Convective rainfall often shows a clear diurnal cycle. The nighttime peak of convective activity prevails in various regions near mountains in the world. The influence of the water vapor and convective instability upon nocturnal peak of convective system is investigated by observed data and numerical experiment. Recent developments in GPS meteorology allow the estimation of Precipitable Water Vapor (PWV) with a high temporal resolution. Additionally spatially dense network has been established in Japan. Composite analysis by PWV and radar data for selected clear days in August 2000 provides the following result.
In the early evening, a high-GPS-PWV region forms over mountainous area due to the convergence of low level moisture causing high frequency of convective precipitation. The region of high PWV gradually propagates toward adjacent plain before midnight by an ambient wind. At the same time, region of convection propagates eastwards simultaneously into the plain. The precipitating frequency correlates fairly well with the GPS-PWV and attains a maximum value at night over the plain.
The composite of numerical experiment also provides similar characteristics in the diurnal cycles of rainfall and PWV peaks. Abundant moisture accumulates over the mountainous area in the afternoon due to the convergence of mountain circulation. The amount of moisture accumulation is greater than local evaporation. The specific humidity greatly increases at the level of about 800 hPa over the plain at night, and the PWV reaches its nocturnal maximum which is consistent with observed PWV. Thus, the nocturnal increase of PWV can be explained by the contribution of moisture advection at this level. The increase in the specific humidity causes an increase of equivalent potential temperature at a level of around 800 hPa; as a result, the convective instability index becomes more unstable over the plain at night. Simulated convective instability is consistent with the diurnal cycle of precipitating frequency in this region.