Chang Ki Kim

A Numerical Study of Sea-Fog Formation over Cold Sea Surface Using a One-Dimensional Turbulence Model Coupled with the Weather Research and Forecasting Model

The formation mechanism of a cold sea-fog case observed over the Yellow Sea near the western coastal area of the Korean Peninsula is investigated using numerical simulation with a one-dimensional turbulence model coupled with a three-dimensional regional model. The simulation was carried out using both Eulerian and Lagrangian approaches; both approaches produced sea fog in a manner consistent with observation. For the selected cold sea-fog case, the model results suggested the following: as warm and moist air flows over a cold sea surface, the lower part of the air column is modified by the turbulent exchange of heat and moisture and the diurnal variation in radiation. The modified boundary-layer structure represents a typical stable thermally internal boundary layer. Within the stable thermally internal boundary layer, the air temperature is decreased by radiative cooling and turbulent heat exchange but the moisture loss due to the downward vapour flux in the lowest part of the air column is compensated by moisture advection and therefore the dewpoint temperature does not decrease as rapidly as does the air temperature. Eventually water vapour saturation is achieved and the cold sea fog forms in the thermal internal boundary layer.

Numerical Simulations of Airborne Glaciogenic Cloud Seeding Using the WRF Model with the Modified Morrison Scheme over the Pyeongchang Region in the Winter of 2016

A model was developed for simulating the effects of airborne silver iodide (AgI) glaciogenic cloud seeding using the weather research and forecasting (WRF) model with a modified Morrison cloud microphysics scheme. This model was used to hindcast the weather conditions and effects of seeding for three airborne seeding experiments conducted in 2016. The spatial patterns of the simulated precipitation and liquid water path (LWP) qualitatively agreed with the observations. Considering the observed wind fields during the seeding, the simulated spatiotemporal distributions of the seeding materials, AgI, and snowfall enhancements were found to be reasonable. In the enhanced snowfall cases, the process by which cloud water and vapor were converted into ice particles after seeding was also reasonable. It was also noted that the AgI residence time (>1 hr) above the optimum AgI concentration (105 m−3) and high LWP (>100 g m−2) were important factors for snowfall enhancements. In the first experiment, timing of the simulated snowfall enhancement agreed with the observations, which supports the notion that the seeding of AgI resulted in enhanced snowfall in the experiment. The model developed in this study will be useful for verifying the effects of cloud seeding on precipitation.

Turbulence in Marine Fog

Selected modeling and observational studies on the role of turbulence on marine fog formation are introduced. There can be basically three types of marine fog. Advection fog forms usually when turbulence generated by wind shear or longwave radiative cooling plays a crucial role in cooling and moistening the air above a cold sea surface after the advection of warm and moist air. In contrast, steam fog would be generated by the turbulence which plays an important role in transporting moisture from a relatively warm sea surface. The transition of stratus cloud into marine fog is another formation mechanism. The bottom of marine stratus clouds can be lowered to form marine fog by the cooling of the air just below the cloud base caused by turbulent heat loss and droplet evaporation, which can also supply moisture. Moreover, stratus cloud top can descend owing to the evaporation of cloud droplets in this region by entrainment of warm and dry air from above the cloud top under strong subsidence.

Radiation in Marine Fog

Studies on the role of radiation on the formation, evolution and dissipation of marine fog are introduced. Cooling of the air above colder sea surface can be caused by longwave radiative flux divergence and if this cooling is strong enough water vapor saturation can occur to form advection fog. Once fog droplets are formed, fog top radiative cooling due to outgoing longwave radiation plays a significant role in developing the fog layer. The dependency of longwave radiation on the fog microphysics is also examined. Contrary to longwave radiation, solar warming is found to be a main cause of fog dissipation.