the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
26 Feb 2021
26 Feb 2021
The interaction of tropical and extratropical air masses controlling East Asian summer monsoon progression
- 1Department of Meteorology, University of Reading, Reading, RG6 6ET, UK
- 2National Centre for Atmospheric Science, University of Reading, Reading, RG6 6ET, UK
- 1Department of Meteorology, University of Reading, Reading, RG6 6ET, UK
- 2National Centre for Atmospheric Science, University of Reading, Reading, RG6 6ET, UK
Abstract. The East Asian summer monsoon (EASM) is a complex phenomenon, influenced by both tropical and mid-latitude dynamics and by the presence of the Tibetan Plateau. The EASM front neatly separates tropical and extratropical air masses as the monsoon marches northwards. Although the different factors behind EASM progression are illustrated in a number of studies, their interactions, in particular between tropical and extratropical air masses, still need to be clarified. In this study we apply Eulerian and Lagrangian methods to the ERA5 reanalysis dataset to provide a comprehensive study of the seasonal evolution and variability of the EASM, and we highlight the dynamics of the air masses converging at its front.
A frontal detection algorithm is used to perform a front-centred analysis of EASM evolution. The analysis highlights the primary role of the sub-tropical westerly jet (STWJ) in controlling the strength and the poleward progression of the EASM front, in particular during Mei Yu, one of the stages of EASM progression. The upper-level mid-latitude forcing acts in conjunction with the southerly advection of low-level moist tropical air, modulated by the seasonal cycle of the South Asian monsoon and by the location of the Western North Pacific subtropical high. The Mei Yu stage is distinguished by an especially clear interaction between tropical and mid-latitude air masses converging at the EASM front. The analysis of composites based on the latitude of the EASM front during Mei Yu reveals the influence of the STWJ on the strength of the mid-latitude flow impacting on the northern side of the EASM front. In turn, this affects the extent of the warm moist advection on its southern side and the distribution and intensity of resultant rainfall over China.
This study shows the validity of an analysis of EASM evolution focused on its front and on the related low-level airstreams, at least in the Mei Yu stage. The framework highlighted shows how the upper-level flow drives the low-level airstreams that converge at the EASM front, thus controlling the shape of EASM progression. This framework provides a basis for studies of climate variability and extreme events and for model evaluation.
Ambrogio Volonté et al.
Status: final response (author comments only)
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RC1: 'Comment on wcd-2021-12', Anonymous Referee #1, 31 Mar 2021
The comment was uploaded in the form of a supplement: https://wcd.copernicus.org/preprints/wcd-2021-12/wcd-2021-12-RC1-supplement.pdf
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RC2: 'Comment on wcd-2021-12', Anonymous Referee #2, 02 Apr 2021
General Comments
The paper examines the structure and evolution of EASM front and the associated synoptic features. The paper is mostly well written and makes a very worthwhile contribution to the topic. However, in the Introduction the paper discusses the recent work by Parker et al. on the progression of the Indian monsoon and says it will adapt their approach to the EASM to study how competing topical and mid-latitude airmass control the progression of the EASM. I don’t think the paper really achieves this. It doesn’t tackle the effects of the airmasses in controlling the way in which shallow convection deepens etc. I was expecting more along these lines. Nonetheless, I found the paper as it stands insightful and original.
Significant Comments
Section 2.1. Using the rainfall from reanalyses can be a problem as the rainfall is the model-generated rainfall. Have you compared the ERA5 rainfall to GPCP (or something similar)? You say that the patterns of ERA5 rainfall are similar to Kong et al. 2017, but what about the cross-sections plotted in Figs. 8 and 14, and the arguments made about the tri-pole structure?
Section 2.2. The EASM front is define by the meridional gradient in the equivalent potential temperature. But later in the paper, you relate the front to the upper jet through thermal wind. Of course, the thermal wind relates horizontal gradients in the potential temperature (not the equivalent potential temperature) to the wind shear. So, what are the implications of defining the front by equivalent potential temperature rather than potential temperature? How much of the frontal gradient comes from gradients in the moisture and how much from gradients in the temperature?
Section 2.2. The front detection methods is based on the meridional gradient in the equivalent potential temperature. This builds in the assumption that the front is oriented zonally. How true is this? Are there any implications for your study? That the fronts are not zonal shows up in Fig. 7 (as you note).
Section 3.1, Fig. 1. You discuss the position of the front with time, but how does the strength of the front and the slope change with time? You don’t explicitly say this, but Fig. 1 shows the EASM to be a warm front.
Figure 2. How reliable is ERA5 for precipitation? How much would this figure change if you used CPCP or something similar? I suspect it would change quite a lot. Also, there are relatively regular bursts in the rainfall prior to June with periods of around 10 days. What are these? I find is surprising that monsoon burst show up in the composite mean.
Line 230-231. “… a feature that has been shown to be associated with the EASM progression …”. Associated in what way? Be more explicit.
Lines 240-241. You talk about a climatological trough over the Korean Peninsula, but the PV = 2 contour is displaced far poleward in panels c and d. Explain what’s going on here.
Lines 243-251. Presumably the EASM front moves because the dilatation axis moves. And the dilatation axis is determined by the strength and location of the WNPSH, among other things. I’d like to see a plot of the deformation and dilatation axes as it would link the EASM to the theory for midlatitude fronts.
Lines 255-262. Why not simply calculate the thermal wind assess quantitatively how well it holds?
Last paragraph of Section 3.3. As the front weakens does is slope more? The frontal slope diagnostics of Papritz and Spengler 2015 might be useful here.
Line 267. Is there really causality here? The thermal wind relation is diagnostic.
Line 269-270. The front is also in the left exit of the upstream jet, which is even more favourable for precipitation.
Line 272. 1000 hPa is below the ground in many places. Isn’t this a problem?
Section 4.2. You start the warm and cold back-trajectories at different heights (900 and 700 mb). Hence much of the difference in the thermodynamic properties of the parcels at the initial time is due to the difference in elevation as opposed to horizontal differences in the properties of the air masses. Why not begin the parcels at a single height, say 850 mb, which is of course the height at which you define the front? How sensitive are you results to this choice?
Line 492. The STWJ doesn’t really **control** the variability - does it?
Technical Corrections
Line 70. Insert “which is a” before “consequence”.
Line 125. What is n? Presumably the number of point identified. Say so.
Line 202 and numerous other places. You refer to the Yangtze River and the valley. It would be helpful to mark this on one of the map. I (and presumably others) am only know the general location of the Yangtze River and a not the specific geography.
Figure 3 caption. mm per what?
Figure 10. Use the same colour scheme as Fig. 9: warm = red and cool = blue. In my pdf, red is too close to brown and blue to close to green.
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CC1: 'Comment on wcd-2021-12', David Schultz, 06 Apr 2021
I have a primary concern about this manuscript, which is how the authors define a front and use the term “front” throughout the manuscript.
1. The authors use a definition of a front developed by Li et al. (2019). However, in neither Li et al. (2019) nor in the present manuscript is the suitability of this quantity assessed. Such a critical assessment in relation to previous definitions of fronts, in general, and the Mei-Yu front, specifically, needs to be carried out.
For example, at the most general, quite a few other studies have examined automated frontal detection methods, but that prior knowledge is not discussed in this manuscript as it would pertain to justifying the authors’ choices in the present manuscript. Some examples include Hewson (1998), Berry et al. (2011), Schemm et al. (2018), Thomas and Schultz (2019a,b), and Catto and Raveh-Rubin (2019). The readers would benefit from a detailed discussion of the advantages and disadvantages of various approaches of automated frontal detection and a justification for these specific choices by the authors. Specifically, the following items need to be discussed.
2. Choice of theta-e. In atmospheric dynamics literature (e.g., Hoskins and Bretherton 1972), fronts are defined by the horizontal gradients in density (expressed through temperature changes). However, the present manuscript uses theta-e, which is a function of temperature and moisture. Thomas and Schultz (2019a,b) have discussed the implications of choosing theta-e over a temperature-based quantity (such as potential temperature). See in particular, Table 2 of Thomas and Schultz (2019b), which presents the advantages and disadvantages of using potential temperature versus theta-e. In the present manuscript, however, the authors did not justify their choice of theta-e over other thermodynamic quantities that would not be affected by moisture. Indeed, Yang et al. (2015) write, “mei-yu rainbands typically consist of a much stronger moisture gradient than temperature gradient”. This statement (and others can be found in other articles, as well) is why a more clear definition of “front” is needed in this manuscript.
3. More specifically, what is the context for the choice of theta-e in terms of the Mei-Yu front? The discussion of the previous literature on Mei-Yu frontal identification is limited. Although theta-e is a useful diagnostic in some studies, in others, it is not appropriate. In fact, Chen et al. (2003) argued the following:
“As the mechanism for frontogenesis was almost unrelated to baroclinity in our mei-yu front case, traditional definitions of front and frontogenesis in terms of horizontal temperature gradient become inappropriate.”
In Wang et al. (2016), because the thermodynamic boundary and the wind-field boundary were often not collocated, the authors diagnosed the wind field (through the vorticity equation) and frontogenesis field (using theta-e) separately.
That agreement on how to diagnose the Mei-Yu front is not apparent from just a small sampling of the literature raises the issue of the appropriateness of the frontal diagnostic used in the present manuscript. Thus, this previous literature raises the issue of how cleanly the airstream boundaries line up (or don’t line up, as the case may be) with gradients in the thermodynamic fields. How do the authors reconcile their picture of the Mei-Yu front that is smooth and simple compared to the previous literature on this topic?
4. Choice of meridional gradient. As detailed in their Table 1b, Thomas and Schultz (2019b) showed studies have used the full gradient of temperature as a frontal diagnostic (e.g., Sanders and Hoffman 2002; Spensberger and Sprenger 2018; Thomas and Schultz 2019a,b). The choice of only using the meridional gradient rather than the full gradient is an unusual one. In neither Li et al. (2019) nor in the present manuscript is the use of only part of the full gradient discussed. Given that the Mei-Yu front may not be purely oriented in a west–east orientation on any given weather map, the authors would not be capturing the full magnitude of the front by only using the meridional gradient. This choice needs to be better justified in the manuscript.
5. In summary, by designation of the front as the meridional gradient of theta-e, the authors obtain results that are overly smooth compared to previous literature that describes the complexity of the Mei-Yu front. Therefore, statements such as those below need to be better qualified.
“The EASM front neatly separates tropical and extratropical air masses” (line 2).
“The Mei Yu stage is distinguished by an especially clear inter- action between tropical and mid-latitude air masses converging at the EASM front” (lines 11–12).
ReferencesBerry G., C. Jakob, and M. J. Reeder, 2011b: Recent global trends in atmospheric fronts. Geophys. Res. Lett., 38, L21812.
Catto, J. L., Raveh-Rubin, S., 2019: Climatology and dynamics of the link between dry intrusions and cold fronts during winter. Part I: global climatology. Clim Dyn 53, 1873–1892.
Chen, G. T., Wang, C., & Liu, S. C. (2003). Potential Vorticity Diagnostics of a Mei-Yu Front Case, Monthly Weather Review, 131(11), 2680-2696.
Hewson, T. D., 1998: Objective fronts. Meteor. Appl., 5, 37–65.
Hoskins, B. J., and F. P. Bretherton, 1972: Atmospheric frontogenesis models: Mathematical formulation and solutions. J. Atmos. Sci., 29, 11–37.
Sanders, F., and E. G. Hoffman, 2002: A climatology of surface baroclinic zones. Wea. Forecasting, 17, 774–782.
Schemm, S., M. Sprenger, and H. Wernli, 2018: When during their life cycle are extratropical cyclones attended by fronts? Bull. Amer. Me- teor. Soc., 99, 149–165.
Spensberger, C., and M. Sprenger, 2018: Beyond cold and warm: An objective classification for maritime midlatitude fronts. Quart. J. Roy. Meteor. Soc., 144, 261–277.
Thomas, C. M., and D. M. Schultz, 2019a: Global climatologies of fronts, airmass boundaries, and airstream boundaries: Why the definition of "front" matters. Mon Wea. Rev., 147, 691–717, doi: 10.1175/MWR-D-18-0289.1.Thomas, C. M., and D. M. Schultz, 2019b: What are the best thermodynamic quantity and function to define a front in gridded model output? Bull. Amer. Meteor. Soc., 100, 873–895, doi: 10.1175/BAMS-D-18-0137.1.
Yang, S., Gao, S. & Lu, C., 2015: Investigation of the mei-yu front using a new deformation frontogenesis function. Adv. Atmos. Sci. 32, 635–647.
Wang, C., Tai-Jen Chen, G., & Ho, K. (2016). A Diagnostic Case Study of Mei-Yu Frontal Retreat and Associated Low Development near Taiwan, Monthly Weather Review, 144(6), 2327-2349.
- AC1: 'Comment on wcd-2021-12', Ambrogio Volonté, 14 Jan 2022
Ambrogio Volonté et al.
Ambrogio Volonté et al.
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