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    Resolution dependence of deep convections in a global simulation from over 10-kilometer to sub-kilometer grid spacing

    Kajikawa Y, Miyamoto Y, Yoshida R, Yamaura T, Yashiro H, Tomita H

    deep convection, high resolution, global simulation, NICAM, cloudy disturbance, resolution dependence

    Resolution dependence of (a) the number of convections and (e) grid distance to the nearest convection core detected over the latitudinal area between 70 S and 30 S. The thin dashed line in (a) indicates a log Δ 4 crossing at the point of Δ14 as a reference. The same as (a) and (e) but for the area between 15 S and 15 N (b, f); 15 N and 30 N (c, g); and 30 N and 70 N (d, h). The horizontal grid-spacing in the series of experiments was set at 0.87, 1.7, 3.5, 7.0, and 14 km.

    The success of sub-kilometer global atmospheric simulation opens the door for resolving deep convections, which are fundamental elements of cloudy disturbances that drive global circulation. A previous study found that the essential change in the simulated convection properties occurred at a grid spacing of about 2 km as a global mean. In grid-refinement experiments, we conducted further comprehensive analysis of the global-mean state and the characteristics of deep convection, to clarify the difference of the essential change by location and environment. We found that the essential change in convection properties was different in the location and environment for each cloudy disturbance. The convections over the tropics show larger resolution dependence than convections over mid-latitudes, whereas no significant difference was found in convections over land or ocean. Furthermore, convections over cloudy disturbances [(i.e., Madden-Julian oscillation (MJO), tropical cyclones (TCs)] show essential change of convection properties at about 1 km grid spacing, suggesting resolution dependence. As a result, convections not categorized as cloudy disturbances make a large contribution to the global-mean convection properties. This implies that convections in disturbances are largely affected organization processes and hence have more horizontal resolution dependence. In contrast, other categorized convections that are not involved in major cloudy disturbances show the essential change at about 2 km grid spacing. This affects the latitude difference of the resolution dependence of convection properties and hence the zonal-mean outgoing longwave radiation (OLR). Despite the diversity of convection properties, most convections are resolved at less than 1 km grid spacing. In the future, longer integration of global atmosphere, to 0.87 km grid spacing, will stimulate significant discussion about the interaction between the convections and cloudy disturbances.