I would also like to thank Dr. Arnold Prause for teaching me most of what I know about the operation of the old LIDAR. 144 7.6 Comparison of LIDAR/SAGE II extinction profiles 149 7.7 Validation of the LIDAR temperature profiles 154 7.7.1 SAWS radiosonde data.
7.18 (a) Comparison of L1DAR monthly mean temperature profile with ECMWF monthly mean temperature profile for the month of November. L1DAR profile averaged over 11 nights in September (b) Comparison of L1DAR temperature profile with C1RA-86 temperature profile for 30 October 1999, no L1DAR data for October 2000 due to cloudy and rainy weather.
List of Tables
List of Acronyms and Symbols
Backscattering coefficient of the volume of the atmosphere. Backscattering coefficient of the volume coming from aerosols. Backscattering coefficient of the volume coming from air molecules. Extinction cross section for a particle. Diameter of the scattering volume Apparent frequency of the wave Optical frequency of the return signal Wavelength.
LIDAR Review
Historical Background
Radiosondes and dustsondes are capable of producing profiles with vertical resolutions of up to 100 m. A single laser pulse can produce an entire profile up to an altitude of (typically) 100 km.
Deployment of LIDARs in Space
The minimum footprint is determined by the diffraction limit of the transmitting telescope, which can be as small as a few mrad. An importance or consideration of each of the two types of application (backscatter and wind Doppler) is the choice of laser beam.
Franco-South African Cooperation and the Durban LIDAR
These studies can also help identify dynamic tracers in the upper troposphere and lower stratosphere. Research on the evolution of ozone in the troposphere was undertaken by both institutions during the 1992 SAFARI campaign.
Scattering Theory
Introduction
Rayleigh Scattering
These three values of the parameter Pr.G show that the fraction of light backscattered from a height of 100 km decreases by a factor of 10 for a height difference of 1 km from 6.5 to 5.5 km. This is a reasonable estimate of the fraction of the intensity of light reflected from a pure molecular atmosphere in the direction of the incident beam.
Mie Scattering
The factor (G~:R) decreases rapidly with increasing altitude; using standard atmospheric data for 15°N obtained from Kent et al. For larger ratios between circumference and wavelength up to approx. 10, constructive and destructive interferences of the partial waves due to Mie scattering will again add up spatially, so that Ps exhibits an oscillatory behavior that slowly damps out as higher modes become involved at higher values of ~ .
Particulate scatter
Raman scattering
The scattering cross section for Raman scattering is two to three orders of magnitude smaller than for Rayleigh scattering and is of the order of 10-34 m2. Raman scattering is important in that the scattered light is frequency shifted by an amount characteristic of the scattering molecule.
Resonance and Fluorescent Scattering
The planetary boundary layer is the layer of the atmosphere where most aerosols are concentrated and is located about 6 km above the earth.
Multiple Scattering
The LIDAR equation
3 is defined as the fractional amount of incident energy that is distributed per th radian in the backward direction per unit atmospheric path length. For aerosols and substances in the lower atmosphere the N; values vary widely in the range from about 108 to 1011 m-3.
The Scattering Ratio
Therefore, we need some other techniques to be able to determine the absolute density of the aerosol layer.
Signal-to-noise Ratio
This includes the fact that the time interval in which the receiver is on (ie open) for a given resolution length is l. The most efficient way to reduce Bb is to reduce nandb because these parameters do not appear in equation (2.19), which assumes that the transmitter and receiver fields are properly aligned.
Use of filters to detect higher altitude aerosols
The new LIDAR uses a wavelength of 532 nm, which is achieved by frequency doubling the fundamental wavelength of the Nd:YAG laser. It should be noted that a smaller band pass removes most of the background noise from the LIDAR return. A gain in the signal can be expected using an ideal filter which has a bandpass wide enough to transmit the peak emission bank of the emitter source with nearly 100% transmittance and yet narrow enough to eliminate most of the night sky emission.
Spatial and temporal coherence of a LI- DAR system
High-resolution LIDARs are coherent in the sense that the return spectral width is a very small fraction of the optical frequency. The spectral resolution in a Doppler LIDAR, for example, is of the order of (2.23) where f\, is the optical frequency of the return signal. No LIDAR can be coherent in the sense that the optical phase of the return signal is in a fixed relationship with the phase of the transmitter.
Spatial Resolution of a Pulsed LIDAR
This is because each scatterer of the scattered source in the extended scattering volume will scatter independently and the phase of the scattered wave will be determined by the exact range on a wavelength scale. Continuously distributed scatterers in a range Za (z; > Zl) will be scattered by the leading edge of the pulse, which starts at a time _!.., and will. Note that equation (2.29) also holds for approximately any pulse shape provided T is the FWHM of the pulse.
Description of the Durban LIDAR systems
The old Durban LIDAR
- LIDAR Specifications
- The receiver
- The dynamic range of the receiver
- Photomultiplier
Each laser shot is detected by a photodiode located at the back of the laser cavity. The light is finally collimated and falls on the photocathode of a photomultiplier tube (PMT). The high voltage is applied to the photomultiplier only when it is to receive the reflected signal from the upper atmosphere. i) Elimination of photomultiplier fatigue due to the passage of intense light. ii) Option to turn on the receiver only after the laser has been triggered.
The new Durban LIDAR
Location of the LIDAR systems
- The transmitter
- The emitter
- The receivers
- The Detector and Data acquisition system
Diameter of beam after expansion R::: 80 mm Table 3.2: Characteristics of the laser for the new LIDAR. Let e be the divergence of the laser beam (diameter d) before entering the Galilean telescope and ( be the divergence of the beam (diameter D) at the exit of the telescope. PMT (Hamamatsu R 1477) of the new LIDAR system is a photodetector with high sensitivity.
Sources of error in the LIDAR measure- ments
- Instrument errors
- Errors induced on the temperature profiles due to aerosols
Channel A allows the sound of the upper atmosphere (from 20 km to 60 km) and channel B the lower atmosphere (from 12 km to 40 km). In the case of Rayleigh-Mie LIDAR, for large distances from the width of the. The scattering volume is also affected by the quality of the optics (lenses are free of aberrations, the aluminum coating on the receiver mirrors is free of distortions, etc.) used in the LIDAR experiment.
Alignment Procedure
Introduction
Alignment of the old LIDAR
- Alignment of the old LIDAR receiver
- Alignment of the mirror and photomultiplier
Nevertheless, the alignment of the mirror is checked in advance to ensure that it was not tilted out of the vertical position. The mirror is then adjusted using the adjustment bolts at the bottom of the bracket. The author assumes that the alignment of the photomultiplier has not changed and is in the focus of the mirror.
Alignment of the laser
Assuming there are no imperfections in the mirror and that the edges are horizontal to the bottom of the mirror, we can safely conclude that the mirror is now pointing vertically upwards.
45 inclined
Searchlight
PVC laser guide
He-Ne laser
- Suggested method to align the prism using the stepper motor
- Alignment of the new LIDAR
- Alignment of the receivers
- Optirnisation of the signal
- Operation of the old LIDAR
- Operation of the new LIDAR
- Summary
The latter is adjusted so that the laser beam falls on the center of the diode. The laser beam after emerging from the optical fiber falls on the parabolic mirror and is reflected vertically upwards. An image of the diaphragm of the He-Ne laser is formed on the plane mirror.
Results and discussion of the old LIDAR observations
Introduction
The old LIDAR results
The moonlight will saturate the photo-omultiplier so that LIDAR returns from low altitudes are not detected. The high water vapor concentration in clouds results in beam weakening from higher altitudes and therefore the signals will not be detected. ii) Between 150 and 230 laser shots were integrated to obtain each LIDAR profile. This technique avoids saturation of the photomultiplier due to strong returns from aerosols at low altitude.
Low Altitude Aerosols
- Raw Data
The persistence of these layers suggests that they are not affected by changes in atmospheric circulation. The atmospheric load at the beginning of sugarcane burning in June is different in July and October. Therefore, the atmospheric load with aerosols would be higher in September than in June.
High Altitude Aerosols
- Raw Data
This is evident due to the continuous loading of the atmosphere with sugarcane burning, which started in early June. In order to understand the behavior of the aerosol layer, five other profiles were taken in August. There are two possible reasons for this: i) The contribution of sugarcane combustion to the atmospheric load has reached its peak in October, rather as originally expected in July and August.
Comparison of Durban old lidar results with SAGE 11
Unfortunately, there was a 10-day difference in the time when SAGE II data was acquired, except for the September data, where the date matched the lidar data exactly. These differences between our calculated d extinction coefficient profiles and the SAGEII extinction coefficient profiles are due to the different dates and locations at which the SAGE II data were extracted. There is an 8 day lag for the SAGE II data compared to the LIDAR data a.
Conclusion
Inversion method used to retrieve aerosol, relative
- Introduction
- Klett Inversion Method
- Determination of the relative density
- Determination of temperature profiles
- Summary
This clearly shows that the temperature of the atmosphere initially decreases with height up to the tropopause (~ 17 km) and then the temperature increases with height in the stratosphere followed by the stratopause (~ 55 km) and then the temperature decreases with height in the mesosphere. The pressure at the top of this layer is adapted to the pressure from the CIRA - 1986 model Prm(Zn+~) for the corresponding month and latitude. The attenuation of the LIDAR return due to aerosols, clouds, haze and fog is difficult to estimate.
Results and validation of the new LIDAR measurements
- Introduction
- Validation of the aerosol measurements by the new LIDAR
- Stratospheric aerosols
- Aerosol measurements by the SAGE 11 experiment
- Inversion Algorithm used by SAGE 11
The vertical transport of smoke continues to be driven by the structuring of the stability of the atmosphere and the horizon transport by. Before proceeding with the validation of the new LIDAR measurements of aerosols, we will give a brief introduction to stratospheric aerosols. Each extinction profile describes an average of the extinction coefficient within spherical atmospheric shells cut by the limb path between SAGEIl and the Sun.
TRRNSMISSION
Calculation of LIDAR extinction coeffi- cients
As shown in Table (7.2), the values of the aerosol backscattering phase function are large after major volcanic eruptions. Chazet et al. (1995) showed that this assumption is representative of the elevation range where the backscattering coefficient is greatest and the obtained Wa values agree with other in situ and LIDAR measurements. Uncertainties in determining the backscattering coefficient of the total volume (3(z)) arise from the following causes:
Comparison of L1DAR/SAGE 11 Extinc- tion profiles
Table (7.3) below shows a comparison of the time of measurement and location of SAGE II with that of the LIDAR. The observed deviations are due to:. i) the measurement technique of SAGE II which horizontally scans the atmosphere between the earth and the sun. ii) large variation of aerosols in the tetroposphere of Durban as the latter is under the influence of biomass burning (including sugar cane burning). and industrial pollution - most of the chemical industries are located south of Durban. iii) the assumption that the backscattering phase function due to aerosol (wa) is constant at and above the reference atmosphere (zref) which is not always true. This comparison suggests that the values obtained for the two experiments are representative of the peak ion profile for the lati tude300S. This is also a good indication of the quality of the Durban LIDAR data.
Validation of the LIDAR temperature pro- files
- SAWS radiosonde data
- The CIRA-1986 climatological model
- The ECMWF data
The comparisons with the mean monthly SAWS radiosonde temperatures are made for the year 1999. The data consists of the station number (68588 for Durban), the year, month and day, the time, the pressure (in hPa), the temperature (in millidegrees Celsius), the relative humidity (expressed as a percentage), dew point temperature (in millidegrees Celsius), altitude (in meters), wind direction (in degrees) and wind speed (in m/s). For our purpose, we worked with the temperature (converted in K) and the altitude (converted to km) from the early morning (1:00 am) data as it corresponds closely to the time of the LIDAR run.
Comparison of LIDAR temperature pro- file with SAWS radiosonde temperature
Caution should be taken when interpreting the standard deviation above 50 km for the mean July LIDAR temperature profile. Finally, we can say that for the lower stratosphere (20 :S z :S . 28 km), the agreement between the mean monthly LIDAR temperature and the mean monthly SAWS temperature profiles is quite good. If the lower stratosphere is laden with aerosol, the LIDAR temperature will be overestimated and be slightly higher than the SAWS temperature.
Comparison of average monthly LIDAR temperature profiles with average monthly
The L1DAR profile in (a) was averaged over 12 nights in July and in (b) the L1DAR profile was averaged over 23 nights in August. Figure 7.t8-: (a) Comparison of L1DAR monthly mean temperature profile with ECMWF monthly mean temperature profile for the month of November. L1DAR profile averaged over 4 nights in November b) Comparison of L1DAR temperature profile with ECMWF temperature profile for 14 December 1999.
Comparison of average monthly LIDAR temperature profiles with CIRA-1986
