Junsei Kondo

Air-sea bulk transfer coefficients in diabatic conditions

On the basis of recent data for the roughness Reynolds number of the sea surface, and using the Owen-Thomson theory on the transfers of heat and mass between a rough surface and the flow above it, the bulk transfer coefficients of the sea surface have been estimated. For a reference height of 10 m, the neutral-lapse transfer coefficient for water vapor is larger by only a few percent than that for sensible heat. When the wind speed at the 10-m height is u10>3 m s−1, the coefficient for sensible heat CH is larger by about 10% than that for momentum CD. For u10<5 m s−1, however, the value of CD exceeds the value of CH, and for u10=15 m s−1 it is shown that CH≈0.8CD. It may be also proposed that 103CD=1.11 to 1.70, 103CE=1.18 to 1.30, and 103CH=1.15 to 1.26 for a range of u10=4 to 20 m s−1. A plot of diabatic transfer coefficients versus wind speed is obtained by using a parameter of the sea-air temperature difference. For practical purposes, the coefficients are approximated by empirical formulae.

A parameterization of evaporation from bare soil surfaces

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 o...

Studies on the bulk transfer coefficients over a vegetated surface with a multilayer energy budget model

Abstract A multilayer energy budget model for vegetation canopy is developed to describe the fluxes of sensible and latent heat exchanged between the vegetated surface and the atmosphere. The model gives satisfactory results when the calculated radiative surface temperature TR and fluxes of sensible and latent heat are compared with experimental data. This model illustrates that values of the bulk transfer coefficients CH and CE are sensitive to meteorological conditions (incident radiation, wind speed, air humidity) when their values are evaluated using TR as the mean surface temperature. Therefore, the use of TR is inadequate to determine the bulk transfer coefficients. As an alternative, the effective surface temperatures for the energy exchange are defined and the values of CH and CE are determined with these temperatures. These values of CH and CE are only dependent on the vegetation parameters: vegetation type and density, the leaf transfer coefficients (or stomatal resistance), and the moisture ava...

A prediction model for snowmelt, snow surface temperature and freezing depth using a heat balance method

Abstract A snowmelt model based on a heat balance method has been developed. This model takes into account both the heat balance at the snow surface and that of the entire snow cover and simultaneously predicts the snow surface temperature and freezing depth. Observed or estimated incident radiation data are required for operation of the model. Calculated amounts of snowmelt and snow surface temperatures were in agreement with those observed. Dependency of snowmelt on several parameters including maximum liquid water content, thermal conductivity and albedo of the snow was examined. It was found that as liquid water content or thermal conductivity increases, snowmelt decreases. Albedo is very influential in evaluating snowmelt. Runoff from a basin having an area of 583 km2 was estimated using the present model, and was verified by the inflow data to a dam.

Aerodynamic roughness over an inhomogeneous ground surface

The aerodynamic roughness parameter z0 over inhomogeneous ground surfaces, such as cities, rural towns and so on, is determined by analyzing the wind data at AMeDAS observatories in the Tohoku and Kanto districts of Japan, by making use of Rossby number similarity theory. It is found that the aerodynamic roughness parameter is proportional to the average size of the roughness elements.A practical method of estimating the aerodynamic roughness parameter over an extensive area with various inhomogeneities is developed. In this method, the Digital National Land Information data bank is employed. As an example, the roughness parameter distribution around Tsukuba Academic City is presented.

Seasonal Variations in the Heat and Water Balances for Nonvegetated Surfaces

Abstract A model is presented for estimating the seasonal variations of evaporation, soil-water content, and soil temperature over nonvegetated land surfaces, especially in arid and semiarid regions. In the model, several types of soil are taken into consideration. Verification of the model has been achieved by comparing the observed and calculated results for the volcanic ash soil surface in Tsukuba and a sand dune field in Tottori. Using the routine data of 30 observatories including moist, semiarid, and arid regions at different elevations in China, the heat and water balances are estimated by the model. At a station in a semiarid region such as Lanzhou, the sensible heat flux is found to be considerably greater than the latent heat flux during the dry season. Both fluxes, however, have comparable magnitudes during the rainy season. The annual mean value of the soil-water content increases with depth, and the rate of increase grows larger as the amount of annual precipitation increases. But for an arid...

Bulk transfer coefficient over a snow surface

The drag coefficient CDand the bulk transfer coefficient for sensible heat CHover a flat snow surface were determined experimentally. Theoretical considerations reveal that CDdepends on the friction velocity u* as well as on the geometrical roughness h of the snow surface. It is found that CDincreases with increasing u* and/or h. The dependency of CHon u* and h is so small that it is possible to consider CHas a constant for practical purposes: CH, 1= 2.0 × 10−3 for a reference height of 1 m. The bulk transfer coefficient for water vapor is estimated at CE, 1= 2.1 × 10−3 for a reference height of 1 m.

Numerical studies on the two-dimensional flow in horizontally homogeneous canopy layers

The two-dimensional equation of motion containing the pressure gradient and Coriolis force is numerically solved for the wind field in and above the layers of a horizontally homogeneous canopy with a vertical distribution of leaf-area densities. The solution shows that, in the case of descending through the canopy, the wind vector turns with an angle which depends on the profile of leaf-area densities. In particular, for the canopy of a forest consisting of upper layers with higher densities and lower layers with smaller densities, the turning is striking; a secondary maximum in wind profile appears in the lower layers.Variations of the aerodynamic parameters for the flow above the canopy are indicated with respect to the leaf-area density. The roughness length varies in such a manner that a maximum appears in intermediate density values, depending on the shape of the profile of leaf-area density. In the case of very dense canopies, the shearing stress acting on the flow above the canopy is determined by the contribution from only the upper canopy elements, but not by that from the lower parts of the canopy.

Heat Budget Analysis of Nocturnal Cooling and Daytime Heating in a Basin

Abstract Nocturnal cooling and daytime heating in a basin were studied on clear and calm days by means of heat budget observations. In the nighttime, drainage flow occurs along the basin sideslope and advects cold air to the boundary layer over the basin bottom (BBL), intensifying the cooling rate of the layer. A nocturnal cold air lake develops in the basin, attaining a depth nearly equal to the topographical depth of the basin. Heat budget analysis of the whole basin surface shows that net radiative flux closely balances with sensible heat flux and ground heat conduction. In the daytime, the BBL is warmed not only by sensible heat flux from the surface of the basin bottom, but also by local subsidence heating. This local subsidence above the basin bottom depresses development of the convective boundary layer until the nocturnal cold air lake vanishes completely. The subsidence velocity increases with time after sunrise. Over the whole basin surface, net radiative flux closely balances with sensible and ...

A model and experimental study of evaporation from bare-soil surfaces

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-rest...