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- Author or Editor: Tomo’omi Kumagai x
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Abstract
This study investigated atmospheric water cycles over several time scales to understand the maintenance processes that control heavy precipitation over the islands of the Maritime Continent. Large island regions can be divided into land, coastal, and ocean areas based on the characteristics of both the hydrologic cycle and the diurnal variation in precipitation. Within the Maritime Continent, the major islands of Borneo and New Guinea exhibit different hydrologic cycles. Large-scale circulation variations, such as the seasonal cycle and the Madden–Julian oscillation, have a lesser effect on the hydrologic cycle over Borneo than over New Guinea because the effects depend on their shapes and locations. The impact of diurnal variations on both regional-scale circulation and water exchange between land and coastal regions is pronounced over both islands. The recycling ratio of precipitation, which can be related to stronger diurnal variation in the atmospheric water cycle that results from enhanced evapotranspiration over tropical rain forests, is higher over Borneo than over New Guinea.
Abstract
This study investigated atmospheric water cycles over several time scales to understand the maintenance processes that control heavy precipitation over the islands of the Maritime Continent. Large island regions can be divided into land, coastal, and ocean areas based on the characteristics of both the hydrologic cycle and the diurnal variation in precipitation. Within the Maritime Continent, the major islands of Borneo and New Guinea exhibit different hydrologic cycles. Large-scale circulation variations, such as the seasonal cycle and the Madden–Julian oscillation, have a lesser effect on the hydrologic cycle over Borneo than over New Guinea because the effects depend on their shapes and locations. The impact of diurnal variations on both regional-scale circulation and water exchange between land and coastal regions is pronounced over both islands. The recycling ratio of precipitation, which can be related to stronger diurnal variation in the atmospheric water cycle that results from enhanced evapotranspiration over tropical rain forests, is higher over Borneo than over New Guinea.
Abstract
Southeast Asian tropical rain forests in the Maritime Continent are among the most important biomes in terms of global and regional water cycling. How land use and land cover change (LULCC) relating to deforestation and forest degradation alter the local hydroclimate over the island of Borneo is examined using the Weather Research and Forecasting (WRF) Model with an appropriate land surface model for describing the influence of changes in the vegetation status on the atmosphere. The model was validated against precipitation data from Tropical Rainfall Measuring Mission (TRMM) satellite 3B42 measurements. A main novelty in this analysis is that the diurnal cycle of precipitation over the island, which is a dominant climatic characteristic of the Maritime Continent, was successfully reproduced. To clarify the impact of the LULCC on the precipitation regimes over the island, numerical experiments were performed with the model that demonstrated the following. Deforestation that generates high albedo areas, such as bare lands, would induce a reduction in precipitation because of reductions in evapotranspiration, convection, and horizontal atmospheric moisture inflow. On the other hand, a decrease in evapotranspiration efficiency without changing the surface albedo could increase precipitation due to an increase in convection and horizontal atmospheric moisture inflow in compensation for the decrease in evapotranspiration. In detail, on the Maritime Continent, through changes in the land surface heating process and land–sea breeze circulation, the LULCC would impact the amplitude of the diurnal precipitation cycle in each region as defined according to the distance from the coast, resulting in changes in the precipitation regimes over the island.
Abstract
Southeast Asian tropical rain forests in the Maritime Continent are among the most important biomes in terms of global and regional water cycling. How land use and land cover change (LULCC) relating to deforestation and forest degradation alter the local hydroclimate over the island of Borneo is examined using the Weather Research and Forecasting (WRF) Model with an appropriate land surface model for describing the influence of changes in the vegetation status on the atmosphere. The model was validated against precipitation data from Tropical Rainfall Measuring Mission (TRMM) satellite 3B42 measurements. A main novelty in this analysis is that the diurnal cycle of precipitation over the island, which is a dominant climatic characteristic of the Maritime Continent, was successfully reproduced. To clarify the impact of the LULCC on the precipitation regimes over the island, numerical experiments were performed with the model that demonstrated the following. Deforestation that generates high albedo areas, such as bare lands, would induce a reduction in precipitation because of reductions in evapotranspiration, convection, and horizontal atmospheric moisture inflow. On the other hand, a decrease in evapotranspiration efficiency without changing the surface albedo could increase precipitation due to an increase in convection and horizontal atmospheric moisture inflow in compensation for the decrease in evapotranspiration. In detail, on the Maritime Continent, through changes in the land surface heating process and land–sea breeze circulation, the LULCC would impact the amplitude of the diurnal precipitation cycle in each region as defined according to the distance from the coast, resulting in changes in the precipitation regimes over the island.
Abstract
Temperature data in the mountain forest regions are often extrapolated from temperature data recorded at base stations at lower elevation. Such extrapolation is often based on elevation differences between target regions and base stations at low elevation assuming a constant temperature lapse rate throughout the year. However, this assumption might be problematic where slope circulation is active and decoupled from the regional circulation. To model the seasonal change in the lapse rate, the authors compared daily maximum (T max) and minimum temperatures (T min) observed at a mountain forest site (Kog–Ma; 1300-m altitude) with those observed at the bottom of the basin (Chiang–Mai; 314-m altitude) in northern Thailand, where slope circulation is active and decoupled from the regional circulation. The difference in T max between Kog–Ma and Chiang–Mai (ΔT max; Kog–Ma minus Chiang–Mai) was relatively unchanged throughout the year. However, the difference in T min between Kog–Ma and Chiang–Mai (ΔT min) changed seasonally. Thus, assuming a constant lapse rate throughout the year could cause large errors in extrapolating T min data in mountainous areas in northern Thailand. The difference ΔT min was related to nighttime net radiation (Rn), suggesting that nocturnal drainage flow affects the determination of ΔT min. This relationship would be useful in formulating seasonal changes in the lapse rate for T min. As Rn data are generally unavailable for meteorological stations, an index that relates to the lapse rate for T min and is calculated from T max and T min data is proposed. This index might be useful for accurately estimating T min values in mountainous regions in northern Thailand.
Abstract
Temperature data in the mountain forest regions are often extrapolated from temperature data recorded at base stations at lower elevation. Such extrapolation is often based on elevation differences between target regions and base stations at low elevation assuming a constant temperature lapse rate throughout the year. However, this assumption might be problematic where slope circulation is active and decoupled from the regional circulation. To model the seasonal change in the lapse rate, the authors compared daily maximum (T max) and minimum temperatures (T min) observed at a mountain forest site (Kog–Ma; 1300-m altitude) with those observed at the bottom of the basin (Chiang–Mai; 314-m altitude) in northern Thailand, where slope circulation is active and decoupled from the regional circulation. The difference in T max between Kog–Ma and Chiang–Mai (ΔT max; Kog–Ma minus Chiang–Mai) was relatively unchanged throughout the year. However, the difference in T min between Kog–Ma and Chiang–Mai (ΔT min) changed seasonally. Thus, assuming a constant lapse rate throughout the year could cause large errors in extrapolating T min data in mountainous areas in northern Thailand. The difference ΔT min was related to nighttime net radiation (Rn), suggesting that nocturnal drainage flow affects the determination of ΔT min. This relationship would be useful in formulating seasonal changes in the lapse rate for T min. As Rn data are generally unavailable for meteorological stations, an index that relates to the lapse rate for T min and is calculated from T max and T min data is proposed. This index might be useful for accurately estimating T min values in mountainous regions in northern Thailand.
