Diurnal Variation of Precipitable Water over a Mountainous Area of Sumatra Island

Diurnal variations in atmospheric water vapor at Koto Tabang, a mountainous area of Sumatra Island, Indonesia, are studied by analyzing the GPS-derived precipitable water, radiosonde data, and surface meteorological observation data. A permanent GPS receiving station was established at Koto Tabang in March of 2001. Radiosonde soundings were carried out at 3-h intervals to study the atmospheric water and energy cycles in the Maritime Continent of Indonesia. A distinct diurnal variation in water vapor is observed. The precipitable water increases during daytime, reaching its maximum in the late afternoon. The results of the radiosonde observations indicate that the increase in water vapor occurs up to an altitude of approximately 3 km. Water vapor is transported by turbulent mixing with the development of the boundary layer in the afternoon. The diurnal cycle of water vapor is affected by the intensity of incident solar radiation. It is suggested that the diurnal variation in water vapor is caused by the horizontal transport of water vapor by local circulation. The diurnal range of the precipitable water on days with heavy rain was larger than that on days without rain. Rainfall often occurs as intensive showers during a short period in the late afternoon and early evening in the Koto Tabang area; daily shortwave radiation is still sufficiently strong to generate the local circulations and to induce large diurnal variation in precipitable water, even on the heavy-rain days.