Michiaki Sugita

Application of self-preservation in the diurnal evolution of the surface energy budget to determine daily evaporation

Evaporation from natural land surfaces often exhibits a strong variation during the course of a day, mostly in response to the daily variation of radiative energy input at the surface. This makes it difficult to derive the total daily evaporation, when only one or a few instantaneous estimates of evaporation are available. It is often possible to resolve this difficulty by assuming self-preservation in the diurnal evolution of the surface energy budget. Thus if the relative partition of total incoming energy flux among the different components remains the same, the ratio of latent heat flux (LE) and any other flux component can be taken as constant through the day. This concept of constant flux ratios is tested by means of data obtained during the First ISLSCP Field Experiment (FIFE); the instantaneous evaporation values were calculated by means of the atmospheric boundary layer (ABL) bulk similarity approach with radiosonde profiles and radiative surface temperatures. Good results were obtained for evaporative flux ratios with available energy flux (LE/(Rn - G)), with net radiation (LE/Rn), and with incoming shortwave radiation (LE/S ↓).

Daily evaporation over a region from lower boundary layer profiles measured with radiosondes

Regional daily evaporation was estimated by means of continuous measurements of the available energy flux at the surface and one or more instantaneous determinations of the evaporative fraction (EF). The data were obtained during the First ISLSCP Field Experiment (FIFE) in northeastern Kansas. EF, which is the ratio of the latent heat flux and the available energy flux, was assumed to be constant during the daylight hours; thus it was determined from only a few instantaneous surface flux values calculated on the basis of radiosonde profiles in the surface layer, together with remotely sensed surface temperature. Comparison of 23 estimated and measured daytime evaporation values showed good correlation (r = 0.97), although the evaporation was underestimated by about 5% on average. In the estimation of daily evaporation, nighttime evaporation must also be considered; measurements by means of the eddy correlation method showed that on average it accounted for some 8% of the total daily evaporation.

Sensible Heat Transfer Parameterization for Surfaces with Anisothermal Dense Vegetation

Abstract The scalar roughness for sensible beat can be directly formulated in terms of the surface temperature. Therefore, in the case of an anisothermal vegetation canopy, the concept of a scalar roughness is ill defined and it may vary greatly depending on the method and scale of measurement of the surface temperature. Through an analysis of the turbulent transport phenomena inside the vegetation canopy, it is shown how this uncertainty and variability of the scalar roughness of a vertically anisothermal surface can be reduced by defining a properly weighted average surface temperature. Implications are discussed for the optimal measurement of surface temperature by remote sensing with radiation thermometers.

An overview of the rangelands atmosphere–hydrosphere–biosphere interaction study experiment in northeastern Asia (RAISE)

Summary Intensive observations, analysis and modeling within the framework of the rangelands atmosphere–hydrosphere–biosphere interaction study experiment in northeastern Asia (RAISE) project, have allowed investigations into the hydrologic cycle in the ecotone of forest-steppe, and its relation to atmosphere and ecosystem in the eastern part of Mongolia. In this region, changes in the climate have been reported and a market oriented economy was introduced recently, but their impact on the natural environment is still not well understood. In this RAISE special issue, the outcome is presented of the studies carried out by six groups within RAISE, namely: (1) Land-atmosphere interaction analysis, (2) ecosystem analysis and modeling, (3) hydrologic cycle analysis, (4) climatic modeling, (5) hydrologic modeling, and (6) integration. The results are organized in five relevant categories comprising (i) hydrologic cycle including precipitation, groundwater, and surface water, (ii) hydrologic cycle and ecosystem, (iii) surface–atmosphere interaction, (iv) effect of grazing activities on soils, plant ecosystem and surface fluxes, and (v) future prediction. Comparison with studies on rangelands in other parts of the world, and some future directions of studies still needed in this region are also summarized.

Regional surface fluxes from satellite-derived surface temperatures (AVHRR) and radiosonde profiles

Radiometric surface temperatures, derived from measurements by the AVHRR instrument aboard the NOAA-9 and the NOAA-11 polar orbiting satellites, were used in combination with wind velocity and temperature profiles measured by radiosondes, to calculate surface fluxes of sensible heat. The measurements were made during FIFE, the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment, in a hilly tall grass prairie area of northeastern Kansas. The method of calculation was based on turbulent similarity formulations for the atmospheric boundary layer. Good agreement (r = 0.7) was obtained with reference values of sensible heat flux, taken as arithmetic means of measurements with the Bowen ratio method at six ground stations. The values of evaporation (latent heat fluxes), derived from these sensible heat fluxes by means of the energy budget, were also in good agreement (r = 0.94) with the corresponding reference values from the ground stations.

Radiometrically determined skin temperature and scalar roughness to estimate surface heat flux. Part I : Parameterization of radiometric scalar roughness

A series of experiments carried out in a pasture field during a growing season, allowed a radiometric determination of the scalar roughness for sensible heatzoh,r. The values ofzoh,r are shown to vary over the range of 10−1–10−7m both diurnally and seasonally, and an existing theoretical model for the estimation of scalar roughness for sensible heat is found to be inappropriate for the precise estimation ofzoh,r. To parameterizezoh,r better, a multiple regression analysis was performed, with predictor candidates such as solar elevation, solar radiationRs, leaf area index LAI, canopy height, the ratio of the solar radiation and the extraterrestrial radiationRs/Re, the ratio of the direct and the total solar radiationRd/Rs, and the roughness Reynolds number among others. The best regression equation which usesRs, LAI,Rs/Re, andRd/Rs is derived withr=0.75; with smaller numbers of predictors, values ofr tend to deteriorate gradually down tor=0.52 when only one predictor, LAI, was incorporated into the equation.

The extent of the unstable Monin-Obukhov layer for temperature and humidity above complex hilly grassland

Potential temperature, specific humidity and wind profiles measured by radiosondes under unstable but windy conditions during FIFE in northeastern Kansas were analyzed within the framework of Monin-Obukhov similarity. Around 86% of these profiles were found to have a height range over which the similarity, formulated in terms of the Businger-Dyer functions, is valid and for which the resulting surface fluxes are in good agreement with independent measurements at ground stations. When scaled with the surface roughness z0= 1.05 m and the displacement height d0= 26.9 m, for the potential temperature this height range was 45 (±31) ⩽ (z − d0)/z0⩽ 104 (±54) and the comparison of the profile-derived surface fluxes with the independent measurements gave a correlation coefficient of r = 0.96. For the specific humidity these values are 42 (±29) ⩽ (z − d0)/z0⩽ 96 (±38) and r = 0.94. In terms of the height of the bottom of the inversion Hi, in the morning hours the upper limit of (z − d0) in the Monin-Obukhov layer is approximately 0.3Hi, whereas for a fully developed ABL it is closer to 0.1Hi. Probably, as a result of the short sampling times and perhaps also of the small gradients under the windy conditions, the exact height range of validity was difficult to establish from a mere inspection of these profiles.

Optimal measurement strategy for surface temperature to determine sensible heat flux from anisothermal vegetation

For a surface covered by vegetation, the scalar roughness for sensible heat zoh (or equivalently, any bulk heat transfer coefficient or surface resistance) depends directly on the way in which the effective or bulk surface temperature θs is measured. One undesirable feature of any such parameter like zoh is that when θs is measured radiometrically under conditions of solar heating of the surface, it tends to depend strongly on solar elevation. An analysis of experimental surface heat flux and related data herein confirms that this dependency can be weakened by making the radiometric surface temperature measurements obliquely, that is, by properly weighting the temperature of the upper layers of the canopy; these are the more direct heat exchangers with the turbulent air than the lower layers. It is also shown, however, that such oblique radiometric measurements produce lower values of θs, which in turn, require larger values of zoh, in the bulk heat transfer equation. Unfortunately, lower θs and larger zoh result in larger relative error in heat flux calculations. Therefore, under certain conditions, it may be preferable to design a compromise strategy and to accept some dependency of zoh on solar elevation, in order to maintain a sufficiently low error level in the calculated heat flux. In any event, this error level can be reduced considerably by making use of air temperature measurements at higher levels above the ground.

A bulk similarity approach in the atmospheric boundary layer using radiometric skin temperature to determine regional surface fluxes

Profiles of wind velocity and temperature in the outer region of the atmospheric boundary layer (ABL) were used together with surface temperature measurements, to determine regional shear stress and sensible heat flux by means of transfer parameterizations on the basis of bulk similarity. The profiles were measured by means of radiosondes and the surface temperatures by infrared radiation thermometry over hilly prairie terrain in northeastern Kansas during the First ISLSCP Field Experiment (FIFE). In the analysis, the needed similarity functions were determined and tested; the main scaling variables used for the ABL were hi, the height of the convectively mixed layer, and Va and θa, the wind speed and potential temperature averaged over the mixed layer. Good agreement (r = 0.80) was obtained between values of friction velocity u* determined by this ABL bulk similarity approach and those obtained by Monin-Obukhov similarity in the surface sublayer. Similarly, values of surface flux of sensible heat H determined by this method compared well (r = 0.90) with the regional means measured at six ground stations. The corresponding regional evaporation values, determined with the energy budget equation, also compared favorably (r = 0.94).

Response of gross ecosystem productivity, light use efficiency, and water use efficiency of Mongolian steppe to seasonal variations in soil moisture

8 [1] The examination of vegetation productivity and use of light and water resources is 9 important for understanding the carbon and water cycles in semiarid and arid 10 environments. We made continuous measurements of carbon dioxide and water vapor 11 fluxes over an arid steppe ecosystem in Mongolia by using the eddy covariance (EC) 12 technique. These measurements allow an examination of EC-estimated gross ecosystem 13 productivity (GEP), light use efficiency (LUE), and water use efficiency (WUE) of the 14 steppe. Daily variations of GEP, LUE, and WUE were associated with daily variations of 15 incident photosynthetically active radiation (PAR), ambient temperature (Ta), and vapor 16 pressure deficit (VPD). The magnitudes of these variations were also dependent on canopy 17 development. On the daily basis, GEP linearly correlated with evapotranspiration rate and 18 PAR. LUE correlated positively with leaf area index, Ta, and soil moisture availability but 19 negatively with the surface reflectivity for short-wave solar radiation. Throughout the 20 growing season, both GEP and LUE responded strongly to precipitation-fed soil moisture 21 in the top 20 cm of the soil. An examination of the responses of LUE and WUE to PAR 22 under different soil moisture conditions shows that when soil water availability exceeded 23 VPD, the steppe was most efficient in light use, whereas it was less efficient in water use. 24 The multivariate analysis of variance also suggests that soil moisture availability, 25 especially water status in the upper 20-cm soil layer with dense distribution of grass roots, 26 is the most significant factor that governs GEP, WUE, and LUE. This study provides a 27 preliminary assessment of the use of available water and light by the Mongolian arid 28 steppe ecosystems under seasonally varying soil moisture conditions. A better 29 understanding of these functional responses is required to predict how climate change may 30 affect arid steppe ecosystems.