The long-term experiment VerTurM (Vertical Turbulence Measurements) was set up for investigating vertical profiles of atmospheric turbulence in the lower atmospheric boundary layer. Measurements up to a height of 400 m have been performed since summer 2009.
The data of the vertical profiles were monitored using 3 different complimentary measurement systems. Ground-layer turbulence is measured by a Laser-Scintillometer. Measurements in the surface layer are provided by 4 ultrasonic anemometers mounted on mast up to a height of 64 m. Turbulence data of the upper part of the profile are monitored remotely using a SODAR-RASS (Sound Detection And Ranging Radio Acoustic Sounding System).
|Mess station||Data acquisition
For Performance, setup, and operation of electro-optical systems atmospheric turbulence has to be considered to correct its effects on the wave propagation. The origin of turbulence in the atmospheric boundary layer (1-2 km height above ground) is thermal heating of the ground. During the day warmer air bubbles will rise and create a vertical mixing of the air. Cooling at the ground and horizontal wind will create temperature fluctuations during the night. The strength of turbulence depends on local weather, i.e. solar radiation, temperature fluctuations and wind speed. Diurnal and seasonal variations arise in the prevailing climate. The strength of turbulence is characterized by the structure function parameter of the refractive index Cn2. It is called optical turbulence.
The strength of turbulence decreases with height above the ground with reducing earth friction. Investigating non-horizontal paths of wave propagation it will be necessary to know the vertical distribution of Cn2. Over land Cn2 is proportional to temperature fluctuation characterize by the structure function parameter of temperature CT2. Additional knowledge of temperature and air pressure is necessary to derive the height (h) distribution of Cn2:
G round layer turbulence is investigated using a Laser-Scintillometer (SLS40, Scintec, Germany) at a height of 1.2 m above ground. Scintillometer measure Cn2 directly from the scintillations of a monitored light source. For the SLS 40 using during the VerTurM-Experiment the operating laser had a wavelength of 670 nm. The horizontal distance between transmitter and receiver belonged to 75 m.
Using ultrasonic anemometers the ultrasonic anemometer temperature is measured with a high temporal resolution of 50 Hz. Analysis of the monitored time series for their variances provide CT2. Four ultrasonic anemometer have been mounted on the meteorological mast at the heights of 4 m, 8 m, 32 m and 64 m providing the vertical profile of Cn2 in the atmospheric surface layer.
|Ultrasonic-anemometer||Meteorological mast (up to 64m hight)|
Sound Detection And Ranging with Radio Acoustic Sounding System.
For determination of the vertical profiles of Cn2 in the lower atmospheric boundary layer above the surface layer a remote SODAR-RASS has been used. It provides Cn2 values between 40 and up to 400 m with a height resolution of 5 m.
The SODAR sent out pulses vertically. They will be back scattered at temperature fluctuations in the atmosphere. Intensity and run time of the back scatter signal will provide information of the height distribution of CT2. The RASS emits radio waves that will be back scattered at the sound wav e of the SODAR system. From the back-scattered signal a height profile of the virtual temperature is received. Pressure values are calculated using the barometric height formula and ground pressure measurements. From the combination of the two systems and the derived height profile of air pressure Cn2 (h) may be evaluated.
The SODAR-system have been also applied for measuring the vertical distribution of the horizontal wind speed using the Doppler-displacement also measurable in a slightly other configuration of the system.
RASS (Radio Acoustic Sounding System)
|SODAR (Sound Detection And Ranging)|
The monitored vertical profiles of Cn2 should be used to verify a newly developed model of the vertical distribution of optical turbulence in the lower atmospheric boundary layer. Additional long-term statistics are evaluated of the vertical profiles of Cn2 dependent on diurnal and seasonal variations and different meteorological conditions.
More over the Fraunhofer IOSB has built up a database data on ground –layer turbulence Cn2 above land as well as water surfaces distributed on certain locations worldwide.