Based on the analysis of observations from a 213-m tall meteorological tower at Tsukuba, Japan, we have investigated the favourable conditions for the predominant existence of large-scale turbulence ...structures in the near-neutral atmospheric boundary layer (ABL). From the wavelet variance spectrum for the streamwise velocity component (
u
) measured by a sonic anemometer-thermometer at the highest level (200 m), large-scale structures (time-scale range of 100–300 s) predominantly exist under slightly unstable and close to neutral conditions. The emergence of large-scale structures also can be related to the diurnal evolution of the ABL. The large-scale structures play an important role in the overall flow structure of the lower boundary layer. For example,
u
velocity components at the 200-m and 50-m levels show relatively high correlation with the existence of large-scale structures. Under slightly unstable (near-neutral) conditions, a low-speed region in advance of the high-speed structure shows a positive deviation of temperature and appears as the plume structure that is forced by buoyancy in the heated lower layer. In spite of the difference in buoyancy effects between the near-neutral and unstable cases, large-scale structures are frequently observed in both cases and the same vertical correlation of
u
components is indicated. However, the vertical wind shear is smaller in the unstable cases. On the other hand, in near-neutral cases, the transport efficiency of momentum at the higher level and the flux contribution of sweep motions are larger than those in the unstable cases.
Vertical diffusion in a stable boundary layer near the surface is not clearly understood. In the present study, the vertical diffusivity of radon and the height below which the concentration of radon ...is high are estimated from observations made at two levels (1.5 and 100 m) on the meteorological tower in the campus of the Meteorological Research Institute, Tsukuba, Japan, in November 2006. Seven 12-hour episodes, in which radon concentration near the surface increased, were averaged to make one sequential dataset. The averaged time sequence was divided into two periods: the radon concentration near the surface increased in the first one (Period I) and was stationary in the second (Period II). The estimated vertical diffusivity was less than 0.05 m2 s-1 for Period I and was at most 0.13 m2 s-1 for Period II. The estimated thickness of the radon-rich layer was less than 50 m for the first three hours in the first period; however, inversion height was approximately 100 m. The height under which radon accumulated was somewhat lower than that at the temperature inversion in period I, which suggests that the turbulent transfer was not dominant in the process to generate the temperature inversion layer in this area. Although the vertical diffusion of radon was different between the two periods, it was difficult to distinguish them with surface observation, employed for conventional Pasquill stability categories classification.
Large-scale turbulence structures in the near-neutral atmospheric boundary layer (ABL) are investigated on the basis of observations made from the 213-m tall meteorological tower at Tsukuba, Japan. ...Vertical profiles of wind speed and turbulent fluxes in the ABL were obtained with sonic anemometer-thermometers at six levels of the tower. From the archived data, 31 near-neutral cases are selected for the analysis of turbulence structures. For the typical case, event detection by the integral wavelet transform with a large time scale (180 s) from the streamwise velocity component (
u
) at the highest level (200 m) reveals a descending high-speed structure with a time scale of approximately 100 s (a spatial scale of 1 km at the 200-m height). By applying the wavelet transform to the
u
velocity component at each level, the intermittent appearance of large-scale high-speed structures extending also in the vertical is detected. These structures usually make a large contribution to the downward momentum transfer and induce the enhancement of turbulent kinetic energy. This behaviour is like that of “active” turbulent motions. From the analysis of the two-point space–time correlation of wavelet coefficients for the
u
velocity component, the vertical extent and the downward influence of large-scale structures are examined. Large fluctuations in the large-scale range (wavelet variance at the selected time scale) at the 200-m level tend to induce the large correlation between the higher and lower levels.
Recently, semiconductor light-emission devices, tunable-diode lasers (TDLs) and light-emission diodes (LEDs), have begun being used as light sources for infrared absorption hygrometers. In order to ...use these semiconductor light emission devices for a two-wavelength infrared absorption hygrometer to measure the humidity inside a cloud chamber containing ice particles and aerosols, two light beams emitted from two devices must be combined into one beam so that the hygrometer measures the same measuring volume with two wavelengths. Incandescent light has a very wide wavelength band including two wavelengths that can be used for infrared absorption hygrometers. We decided to develop an infrared absorption hygrometer to measure the humidity inside a cloud chamber containing ice particles and aerosols using an incandescent light source and subsequently fabricated a prototype hygrometer to measure humidity at normal temperature and ordinary pressure. This prototype hygrometer has an optical beam splitter and two optical filters to measure two wavelength bands of infrared, the water vapor absorption band at 1.9μm and a nonabsorption band at 1.6μm. The prototype needs no rotating filter motor that may cause mechanical vibration, and it has variable measuring path lengths up to several meters. The calibration data demonstrate that the hygrometer has a measurement accuracy within ±5% for relative humidity when the temperature varies within two degrees. In the future, we will improve the accuracy and stability for measuring humidity and examine how to install the hygrometer in the cloud chamber.
Simulation of carbon dioxide (CO
2
) at hourly/weekly intervals and fine vertical resolution at the continental or coastal sites is challenging because of coarse horizontal resolution of global ...transport models. Here the regional Weather Research and Forecasting (WRF) model coupled with atmospheric chemistry is adopted for simulating atmospheric CO
2
(hereinafter WRF-CO
2
) in nonreactive chemical tracer mode. Model results at horizontal resolution of 27 × 27 km and 31 vertical levels are compared with hourly CO
2
measurements from Tsukuba, Japan (36.05°N, 140.13
o
E) at tower heights of 25 and 200 m for the entire year 2002. Using the wind rose analysis, we find that the fossil fuel emission signal from the megacity Tokyo dominates the diurnal, synoptic and seasonal variations observed at Tsukuba. Contribution of terrestrial biosphere fluxes is of secondary importance for CO
2
concentration variability. The phase of synoptic scale variability in CO
2
at both heights are remarkably well simulated the observed data (correlation coefficient >0.70) for the entire year. The simulations of monthly mean diurnal cycles are in better agreement with the measurements at lower height compared to that at the upper height. The modelled vertical CO
2
gradients are generally greater than the observed vertical gradient. Sensitivity studies show that the simulation of observed vertical gradient can be improved by increasing the number of vertical levels from 31 in the model WRF to 37 (4 below 200 m) and using the Mellor–Yamada–Janjic planetary boundary scheme. These results have large implications for improving transport model simulation of CO
2
over the continental sites.
This note describes a platinum coating method of a tungsten wire for hot-wire anemometry. The resistance deviation of the coated tungsten wire stayed within ±2% during a half year. Using sensors with ...the coated tungsten wire, we obtained expected results of turbulent intensities and spectra in the turbulent boundary layer. This platinum coating method makes the tungsten wire sensor stable.
PDF
