Variability Of Sensible Heat Fluxes Research Articles

都市気候に及ぼす河川水の熱的影響に関する実測研究 : 隅田川における熱収支と周辺影響の検討

Water bodies in urban area have been expected to mitigate the urban heat island as well as green space. In this paper, we showed the results of observations about heat budget at river water and moving observations to clarify horizontal extent of its thermal effects. Diurnal variations of sensible heat flux estimated by several methods corresponded closely with each other, though these values didn't exceed 100W/m2 even in the peak about midday. Estimated heat storage term occupied more than 90% of net radiation during daytime. According to the numerical simulation, observed water temperature variation required equivalent heat conductivity for actual stirred water about forty times as much as that of stagnant water. This heat balance is completely different from that of green space characterized large latent heat flux. On the other hand, horizontal sensible heat flux from adjacent built-up area to the river space also estimated by eddy correlation method at riverside terrace. Turbulent heat flux across the river-edge amounted to 250W/m2 in daytime, which greatly surpassed vertical sensible heat flux at water surface. The horizontal limits of river-effect along the street crossing the river were about 300m from river-edge, and there was no clear relation between this limits and wind speed.

FACTORS INFLUENCING THE VARIABILITY OF MIDWINTER SNOW-DEPTH DECREASE IN THE NORTHERN GREAT PLAINS OF THE UNITED STATES

Meltwater from snow provides an important source of fresh water for agriculture, domestic use, and hydroelectric power in many regions of the world. In the winter and early spring, however, rapid ablation of the snowpack can lead to environmental hazards such as flooding. Most snowmelt research has relied on measured data, which have limited the spatial and temporal resolution of the studies. Generally, these studies have been conducted at single locations over periods of a few days to a few weeks. This project investigates snowdepth changes on a more expansive scale by using a one-dimensional mass- and energy-balance model (SNTHERM) to compute the convective and radiative fluxes associated with large snow-depth change events. Midwinter snow-depth changes and the variability in snow-depth changes are evaluated for a 25-year time period over the northern Great Plains region of the United States during February. Statistical analyses indicate that variations in sensible heat flux have the greatest influence on midwinter variability of snow-depth changes. This is shown by the consistently strong relationship between modeled sensible heat-flux values and changes in snow depth over the entire study period and at nearly every station. Mean daily temperature followed by mean daily dew-point temperature show the next strongest statistical relationships with snow-cover ablation. Radiative fluxes and net solar radiation in particular, have low correlations with changes in snow depth and are therefore found to have little impact on snow-depth change variability during the month of February. Although net solar radiation is not highly correlated with the variability of decreases in depth, its energy input is nevertheless an important source of energy for ablation processes. [Key words: snow melt, modeling, Great Plains.]

小都市における市内外の拡散係数 : 都市における大気の拡散係数に関する測定(その1)

Daily variations of sensible heat fluxes and vertical eddy diffusivities were obtained in early summer in Utsunomiya, 100km north of Tokyo. Observing sites were at 15m and 25m in hight above the surface in urban area and at 13m and 25m in hight above the surface in residential area. Vertical eddy diffusivities, Dz were measured with two different methods, which are the measure- ment with autocorrelation function and the measurement derived with relationship between heat flux and temperature difference between zl and z2 in hight. The values of Dz at higher places and lower places in residential area are equal : yet the values of Dz in urban area are not equal due to the effects of surrounding environment. Daily variations of Dz in urban and rural area are 0.1-1.0 (m^2/s).

Daytime variation of sensible heat flux estimated by the bulk aerodynamic method over a grass canopy

A study was performed to evaluate the bulk aerodynamic method for estimating daytime variation of sensible heat flux ( H A) for a wetted and non-wetted grass canopy under clear and cloudy days. The aerodynamic resistance applied was corrected for atmospheric stability using the Oregon State University One-Dimensional Planetary Boundary Layer model stability function. The performance of the bulk aerodynamic method was tested with indirect measurements of sensible heat flux ( H B) derived from the Bowen ratio energy balance method on 20-min time intervals. Results indicate that there was a good agreement between H B and H A with a coefficient of correlation of 0.93 and slope of the regression line through the origin of 0.96.

Atmospheric turbulence within and above a douglas-fir stand. Part II: Eddy fluxes of sensible heat and water vapour

This is the second paper describing a study of the turbulence regimes and exchange processes within and above an extensive Douglas-fir stand. The experiment was conducted on Vancouver Island during a two-week rainless period in July and August 1990. Two eddy correlation units were operated in the daytime to measure the fluxes of sensible heat and water vapour and other turbulence statistics at various heights within and above the stand. Net radiation was measured above the overstory using a stationary net radiometer and beneath the overstory using a tram system. Supplementary measurements included soil heat flux, humidity above and beneath the overstory, profiles of wind speed and air temperature, and the spatial variation of sensible heat flux near the forest floor. The sum of sensible and latent heat fluxes above the stand accounted for, on average, 83% of the available energy flux. On some days, energy budget closure was far better than on others. The average value of the Bowen ratio was 2.1 above the stand and 1.4 beneath the overstory. The mid-morning value of the canopy resistance was 150–450 s/m during the experiment and mid-day value of the Omega factor was about 0.20. The daytime mean canopy resistance showed a strong dependence on the mean saturation deficit during the two-week experimental period. The sum of sensible and latent heat fluxes beneath the overstory accounted for 74% of the available energy flux beneath the overstory. One of the reasons for this energy imbalance was that the small number of soil heat flux plates and the short pathway of the radiometer tram system was unable to account for the large horizontal heterogeneity in the available energy flux beneath the overstory. On the other hand, good agreement was obtained among the measurements of sensible heat flux made near the forest floor at four positions 15 m apart. There was a constant flux layer in the trunk space, a large flux divergence in the canopy layer, and a constant flux layer above the stand. Counter-gradient flux of sensible heat constantly occurred at the base of the canopy. The transfer of sensible heat and water vapour was dominated by intermittent cool downdraft and warm updraft events and dry downdraft and moist updraft events, respectively, at all levels. For sensible heat flux, the ratio of the contribution of cool downdrafts to that of warm updrafts was greater than one in the canopy layer and less than one above the stand and near the forest floor.

Experimental study of heat and water vapor transfer processes in the planetary boundary layer (PBL)

The turbulent structure of the planetary boundary layer (PBL) is investigated based on measurements from a 213 m meteorological tower. It is found that the sensible heat flux decreases with height, resulting in a temperature increase during the daytime. On the other hand, the water vapor flux is almost constant with height and the diurnal variation of specific humidity is not so clear. Time variations of sensible heat flux and water vapor flux are closely correlated to the variation of net radiation and some differences are seen in their response times. Transient processes in the mixed layer including those related to the moisture structure are also investigated during periods of development and disappearance of the layer.