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An accident occurred when an aircraft landed at Narita International Airport, Japan, on 20 June 2012. The aircraft encountered rapid changes of winds together with strong turbulence, although the weather was fair. In the present study, a two‐domain nested regional weather prediction models are used. The results in the outer domain show that southwesterly winds associated with a synoptic extratropical cyclone were locally accelerated to the southwest of the airport resulting in strong vertical shear. The simulation in the inner domain reproduces horizontal convective rolls, which are similar to those observed by a Doppler lidar at the airport. The wind velocity component parallel to the runway had a spatial variation of about 10 m s −1. The present approach using a large eddy simulation is useful for clarifying environments and features of horizontal convective rolls and forecasting low‐level wind shear associated with them, which can be a significant risk for aircraft. Plain Language Summary: Synoptic/mesoscale fronts and downbursts (strong downdrafts from a thunderstorm and diverging winds after they hit the ground) are known to be serious threats to airplanes during takeoff and landing. However, it is less known that there is another type of danger under fair weather conditions. An aircraft landing at Narita International Airport, Japan, in the afternoon of 20 June 2012 encountered rapid changes of wind and turbulence and made a hard landing, resulting in several injuries and damage to the fuselage. A Doppler lidar, a remote sensing device for monitoring low‐level winds at the airport observed alternating regions of strong and weak winds aligned southwest–northeast, perpendicular to the runway. In order to clarify the detailed structure and mechanism of these disturbances, numerical simulations using a fine‐resolution (horizontal resolution: 100 m) nested in a coarse resolution (horizontal resolution: 1 km) are made. The simulations show that horizontal convective rolls, which occur in convective boundary layers with strong vertical shear (vertical gradient of horizontal wind speed) were prevailing. These rolls are associated with the rapid changes in wind speed and turbulence similar to those observed by the Doppler lidar and flight recorder. The present study demonstrates that the large eddy simulation is useful for clarifying the structure and mechanisms of horizontal convective rolls, which caused the aircraft accident, and also for forecasting them. Key Points: Horizontal convective rolls that caused an aircraft accident in a fair weather condition were successfully reproduced by a numerical modelCharacteristics of the rolls and turbulence as observed by a Doppler lidar and flight recorder are clarified based on the model resultsCapability of accessing and operationally predicting potentially hazardous rolls by use of a large eddy simulation is demonstrated [ABSTRACT FROM AUTHOR]

An intense rainband associated with Typhoon 1326 (Wipha) induced a fatal debris flow on Izu Oshima, Japan, on October 15 - 16, 2013. This rainband formed along a local front between the southeasterly humid warm air around the typhoon and the northeasterly cold air from the Kanto Plain. In this paper, the Japan Meteorological Agency Nonhydrostatic Model was optimized for the "K computer", and ultra-high-resolution (500 - 250 m grid spacing) numerical simulations of the rainband with a large domain were conducted. Two of main factors that affect a numerical weather prediction (NWP) model, (1) grid spacing and (2) planetary boundary layer (PBL) schemes [Mellor-Yamada-Nakanishi-Niino (MYNN) and Deardorff (DD)], were investigated. Experiments with DD (Exps_DD: grid spacings of 2 km, 500 m, and 250 m) showed better reproducibility of the rainband position than experiments with MYNN (Exps_MYNN: grid spacings of 5 km, 2 km, and 500 m). Exps_DD simulated distinct convective-scale up/downdraft pairs on the southeast/northwest sides of the front, whereas those of Exps_MYNN were not clear. Exps_DD yielded stronger cold pools near the surface than did Exps_MYNN. These differences in the boundary layer structures likely had a large impact on the position of the front and the associated rainband. Exps_DD with the 500-m grid spacing showed the best precipitation performance according to the Fractions Skill Score. To check other factors which influence precipitation forecast, model domain sizes, lateral boundary conditions in nesting simulations, and terrain representations were investigated. In the small domain experiments, the rain-planband shapes were very different from the observations. In the experiment using a nesting procedure, the deterioration of the forecast performance was acceptably reduced. The model with fine terrains reproduced the intense rain over the island. These results demonstrate that the ultra-high-resolution NWP model with a large domain has the possibility to improve predictions of heavy rain. [ABSTRACT FROM AUTHOR]