and fog frequency over the mid-latitudes. Seasonal variations in the cloud top height of mid-latitude MBLCs as well as the differences in these seasonal variations between the Northern and Southern hemispheres were determined. For example, over the North Pacific, the cloud top height is high in winter (up to 1800 m) but low in summer (down to 800 m), whereas in the Southern Hemisphere, the seasonal variation is not well defined, with heights ranging from 1300 to 1500 m. While clear seasonal variations in the frequency of fog occurrence are found over the North Pacific and the northwest Atlantic, the fog frequency over the Southern Ocean is almost constant irrespective of the season. High correlations were found between the MBLC top height and stability indexes and between the fog frequency and some surface parameters such as temperature difference between the surface air and the sea surface. The latitudinal variations in the cloud top height of MBLCs in summer and winter over the Southern Ocean were compared with those over the North Pacific. The difference in cloud top heights between nighttime and daytime is also presented. [ABSTRACT FROM AUTHOR]