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How Can the International Monitoring System Infrasound Network Contribute to Gravity Wave Measurements?

Atmosphere 2019, 10(7), 399; https://doi.org/10.3390/atmos10070399

by Patrick Hupe^1,*

, Lars Ceranna¹ and Alexis Le Pichon²

Reviewer 1: Anonymous

Reviewer 2: Anonymous

Reviewer 3: Anonymous

Atmosphere 2019, 10(7), 399; https://doi.org/10.3390/atmos10070399

Submission received: 10 June 2019 / Revised: 5 July 2019 / Accepted: 12 July 2019 / Published: 16 July 2019

(This article belongs to the Special Issue Atmospheric Acoustic-Gravity Waves)

Round 1

Reviewer 1 Report

Based on the International Monitoring System (IMS) infrasound network, the Progressive Multi-Channel Correlation Method (PMCC) is used for re-processing up to 20 years of IMS infrasound recordings in order to derive GW detections. From my own perspectives, the techniques and datasets appear to be very cutting-edge and promising. With that being said, my main concern is that some of the technical points may still be very hard to follow for the general readers, and it is necessary to give more details and clarify them. For example, can all the detected wave signals be explained by the physics of the trapped gravity waves? What gravity wave parameter can be obtained from the network (in addition to wave period and wave front direction)? How reliable are the obtained parameters and is it possible to estimate their errors? Furthermore, I also have comments/concerns on the study of the potential source mechanism. Those questions are listed as below, and I hope that they could help the authors improve the manuscript.

Major comments

(1) My biggest concern for this study is whether all the detected wave signals can be explained by the physics of the trapped gravity waves or by the gravity wave dynamics in general. In particular, have the authors verified the signals with polarization relation and dispersion relation? Are they consistent with the linear gravity wave theory? Is it possible that any other physical process (e.g., turbulence, convection, balance motions at smaller scales) has any impact on the detections and the related obtained results? To be more rigorous, I think that it is very important for the authors to clarify it.

(2) I assume that in this manuscript the observed period refers to the absolute period, which is relative to the ground. In this case, I am wondering whether Doppler effect has any impact on separating GWs from acoustic-GWs, especially when background wind is strong. For example, is Figure 2b really all for the GW detections? Please clarify.

(3) What gravity wave parameter can be obtained from the network (in addition to wave period and wave front direction)? How reliable are the obtained parameters and is it possible to estimate their errors? In particular, in order to improve the gravity wave parameterization schemes, it is very important to understand the distribution of momentum fluxes in the space of phase velocity relative to the ground, as it determines the approximate altitude at which GWs deposit their momentum, and the associated interaction between mean flow and GWs. Please clarify.

(4) Trapped gravity waves have very weak vertical energy flux and very weak vertical momentum flux. Can this be verified in this work? Also, I don’t quite understand why only trapped GWs cause traceable pressure fluctuations at the surface? What about the downward propagating GW signals which can be generated by the strong wind shear or upper-level jet imbalance? What about other physical processes in addition to GWs? Please clarify.

(5) If those waves are really trapped and horizontally propagating, what are the main maintenance mechanisms for the trapped GWs? Can the background wind/temperature profile support the trapped GWs? Please clarify.

(6) I think that there is still room to improve the discussion on the potential source mechanism in this manuscript. Currently, the discussion is mainly based on the Lightning maps (Figure 12, which mainly shows the activity of deep convection instead of all the convection processes) and the obtained azimuth. Generally speaking, to be more accurate, I believe that the authors should utilize a linear ray tracing model in order to track the source and the propagating characteristics of the observed GW wave packet during its entire life cycle. For example, the Gravity Wave Regional or Global Tracer (GROGRAT) model (e.g., Marks and Eckermann 1995) could be a very good tool to track the GWs based on the WKB assumption. Please clarify.

Minor comments

(1) Line 53: trapped GWs waves -> trapped GWs

(2) Figure 3: If possible, I think that it is better to show the approximate latitude change in this plot.

(3) Figure 3: Some of the stations in the tropical region (e.g., IS21) do not have any apparent seasonal variation. Why is that? Please clarify.

(4) Lines 181-183: How can one separate GW signal from the turbulent noise? Please clarify.

(5) Figure 9: If the gap in (a) can be filled after a new re-processing, why not just use this technique for this figure. I don’t understand why the authors use an old technique here. Please clarify.

Reference

Marks, C. J., and S. D. Eckermann, 1995: A three-dimensional nonhydrostatic ray-tracing model for gravity waves: Formulation and preliminary results for the middle atmosphere, J. Atmos. Sci., 52, 1959–1984.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Review of the paper by Patrick Hupe1, Lars Ceranna , and Alexis Le Pichon “ How can the International Monitoring System Infrasound Network Contribute to Gravity Wave 4 Measurements?”

In this paper the authors give an answer asked in the title. The obtained results of continuous measurements of atmospheric gravity wave (GW) parameters (back-azimuth, periods, signal-to-noise ratio) using a network of infrasound station over the globe are essentially supplement the existing GW climatology and may improve weather prediction models. I found these results novel and recommend to publish this paper in the journal “Atmosphere”, but after correcting the shortcomings of the article that I give below.

1.One of the conclusions made from 10-year observations is that seasonal variations in the azimuths of GW arrivals and number of detections is associated with deep convection and predominant tropospheric wind directions. The wind was measured at 10 m height. However, I didn’t find the explanation of the mechanism of the wind effect on GWs. For instance, in the earlier work by Herron and Tolstoy (Tracking jet stream winds from ground level pressure signals J. Atmos. Sci. 1969, V.26, p.266-269) the correlation was found between direction of propagation of low-frequency pressure perturbations (periods 30-90 min) near ground and jet stream winds in the tropopause. These perturbations was suggested to be dragged along by tropopause winds. What is the cause of correlation between 10-m winds and pressure perturbations in the paper?

2) Line 232. The assumption “…The seasonal variation at IS02 proves that the detections represent trapped lee waves downstream of the southern Andes…” raises the question of whether lee waves are propagating gravity waves with nonzero phase speed or with zero phase speed relative to the mountains?

3) In the Introduction (line 31) the statement “Due to the exponential density decrease with altitude, the amplitude of a vertically propagating wave increases, hence energy and momentum ...” is necessary to correct, because energy density and momentum decrease with altitude, tending to zero at infinity.