Laser scintillometer data location no. 16 in CASES99 network of flux towers

version 1.0

1. Authors and data source:

Wageningen University (the Netherlands)
Oscar Hartogensis, Bas van de Wiel


2. Description of the sensor

2.1. Sensor Type
Split beam laser scintillometer, the SLS20 by Scintec (http://www.scintec.com).

2.2. Theory in a nutshell:
The SLS20 is made up of a transmitter and a receiver. In the transmitter a beam of red 
light is split up into two parallel beams distinguishable by a different polarisation 
which is emitted in the direction of the receiver placed at a distance of 100 to 250m at 
the same height as the transmitter. At the receiver the intensity fluctuations of both 
beams and their correlation is analysed. Following wave propagation theory in weakly 
scattering turbulence from these the structure parameter of the refractive index Cn2 
and the inner scale of turbulence can be determined.
The inner scale is closely related to the dissipation rate (E) of the kinetic energy, 
which follows Monin-Obukhov scaling to give U*.
In the near infra-red and red part of the electromagnetic spectrum fluctuations in the 
refractive index are mainly caused by temperature fluctuations, i.e. Cn2 can be 
expressed into Ct2, the structure parameter of temperature, which follows Monin-
Obukhov scaling to give T*.
With T* and U* known, the sensible heat flux can be calculated.

2.3. Location:
Location no. 16 in CASES99 network of flux towers; co-ordinates N37’38.611’ 
W096’44.233’

2.4. Set-up:
Transmitter and receiver were installed on tri-pods at a height of 2.46m. Path length 
between transmitter and receiver was 112m.
Receiver was set-up at 40m south of tower no.16; the scintillometer path had a NE to 
SW direction.
A small field survey had been conducted to establish the extend of the sloping terrain 
along the scintillometer path. In the table below the heights are given relative to the 
surface at the scintillometer transmitter, which is indicated by (*); the receiver is 
indicated by (**). The table represents a grid around the scintillometer path with 
intervals of 50 m between the points.

-0.45		-0.15		0		0.4		0.95
0.15		-0.05		0 (*)		0.25		0.6
0.35		0.3		0.45		0.25		0.3
1.45		0.95		0.9 (**)	0.85		1.1
		0.4		1.8		2.2		1.85






Data was logged and online processed on a laptop with Scintec software. 


3. Description of derived variables

The Scintec software processes the raw measurements to give 6 second Cn2 and the 
inner scale. Given indicative values of temperature and pressure the software also 
produces a sensible heat flux for stable and unstable Monin-Obukhov scaling 
relationships.
Here, a pressure of 1000hPa and a temperature of 10 degrees Celsius were used. 
Given the fact that these are not exact and that the scaling relationships used under 
stable stratification are under debate they should be seen as indicative sensible 
heatflux estimates only. The “hard” scintillometer data is made up of Cn2 and the 
inner scale.
Invalid or no data are given as a dummy “-9999”.

The columns in the data files are labelled; here follows an overview of all the 
variables with their label names and units:

YYYYMMDDHHMMSS : Time in CASES99 standard time format [UTC]
DOY : Day of Year [CDS]
HH,MMSS : Decimal time, where minutes and seconds are expressed as a fraction of 
an hour [CDS]
siglogX : The standard deviation of the logarithm of the amplitude in channel 1 
calculated from the measured intensity data.[-] 
siglogY : The standard deviation of the logarithm of the amplitude in channel 1 
calculated from the measured intensity data.[-] 
logCor : The correlation of the logarithms of the amplitude in channels 1 and 2 
calculated from the intensity data. [-] 
error : percentage of error free diagnosis data; since raw 6s analyzed data is recorded 
this value is either 100 for good data or 0 for bad (no) data[%]
Cn2 : structure function parameter of the refractive index [10^12m^-2/3]
lo : inner scale of refractive index fluctuations [mm]
Ct2 : structure function parameter of temperature [K^2m-2/3]
Diss : rate of kinetic energy dissipation [m2/s3]
H_st : downward turbulent heat flux, calculated using Monin_Obukhov similarity for 
stable conditions [W/m2]
Tau_sta : downward turbulent flux of momentum , calculated using Monin_Obukhov 
similarity for stable conditions [N/m2]
L_stable : absolute value of the Monin_Obukhov length L, using the Monin_Obukhov 
similarity for stable stratification [m]


4. Sample period

Sample period = 6 seconds
Raw 6second data were stored.

The data are cut up into night-files running from 17:00 to 10:00 next morning local 
(CDS) time. In the filenames the date of the beginning of the night is given. The files 
are in plain ASCII text; the columns are space separated.


5. Data availability

Laser scintillometer data are available between 0930 17:00 and 1027 10:00 local 
(CDS) time. During some nights there have been power or alignment problems which 
cause the night data to be incomplete:
night from 1001 to 1002 between 5:00 and 10:00 local time
night from 1007 to 1008 between 5:00 and 6:15 local time
night from 1016 to 1017 between 1:00 and 2:30 local time
night from 1017 to 1018 between 0:00 and 10:00 local time
night from 1023 to 1024 between 2:30 and 10:00 local time