delft university of technology 1 cesar science day, 19 june 2013 albert oude nijhuis dynamics of...

DelftUniversity ofTechnology 1

CESAR Science Day, 19 June 2013


Albert Oude Nijhuis

Dynamics of turbulence in precipitation: Unravelling the eddies



• UFO project• Retrieval of wind field with IDRA• Retrieval of the Eddy Dissipation Rate• The Inertia Effect• Outlook

Contents



UFO Project



UFO Project

• The combination of LIDAR and RADAR can be used to detect wake vortices in all weather conditions.

• Quantitative information on turbulence, e.g. the eddy dissipation rate, can be retrieved from the Doppler moments, velocity and spectral width, from LIDAR and RADAR instrument.



Retrieval of wind field with IDRA

In contours: Doppler velocitiesHorizontal wind vectors areobtained via a least sqaures fit.




Model from Doviak and Zrnic (1992)



Retrieval of the eddy dissipation rate

The eddy dissipation rate (EDR) is based on the assumption of homogenous isotropic frozen turbulence. In the inertial sub-range the dissipation goes with a -5/3 power.

Figure from O'Connor (2010).




Several retrieval methods are possible:• 1. Use the Doppler spectral width. Subtract the fall speed

distribution and compute the turbulence spectral width. Then calculate the EDR.

• 2. Use a sequence of Doppler velocities and compute the variance. Use an analytic relation to compute the EDR.

• 3. Use a sequence of Doppler velocities, and compute the vertical velocity energy density spectrum (FFT). Fit the slope from the loglog plot, which can be used to compute the EDR.

The eddy dissipation rate (EDR) is based on the assumption of homogenous isotropic frozen turbulence. In the inertial sub-range the dissipation goes with a -5/3 power.




EDR from Doppler spectral width

TARA (S-band RADAR) measures the vertical Doppler verlocity at Cabauw.

EDR from Doppler velocitiy sequence

O'Connors method(from the variance)

LSF method 1

LSF method 2

Different retrieval methods lead to different answers!




EDR from Doppler spectral width

EDR from the spectral width is quite a challenge, as it is depending on the drop size distribution!




EDR from Doppler velocitiy sequence

O'Connors method(from the variance)

The basics




Figure from O'Connor (2010). Power spectrum for a LIDAR instrument.

Power spectrum from TARA (S-band RADAR). +/- 60 min. of data.

For the RADAR the -5/3 power law seems not to be satisfied for the shorter spatial scales.



The inertia effect

• Large droplets have inertia. They need some time to participate in the turbulent motion.

A very simple model can be used (Gorelik, 1964):- For each drop size there is a minimum length scale Lmin.- For turbulent motions larger than Lmin the drop is a perfect tracer.- For turbulent motions smaller than Lmin the drop will not participate in the motion.- For Lmin we use the approximation:



The inertia effect

• Large droplets have inertia. They need some time to participate in the turbulent motion.

Power spectrum from TARA (S-band RADAR). +/- 60 min. of data.

Theoretical power spectrum,with and without the inertia effect. Inertia effect??



Benchmark: plot vs. height

Spectral widthFrom variance, method from (O'Connor, 2010)

Spectrum lsf method 1 Spectrum lsf method 2



Comparing retrieval methods

LSF 2 (y-axis) vs. O'Connor

LSF 1 (y-axis) vs. O'Connor



Outlook

• Benchmark / validate EDR retrieval methods• Investigation of the inertia effect. Potentially it may be

possible to retrieve the drop-size distribution.• The addition of a Doppler LIDAR on CESAR would be helpfull.

Then the LIDAR+RADAR combination performance for all weather conditions can be assessed.

delft university of technology 1 cesar science day, 19 june 2013 albert oude nijhuis dynamics of...

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