Resolution dependence of nonhydrostatic models in simulating the formation and evolution of low-level clouds during a "Yamase" event

In this study, numerical simulations of 'Yamase' clouds were conducted using a nonhydrostatic model (NHM). Yamase clouds are typical maritime boundary-layer clouds that frequently appear over the sea off the east coast of the Sanriku District (northeast Honshu Island, Japan) during the summer season. The simulation period was from July to August 1993, when cool easterly winds (termed Yamase winds) prevailed. The NHM, with a horizontal grid of 40 km (NHM40) was integrated, nested within global objective analysis data, for one month by adopting sponge boundaries widely to lateral grids and upper layers, in order to reproduce a synoptic situation. The synoptic weather pattern is precisely reproduced in the long term integration. Then, in order to investigate the formation and evolution processes and structures of Yamase low-level clouds, multi-nesting simulations were conducted using the results of the one-month integration. The characteristic features of simulated Yamase clouds in terms of horizontal distribution, cloud shape and structures, and cloud water content, as well as the dominant mechanisms of cloud formation and evolution, strongly depend on the horizontal resolution of NHMs. The NHM with the high resolution of 100 m (NHM01), simulates convective structures similar to those observed from satellite images. Furthermore, the simulated liquid water path is close to the values observed in other Yamase cases. These results indicate that NHM01 reproduces the characteristic properties of the low-level clouds. In contrast, in simulations using the NHM with the low-resolution of 10 km (NHM10), the mixed layer is too moist and its height too low compared with observed data, and the liquid water path is overestimated. These errors are brought from the fact that the interaction between cloud radiation and formation at night time is too strong compared with the case of NHM01. Vertical heating profiles related to radiation, condensation, and evaporation in NHM10 are different from those generated by NHM01. This study suggests that improved parameterizations, such as a partial condensation scheme, and an adequate scheme of the buoyant production, should be introduced in low-resolution models such as NHM10.