1. Remote Sensing  Remote Sensing: The quantitative and qualitative observation and interpretation of the interaction of some form of wave energy (electromagnetic or sound) with the atmosphere.  Sensors are categorized as: l Active: Sensor emits wave energy and detects a return of the emitted energy after interaction with atmospheric constituents. l Passive: Sensor only detects energy emitted, scattered, or reflected by atmospheric constituents.

2.  Remote Sensors are needed to provide space and time coverage of the atmosphere to the degree not capable by traditional (in situ) sensors.  Interactions of the signal with the atmosphere: l Spreading losses l Scattering losses l Reflection losses l Absorption losses

3.  Degree of loss depends to a great extent on the wavelength of the signal. l Acoustic waves: Interact strongly with gaseous portion of the atmosphere. Weaker interaction with aerosols (unless in high concentration to significantly affect density). Aerosols are suspensions of minute particles in the atmosphere. Density differences affect acoustic waves greatly. Have short ranges.

4.  Electromagnetic waves: l Weaker interaction (than acoustic) with gaseous portion in visible and radio wavelengths. l Absorption of certain bands in visible and IR spectrum is important. l Interact more strongly with aerosols (closer the size of the wavelength to the size of the aerosol - the stronger the interaction).

5. Weather Radar l Emits electromagnetic waves of a particular wavelength. Wavelength used is determined by the desired target. l The shorter the wavelength, the greater the attenuation that occurs and the shorter the effective range.

6.  Receives returning reflected energy Must be amplified l Orientation of antenna determines azimuth angle to target. l Time of travel for emitted energy to return determines range from radar location. l Intensity of returned energy determines strength (size or concentration) of target. Typical usable range is about 200 km for detecting precipitation in the troposphere. Radar signal travels in nearly a straight line. Earth curves away from the signal. Signal path affected by refractive properties of atmosphere. –May produce anomalous propagation.

7.  Consider a 3 GHz, 10-cm radar wave with a pulse duration of 2  s (0.000002 seconds).  This microwave oscillates at 3,000,000,000 cycles per second (3,000 MHz).  The radar listens for returning echoes for 0.000999 seconds before transmitting another pulse.

8.  The 2  m pulse will contain 6000 cycles and will be 300 meters long (along its path of radiation).  Consider that the beam is focused to 1 degree diameter.

9.  At a range of 50 km, the beam will spread to a diameter of 873 meters since: where: S = arc length, or width of spread, R = radius, or distance along beam,  = angle of spread in radians  So:  The pulse will reflect off a volume of raindrops (or other targets) that is about 873 meters in diameter with a depth of 150 meters (half of the 300 meter pulse length which is folded by reflection).

10.  In the case of precipitation, the radar illuminates a large number of individual targets. The average returned power from this volume of raindrops is given by: where:  The summation is over the volume from which power is returned simultaneously.

11.  For spherical targets uniformly distributed over the volume illuminated with an incident wavelength,, large compared to the radius, a, of the target, the radar back-scattering cross section,  b, of the volume is given byRayleigh’s law to be:  Returned power is strongly dependent on the radar back-scattering cross section which is dependent on the 6th power of the size of the raindrops.  The returned power, intensity, is expressed in decibels of reflectivity.

12. l 20 - 30 dB: Weak, steady rain l 30 - 40 dB: Showers l 50 dB or more: Severe storms.  The intensity for each range interval moving outward from the radar (perhaps 1 kilometer in length) is stored in a bin. After a number of successive pulse transmissions (e.g., 32), the bin values are averaged, resulting in this for our example.

13.  Thus, each bin receives a single intensity value for all the precipitation falling within that volume, regardless whether some reflects high intensity and some reflects low intensity.

14. Doppler Radar  Named after Christian Doppler, Austrian physicist who discovered the principle.  General Types: l Continuous: Frequency shift between emitted signal and returning signal is a measure of the target radial velocity. l Pulsed: Measures phase shift between emitted pulse and returning signal Phase shift is related to the radial velocity of the target. Measures return time of pulse energy which gives range. Sorts velocities in range bins, averages, and displays velocities according to color/shades.

15.  If the emitted frequency is “ ” and the wavelength is “ ” and “c” is the speed of light, then:

16.  When a target is in motion with radial velocity (towards or away from the radar) of, the reflected signal has a frequency of ’ given by: where + means target is moving towards the radar, and - is target moving away.  The difference in frequency, the Doppler shift, is:

17.  If is expressed in knots,  in cm, and D in Hz, then:  The 144 o shift relates to about 10 m/s.

19.  Typically, clutter (from buildings, trees, etc. which would indicate zero velocity) is picked up in the range bins and must be removed before averaging occurs.

20.  Nexrad (WSR-88D)(~164) l Pulsed Doppler (~1300 pulses emitted each second) l During course of 1 hour, pulses are transmitted for a total of about 7 seconds and radar listens for returning signal about 59 minutes and 53 seconds. l 10 cm (S-Band) l Can operate in “Clear Air” or “Precipitation” mode Precipitation Mode: When precipitation is occurring. Rotation rate is faster. Moves through increasing elevation angles from 0.5 o up to 19.5 o to obtain a volume scan (volume coverage pattern) for precipitation. Clear Air Mode: Rotation rate slower. Samples volume of air longer. Can detect airborne dust and particulate matter. Better at detecting snow. Volume coverage pattern angles only go up to 4.5 o.

21. Terminal Doppler Weather Radar  FAA  5 cm (C-Band)  41 Major airports (may rise to 47)  Primarily to detect microbursts and wind shear

22. Airport Surveillance Radars  ASR-9 Weather system Processor  ASR-11 l Digital Survellance Radar with monopulse second surveillance radar l Has weather system processor

23.  Installed at airports which do not have TDWR l Approximately 34 operational and 3 support systems l Weather processor provides warnings of wind shear and microbursts near runways l Used to predict arrival of gust fronts and storm track motion. l Operated in KU band (14 - 16 GHz)

24. Wind Profiler  Vertical pointing Doppler radar  Types: UHF (300-3000MHz) 915 MHz (33 cm), 404.37 MHz (74 cm), VHF (30 - 300 MHz), 50 MHz (600 cm)  Fluctuations in atmospheric density cause reflection of signal  Three beams are used to determine components of wind along the beams.  U, V, W components are determined from the beam components.

25.  Height of wind is determined from elapsed time for returning signal.

26.  Angle of beam is achieved by delaying pulse to individual antennas across the profiler.  Typically operates in 2 modes: low mode - 1 2/3 microsecond pulse for low levels, 6 2/3 microsecond for higher altitudes.

28. Palestine, TX, profiler data

29. Laser Radar (Lidar/Ladar)  Light Detection And Ranging: Operate in the ultraviolet, visible or infrared portion of the spectrum.  Operation may be Continuous (always on) or Pulsed (like a strobe light).  Returned echo detected either: l Incoherently: Without reference to the emitted signal Detects backscattered radiation Range can be determined by –Focusing the energy at a particular range –Measuring the angle the reflected light makes with a baseline. E.g., for determining cloud height. Concentration level of target is determined from the intensity of the returned signal.

30. l Coherently: With reference to the emitted signal. Detects backscattered radiation Range is determined by measuring the time of return of reflected signal. Speed of target is determined by frequency shift between transmitted and returned signal. Concentration level of target is determined by the intensity of the returning signal.

31. l Medium for different types of lasers include: gases (Helium Neon, Zenon Fluoride); solid state diodes, dyes and crystals (ND:YAG = Neodymium: Yttrium Aluminum Garnet) l Photomultiplier tubes detect the backscattered radiation and convert the quanta of light into electric currents and then into photocounts, digital values that can be stored on a computer. Currents generated are on the order of picoamps (1 pA = 10 -12 A). l A 60W light bulb draws 0.5 Amps.

32. l Photocounts received at fixed time intervals after a lidar pulse. Fixed time intervals represent heights above the lidar unit when aimed vertically.

35.  Used in detection of Clear Air Turbulence, aerosols, concentrations of gases (water vapor, carbon dioxide, methane, chloroflurocarbons, ozone), Clouds, temperature profiles, winds, etc.

36. Differential Absorption Lidar (DIAL)  Based on the fact that absorption of wavelengths of light by constituents in the atmosphere is different for different wavelengths.  Measures intensity of returned lidar signal at the “on line” frequency as compared to the “off line” frequency.  “On line” is a frequency that a particular gas in the atmosphere will absorb the the lidar energy. “Off line” is where it does not absorb the energy.  Gives a measure of the presence/concentration of a particular gas which absorbs at the on line frequency.

39.  LITE Experiment: Lidar Technology Experiment. l Flew on shuttle mission STS-64 in Sep. 1994. l Demonstrated use of perating a lidar from space to detect clouds and aerosols.  GALE: Giant Aperture Lidar Experiment l A layer of alkali metals exist from 80-120 km in atmosphere from meteorites. Sodium atoms when radiated with 589nm light fluoresce. Lidar’s receivers measures this fluorescence. l By shifting the wavelength of transmitted light a tiny amount, will create a Doppler shift due to the motion of the sodium atoms which will then give a measure of wind.

40.  GALE results: l Notice how the wind changes from 40m/s westward (-40) near 90 km to 80 m/s eastward near 97 km at 8:30 UTC.

41.  Pressure profiles can be determined based on the amount of “pressure broadening” of the actual absorption vs. wavelength curve as compared to a standard absorption vs. wavelength curve for the particular gas under study.  Temperature profiles can be determined in two ways, one by the sodium resonance-fluorescence scattering as in the GALE system, secondly by Rayleigh-scattered light. Rayleigh scattered light is responsible for the blue sky observed on clear days.

42.  Lidar photocounts in range bins can be related to atmospheric density by: where, b is a constant which depends on the individual lidar system, the type of scattering (Rayleigh) and the transmission of the atmosphere, z is the height, N(z) is the number of photocounts at each height.  Once the density profile is known, temperature is determined assuming hydrostatic equilibrium.

43.  Clear Air turbulence is detected by sudden changes in the velocity of aerosols as measured by the phase shift between the transmitted and returned laser energy.  Pollution: l Lidar can track pollutants by detecting the backscattered energy from the pollutant aerosol l Utilizes a specific wavelength for different types and sizes of pollutants.

45. Sodar  Detects sound energy which has been scattered by density differences in the atmosphere.  Ranges: ~300 m  Studies near-surface boundary layer moisture, temperature, wind variations.

46.  Cut-away of a Sodar unit.  Some systems have a series of sound units pointed upward.

47. ..

48. Radio Acoustic Sounding System (RASS)  Combines Wind Profiler and Sodar  Sodar transmits soundwave vertically.  Wave front compressions and rarefactions of air provides a target the wind profiler can detect.  The rate of movement of a sound wave is dependent on the virtual temperature of the air through which the wave moves.

49. l T v =Virtual Temperature l M=Molecular weight of air molecules l Y=ratio of specific heat l R=Universal gas constant l V ac =acoustic speed of sound wave front  Temperature profile determined from data taken during first 10 minutes of each hour  Wind profiles are determined from the next 50 minutes of data

52.  End