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Takeshi Yamazaki
,
Junsei Kondo
,
Tsutomu Watanabe
, and
Takeshi Sato

Abstract

A heat-balance model having a canopy of one or two layers has been developed. The calculated fluxes using the present model were found to be in agreement with measurements from a rice paddy field, an orchard, and the calculated fluxes of a multilayer model. Although there was some difference in the calculated fluxes between the one- and two-layer model was the one-layer model was found to be sufficient when the dependency of the radiative ground-surface temperature on the viewing angle was not considered.

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Junsei Kondo
,
Nobuko Saigusa
, and
Takeshi Sato

Abstract

A simple model of evaporation from a bare soil surface is developed. This model combines two processes of water vapor transport: one is the vapor transport in air expressed by the bulk formula, and the other is molecular diffusion of vapor in the surface soil pore with the vapor being carried from the interior of the soil pore to the land surface. The resistance to the vapor diffusion in the soil pore is expressed using a new parameter, estimated by experimental means.

General formulation of the so-called “surface moisture availability” is expressed with this model. The formulation shows that the “surface moisture availability” depends not only on volumetric soil moisture, but also on wind velocity, and on the ratio of the specific humidity of the air to that of the saturation value at the soil surface temperature. This dependence agrees with experiments performed with loam and sand under various conditions.

In the evaporation parameterization used in current numerical simulations, the humidity of the air adjacent to the water in the soil pore, which is determined thermodynamically, is often substituted for the land surface humidity. The present study suggests, however, that such a parameterization is invalid except for saturated conditions.

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Junsei Kondo
,
Nobuko Saigusa
, and
Takeshi Sato

Abstract

A model is constructed for estimating evaporation from bare-soil surfaces. In the model, the evaporation is parameterized with the soil-water content for the upper 2 cm of the soil (Kondo et al.), and the heat and water transport within the soil layer below 2 cm is explicitly described by the heat conduction and moisture diffusion equations.

Experiments on evaporation from loam packed in pans are also carried out. The present model well simulates the observed evaporation and vertical profiles of soil temperature and water content.

Long time simulations of evaporation by the present model an compared with the force-restore method and the bucket model for a drying period of over several months. The decrease in evaporation rate for the bucket model is comparatively small. However, the evaporation by the present model and the force-restore method decreases rapidly several days after the beginning of the drying period. Differences between the evaporation by the present model and that by the force-restore method appear after approximately 10 days from the onset of the drying period.

The sensitivity of the present model on the wind velocity is tested. The evaporation is sensitive to the wind velocity when the soil is wet, but less sensitive when the soil is dried because the soil resistance to the water transport becomes large as compared with the aerodynamical resistance.

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Shinji Matsumura
,
Takeshi Horinouchi
,
Shiori Sugimoto
, and
Tomonori Sato

Abstract

The Pacific–Japan (PJ) teleconnection pattern leads to a meridional precipitation dipole between the subtropics and East Asia in the summer of El Niño decaying years. However, observational analysis and model experiments indicate that increased sea surface temperature (SST) in the Kuroshio–Oyashio Extension (KOE) region, where a strong northward-decreasing SST gradient occurs, induces a northward shift of baiu rainfall with the subtropical jet, forming a tripolar precipitation anomaly pattern over the northwest Pacific. In July, the leading empirical orthogonal function (EOF) mode for precipitation represents the meridional dipole between subtropical and baiu rainfalls, while EOF-2 features the northward-shifted baiu rainband. The PJ atmospheric forcing increases KOE SST, which enhances northward shifts in the subtropical jet and baiu rainband, resulting in oceanic forcing over the KOE. In experiments using a regional climate model the subtropical jet response to KOE SST forcing explains about 20% of the anomalies in the control experiment. The northward-shifted baiu condensational heating also enhances the northward shift of the subtropical jet with a northeastward extension. In August when the baiu rainband disappears, the northward shift of the subtropical jet is enhanced mainly by KOE SST variability. During the summer, as a result of increased KOE SST and the northward-shifted baiu heating, cyclonic anomalies intensify over the Sea of Okhotsk and anticyclonic anomalies intensify over the northwest Pacific, affecting the western Pacific subtropical high. The results suggest that air–sea interaction over the KOE must be considered to better understand the summer northwest Pacific climate.

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