What is remote sensing?

1. What is remote sensing? • Remote sensing is defined as the way to infer about the objects from distance i.e size, consentration, content etc.

2. The interaction of electromagnatic waves with the objects modifies the incident wave; • The resulting signature depends on the composition and structure of the medium; • The principle of measurements of the Atmospheric parameters i.e. temperature and humidity is the interpretation of measured radiation in the specific spectral intervals which are sensitive to the constituent;

3. In the infrared and microwave regions of the spectrum atmospheric constituents absorbs the radiation; then emitAccording to Kirckhoff’s law; • Since the emitted radiance is a function of distribution of objects, measurements of radiance gives information about them.

4. Absorption and Transmission of Monochromatic (spectral) Radiation: • The amount of energy, radiance, crossing a differencial • area dA in a time integral dt and wavenumber v is given as: L= dE / Cos dA dt d d its unit is W/m2 sr cm-1

5. d d • is a solid angle and is defined as:

6. Absorption: Attenuated exit beam Absorbing medium X L dx Incident beam Figure-.. Absorption through a Medium

7. When a monochromatic radition in Figure- whose radiance is L peneterates into absorbing medium (non scatering) the fractional decrease is: d L/ L= -kdx Where is the density of medium, k is the spectral absoption Coefficient.

8. When integrating equation ... between 0 and x, becomes: Where L(0) is the radiance entering the medium at x=0, is called optical depth. And is called transmittance.

9. Black Body Radiation: Blackbody radiation field is characterised as: • Isotropic and nonpolarized; • Independent of shape of cavity; • Depends on only temperature (T). • In a perfect blackbody emisivity is equal to unity due to thermodynamic equilibirium.

10. Blackbody radiation E Greybody radiation  The ratio of emitted radiance by an object to the radiance emitted by a blackbody at the same temperature is called emisivity (). = 1 for blackbody. < 1 for greybody.

11. Planck Law: To explain the spectral distribution of radiance emitted by solid bodies, Planck found that the radiance per unit frequency emitted by a blackbody at temperature (T) is given as: Where h is Planck const, k is Boltzmann const.

12. Stefan Boltzmann Law: It gives the total radiation of a cavity (blackbody) not spectral distribution of radiation. When Planck function (...) is integrated 0(zero) to infinity (), S-B is given as: E(Exitance)=E4 Where  is stefan Boltzmann constant.

13. What wavelengthg gives maximum energy E  Wien Law: • When Planck equation is differentiated w.r.t wavenumber () or wavelength () and equated to 0(zero), one can find maxfor a given temperature (T) called Wien Law. max =0.2897/T (cm)

14. The radiative temperature of the sun surface is about 5780 K. After applying Wien law, maximum Planck radiance is obtained at the wavelength (max) of 0.50 m which is the center of the visible region of te spectrum. • On the other hand eath’s atmospheric temperature is about 255 K. Maximun emitted energy takes place around 11 m which is infrared region (Figure...).

15. Brightness Temperature (or equivalent blackbody temperature) is the temperature estimated by inverting Planck function.

16. Gaseous Absorption: • In the atmosphere, the absorption of radiation is mostly due to gases. • Major interest for the transfer of radiant energy is the value of absorption coefficient (spectroscopy). • Total energy of a molecule consists of rotation, vibration, electronic, and translation. E=Erot+Evib+Eelc+Etrans

17. Absorption or emmision occurs when molecule changes from energy level E1to E2 with a frequency f = (E1- E2) / h. Where h is a planck constant. Significance in the spectrum: • rotational energy in the microwave and far IR regions; • vibrational energy in the near IR region; • electronic as well as vibrational and rotational energy in the visible and UV regions.

18. In order to posses rotational energy (interact with the elecrtromagnatic field), molecules shall have dipole moment. • Atmospheric such gases important for satellite meteorology as CO, N2O, H2O and O3have dipole moments while N2, O2 , CO2 and CH4 do not. • However, as CO2, and CH4 vibrate electrical dipole moment is produced and so rotational interaction take place. Therefore, vibration-rotation interaction takes place with the incident wave.

19. No static and dynamic electric dipole So no interection with the incident radiation So no absoption (Figure-...). O-C-O =7.46 m (Wavelength of vibration) Symetric Stretching Dipole occurs due to bending Vibration (vibration-rotation) so Absoption takes place (Figure-...). =14.98 m O-C-O Bending O-C O =4.26 m Asymetric Stretching Figure-... Vibration modes of CO2

20. Temperature Sounding and measurements of some trace gases • in the atmosphere are based on vibrational transition. For example; • The 15 m and 4.3m of CO2bands are used for temperature soundings. • The 6.3 m H2O band is used for water vopour. • The 9.6 m O3 band is used for the total ozone measurements.

21. Figure-... IR transmittance of several gases in the atmosphere (after Kidder,S.Q)

22. Scattering: Radiation scattered from a particle depends on: • Size; • Shape; • Index of refraction; • Wavelength of radiation; • View geometry.

23. Fundemental two types of scattering are Mie and Rayleight. For Rayleight scattering,  >>  For Mie scattering, ≈ Where  is particle size. • Scattering properties of such aerosols as smoke, dust, haze in the visible part of the spectrum and of cloud droplets in the IR region can be explanined by Mie scattering, while of air molecules in the visible part can be explained by Rayleight Scattering (Figure-...).

24. Figure-... Scattering Properites of atmospferic Contitiuens (after Kidder, S.Q).

25. Scattering phase function determines the direction in which the radiation is scattered. As the size parameter (x=2πr/) inreases, more scattering takes place in the forward direction (Figure-...). Figure-...Scattering phase function of water droplets (after Kidder, S.Q).

26. Radiative Transfer Equation (RTE): • Consider a volume of gas (Figure-...) where absorption and emission takes place but no scattering, energy transfer equation can be written as: Where first therm on the right hand side is abrorption within dx and second term is emission within dx.

27. Applying Kirchhof law and some manpulation and integration finally we get: (...) • The first term on the right hand side is the radiance at the Boundary multiplied by the transmitance from the boundary to a. Second term is the contribution due to emission from the medium in the direction of incident wave.

28. where and • Similar equation can be computed for the emitted radiance in the atmospfere with  zenith angle as: is called weighting function.

29. Figure-... NOAA HIRS weighting functions.

30. Radiance at the TOA Surface contribution Atm layer cont. z TOA , B(T) dz τ L(0)=εs,νB(T) surface So equation (...) can be written in a more compact form as:

31. Brain Storming! Previous RTE (Figure-...) is driven in cartesian coordinate system drive it in presure coordinate system in p by using hydrostatic Equation. Where q is mixing ratio and g is gravity.

32. Processes of Atmospheric Radiation: transmitted absorbed, emitted and scattered by aerosols and molecules reflected absorbed &scattered emitted transmitted transmitted reflected emitted reflected emitted Land transmitted absorbed absorbed Ocean Figure-... Process of Atmospheric Radiation

33. Sensors: • Sensors are the devices for detecting the photons. The critical part of the sensors is the detectors which works based on photoelectric effect. That is, There will be an emmision of negative particles (electrons) when negatively charged plate is subject to a beam of photons. • The electrons then can be made to follow,collected and counted as signals. • The magnitude of electric current (number of photoelectrons per unit volume is directly proportional to light intensity.

34. Thus, the change of electric current can be used to measure the change in the photons (number, intensity) which strikes the plate during the given time interval. Photon beam C Negatively Charged Plate R C Figure-... Shemetic view of a detector.

35. Remote Sensor Types: Microwave radiometer Magnatic ensor Gravimeter Fourier spectrum non-imaging non-scaning passive Monochrom IR imaging Camera TV camera imaging image plane scanning Solid scanner Sensor types scanning Optical mechanical scan. object plane scanning Microwave radiometer non-scaning non-imaging Microwave radiometer Microwave altimeter aktive image plane scanning Passive phased array radar scanning imaging Real aperture radar object plane scanning Synthetic aperture radar

36. Passive sensors: Radiation comes from the external sources. • Active sensors: Radiation is generated within the sensor. • Non-imaging: Measured radiation received from all points in the sensed target and integrated. • Imaging: Radiaiton is received from a specific points (pixels) in the target end result is an image(picture). • Sensors which instantaneously measure radiation coming from entire scene called framing systems e.g eye, camera; if the scene is sensed point by point along successive lines over finite time called scanning systems. • The size of scene which is determined by the aperture and optics called field of view (FOV).

37. Radiometer is the general term for any instrument which quantitatively measures the EM radiation in some interval of EM spectrum. • When the radiation is light from the narrow visible band, the term photometer is used. • If the sensor includes such components as prism or difraction grating which can break incoming radiation into discrete wavelengths and despers them to detectors called spectrometer. • Spectroradiometer implies that dispersed radiation is in bands (Δλ) rather than discrete wavelenght (); most space sensors are of this type.

38. Retrieval Method: Retrieval methods can be classified in three general categories: • Physical; • Statistical and; • Hybrid.

39. To predict the obserable parameters from arbitrary model parameters called forward problem; on the other hand, invers problem is to infer the model from observed parameters, • Invers problems are “ill posed”; that is, the solution is not unique.

40. Example: Estimation of L(z) from known temperature profıle, T, is a forward problem, while estimation of T profile from satellite measurement of L(z) is an invers problem.

41. Temperature Sounding Retrieval: Physical Retrieval: • It is based on itiration of RTE by using first guess NWP profile until the desired solution is optained. Statistical Retrievals: • The siplest is to make regression between radiosonde sounding called training data and measured radiance.

42. Hybrid Retrieval: RTE can be written in the dicrete formas: After putting surface contribution into Summation and replacing Li by Ri. in maxrix notation Where W is a matrix containing discrete weighting function.

43. Assuming linearity and introducing basis function by matrix inversion; b=A-1R (...) • Which is an exact solution of RTE. However not a satisfactory solution because it is ill conditioned; smaller error in R results in larger errors in B. Trying least square fit of Σ(R-Σab)2 gives the solution as: b=(AT.A)-1.AT.R (...)

44. Which is better solution than equation (...), yet it can be improved by applying other methods (e.g. minimum variance)

45. Wind Retrieval: • Atmospheric wind retrieval from the images is based on cross correlation method of successive three images. center of search area cc pick Wind wector VH+1/2 VH-1/2 cc pick 32x32 pixel H 96x96 96x96 Image 2 (target image) Image 1 (H-1/2) Image 3 (H+1/2) • First, cross-correlation matrix produced between target image and Last image (H+1/2) to find the correlation peak .

46. The distance between the center of search and the cc peak is the cloud tracking wind vector. • In order to eleminate the spurious (false) peak cc is estimated between the target image and first image (H-1/2). If cc peaks are not symetric, it is rejected. If not, wind speed is claculated as: V=1/2(VH-1/2 +VH+1/2 ) • Wind direction is the vectorial sum of two winds: VH-1/2 V VH+1/2

47. Height assignment: After finding the most contributing cloud cluster to cc peak, its top temperature is used to assign height to V by means of NWP forecast temperature profile. CO2 slicing method can also be used for height assignment gives better solution. • Sea surface wind speed and direction are estimated by means of scatterometer which works like a radar. • Tracking winds can also be retrieved from atmospheric soundings data.

48. CO2 Slicing: • Radiance measurements around 15m CO2 band ( e.g HIRS sounder) allows to detect clouds at various level. In the center of band, upper level clouds and at the wings lower level clouds are detected (Figure …). • Cloud amont and level (top pressure) can be estimated by means of RTE. • Effectively detects thin cirrus clouds which are missed by IR window and VIS channels.

49. Radiance from the partly cloudy regions are given by L=Lcd+(1-)Lcl where  is a fractional cloud cover, Lcd is radiance from clouds and Lcl is radiance from clear air. • The cloud radiance is given by Lcd=Lbcd+(1-)Lcl where Lbcd is radiance from completely opaque cloud. After expresing Lcl and Lbcd in the form of RTE and some algebric manipulation, it becomes: (…)

50. Where  is called effective cloud amount. Lcl estimated from known temp and moisture profiles and L is a satellite measurement (e.g HIRS). Right hand side is calculated from known temp profile and atmospheric transmitance profile. • Representing left hand side with, L, and right hand side, B, and taking the ratio of two spectral channels which see the same FOV (e.g 14.2m/14.0m or 14.2m/13.3m), it becomes: (…)