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    Atmospheric and hydrospheric sciences

    Diurnal pattern of rainfall in Cambodia: its regional characteristics and local circulation

    Tsujimoto K, Ohta T, Aida K, Tamakawa K, Monichoth So Im

    Diurnal pattern of rainfall, Local circulation, Land breeze, Sea/lake breeze, Katabatic wind, Anabatic wind, Asian summer monsoon, Cambodia, Indochina Peninsula, Tonle Sap Lake

    Diurnal pattern of rainfall frequency for each region N, S, M, and C as the date–hour cross-section in 2011. The percentage of the number of “rainy” stations against the total number of stations in each region is shown with a blue shade. At region C, there is only one gauge, and thus the percentage is either 0 or 100. The horizontal red lines indicate 1 March, the onset and withdrawal dates of the summer monsoon season, and 30 November. The vertical yellow shades indicate hours with peak rainfall frequency in each season in each region, whereas the horizontal orange broken lines indicate days with prominent rainfall with no diurnal cycle or in different hours. The geopotential height (Φ) at 850 hPa and zonal winds (u) at 700 hPa (gray line) and 850 hPa (black line) as the areal averages over Cambodia (102–108°E and 10–15°N) at 7:00 LT (0:00 UTC) from JRA-55 are also shown for each day in the left panels

    We analyze the hourly rainfall data of 30 rain gauges in Cambodia from 2010 to 2015 to focus on the diurnal pattern of rainfall and its regional characteristics, with the underlying mechanisms inferred from the observed data. The observed annual rainfall in inland Cambodia ranges from 1,087 mm to 1,528 mm on station-average. Approximately 5–20% of the annual rainfall occurs during the pre-monsoon season, 50–78% during the summer monsoon season, and 12–36% during the post-monsoon season. During the pre-monsoon season, rainfall is dominant on the coast and over the Cardamom Mountains, with a maximum in the afternoon. The rainfall amount is smaller around the Tonle Sap Lake. During the summer monsoon season, rainfall is larger in the northern region and smaller in the western region in inland Cambodia, in both amount and proportion to annual rainfall. The rainfall amount on the coast is distinctively large. The diurnal rainfall maximum occurs in the early afternoon in the Cardamom Mountains, in the afternoon on the plain at the southwestern side of the Tonle Sap Lake, in the evening on the wide area of the northeastern side of the lake, and in the early morning on the coast. The clear regional characteristics in the diurnal rainfall pattern suggest significant effects of local features, even during the Asian summer monsoon season. During the post-monsoon season, rainfall is larger on the southwestern side of the Tonle Sap Lake with dominant nocturnal rainfall. These diurnal patterns are, however, not clear on some days, and analysis of the synoptic-scale atmospheric condition suggests the effect of the large-scale low-pressure system and disturbances on the appearance of the clear diurnal rainfall pattern. The effect of land–lake and mountain–valley circulations on forming the diurnal rainfall pattern is also implied from ground-observed meteorological data, although further numerical studies are required to examine the detailed mechanisms. The study of local effects on rainfall with consideration of the land-surface dynamics may aid flood and drought management in Cambodia by facilitating a greater understanding of its rainfall pattern.