the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Available potential energy of the three-dimensional mean state of the atmosphere and the thermodynamic potential for warm conveyor belts
- 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
- 2Institute of Meteorology, Freie Universität Berlin, 12165 Berlin, Germany
- 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
- 2Institute of Meteorology, Freie Universität Berlin, 12165 Berlin, Germany
Abstract. Much of our understanding of the atmospheric circulation comes from relationships between aspects of the circula- tion and the mean state of the atmosphere. In particular, the concept of mean available potential energy (MAPE) has been used previously to relate the strength of the extratropical storm tracks to the zonal-mean temperature and humidity distributions. Here, we calculate for the first time the MAPE of the three-dimensional time-mean state of the atmosphere including the effects of latent heating. We further calculate a local MAPE by restricting the domain to an assumed eddy size, and we partition this local MAPE into convective and nonconvective components. Local nonconvective MAPE has a similar spatial pattern to the Eady growth rate, while local convective MAPE has some similarities in spatial pattern to a high percentile of instanta- neous convective available potential energy. Furthermore, the maximum potential ascent associated with nonconvective local MAPE is strongly related to the frequency of warm conveyor belts (WCBs) which are ascending air streams in extratropical cyclones with large impacts on weather. This maximum potential ascent can be calculated based only on mean temperature and humidity, and it also skillfully identifies the necessary conditions for WCBs at a given location on a specific day. These advances in the use of MAPE are expected to be helpful to connect changes in the mean state of the atmosphere, such as under global warming, to changes in important aspects of the extratropical circulation.
Charles Garrison Gertler et al.
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2022-904', Anonymous Referee #1, 21 Oct 2022
Summary
The authors calculate 2d maps of mean/moist available potential energy (MAPE) from time-averaged reanalysis data. They then partition the MAPE in two different ways: 1) Non-convective MAPE is found by restricting the vertical rearrangement of parcels, and the remaining MAPE is convective MAPE, and 2) A local MAPE is calculated by restricting the distance over which the parcels can be rearranged. The authors then connect the non-convective local MAPE, and associated ascent, with warm conveyor belts (WCB), including example instantaneous snapshots as well as time-averaged data.
The calculations of MAPE are well described and the partitioning into different components is interesting. However, the interpretation of the results, in particular the explanations in terms of CAPE and WCBs are lacking in depth and insight. This could be a very good paper, and the calculations of MAPE are interesting in their own right, but to justify some of the statements in the abstract and having “warm conveyor belts” in the title, further work is needed. So, I have recommended major revisions
Major Comments
The connection of convective MAPE to a high percentile of instantaneous CAPE looks weak. Is there a reason you didn’t also calculate CAPE from the mean state? In DJF the contour chosen seems to somewhat match the regions of high MAPE, but since only one value is contoured it looks like the value has been chosen to fit rather than an actual correspondence. It would be useful to show more values in the contours.
In JJA the MAPE has very little correspondence to the contour shown and looks more like it picks out regions of more tropical/convective storms: western pacific and Atlantic tropical storms and the Indian monsoon. I think the DJF map could also have similar explanations. My knowledge of Southern Hemisphere meteorology is not so good, but the local maxima seem like they could also relate to tropical storm regions. Also, there appears to be a strong signature of the African Easterly Jet in DJF. It may be useful to explain the convective MAPE in this way rather than just in terms of CAPE. Rather than connecting MAPE to “instantaneous atmospheric convection”, it could be connected with convectively driven storms.
The statement in the abstract that the maximum potential ascent in the MAPE calculation “skilfully identifies the necessary conditions for WCBS” is misleading. All that has really been shown is that there is some correlation between WCB genesis regions and the ascent in the MAPE calculation. To me, it looks like the MAPE picks out the storm tracks, and because WCBs are associated with storms there is some relation.
The idea that WCBs will relate to ascent in the MAPE calculation makes sense as they are the ascending air in extratropical cyclones and therefore will relate to this instability but the results don’t show that MAPE is adding any value to this. The authors state that they interpret the non-convective local MAPE as energy available for the “generation of large-scale eddies through moist baroclinic instability” and show that it has a similar pattern to the Eady growth rate. So my question is what does value does MAPE add over the Eady growth rate (which is much easier to calculate) in predicting/explaining warm conveyor belts? I wonder if this could be shown by relating the ascent predicted in the MAPE calculation with the actual ascent in the WCB trajectories.
I would also like some discussion of the large areas where there is ascent predicted by the MAPE calculation, and presumably large Eady growth rate, but no warm conveyor belts. Presumably this is just related to where cyclones do and don’t actually form, but if the MAPE calculation or some additional variable can’t predict this then I don’t see how it can be described as skilfully predicting warm conveyor belts.
Minor Comments
- P3L60 – “More than half of extratropical cyclones are associated with a WCB in northern hemisphere winter…”. This seems very low. I would expect most cyclones to be associated with a WCB. One reason this could be so low is that the Madonna et al. (2014) climatology uses the strict 600 hPa criteria for identifying WCB trajectories and so miss weaker WCBs. Madonna et al. (2014) acknowledge that they are not aiming to identify the full WCB airmass in their climatology and test the sensitivity of using a 500 hPa threshold instead. As far as I can tell, Madonna et al. (2014) doesn’t go into quantifying the co-occurrence of WCBs with cyclones, so I don’t know where this number, or the other details about WCBs and cyclones in the sentence, has come from.
- Figure 2 – The caption states that (a,b) shows 796 hPa but the figure says 864 hPa.
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RC2: 'Comment on egusphere-2022-904', Anonymous Referee #2, 21 Nov 2022
In this manuscript the authors extend the traditional view of MAPE into three dimensions, and examine its local variations. They further decompose MAPE to its non-convective and convective components and show that the former accounts for most of the total MAPE, and can be expressed via linear baroclinic theory, in the form of the Eady growth rate. Lastly, the authors show that the maximum potential ascent associated with non-convective MAPE is linked to WCB. Deriving local MAPE is an interesting exercise, and in the context of WCB, it seems that one could retrieve new physical understanding that links Eulerian and Lagrangian perspectives of the mid-latitude flow. It is unfortunate, in my opinion, that the authors do not further investigate such avenue to yield new physical understating of the system. Instead, through most of the paper, the authors focus on results which do not necessarily allow us to learn new physics on the mid-latitude flow, or simply describe how MAPE behaves spatially. Related to the above point, the paper is rather technical, and in several cases repetitive, and the writing is not concise; in several places this only diverts the reader from the main take-home message.
Major comments:The authors show that mid-latitude MAPE, which follows non-convective MAPE, basically describes the baroclinic zones in the mid-latitudes, which one could also retrieve from Eady growth rate. Why is it thus necessary to thoroughly discuss the derivation of MAPE (separating its convective and non-convective components) and its spatial patterns? What new information have we acquired here on the mid-latitude flow? On the other hand, the results that links MAPE to WCB are interesting, as it allows us to learn how such events are created and how they are linked to the mean state in the atmosphere. My suggestion is to further explore this link, and provide a new piece of physical understating.The introduction and method sections are considerably long, and include large amount of details. In my opinion this only diverts the reader from the main take home message as the reading becomes cumbersome. For example, in the introduction the authors not only discuss the results but also the methods. Furthermore, that exact algorithm used to calculate the mean state (e.g., divide-and-conquer), is an unnecessary detail in my opinion. There are other examples of that throughout the method section.Although the authors chose to show results from both DJF and JJA, the discussion on Figs. 3-7 is almost entirely limited to DJF. Either remove panels which you do not discuss, or extend your discussion to JJA as well. Specifically, why does the structure of MAPE in JJA does not follow that of DJF? Why does MAPE maximizes in land?Minor comments:- You chose to analyze Era-Interim, what about other reanalysis products? How do you know that your results are not product dependent?- In several locations throughout the manuscript the term “northern/southern hemisphere” is lacking capital letters (e.g., lines 58-59).-line 148: recognize - > recognized
Charles Garrison Gertler et al.
Charles Garrison Gertler et al.
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