Windprofiler Radars and detection of STE events

W.K. HockingUniversity of Western Ontario

Radar.

Small airport radars to giant dishes…

Purpose – to create a narrow beam of radiation, of VERY high intensity.

Japan MU radar

Looks like a dish…

Zoom in…

> $10M for 1 radar in 1985!!

With a dish, we can move itaround …

But even if we cannot do that with our radar, we CAN steer the beam by feeding different signals to different antennas – the whole beam can be steered in a fraction of a second!

The Canadian approach…

Effective, and much cheaper…

Require:

High Power – 40 kW in short bursts

c.f. Japan and Germany – 1 MW.

But our system is still effective, and by spreading multipleradars across the province, we gain great meteorologicaladvantage.

Receivingand DigitizationSystem

BeamSteeringUnit

Highly sensitive receivers are needed – transmitted signal is kilovolts, but received signal is microvolts!

Special signal processing is required – coded pulses, coherent integration, spectral fitting…

Measure winds, turbulence, backscattered power.

Armin Dehghan, poster

Relation to Ozone?

Radar/Ozone

Why is there enhanced scatter at the tropopause?

1.Specular Reflectors

Sheets….

Common in regions of HIGH STATIC STABILITYe.g. tropopause.

Turbulence

When people talk about turbulence,they often talk about “eddies”.

We often envisage turbulent particletrajectories to be elliptical in shape...

“Eddies”(Laboratory photographs of tracers)

Due to the velocity shears (differential velocities)in the fluid, even initially isotropic objectsare stretched and torn apart ...

Computer simulations showing vorticitystrings in a patch of turbulence (Werne, Fritts et al.)

There are also various stages of turbulence - developing,steady-state, and decaying.

Enhanced VHF signal due to

EITHER specular reflections or turbulence

Yamadaet al., GRL, 2001.

Time

sequence.

It may appear in patches of limited extent...

Cause of upper level turbulence – Gravity Waves.

Gravity waves propagating upward increase In amplitude as they encounter regions of high static stability.

Can result in turbulent layers just above the tropopause.

t=0 t=t1

z z

xx

--------- Layering

Energy deposition - at the smallest scales, large wind-shears produce fluid frictional heating (depositionof kinetic energy).

We talk about the “energy dissipation rate”

Energy may also be dissipated by deposition of potential energy.

Turbulence may in fact also function as a storage mechanism of energy.

C. Diffusion...

Diffusion is often represented by an “eddy diffusion coefficient”,using an analogy with molecular diffusion.

But this grossly over-simplifies the process of turbulent diffusion in the atmosphere.

Different diffusion mechanisms apply at different scales.

4. What are the unique aspects of turbulence pertaining to the atmosphere?

In the atmosphere, turbulence is:

Spatially and temporally variable and intermittent

Frequently anisotropic

Driven by multiple phenomena, some of which themselves are scale dependent.

Subject to unique measures of stability (Richardson number compared to Reynolds number, Froude number etc.)

Species-dependent (ions compared to electrons compared to neutrals)

Layered structures higher up ...

Fully developed turbulence (FM-CW radar,Eaton et al.)

LAYERING in Turbulence

Czechowsky et al.

Relevance here?

Homogeneous Turbulence:

cmolecular diametermean free path

K = Lv, L = “eddy size”, v = associated velocity.

K = L2/T, L = dimension, T = time scale.

L = (KT)1/2

K = 1-10 m2s-1, T = 4 days,

D ~ 2 km.

????

But does this apply in the case of layered turbulence?

K = K(Layer depth, Klayer, Freq. of Occ., Spatial variability)

K = c1./N2, K=Lv etc no longer apply!

Current models grossly oversimplify K.

Relevance to STE.

How does ozone cross the stable tropopause layer?

Need for turbulent transport?Turbulent layers at the tropopause?

Is FlexPart always valid? Do we need to “open the tap” first?

Mohammed Osman

Jet Stream can generate gravity waves.Production of small scale turbulence?

The End

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