In case the path length has been set incorrectly during the installation of an LAS scintillometer, WINLAS can correct the Cn2 data.
In order to do so please use the following procedure.
After entering the relevant info in the parameters section, enter the path length setting set with the potentiometer of the LAS receiver in meters and enter the correct path length in the input field below.
Select ‘Run…’ in the WINLAS file menu.
WINLAS will now process the Scintillometer Cn2 data using a correction algorithm for the actual path length.
WINLAS corrects the path length in the following way:
Using the following equation to derive the intensity fluctuation data from the recorded Cn2 values calculated by the LAS using the incorrect path length setting.
(Wang et al., 1978)
Aperture diameter ~ 15cm
Initial path length
Variance of log intensity
The equation is re-written to yield the variance of the intensity fluctuations:
And finally re-calculate Cn2 with the correct path length:
End of FAQ.
A scintillometer measures the path weighted structure parameter of air, Cn2, using an optical transmitter and receiver.
In certain cases of relatively high Cn2 values the signal can become saturated depending on the diameter of the lens, wavelength and path length.
The so called saturation limit for Cn2 can be derived using the following formula (Ochs and Hill 1982)
Cn2 < 0.18.D5/3.L-8/3.λ2/6
the diameter of the scintillometer
[0.15m or 0.3 m]
the path length
the emitted wavelength
In the calculation tool below, you can calculate the saturation limit for the LAS and X-LAS scintillometer as a function of Path length.
Once the LAS has been installed and properly aligned the Path Length dial knob at the receiver control panel must be set for the correct distance between the transmitter and the receiver. The Path Length dial knob has 10 turns maximum with a vernier counter and a locking mechanism.
These graduations are NOT in units of distance! The precise path length must first be converted to a dial knob setting (Pot) using the following relationship for the LAS. The equations below can be used to find the correct Potentiometer setting as a function of pathlength for the LAS and X-LAS.
In addition you can use the calculation tool below to calculate the correct potentiometer setting for the (X)LAS.
The LAS and X-LAS scintillometer can be connected to the CR10x and CR1000 data loggers from Campbell Scientific.
Configuration examples for these data loggers can be viewed on this page, here:
Campbell CR10x Data Logger
A (X)LAS scintillometer can be connected to a Campbell CR10x data logger using a 2:1 voltage divider like VDIV2.1 from Campbell. The reason for this is the fact that the LAS has an output of 0..-5V and the standard input range of the CR10x is ± 2.5V.
The procedure is as follows:
Collect data using the data logger and check for normal operation of the Scintillometer data collection.
Campbell CR1000 Data logger
The connection procedure for the CR1000 is similar to the CR10x. The same connection to the terminals for differential measurement of the LAS signals can be used. However, the input range of this data logger is ± 5V so a voltage divider is not required.
A example configuration for the CR1000 in LoggerNet format can be found here: LAS_CR1000.CR1
Point measurements (alternative methods to LAS)
The LAS MkII Large Aperture Scintillometer provides continuous measurements over path lengths from 100 m up to 4.5 km. It is the only scintillometer currently available with a built-in display and control-pad. It has internal digital processing to make calculations in-situ and to store data and results. Measurements are on a comparable scale to the pixel size of satellite instruments, making LAS MkII ideal for ground-truthing applications. The LAS MkII is convenient to use due to low power consumption, integrated data logger, accurate reference time from GPS, and works reliably in cold environments.
The advantages of the LAS MkII are the following:
Yes you can, but there are a few points you have to take care with as the analogue data output has changed in format.
See appendix H of the latest manual, for details of the conversion of the analogue voltage output, UCn2. Note, the manual is written for the improved LAS MkII with GPS, so there will be differences in the hardware, and there are a couple of data formatting issues that are different (voltage ranges).
Most likely the format of your UCn2 data is incorrect for the latest version of Evation, as the software was developed to work with the LAS MkII, which has a different analogue voltage output (positive instead of negative).
For the LAS MkI, the Cn2 voltage output, UCn2, is output as -5 to 0V, where:
-5V analogue voltage output is equivalent to a Cn2 of 1 x 10^-17 [m-2/3]
0V is equivalent to 1 x 10^-12 (m^-2/3)
Cn2 (m^-2/3) = 10^(-12+ UCn2) (this applies to the LAS150, not to the LAS MkII !)
The latest version of the Evation software V2R5 and the new manual (vs1511) which explains the use of Evation, are available online, under downloads:
[The manual is now very comprehensive and covers all the recent improvements to the LAS MkII, such as the time from GPS, new firmware and processing software. It also includes clearer instructions for operation of the instrument and software, and several appendixes to cover additional information such as logging the serial or analogue output with an external logger, such as from Campbell Scientific.]
To derive fluxes of sensible heat (H) from the LAS measurements (Cn2), one needs to know the height of the LAS beam above the ground, also known as the effective height. Because the flux is almost linearly related to the height it is important to determine the effective height as accurate as possible (see Appendix F of the LAS manual). Over flat terrain it is relative easy to determine the LAS height: take the average of the height of the transmitter unit and receiver height (i.e. the height between the centre of the beam and the ground).
Over non-flat terrain it is a bit more complicated, because now we also need to consider the path-weighting function of the LAS. This weighting function reveals that the centre of the LAS path contributes more to the measured Cn2 data, than near the transmitter and receiver units. This calculation is easily done by using the effective height calculator built-in to the Evation software, which takes care of the weighting function.
Note: for very long path lengths (> 5km), such as when using the XLAS it is also important to consider the earth’s curvature (decreases the height in the centre of the path by approx. 2 m over a path length of 10 km.
More detailed information of deriving the effective height of scintillometers over complex terrain and the effect of atmospheric stability on the effective height can be found in: Hartogensis et al., Derivation of an Effective Height for Scintillometers: La Poza Experiment in Northwest Mexico, Journal of Hydrometeorology, 2003.
Yes, the LAS transmitter and receiver unit can be placed inside behind glass or Perspex windows, preferably at normal incidence to minimise light loss and refraction of the beam. However, it must be noted that windows absorb a fraction of the light beam (~8 to 25%) thereby limiting the maximum path length of the LAS or XLAS.
An error in the path length L of 1% results in an error of 3% in Cn2 (and thus H). This shows that the path length should be determined accurately.
The effective height or height of the LAS beam should be measured to 1 cm (a tape measure can be used for this).
The measurement principle of the LAS is based on the scattering of EM radiation by the turbulent atmosphere that result in fluctuations of the intensity of light. The turbulent eddies that produce these scintillations have a size of the order of the aperture diameter of the LAS (or XLAS). The figure below shows that in general these fluctuations lie mostly between 1 and 10 Hz (exact positing of the curve with respect to the x-axis is slightly dependant on the crosswind). The bandwidth of the LAS electronics is set around these fluctuations (0.1 Hz to 400 Hz). In this way electronic noise (> 400 Hz) and low frequency fluctuations related to absorption by the atmosphere (< 0.1 Hz) are removed.
Figure 1: Theoretical spectrum of a 0.15 m LAS (path length = 1 km, wind speed = 1.5 m/s).
Any type of fluctuations, e.g. caused by tower vibrations that lie within this bandwidth, in particular the ones that lie between 0.5 to 10 Hz can have significant effects on the measurements. It is therefore, strongly recommended to use stable and robust mounting platforms for the LAS units.
Yes, this is possible by logging the analogue output with a fast (500 Hz) data logger, and looking at the shift in the peak of the scintillation spectrum (the scintillation power spectrum shifts linearly along the frequency domain as a function of the crosswind). In the optical microwave sintillometer, the raw data is already logged at 500Hz, so this could be implimented. Built-in data processing to calculate the crosswind may be added in the future.
See the following publication for more information: van Dinther, D., O. K. Hartogensis, and A. F. Moene, 2013: Crosswinds from a single-aperture scintillometer using spectral techniques. J. Atmos. Oceanic Technol., 30, 3–21.
The LAS MkII includes an inbuilt data logger, but sometimes you may which to add the data to an existing meteorological station with its own data logger.
The LAS transmitter and receiver both have multiple analogue output signals, which can be measured by most standard data loggers. These signals allow the user to monitor the internal temperature as well as some raw signals to check the performance of the electronics. In most experiments these signals don’t have to be measured.
For general flux measurements two signals are important: the Cn2 signal and Demod signal. Both signals are measured at the receiver unit. The first signal, re-scaled Cn2, provides information of the turbulent intensity of the atmosphere and is used to derive the sensible heat flux (H). It’s range lies between 0 to 2.4 V for the LAS MkII (-5 and 0 Volt for the LAS150). The second signal: the demod signal is a measure of the signal strength and it’s range lies between 0 to 2 V for the LAS MkII (-2 and 0 Volt for the LAS150). The more positive (more negative for the LAS150), the more signal the receiver has. In general the signal strength depends on the distance between the transmitter and receiver and the opacity of the atmosphere.
The reason it is advised to measure the demod signal is that it can help with the interpretation of the Cn2 signal. In some cases the Cn2 can be difficult to understand, e.g. during rainy and foggy periods, while the demod signal shows clearly whether or not the receiver has a signal, or some signal is lost due to the weather.
The LAS instrument provides the structure parameter of the refractive index of air, Cn2. The latter can be considered as a parameter that describes the turbulent intensity of the atmosphere, in particularly related to the turbulent temperature fluctuations. This is way the LAS can be used to measure the sensible heat flux. However, the derivation of the sensible heat flux requires some steps. In each step additional meteorological data is required (see also processing data in the LAS manual):
Step 1: from Cn2 to CT2 requires data of:
Step 2: from CT2 to the sensible heat flux H requires data of:
Step 3: from H to evaporation requires data of:
In additional the gravitational acceleration, surface roughness and sensor heights are required.
It is recommended to have the additional data at the same measurement interval as the LAS data.
Step 4: selection of unstable or stable solution H:
For land surface the typical diurnal course of H shows positive values during the day and negative values at night. Explanation: during (sunny) daytime conditions (roughly between sun rise and sun set) the earth’s surface heats up the atmosphere from below. This means H is pointed upward and defined positive. This situation is known as the unstable period. At night (roughly between sun set and sun rise) the surface cools due to long wave radiative cooling. As a result heat from the atmosphere is transported downwards to the surface. Hence, H is negative. This situation is defined as the stable period. The LAS is able to measure the magnitude of the sensible heat flux (H) but not the sign, i.e. is H directed upward (> 0) or downward (< 0)?
There several ways to choose either the unstable or stable solution of H:
Net radiation: During most situations when the net radiation is positive, the atmosphere is unstable. Once the net radiation becomes negative the atmosphere becomes stable. Note that this option is not applicable over intensively irrigated fields.
Global/solar radiation: Although less accurate than net radiation data, but still useable. When the global radiation is higher than approximately 20 Wm-2, the atmosphere is unstable. When it drops below 20 Wm-2, assume stable conditions. The exact values are site/surface dependant.
Temperature profile data: for example air temperature data collected at 0.25m and 3m height. During daytime close to the surface (0.25m) it is warmer than higher up in the atmosphere (3m), i.e. unstable conditions (dT/dz < 0). At night the situation is opposite, cold close to the surface and warm at higher levels, the condition is stable (dT/dz >0). This method is the most reliable one, but requires accurate temperature measurements.
Cn2 data: During clear sunny days the Cn2-signal shows a very distinctive behaviour. Every time the atmosphere changes transition, the Cn2-signal drops to a very small value (® 1e-17). By determining the exact time when this occurs, the average time periods of unstable and stable conditions can be simply determined. During cloudy conditions the exact transition times are difficult to detect and is therefore difficult to automate.
The LAS manual shows a terrain classification for typical landscapes with corresponding surface roughness lengths. General meteorological literature can provide more detailed information of surface roughness length for specific surfaces and/or crops.
The surface roughness can also be determined experimentally, using e.g. eddy-covariance stations or from wind profile measurements.
This has been done, see publication by: McJannet, D. L., F. J. Cook, R. P. McGloin, H. A. McGowan, and S. Burn (2011), Estimation of evaporation and sensible heat flux from open water using a large‐aperture scintillometer, Water Resour. Res., 47, W05545, doi: 10.1029/2010WR010155.
But there are several reasons why LAS measurements of open water is rather complicated:
To measure both evaporation / latent heat flux and sensible heat flux directly (and without the restrictions when using a LAS on it own), a combined optical and microwave scintillometer can be used. This system measures both CT2, Cq2, and the co-variant term, CTq, between them, so no assumptions are made. Along with meteorological data from the receivers side mounted weather station, latent heat and sensible heat fluxes are calculated internally. See the Optical Microwave Scintillometer system page for more details.
How frequent the alignment has to be checked is dependent on the installation set-up. Tripods fixed in the ground can have the tendency to move, in particularly as the soil can become soft after periods of rain. In that case the alignment has to be checked at a regular interval. Once steel constructions are used on top of buildings, or tripods on a concrete foundation, the setup is much more stable and has to be checked less frequently.
In-case one has the ability to check the data in real-time, the demod signal can help to monitor the alignment. A slow decreasing trend of the demod signal can suggest possible changes in the optical alignment (ignoring degradation of the LED).
For a long term installation, stable steel constructions such as a steel frame or post, on a concrete foundation, are recommended to avoid vibration or miss-alignment. If tripods are used for field work, they should be fixed or tied to the ground when left unattended for a period to avoid damage from storms nocking the tripod over. To do this a ground anchor can be screwed into the ground under the tripod, and the tripod tied to this using a ratchet or cargo strap. Insure the tripod feet are firmly pushed into the ground first.
If there are animals in the area protect the equipment from being pushed over, disturbed, or the cables chewed, by surrounding the installation with electric fencing.
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