|2nd Review for |
Analyzing the turbulence in the Planetary Boundary Layer by the synergic use of remote sensing systems: Doppler wind lidar and aerosol elastic lidar
G. Moreira et al
This manuscript presents results from the SLOPE campaign in Granada, Spain, in which the objective was to obtain closure between remote sensing and in-situ measurements. For this manuscript, the focus is on characterising the planetary boundary layer using a Doppler lidar, multi-wavelength lidar (MULHACEN), and a profiling microwave radiometer, all operating at high temporal resolution (2 seconds). The authors investigate the use of fluctuations in aerosol number density from the elastic system (EL), vertical velocity fluctuations obtained from the Doppler lidar (DL), and potential temperature profiles retrieved from the microwave radiometer (MWR), to identify the boundary layer height (PBLH).
As stated in the first review, some of the methodology is relevant, and the influence of random error introducing extra noise in higher-order moments is explored using suitable techniques, but a major issue was that the manuscript did not have a concrete focus and conclusion, and without these, did not present anything new.
The authors now state that the focus of the paper is 'the synergetic combination of information from different remote sensing systems that are sensitive to different tracers'. This would present something new and useful to the scientific community but there is minimal and insufficient discussion presented in the manuscript in its current state. The comments outlined in the first review have not yet been adequately addressed, and so the manuscript is not yet ready for publication.
The authors state that the focus of the paper is on the synergistic combination of different methods to determine PBLH, but there is still insufficient discussion in the main text. There is little suggestion on how the various retrieved parameters could be combined, or how the relative magnitudes of their uncertainties could influence the combination.
"The EL and DL parameters are calculated over 1-hour periods. Is this 1-hour timescale suitable during rapidly varying conditions such as during the morning growth of the boundary layer?" This question is asking whether a 1 hour timescale is suitable when, during the morning growth, a particular region may have been calm for 30 minutes, and then strongly turbulent for 30 minutes. If changing the timescales has an impact, then this is important information to include in the manuscript, e.g. how does the noise reduce the integral time scale, and is this SNR-dependent? The abstract states that there is "low influence of noise", so how can both od these statements be correct?
"What is the impact if you change the averaging period, and why was 1-hour chosen when the MWR data are averaged over 30 minutes?". The authors state that using a different timescale for MWR parameters does not interfere in the analysis, yet the focus is the synergistic combination of the various retrievals? The MWR retrieval is much smoother in time than the lidar retrievals, so what is the likely impact when combining them?
The manuscript requires a much more rigorous but short description of the processes driving turbulent mixing in the boundary layer.
The response from the authors does not satisfy this requirement, is far too long, and contains many factual errors. For instance, air temperature is not directly related to RN and Sw'. The abstract states (lines 22-24) "Furthermore, we show how some meteorological variables such as air temperature, aerosol number density, vertical wind speed, relative humidity and net radiation might influence the turbulent PBL dynamic" but the main text does not discuss how any of these parameters influence the PBL, and in any case it is not clear to me how, in most situations, the aerosol number density would influence the turbulent PBL dynamic.
DL analysis: The time-height plots provided in the supplementary material are not satisfactory. The upper panel displays vertical velocity, not wind speed and appears to have been drastically smoothed or averaged compared to the lower panel, which is not backscatter but potentially attenuated backscatter. From the system configuration information provided by the authors (telescope focus) it is unlikely that the 'attenuated backscatter' field has been corrected for the telescope focus, hence the request to provide the signal (SNR+1) instead. Please plot both the vertical velocity and signal (SNR+1) at the original resolution without averaging. If you plot the SNR, you can then see that all velocity data above about 2 km is likely to be noise.
"EL analysis: 'Is it safe to assume the two-way transmittance is negligible?" and "what are the typical molecular, aerosol and total extinction values for the cases shown here?" The two-way transmittance at a wavelength 532 nm is less than 0.98 by 2 km above sea level from molecular extinction alone. The AOD is also > 0.1 for 19th May 2016, and over 0.3 for 8th July 2016 (at a wavelength of 500 nm, from AERONET data). These values are not negligible. They may not be sufficient to impact the results, but whether this is the case should be discussed. Does this attenuation impact the noise characteristics and the integral time scale? The new figures 10 and 14 do not aid the interpretation and are not necessary for the manuscript.
Supplementary material, Figs. 5 and 6, do not show time-height plots, only two profiles, so it is still not possible to evaluate whether these parameters provide a reliable guide to the boundary layer development.
Doppler lidar and Elastic lidar analysis: Since you make some effort to quantify the influence of noise on the statistical parameters derived from these two systems, it would be beneficial to discuss how this impacts your interpretation, e.g include time-height plots of the correction factor or relative correction, relative importance in determining PBLH, how much temporal averaging is required to obtain good results. You state that your objective is 'to approach a synergetic combination', hence discussing how the influence of noise impacts your interpretation is vital, otherwise there is nothing new presented in this manuscript.
Case study 2: Did you try cloud-screening EL data before calculating EL parameters? The PBLH from EL would agree much better with PBLH from MWR in Figure 13, and maybe Figure 14 (it is hard to tell with the scales used). Clouds should also be visible in DL data. The authors response is "No, any cloud-screening method was not applied before calculating the EL parameters". What happens if you do attempt a simple cloud screening procedure. This is simple to apply and would presumably be used in any synergetic combination?
If the DL telescope focus is set to 800 m then what method do you use to obtain attenuated backscatter profiles from the (SNR + 1) profile? Therefore, in the supplementary material it would be more appropriate to present the time-height plots of vertical velocity and (SNR + 1), which was what was originally requested.
Line 32: How do the variables interfere in the process?
Lines 40-42: Please check and reformulate these sentences. Surface heating is still unlikely to directly impact the upper troposphere. The convective boundary layer does not reach the upper troposphere.
Lines 226-229: The methodologies are not used synergistically, even though this is the focus of the paper, and it is not shown or discussed how each variable 'influences' the turbulent PBL behaviour.
Many of the figures still have very short captions without enough information. Please include the instrument name, date and the location in the caption.
Figure 4: Is this autocovariance from DL?
Figures 5,7: Profiles from which instrument, and from which location? At what time, and on what day? What height is the surface? Please include this information in the caption
Figures 5, 7-10,12-14: The captions do not state which instrument the data comes from. Please include the instrument names and the location in the caption. Where applicable, state which data comes from which instrument.
Figures 11, 15, 16: The caption states 'elastic lidar data'. Please include the instrument name and the location in the caption.