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Radar Observation in Clear Sky

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  1. Radar Observation in Clear Sky 이동인 부경대학교 환경대기과학과

  2. Air Pollutant Dispersion Using a Doppler Radar in Clear Sky Nov 23, 2000 Prof. Leedi

  3. Remote Sensing Techniques • Lidar: tridimensional distribution of some air pollution concentration, air motion • Sodar: easily the dynamic and thermodynamic structures of the low atmospheric layers • Wind profilers: height of stratified layers, convection in clear air Radar Observation in Clear Sky

  4. Isolated Pollution Sources • Lidar, Sodar techniques : • Sodar : • Restricted to observing along fixed directions around the vertical • Lidar : • More disadvantages --- measurement problems for plumes are more acute 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 > • 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 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 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. For a well developed turbulence • 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 Radar Observation in Clear Sky

  15. Artificial tracers • 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 Radar Observation in Clear Sky

  16. Radar 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. Basic Doppler radar data • 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 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. 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 perpendicular horizontal direction x and y • the stretching deformation : • the shearing deformation : Range Marks Velocity Spectra Radar Observation in Clear Sky

  23. Range Marks Velocity Spectra Downward 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 ; 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) Radar Observation in Clear Sky

  29. Z= 0.068 M1.94 Z= 3.6  107 D03.16 where : D0 is defined by : Z : ( mm6m3), M : ( gm3 ), and D0 : ( m ) N(D) dD is the number of droplets with diameter between D and d+dD. Radar Observation in Clear Sky

  30. 2) Geometry and dynamic structure Observed plume target : Relatively narrow target with small amplitude beam scans can’t determinethe Doppler velocity field unambiguously. 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 Radar Observation in Clear Sky

  31. 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 Radar Observation in Clear Sky

  32. Radar Observation in Clear Sky

  33. Horizontal planes of a chaff bundle 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) Radar Observation in Clear Sky

  35. 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 airflow conditions End Radar Observation in Clear Sky