Scintillometry: A brief Review

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Scintillometry A brief Review

• Henk de Bruin
• Based on work by Wouter Meijninger, Oscar
  Hartogensis, Andreas Lüdi, Frank Beyrich, Wim
  Kohsiek, Arnold Moene (ET)
• Christian Mätzler and Lorentz Martin (R) and
  others

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Measuring sensible heat, evaporation and rain on
km scales
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Objectives long-path scintillometry

• Meteorological and hydrological models need as
  boundary conditions the surface fluxes of
  sensible heat and water vapour as well as
  precipitation
• The horizontal spatial scale of a grid box used
  in these models is at least several kilometres
• At this scale most land surface are heterogeneous
• Conventional techniques concern point
  observations
• There is a need for an observation technique for
  heterogeneous terrain applicable on km scales.

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Objectives

• Measurements of spatially average fluxes of
  sensible heat and water vapour (evaporation) on
  kilometre scale
• Measurements of spatially average rainfall rate

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• What is a scintillometer?

Path length L, Aperture D, Wavelength l
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What is Measured?

• The variance of the logarithm of the light
  intensity received by the detector
• This variance is caused by fluctuations of the
  refraction index of the air, which in their turn,
  are due to turbulent fluctuations of temperature
  and humidity
• In case of rainfall extinction coefficient

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Scintillations and a Nursery Rhyme
Twinkle, twinkle, little star, How I wonder what
you are! Up above the world so high, Like a
diamond in the sky. Twinkle, twinkle, little
star, How I wonder what you are!
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Example of scintillations
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What causes scintillations?
Huygens-Fresnel Defraction Effects
l ltlt L
L l/2
Fresnel zone
F
Detector
L
Plane wave front
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Turbulent Eddies cause 'Huygens-Fresnel effects
Eddies of size S act as slit
L l/2
S
Detector
L
If S ? F, than dispersion effect plays a role If
S gtgt F geometric optics applies F is Relevant
Length Scale in scintillometry
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Large Aperture Scintillometer (LAS)
If aperture D of detector gtgtF Scintillations of
size F are smoothed out So when D gt F ,
scintillometer sees eddies of size D only
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Overview different scintillometer types
If saturation corrections are needed Hill
spectrum is needed also
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Scintillometers (XLAS, LAS and DBLS)
XLAS 1 - 10 km
LAS 0.2 5 km
DBLS 25 200 m XLAS eXtra LAS LAS
Large Aperture Scintillometer DBLS Displaced
Beam Laser Scintillometer
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Radio Wave Scintillometer
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Scintec BLS2000 (XLAS) dual transmitter type
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weighing function of LAS and RWS

(X)LAS
RWS
R
T
Normalized distance
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eddy sensitivity of the LAS

Relative weight
eddy size/D
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eddy sensitivity of the MWS

Relative weight
eddy size/F
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Basic expressions for the LAS and RWS
The variance of the logarithm of the light
amplitude received by the detector provides Cn2.
F gt D
D gt F
Large Apeterure Scintillometer l about 1 micron
Radio Wave Scintillometer l about 1 cm or 30GHz
LAS (optical)
RWS (radio-wave)
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Schematic Picture n spectrum
Inertial sub-range
3-D spectrum
structure parameter
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Advantage of scintillometry

• scintillometers determine weighed path average
  values of CT2 and Cq2

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Fluxes derived from Scintillometry
Sensible heat flux
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Urelevant ??
Free convection conditions, i.e. sunny and calm
day
after some algebra
Finally
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Urelevant ??
near neutral conditions, i.e. cloudy and windy day
General daytime conditions
k 0.4 c1 and c2 empirical constants L Obuhkov
length
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Water vapour flux
Latent heat flux
similarity between T and q and same dimensional
arguments leads to
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Structure parameter of refraction index

• AT and AQ depend on wavelength l
• For 'optical' wavelengths, l? 1 micron, n
  fluctuations primarily due to temperature
  fluctuations (T ')
• When l ? 1 cm (radio wave, microwave) n
  fluctuations primarily due to water vapour
  pressure fluctuations (Q ')
• In both cases a cross T-Q term plays a role.

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Typical values 3 terms of Cn2
Very good Assumption
Often, but not always
More research needed!! Derivable from LAS-RWS??
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Simplified Picture
LAS gives CT2 and RWS gives CQ2 Once CT2 and
CQ2 are known, the sensible heat flux (H) and
evaporation (E) can be estimated using the
Monin-Obukhov Similarity Theory. Details given
before
During daytime free convection scaling
dominates!!!! and then life becomes so easy.
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Does it work?

• Some results field experiments

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Flevoland Experiment 1998
(2 km)
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Land use
Fractional area crops each 25 (isotropic
conditions)
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Flevoland Results
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• Why does a LAS work over heterogeneous terrain,
  while using scaling for homogenous conditions???

Concept Needs Further Validation!!
In this region one needs footprint analyses
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Cabauw XLAS (path length 10 km)
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Cabauw XLAS ? Aircraft
Data was collected during the RECAB field
experiment
Moene at al. 2006
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LITFASS-2003, Lindenberg, Germany
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LITFASS-2003 The measurement strategy
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LITFASS-2003 Fluxes over Different Surfaces
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LITFASS-2003 Area-averaged Fluxes (I)
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LITFASS-2003 Area-averaged Fluxes (II)
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Energy balance closure
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Conclusions flux part

• Flevopolder, Cabauw and LITFASS 2003 field
  campaigns revealed that a combined long-path
  optical- radio-wave scintillometer (ORS) system
  provides reliable area averaged fluxes of
  sensible heat and water vapour over heterogeneous
  terrain
• But many more validation experiments under a
  wider range of conditions have to be carried out

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• Rainfall with radio wave scintillometer or LAS

General remark success of flux measurements with
LAS and RWS is primarily based on fact that they
use inertial sub-range spectra, which appear to
be universal (Kolmogorov-Corrsin_Obuhkov) In
case of rainfall universal spectra do not exist.
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Rainfall drop - size distribution depends on
rainfall type.After Mätzler and Martin (2003)
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Rainfall rate depends on fall velocity, which
depends on drop-size.After Mätzler and Martin
(2003)
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Extinction Coefficient versus rainfall Rate for
38 and 94 GHzMätzler and Martin (2003)
Mie theory extinction scattering absorption, bac
k-scattering asymmetric scattering
94 GHz
38 GHz
Model (ignore here)
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Results field experiment near Bern for 38 GHz
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Results field experiment near Bern for 94 GHz
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Results field experiment near Bern for optical
wavelengths
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Results field experiment near Bern extinction
at 38 GHz versus extinction at 94 GHz
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Conclusion rainfall study

• Rainfall observations with a 38 GHz RWS appears
• to be promising, but transmitter stability and
• rainfall heterogeneity along path are unsolved
  issued
• Study concern water droplets and rain. Snow,
  hail and melting snow/hail are not dealt with
• By using a 38 GHz and a 94 GHz scintillometer
  information on rainfall type can be obtained.
• Rainfall rate derived from extinction of a LAS is
  not promising, but.... may be spectral analyses
  of the LAS signal might provide better results.