Improved ozone DIAL retrievals in the upper troposphere and lower stratosphere using an optimal estimation method
Summary (2 min read)
1. INTRODUCTION
- The upper troposphere and lower stratosphere (UTLS) extends from about 6 km to 25 km in height and plays a significant role in the atmospheric climate system.
- Providing a single ozone profile with a full uncertainty budget using both sets of measurements can significantly improve their measurements of ozone in the UTLS [13] [14] [15] .
- The OEM also provides averaging kernels of the retrievals, which allows comparison of the profiles with other measurements which can account for differences in vertical resolution, such as when compared to space-based measurements.
- Moreover, the state vectors and the b parameter quantities are defined and a brief overview of the lidar's specifications is given.
2. METHODOLOGY
- In the DIAL system, two wavelengths are simultaneously transmitted to the atmosphere.
- Any corrections due to nonlinearity of the counting system (because of saturation) should also be applied to the raw counts.
- The authors use the lidar equation as the forward model, where the raw counts are the measurements.
- The geometrical overlap is O(z), and β(λ i , z) are the atmospheric backscattering coefficients which includes both molecular and aerosol terms.
A. Implementing the OEM for the OHP lidars
- Knowing the measurements vector and its covariance matrix S , and using an a priori profile and its associated covariance matrix S a , the OEM calculates an optimal a posteriori state by minimizing a cost function: EQUATION.
- The iteration stops when the cost function decreases to a value much smaller than the number of measurements.
- The relation between the retrieved state and the true state is described by the averaging kernel of the retrieval.
- The altitude above which the background counts are determined is different for tropospheric and stratospheric lidars.
- The overlap function, Rayleigh cross sections, and ozone absorption cross sections are assumed as b parameters.
3. OEM OZONE RETRIEVAL IN THE FREE TROPOSPHERE AND STRATOSPHERE
- In this section the authors present the result of combining the two lidar measurements to retrieve a single ozone profile.
- As a result the red line (which indicates the noise of measurements) for the analog channels is more structured then for the digital channels. of 12 July 2017 is retrieved from 2.6 km to 42.2 km altitude.
- In Fig. 3 the OEM retrieval is plotted against the traditional stratospheric and tropospheric retrievals.
- The Rayleigh-scatter cross section uncertainty for both tropospheric and stratospheric lidars has a significant contribution.
- Ozone density profiles are also retrieved for the nights of 14 July 2017, and 26 July 2017, each of which also had coincident ozonesonde measurements.
4. SUMMARY
- The authors have introduced a first-principles OEM retrieval for tropospheric ozone profiles, as well as for a combination of tropospheric and stratospheric ozone profiles.
- The results from their implementation of the OEM are summarized below.
- The combined stratospheric-tropospheric DIAL OEM retrieval calculate a single ozone profile consistent with all the measurements.
- For the tropospheric retrievals, the traditional method and their OEM retrieval produce similar results.
- At the region where the tropospheric and stratospheric lidar measurements overlap, the OEM can smoothly makes a transition from one lidar system's measurements to the other system's measurements.
5. CONCLUSIONS
- The authors used simultaneous tropospheric and stratospheric lidar measurements with the OEM to retrieved ozone profile from 2.5 km to above 40 km.
- The OEM method has no need for "gluing" or "merging" the tropospheric and stratospheric measurements, as all measurements are simultaneously considered when retrieving a single ozone profile.
- The authors forward model has been tested under clear sky conditions.
- In the UTLS region, clouds and significant aerosol loads can exist.
- Furthermore, as their methods allows the calculation of averaging kernels for the retrieval, the authors are interested in comparing their retrievals with satellite measurements, as well as processing more of the OHP measurements.
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...Improvement of the lidar data processing and removal of this potential bias will be investigated in future work involving optimal estimation techniques (Farhani et al., 2019)....
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...Improvement of the lidar data processing and removal of this potential bias will be investigated in future work involving optimal estimation techniques (Farhani et al., 2019). Future tropospheric ozone lidar campaigns for NDACC lidars would be required to assess the new technique and fully characterise any residual biases. MLS and SABER ozone profiles agree with the profiles produced by lidars and ECCs from about 20 to above 40 km. Below 20 km, both sets of satellite profiles deviate significantly from the lidars and the ECCs. Above 40 km, ozone measurement uncertainties become large for the lidars, and differences increase while their significance goes down. The assessment of the uncertainty budget for ozone concentration profiles for each instrument showed that the reported measurement uncertainties for both LiO3S and NASA STROZ are well characterised and realistic. The reported measurement uncertainty estimates for ECCs from Tarasick et al. (2016) appear too optimistic for the sondes launched during LAVANDE....
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Frequently Asked Questions (15)
Q2. What are the future works in "Improved ozone dial retrievals in the upper troposphere and lower stratosphere using an optimal estimation method" ?
The authors are planning to augment their forward model to allow for inclusion of aerosols, as well as other trace gases.
Q3. What can be retrieved along with ozone profiles?
Other atmospheric and systematic parameters such as air density, the dead time of the system, and the background counts can be retrieved along with ozone profiles.
Q4. What is the output energy of the XeCl excimer laser?
This laser has an output energy of about 200 mJ for the “on-line” channel, while the “off-line” wavelength at 355 nm is generated by the third harmonic of a continuum Nd:YAG laser with an energy of 50 mJ at 50 Hz.
Q5. What is the main concern for stratospheric ozone measurements?
for stratospheric ozone measurements, choosing a laser that can reach higher altitudes in the stratosphere is the main concern [11, 20, 21].
Q6. How long was the a priori covariance matrix used?
To generate a full length a priori covariance matrix for both air and ozone density profiles at altitudes below 12 km, a tent function with a correlation length of 300 m was used.
Q7. What is the ozone uncertainty at the bottom of the retrievals?
For stratospheric measurements, the ozone uncertainty has its maximum of 4% at the bottom of retrievals, which is higher than the calculated uncertainty of 2% in [36] uncertainty budget.
Q8. What is the a priori value for the background counts in the troposphe?
In order to determine the background counts in both the tropospheric and stratospheric measurements, the mean of the counts above a specific height is calculated and used as the a priori for the “off-line” channels, since SIN is negligible in these channels.
Q9. What is the effect of SIN in the on-line channel?
as the laser power in the “online” channel is about 2 times stronger than the laser power in the “off-line” channel, the effect of SIN in the “on-line” channel is more pronounced.
Q10. What is the definition of a perfect retrieval?
A perfect retrieval, in the sense all the information comes from the measurement with no effect from the a priori state, has averaging kernels equal to one, where the first term of the above equation becomes zero.
Q11. How many different channels did the authors use for the stratospheric ozone retriev?
The authors used four different channels for tropospheric ozone retrievals, and eight different channels for the stratospheric-tropospheric ozone retrievals.
Q12. What is the frequency of the Continuum Nd:YAG laser?
The transmitter for the tropospheric system uses the fourth harmonic of a Continuum Nd:YAG laser (266 nm) frequency shifted by Raman Stimulated Scattering in a D2 high pressure cell [34].
Q13. What is the effect of the gases in the final ozone retrievals?
in most cases, for both stratospheric and tropospheric ozone studies the effect of these gases in final ozone retrievals is negligible.
Q14. What is the trade-off between the traditional and the stratospheric retrieval?
The trade-off is that the uncertainty of the retrieval in the traditional method is smaller, so that at 11 km it is 4.5% as opposed to the OEM retrieval which has a larger uncertainty of 7.5%.
Q15. What is the correlation length for stratospheric lidar measurements?
This choice of correlation length is because above 12 km, the stratospheric lidar measurements have the most weight and the retrieval grid for these measurements starts at 300 m.