Radar Observation in Clear Sky

이동인부경대학교 환경대기과학과

Air Pollutant Dispersion Using a Doppler Radar

in Clear Sky

Nov 23, 2000

Prof. Leedi

Radar Observation in Clear Sky 3

Remote Sensing Techniques

• Lidar:Lidar: tridimensional distribution of some air pollution concentration, air motion

• Sodar:Sodar: easily the dynamic and thermodynamic structures of the low atmospheric layers

• Wind profilers:Wind profilers: height of stratified layers, convection in clear air

Radar Observation in Clear Sky 4

• Lidar, Sodar techniques :Lidar, Sodar techniques :– Sodar :Sodar :

• Restricted to observing along fixed directions around the vertical

– Lidar :Lidar :• More disadvantages --- measurement

problems for plumes are more acute

Isolated Pollution Sources

Radar Observation in Clear Sky 5

< Radar >

• Never used for air pollution studies– Tool for observation and measurements of

reflectivity and velocity fields in precipitating particles

– Because precipitating particles are the main scattering cause for the electromagnetic waves of meteorological radar

Radar Observation in Clear Sky 6

< Object of this class >

• Discuss the ability of– radars to provide observations – measurements useful for air pollution

modeling– especially in the case of an isolated pollution

source as a plume

• Radar tracers problem

• Determination of the characteristics of environmental mean field and plume structure by radar

Radar Observation in Clear Sky 7

< The Radar Tracers >< The Radar Tracers >

• Tracers : Natural and Artificial– Convenient to the wanted observation

• For observing the mean wind field– Wind direction ( x, y, w ) and Wind

speed

• For an isolated pollution source– Plume and Large point source

Radar Observation in Clear Sky 8

The Natural Tracers

• For meteorological radar observations

• Precipitating hydrometeors– rain, snow, hail etc…

• Non precipitating hydrometeors– droplets or tiny ice crystals in clouds

• All of these are close to the limit of radar detection

Radar Observation in Clear Sky 9

Meteor. Radar Wavelengths

• Between 3 and 10 cm– Hydrometeor size to wavelength ratio is

small (except for hail)• radar backscattering is situated in the Rayleigh

scattering region

• The backscattered intensity is proportional to the inverse of the fourth power of the radar wavelength and the sixth power of the scatterer diameter

Radar Observation in Clear Sky 10

Radar Reflectivity EquationRadar Reflectivity Equation

• The radar reflectivity ( the average radar backscattering cross section of the target by unit volume )

•

: Summation for the radar pulse volume V• D: Equivalent diameter of the scatterer : Dielectric factor of the scatterer

• Radar is very sensible to large hydrometeors• Sensibility of detection increases for decreasing wavelengths

V

DVK

62

4

51

Radar Observation in Clear Sky 11

Radar Detection• Hydrometeor:

– Precipitation rate, liquid water content, etc…– Many data on air motions

• Natural Tracers :– Insects, turbulent fluctuations of air refractive

index• Insects in dry air at positive temperature

generally have a proper motion relative to the air very slow

• Their motion relative to the ground can be used as an approximative air velocity measurement

Radar Observation in Clear Sky 12

– Their spatio-temporal distribution exhibits a close dependency on the thermodynamic structure of the atmospheric boundary layers

• Turbulent fluctuations of air refraction index (n) are an important and nearly universal radar tracer.

• In a turbulent mixing process, the air parcels rapidly displaced keep temporarily their identities;

– The pressure undergoes a continuous equilization with the environment

– The potential temperature and the specific humidity are preserved

Radar Observation in Clear Sky 13

This result is to create inhomogeneities in the refractive index field

– The stronger the turbulence and the sharper the initial gradient in temperature and humidity, the stronger will be the refractive index inhomogeneities

• Detection of such fluctuations are accessible to high power, high sensibility radars using wavelengths equal to or larger than about 10cm

Radar Observation in Clear Sky 14

• If the half radar wavelength is included in the inertial subrange,– the radar reflectivity is proportional to the

structure constant of the refractive index Cn2

– () 0.38 Cn2 1/3

• This tracer is almost always available --- detection – Data are obtained on the turbulence intensity, on

many dynamic and thermodynamic structures of small and medium scales

• stratification, waves of diverse kinds, convective cells, etc…

• obviously on the mean and turbulent air motions

For a well developed turbulence

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• When no natural tracer (detectable by the used radar)– Use of artificial tracers like chaff

• Resonant passive electromagnetic dipoles

• Metallised glass fibre cut to a length around the half wavelength

• Terminal velocity is smaller than 0.3 m/s

– Consequently chaff displacements follow reliably the atmospheric motions

Artificial tracers

Radar Observation in Clear Sky 16

Radar reflectivity value of chaffRadar reflectivity value of chaff

• For N dipoles by unit volume, randomly oriented and homogeneously distributed in the radar pulse volume,– () = 0.18 N 2

• Radar reflectivity is proportional to the number of dipoles by unit volume since all scatterers have same radar backscattering cross section

• Chaffs are not really randomly oriented, owing to aerodynamic effects, they tend to fall horizontally

Radar Observation in Clear Sky 17

• The main quantities measured by a coherent (Doppler) radar– Radar reflectivity which is proportional to the

number and radar backscattering cross section of the scatterers situated inside the pulse volume

is deduced from the measurement of the average received power from the radar equation

2r /P rC

Basic Doppler radar data

rP

Radar Observation in Clear Sky 18

• C : a constant depending on the radar technical characteristics , r : radar - target distance– the mean radial Doppler velocity :

– the radial Doppler velocity variance :

– the radial Doppler velocity spectrum : S(V)

• Three parameters : three moments of Doppler spectrum

• If knowing S(V), it can calculate – But these three quantities can be obtained derectly,

without spectrum determination in Doppler radar.

dVVSwith

V

Vmmmm

mmmP

n

nv

r

0

12

,

2

2

0

10

2,,

vrVP

V

2

v

Radar Observation in Clear Sky 19

Determination of mean field characteristics

• The useful data for modeling concern the physics and dynamics of the local atmosphere.

• The informations obtained from radars are qualitative and quantitative.– their usefulness depend on the peculiarities of each case

Radar Observation in Clear Sky 20

1) Physical structure

– The data provided by the radar are:• The tridimensional structure of clouds and precipitaion :

– Size and spacing of the cells, some informations on the nature of the hydrometeors at each level

– Particularly the height of the clouds and the convective field top is determined

• The reflectivity field :– Under some conditions relative to the knowing of the hydrometeor

nature, reflectivity field can be converted into physical quantities describing the scattering medium :

» Precipitation rate R, water content M or characteristic diameters of the particles such as median volume diameter D0

• From Doppler spectra measured at the vertical of the radar :– Accurate precipitation size distributions are obtained with their

incidental anomalies and their temporal variations

Radar Observation in Clear Sky 21

2) Dynamic structure

– From Doppler velocities:• Vertical air velocities can be determined at the vertical of the

radar by correcting the Doppler values for the terminal fall velocity of the tracer in still air.

• The other parameters describing the environmental dynamic field can be measured by different methods.

» When two Doppler radars (or more) are available, the tridemensional velocity field is obtained by combining the radial velocity measurement of each radar.

• With only one Doppler radar, dynamic characteristics of the environmental field can be determined from azimuthal scanning at constant elevation.

– This method : VAD (Velocity Azimuth Display), Browning and Wexler, 1968

Radar Observation in Clear Sky 22

VAD Method

• Calculate– the vertical profile of the horizontal wind in velocity and

direction

– the vertical profile of the mean vertical air velocity

– the horizontal divergence : • are the mean wind velocity along the two perpendicula

r horizontal direction x and y

– the stretching deformation :

– the shearing deformation :

yv

xu

vandu

yv

xu

xv

yu

Velocity Spectra

Range Marks

VrTime wVelocityVerticalDownward

DivergenceHorizontal

uv

w

Radar Observation in Clear Sky 23

Velocity Spectra

Range Marks

VrTime

uOnly

VelocityHorizontal

u

uDownward

Vr

VelocityVerticalplus

VelocityHorizontal wu

Upward PSIonshownnot

Downward

Vr

Vr

ShearingnDeformatio

StretchingnDeformatio

y

vx

u

x

vy

u

DisplayPSI DisplayVAD

Radar Observation in Clear Sky 24

Doppler velocity fluctuations along the VAD scanning circles

• Different informations concerning the field of turbulence are attainable (Wilson, 1970)– An estimate of the horizontal turbulent kinetic

energy– An estimate of the turbulent field isotropy– Estimates of the momentum fluxes ; '','','' vuwvwu

Radar Observation in Clear Sky 25

Determination of Plume Characteristics

• Among the various encountered plume conditions, the main two cases are dry plumes and wet plumes

• 1) Physical structure– Most dry plumes are constituted of gaseous

pollution, dry aerosols or dust with size smaller than ten m.

– Radar detection without artificial tracers is not possible in this case.

Radar Observation in Clear Sky 26

• Dry plumes including larger particles and wet plumes with hydrometeor sizes larger than several ten m are detectable with millimetric radar.

• Then the physical structure of the plume can be analysed from the measurement of the radar reflectivity distribution.– The interpretation of the data as physical quantities

characterizing the scattering medium ( such as volumic, massic, or numerical concentration of scatterers ) is possible only if relations between the radar reflectivity and the physical quantities are available.

• For the plumes, in most cases, no general relation is usable.

Radar Observation in Clear Sky 27

• Particular case for each plume : set up a specific relation– Needs to have sufficient knowledges on the properties

of the scattering medium ( size distribution, dielectric factor, etc.) to calculate a relation.

• Such knowledges can be obtained from simultaneous (in time) and concordant (in space) measurements of the scattering medium properties by instrumented aircraft and of the radar reflectivity by radar.

• The case of wet plumes developed from the same physical processes as natural clouds – Physical properties of the convective air (humidity and

condensation nuclies) are similar to those of the environmental air

Radar Observation in Clear Sky 28

– Only the convection cause is artificial,

– The microstructure of the associated cloud is nearly the same as in natural clouds : same relations can be used.

• Inversely, the plumes associated with wet cooling towers in presence of anomalous droplet size distributions (large “primage” droplets) cannot be treated like natural clouds, particularly for the radar reflectivity.

• For examples in wet plumes with granulometric distributions close to natural clouds and droplets smaller than 150 m ,– Mean radar reflectivity factor is approximately related

to liquid water content M and mean volume diameter D0 by the relationship (Sauvageot and Omar, 1982)

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• Z= 0.068 M1.94

• Z= 3.6 107 D03.16

– where :

• D0 is defined by :

• Z : ( mm6m3), M : ( gm3 ), and D0 : ( m )

– N(D) dD is the number of droplets with diameter between D and d+dD.

dDDDD

NM D3max

min6

dDDD

DNM

D30

min62

Radar Observation in Clear Sky 30

2) Geometry and dynamic structure

• Observed plume target :– Relatively narrow target with small amplitude beam sca

ns can’t determine the Doppler velocity field unambigu

ously.– Case a plume undetectable directly by radar

• Observations from artificial tracers released in and around the plume source and use as lagrangian tracers

– Convenient for thermal plume : Tracers permit to visualize the entire volume occupied by the convective air

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The motions of chaff bundles released from the heat source

• Sectoral scans observation by conventional radar – An accurate determination of plume boundary

– A measurement of the maximum height of the plume

– Measurements of the turbulence inside the plume

• Calculation of the statistical moments of chaff distribution – An objective determination of the plume axis

– An estimate of the mean air velocity along the plume

– An estimate of the axes of the dispersion ellipsoids

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EAST0

500

mZ

AXISPLUME

50010001500WEST

3

1

2

4

5

bundlechaffaofplanesVertical

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Horizontal planes of a chaff bundle

NORTH

0

500

EAST50010001500

WEST

XY

5X

Y

4X

Y

3X

Y2AXISPLUME

XY

1

Radar Observation in Clear Sky 34

• When the plume is detectable by radar (without chaff )– Outlines and turbulence inside plume can be

directly obtained

• An approximatel determination of the velocities in the plume– Can be done only from the observation of the

motions of particularities in the plume reflectivity distribution (relative maximum or minimum)

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• For dry or wet plume, interactions with environmental clouds : – Usefulness of radar :

• Provide data of environmental mean field

• Permit an accurate observation of the plume

– Decrease the efficiency of observations• When plume echoes are diluted among natural

clouds or precipitation echoes

• In order to create observable modifications in the reflectivity field : Discontinuous chaff release

Radar Observation in Clear Sky 36

• Usefulness : meteorological Doppler radar as a tool for air pollution modelling

• Several type of tracer :– Hydrometeors , turbulent fluctuations of the air

refractive index, artificial tracers (chaff)

• Using tracer, measure the quantitative and qualitative results of meteorological environment :– Tridimensional fields of reflectivity, mean air motions

and turbulence, precipitating particle size distributions and cloud or precipitation water content

Summary

Radar Observation in Clear Sky 37

• When artificial clouds or interactions between plumes and natural clouds are concerned,– Statistically significant data on reflectivity (on

microphysical parameters such as liquid water content, mean volume diameter, etc.) in natural and totally or partly artificial clouds and to monitor different aspects of the local meteorological field perturbations.

• In dry plume, chaff techniques :– Accurate determination of lateral limits and maximum height

of the plume at almost all atmospheric conditions– Measure air velocity and some other parameters concerning a

irflow conditions

End