Temperature, humidity, wind and radiation measurements

At the highest monitoring level, 115 m above the ground, wind speed (Vaisala WAA15A), wind direction (Vaisala WAV15A) and air temperature/humidity sensors (Vaisala HMP35D) are mounted along with the air sampling tube at the end of a 4.4 m long instrument arm. The arm projects toward north. The analog signals of the meteorological sensors are digitized by means of a 12 bit A/D converter and are transmitted to the data acquisition computer via an RS232 serial link. Proper shielding of the cables and sensors, as well as digitizing of the signals, are essential to avoid picking up of noise in the long cables, which may be caused by the nearby high power antennas. Grounding is also important to minimize the possibility of damage from lightning.

The meteorological instrumentation at 82 m above the ground is similar, but a wind direction sensor is not installed there. In April 1997 a sonic anemometer was installed at this level for eddy flux measurements (see Section ).

At the 48 m level, the flow distortion caused by the large diameter of the tower (7.75 m) makes it impractical to build a mounting arm long enough to avoid its influence on the meteorological instruments. Instead, two 2.5 m long instrument arms were mounted on opposite (north and south) sides of the tower (Fig. ).

Figure: Theoretical laminal wind flow pattern around the cylindrical body of the tower in the cartesian coordinate system. The two mounting arms are also indicated. The diameter of the tower is 7.75 m, while the length of the mounting arms is 2.5 m. The length of the arrays are proportional to the wind speed. Wind direction is 30.

Anemometers are installed on both arms and a temperature/humidity sensor and air sample inlet tube are mounted only on the north arm. The flow distortion caused by the tower influences both anemometers and can cause wind speed distortion in excess of ±35% of the real wind speed (Fig. ).

Figure: Wind direction dependency of the wind vector deflection caused by the tower at 48 m. The undisturbed wind speed was 4 m s. The configuration of the mounting arms are shown in fig. .

The measured wind speed is corrected based on the theoretical laminar flow pattern around a cylindrical body using wind direction information from the 115 m level. The two components of the horizontal wind speed around a cylindrical body can be written in the cartesian coordinate system as:

$$u=\left\vert v_{0}\right\vert \left\{ \cos \varphi _{0}\left... ...hi _{0}\sin 2\left( \varphi _{0}-\varphi \right) \right\} \: ,$$ (2.1)

$$v=\left\vert v_{0}\right\vert \left\{ \sin \varphi _{0}\left... ...hi _{0}\sin 2\left( \varphi _{0}-\varphi \right) \right\} \: ,$$ (2.2)

where is the undisturbed wind speed, is the angle of the wind vector ( -wind direction), is the radius of the cylinder, and are polar coordinates of the point of interest (Nagy, 1989). The cartesian coordinate system's axis points towards east and axis points towards north. The meaning of the wind direction corresponds to the meteorological standard.

Tower-induced von Kármán vortices can also distort the average wind vector in the downwind direction, thus only the wind speed value of the anemometer located on the upwind side of the tower is used for the reconstruction of the wind speed value. The reconstruction routine uses the measured and to calculate based on eq. and eq. as follows:

$$v_{0}=\frac{v_{48}}{\sqrt{1-2\frac{R^{2}}{r^{2}}\cos 2\left( \varphi _{0}-\varphi _{b}\right) +\frac{R^{4}}{r^{4}}}}\: ,$$ (2.3)

where is the (distorted) wind speed value of the anemometer located upwind and is the angle of the boom (90 or 270 for the northern and southern side anemometer, respectively).

Figure: Monthly average wind profiles derived from wind speed data measured during daytime for 1997. The dashed lines represent the profiles from the uncorrected data, while the solid line shows the profiles after corrections.

The same procedure is applied at 82 m and at 115 m without the possibility to distinguish between more or less disturbed signal of two anemometers, but the shadow effect of the tower is not so severe here, since the radius of the tower is considerably smaller than at 48 m (see Fig. ).

Figure illustrates the corrected and uncorrected wind speed data for six months. Comparing to a theoretical log-linear wind profile (Webb, 1970; Stull, 1988; Weidinger et al., 2000) the corrected data give considerably lower root mean square error than the uncorrected profile and the corrected data exhibits a more realistic wind profile.

In sunny weather, solar heating of the TV tower body may cause overestimates in the air temperature at the 48 m level, when the wind blows from the SW-SE sector. The temperature is corrected based on the assumption that during daytime unstable atmospheric conditions the mean temperature at 48 m should fit the lapse rate determined by the 10 m and 82 m temperature values. Based on Monin-Obukhov similarity, an approximate relationship between the difference in temperature at 10 m and 82 m and the deviation of the 48 m temperature from a linear interpolation of temperature with height is:

$$t_{48}\cong t_{10}-(t_{10}-t_{82})\frac{48-10}{82-10}-0.3(t_{10}-t_{82})\: .$$ (2.4)

The applied correction procedure preserves the variability of the observed temperature time series at 48 m while correcting the smoothed mean value during the critical time periods. The variability (the difference between the smoothed mean value and the measured values) then added to the adjusted mean value to estimate the real temperature value at 48 m.

Figure shows the uncorrected and corrected temperature minus the values calculated from for three months in 1997. The bias in uncorrected temperature for southerly wind directions is obvious from the figure. As it is expected, the correction has no effect for non-critical wind directions. The figure shows that the undisturbed temperature values are somewhat higher than expected from eq. , so the correction routine adjust the erroneous values in such a way that the result will exhibit similar behaviour like the correct ones. The humidity sensor measures the relative humidity for its actual environmental condition, therefore the measured (uncorrected) temperature is used for calculation of the absolute humidity.

Figure: The uncorrected and corrected temperature minus the values calculated from eq. (t ) for three months in 1997. Upper plots: uncorrected data; lower plots: corrected data.

Relative humidity also needs correction since the Vaisala sensors may suffer from zero and span drift. Since span drift can not be recognized easily (L. Haszpra, personal communication), efforts were done to correct for the effect of zero shift. This was attained by substracting a specific value from the relative humidity data determined from long term data selecting saturated (i.e. foggy) conditions. Thus the corrected data can not exceed 100%.

For measurements at 10 m above the ground, a mast was erected about 70 m from the transmitter building and tower. Wind speed, temperature, and humidity sensors are the same as the ones mounted at the tower. Instruments for radiation measurements are mounted at 2 m height on the mast, including a global solar radiation sensor (Kipp & Zonen model CM2), a radiation balance sensor (REBS model Q*6) and a photosynthetically active photon flux density (PPFD) sensor (LI-COR model LI-190SZ Quantum Sensor). Sensors for soil temperature and soil heat flux (Campbell Scientific model HFT-3) have been installed recently near the mast.