[1] Through laboratory experiments generating unidirectional water flows, we examine the process of interaction between two moving barchans (sandy bed configurations in a crescentic plan-shape), which may take dozens of years for barchan dunes in the nature. Three types of the interaction were observed. The first type was absorption of two barchans into one. The second was that the upstream fast barchan rode on the downstream slow barchan and simultaneously a newly born barchan was ejected from the lee side of the slow barchan. The third was that the downstream slow barchan was split into two before the upstream fast barchan touched the downstream. The type of the interaction was determined by both relative and absolute size of two barchans.

The authors investigate trends in precipitation extremes using daily precipitation data from Southeast Asian countries during 1950s to 2000s. Number of wet days, defined by a day with at least 1 mm of precipitation, tends to decrease over these countries, while average precipitation intensity of wet days shows an increasing trend. Heavy precipitation indices, which are defined by precipitation amount and percentile, demonstrate that the number of stations with significant upward trend is larger than that with significant downward trend. Heavy precipitation increases in southern Vietnam, northern part of Myanmar, and the Visayas and Luzon Islands in the Philippines, while heavy precipitation decreases in northern Vietnam. Annual maximum number of consecutive dry days decreases in the region where winter monsoon precipitation dominates. Decrease of precipitation event in the dry season is suggested in Myanmar.

A self‐accelerating current is a particle‐driven gravity flow moving on a sloping bottom whose velocity increases in the downstream direction as a result of increasing suspended sediment concentration due to sediment entrainment from the bed. This implies that the net balance between deposition from the current onto the bed and erosion into the flow must be favorable to the latter; thus, a larger mass of particles is being picked up into suspension than is settling out. The self‐accelerative stage cannot continue indefinitely. Either the downstream bed slope drops off to the point where self‐acceleration cannot be maintained or an autosuspensive stage may be reached where the net balance between deposition and erosion is zero and the channel bed is partially or completely free of alluvium. Once such a state is reached on a constant bed slope, the current can persist indefinitely without any external supply of energy other than the potential energy offered by the slope itself. This paper documents experimental turbidity currents composed of lightweight plastic particles ranging from 20 to 200 μm with a specific density between 1.3 and 1.5. These particles were either noncohesive or slightly cohesive. The experiments were performed in a 15‐m long flume with a bottom slope of 0.05. Self‐acceleration of the head of the flow was achieved in some of the tests reported here. Measurements of velocity and suspended sediment taken at different stages of head evolution document this self‐acceleration. In addition, these measurements are in agreement with previous empirical studies relating to head thickness, concentration, velocity, and water depth. Stratigraphic analysis of the deposit shows the key role bed material plays in determining whether a given turbidity current will or will not accelerate. This factor ties the dynamics of a self‐accelerating current to the existence of deposits laid down by antecedent currents. The conditions of the present tests appear to fulfill previous autosuspension criteria relating to flow velocity, particle settling velocity, and bed slope. Densimetric Froude number similarity analysis is used to estimate equivalent parameters for field scale turbidity currents.

The collision processes of two crescentic dunes called barchans are systematically studied using a simple computer simulation model. The simulated processes, coalescence, ejection and reorganization, qualitatively correspond to those observed in a water tank experiment. Moreover we found the realized types of collision depend both on the mass ratio and on the lateral distance between barchans under initial conditions. A simple set of differential equations to describe the collision of one-dimensional (1D) dunes is introduced.

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Antidunes and their sedimentary structures can be useful in reconstructing paleo-hydraulic conditions, especially for large discharge events. However, three-dimensional (3D) antidunes in sand-sized sediments have not yet been studied extensively, as compared to either two-dimensional (2D) antidunes or antidunes in gravel-sized sediments. In this study, we estimated formative conditions of gravel step-pool morphologies and applied them to the formation of 3D antidunes over a sand bed. Formative conditions are expressed in terms of a relationship between the water discharge per unit width and the bed slope. Flume experiments demonstrated that 3D mound-like antidune configurations and their associated internal sedimentary structures could be preserved. Internal sedimentary structures were characterized by shallow lens-like structures whose bases were erosional. Although gently-dipping concave-upward lamination was dominant, convex-upward lamination was occasionally observed. The dimensions of lenticular lamina-sets can be used to estimate antidune geometry. Thus if 3D antidunes can be interpreted in the stratigraphic record, it is possible to estimate the paleo-hydraulic parameters such as water discharge and bed slope more precisely than previously.

Barchans are crescent-shaped dunes that form under unidirectional wind in areas of limited sand supply. The recent development of flume experiments and computer simulations has renewed interest in the interaction dynamics of two or more barchans. From the flume experiment, four distinguishable types of collision patterns between two barchans have been observed: coalescence, ejection, split and reorganization. We have proposed a simple cellular model for numerical simulations of dune dynamics, in which saltation and avalanche are elementary processes. In the present paper, we first describe the model in detail. The model reproduces three types of collision patterns: coalescence, ejection, and reorganization. The largest reason for a split pattern not to occur is the lack of an effect of the flow separation at the brink line of dunes and the recirculation bubble that it produces. We then model the effect of the recirculation bubble by assuming that strong erosion occurs at the reattachment point of the separation flow. The strong-erosion model successfully reproduces all the collision patterns. Thus, three elementary processes -saltation, avalanche and strong-erosion -are sufficient for a phenomenological description of the interaction dynamics of aqueous barchans. It is also shown that the type of collision is determined by competition between the filling-up of the interdune between two barchans and the change in height of each dune.

This study investigated the characteristics of the rainfall associated with tropical cyclones (TCs), using the TC best-track data and daily rainfall data from 15 meteorological stations for the period 1961−2008 for the coastal region of Vietnam. In addition to investigating the TC rainfall amount, we estimated the TC rain ratio and the ratio of TC heavy rainfall days (TC_R50) and interpreted these parameters for El Niño and La Niña years. Our results show that the maximum TC rainfall occurs from July to September in the northern region, whereas the total rainfall at southern stations is mainly composed of non-TC rainfall. The TC rainfall amount is concentrated in the central region, with a peak in October-November. The TC rain ratio varies from 0 to ~25%, showing a maximum value in the region of 16°N−18°N in September. The mid-central region of Vietnam has maximum TC_R50 ratio in September-October corresponding to its highest TC frequency in the same period. During El Niño (La Niña) years, the TC rain ratio and TC_R50 ratio in the central region show a significant decrease (increase) in October-November. The La Niña phases more strongly affect TC rainfall than the El Niño phases, particularly in central Vietnam. IntroductionTropical cyclones (TCs) regularly threaten many countries, bringing dangers such as disastrously heavy rainfall and flooding. Several studies have examined the rainfall associated with TCs (hereafter called TC rainfall). For example, Rodgers et al. (2000) estimated TC rainfall in the North Pacific using Special Sensor Microwave Imager (SSM/I) observations for an 11-year period. They found that TCs contributed 12% of the climatological rainfall over the western North Pacific (WNP) from June to November. They also showed that TC rainfall increased from 12 to 18% during El Niño events. Englehart and Douglas (2001) investigated the role of tropical storms over the eastern North Pacific in the rainfall climatology of western Mexico and showed that tropical storm-associated rainfall normally constitutes 20 to 60% of rainfall along Mexico's Pacific coast. In more extreme cases, they found that such rainfall can contribute as much as 25 to 30% to seasonal rainfall totals in western interior locations. Gleason (2006) estimated the characteristics of TC rainfall in the United States between 1950 and 2004. They found that coastal and near-coastal regions received 8 to 16% and 4 to 12% of the precipitation by TCs, respectively. Ren et al. (2006) reported that the ratio of annual TC precipitation to total annual rainfall is 20 to 30% in most of Taiwan and along the coast of China south of 25°N, using rain gauge data over China during 1971−2004. Kubota and Wang (2009 investigated the effect of TCs on seasonal and interannual rainfall variability over the WNP by analyzing rainfall data at 22 rain gauge stations. They showed that along 125°E and between 18°N and 26°N, TC rain accounts for 50 to 60% of the total rainfall during the TC season from July to October. In addition, they described some cha...