The formation of channel networks and the landscape evolution

Channel networks on Souya Hill	Channel bifurcation on a river in Picton, Australia

Erosion by surface flow changes the shape of the earth's surface while the shape of the earth's surface works as a boundary condition determining the movement of the flow. The interaction between the flow and the earth's surface sometimes causes instability of a uniformly-eroded flat plateau surface. As a result, regularly-spaced gullies (initial channels) are formed on the initially flat unchannelized plateau surfaces, and evolve into channel networks with complex geometry. The formation of gullies increases surface erosion and sediment yield, and results in serious soil loss and sediment disasters. It is important to study the physics involved in the formation of channels and channel networks from an engineering point of view.

The formation of fluvial bedwaves

Ripples in the Abukuma river	Dunes in a laboratory flume

A variety of bedwaves such as fluvial bars, dunes and ripples are formed in rivers. The formation of fluvial bedwaves is an example of the interfacial instability between river flow and beds composed of sediment. Dunes and ripples increase bed resistance, so that it is of great importance to know how large bedwaves are formed under what conditions for the prediction of bed resistance. In addition, fluvial bars accompanies riffles and pools that enrich the biological environment in rivers. We are working on research as to how bedwaves respond to the variation of flow and other conditions.

Turbidity currents and the formation of submarine canyons

Submarine canyons in the Kii Strait

Around continental margines between continental shelves and slopes under the sea, there are canyons similar to ones observed on land. They are formed by density flow called a turbidity current. Turbidity currents contain high concentration of suspended sediment; and therfore they are heavier than the surrounding water. Turbidity currents are self-accelerated by the increase of density due to the entrainment of bed sediment. We are studying the dynamical features of turbidity currents and the formation processes of submarine canyons.

Subseasonal forecast skill associated with land initializations in global climate models

Land-atmosphere interactions in monsoon regions

West Sahelian monsoon

The simulation of the onset of the West African monsoon (WAM), associated with the northward shift of the Intertropical Convergence Zone (ITCZ) occurring in May, June, and July, is a challenging task for atmospheric general circulation models (AGCMs), because of complex water and energy balance through the land-atmosphere interaction as well as atmospheric processes. We provide evidence that a combination of state-of-the-art global fields of reanalyzed soil moisture anomalies and a sufficiently high-resolution (∼50 km) AGCM produces a successful simulation of the northward shift of the ITCZ and

Dynamical downscaling with regional climate model

The diurnal characteristics of summer rainfall in the contiguous United States and northern Mexico were examined with the United States reanalysis for 5 years in 10-km horizontal resolution (US10), which is dynamically downscaled from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) Global Reanalysis 1 using the Regional Spectral Model (RSM). The hourly precipitation outputs demonstrate a realistic structure in the temporal evolution of the observed rainfall episodes and their magnitudes across the United States without any prescriptions of the observed rainfall to the global reanalysis and the downscaled regional reanalysis. Nighttime rainfall over the Great Plains associated with eastward-propagating, mesoscale convective systems originating from the Rocky Mountains is also represented realistically in US10, while the original reanalysis and most general circulation models (GCMs) have difficulties in capturing the series of nocturnal precipitation events in summer over the Plains. The results suggest an important role of the horizontal resolution of the model in resolving small-scale, propagating convective systems to improve the diurnal cycle of summer rainfall.

Statisticalindex to quantify similarity among ensemble members

This study reveals the mathematical structure of a statistical index,
, that quantifies similarity among ensemble members in a weather forecast. Previous approaches for quantifying predictability estimate separately the phase and shape characteristics of a forecast ensemble. The diagnostic, on the other hand, characterizes the similarity (across ensemble members) of both aspects together with a simple expression. The diagnostic
is thus more mathematically versatile than previous indices.

Cloud classification by using infrared satellite sensors

A new two-dimensional threshold diagram (2D-THR) has been developed based on maximum likelihood cloud classification results, which can readily be applied for Multi-functional Transport Satellite (MTSAT) split window datasets. Because 2D-THR was trained using northern summer 2010 data for Japan and its surrounding area, it is typically suitable only for summer. Comparison of snapshot cloud type distributions showed that 2D-THR images and the corresponding night-time microphysical colour composite images as well as 2D-THR images and Japan Meteorological Agency (JMA) cloud type images are in good agreement. A time series inter-comparison of the hourly 2D-THR cloud classification results with the JMA cloud type classification data product was performed by calculating spatial correlation of cloud percentage for 1◦ x 1◦ grid cells. For cumulonimbus, high-level, middle-level and low-level clouds over tropical and subtropical areas in the northwestern Pacific Ocean region, the spatial correlation between 2D-THR and JMA is moderate. Thus, 2D-THR cloud classification algorithm can be applied in both regions.

Extreme weather events

Line-shaped rainbands

Line-shaped rainbands (LRBs) are severe storm events that cause torrential rainfall across the mid-latitudes and are difficult to accurately predict because of their complex physical processes. We examined the occurrence of LRBs in boreal summer in northern Japan over the period 1990–2010. In years with anomalously many LRBs, the development of LRBs over northern Japan was driven by a combined positive anomaly of warm sea-surface temperatures and westerly moisture flux associated with the prominent pattern of the surface pressure anomalies between the main island of Japan and the Sea of Okhotsk. In contrast, the number of LRBs were significantly small with opposite characteristics.

Fig-6: Frequency of LRBs in Hokkaido region	Fig-7: Place of occurrence of LRBs

Atmospheric Blocking

Atmospheric blocking is one of the main causes of extreme meteorological events such as spells of extremely cold weather, heat wave and continuous heavy rainfall in the mid-latitudes. The European Centre for Medium-Range Weather Forecasts Reanalysis Dataset (ERA-40) was analyzed to clarify both the characteristics of the latitudinal surface temperature field and the frequency of atmospheric blocking (FAB) in December, January, and February between 1960 and 1999. In years with a small meridional surface temperature gradient between the low and high latitudes, the FAB was larger than the average for the 40-year climatology, as well as persisting for longer and having a longitudinally larger spatial scale. A noticeable characteristic of these years was a larger dominant eddy scale length at the 500-hPa level compared to the average for the climatology. The amplitude of the dominant eddy scale length was also enhanced.

Land Surface Modeling

Combination of lumpled and land surface models

The main objective of this study is to propose a new rainfall-runoff method with a theory of soil water capacity associated with hydrological information with a land surface model. Runoff in mountainous basins is uniquely determined by the effective rainfall amount which is based on a water-holding capacity of basins. The effective rainfall and direct runoff is strongly influenced by initial soil moisture content of the basins. Here in this paper, we showed a new estimationmethod of rainfall-runoff process by using the initial soil moisture condition based on a land surface model and the Automated Meteorological Data Acquisition System (AMeDAS). This paper confirmed that the system which we proposed would contribute to better runoff predictions in mountainous basins.

Land surface model with human impact modules over Hokkaido, Japan

Water is very important for human and it have helped the development of human life until now. However, the amount of water we can use is limited and obtaining and sustaining the water resource is very important for keeping the level of life and living comfortably. Recently, people found that some human activities such as irrigation and charging river discharge to dam impact on water sustainability. So, considering and estimating the influence of human impact is very important and a model ‘MATSIRO considering human impact’ was developed for realizing them (Pokhrel et al. 2012 ). Purpose of this thesis is quantifying the human impact on land surface and estimating the human impact on land surface by applying this model for global into Japanese scale. Because it is agriculture that need water mostly, analysis area was decided as Hokkaido ‘have the largest agricultural area’ in Japan and this model succeeded to estimate the increase of evapotranspiration and decrease of surface temperature in agricultural area. Model also estimated the amount of montyly dam water releace at Taisetsu dam.