the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Nov 2024
Variation in shortwave water vapour continuum and impact on clear-sky shortwave radiative feedback
Abstract. This work assesses the impact of the current differences in the strength of the shortwave water vapour continuum on clear-sky calculations of shortwave radiative feedback. Four continuum models were used: the MT_CKD (Mlawer-Tobin-Clough-Kneizys-Davies; versions 2.5, 3.2 and 4.1.1) and CAVIAR (Continuum Absorption at Visible and Infrared Wavelengths and its Atmospheric Relevance) models. Radiative transfer calculations were performed with the ECMWF radiation scheme (‘ecRad’). The correlated k-distribution gas-optics tables required for ecRad computations were trained with each of these continuum models using the ECMWF software tool. The gas-optics tables trained with the different continuum models were used alternatively in the shortwave. The atmosphere configuration was; fixed surface temperatures (TS) between 270–330 K, fixed relative humidity at 80 %, a moist adiabatic lapse rate for the tropospheric temperature and an isothermal stratosphere with the tropopause temperature fixed at 175 K. At TS =288 K, it was found that the revisions of the MT_CKD model in the shortwave over the last decade have a modest effect (~0.3 %) on the estimated shortwave feedback. Compared to MT_CKD 4.1.1, the stronger CAVIAR model has a relatively greater impact; the shortwave feedback is ~0.006 W m-2 K-1(~1.6 %) more positive. The uncertainty in the shortwave feedback increases up to 0.008 W m-2 K-1(~2.0 %) between the MT_CKD models and 0.018 W m-2 K-1(~4.6 %) between CAVIAR and MT_CKD 4.1.1 models at TS≈300 K. Constraining the shortwave continuum will contribute to reducing the non-negligible shortwave feedback uncertainties at higher TS.
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RC1: 'Comment on egusphere-2024-3051', Anonymous Referee #1, 20 Dec 2024
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This manuscript evaluates how different shortwave water vapor continuum models impact the calculation of clear-sky shortwave radiative feedback, computed using ECMWF’s ecRad radiative transfer code. The authors test three versions of the MT_CKD continuum model (v2.5, 3.2 and 4.1.1) as well as one version of the CAVIAR continuum model applied to ecRad. It is known that the strength of the SW water vapor continuum differs greatly across continuum models, but the impact of this on SW feedbacks has not been tested before (while such analysis has been done for the LW). Results presented here show that at a baseline temperature of 288K, the choice of MT-CKD continuum model version has a negligible effect on SW feedback. CAVIAR leads to a relatively stronger SW feedback than MT_CKD, but still only by a few percent. At moderately warmer baseline temperatures the differences in SW feedback across continuum models is larger. The manuscript is well written and fills a gap in the literature that will interest multiple groups of ACP readers. However, I think the importance of the results can be better motivated and there are some areas of the text where some minor clarifications would be helpful. I therefore recommend a minor revision.
General: My biggest concern with the manuscript is, to a reader coming from the GCM user community, the uncertainty in SW feedback associated with the different continuum models is quite small relative to other sources of uncertainty and to the overall spread in these feedbacks across GCMs, and certainly relative to overall net LW+SW feedback spread. Should the reader’s takeaway be that continuum choice doesn’t really matter, relatively speaking? Or is there a reason to care? The authors should spend some time addressing this in order to improve motivation of the work. In my mind, this work matters because the continuum is rooted in fundamental physics and observations. Therefore, in some respects, this is a source of feedback uncertainty that we can constrain. That is not true for many other sources of uncertainty.
Abstract: The result in Figure 4b and c, showing the varying dependency on surface temperature, and the author’s spectral explanation of this result, is really interesting. I think some summary of this is worthy of inclusion in the abstract. The result is much more nuanced than just “uncertainty is larger at warmer temperatures” as I assumed before reading this.Line 91-92: The authors should briefly summarize the results of the studies that investigated the effect of contiuum on LW feedback. It would help put this work into context. My sense is LW feedback is similarly insensitive to continuum magnitude at present-day temps (288K). Also, it would be helpful to understand how the range of continuum strength studied here compares to, for instance, the variations in continuum used by Koll et al. 2023.
Figures 2 and 3 and surrounding text seem unnecessary and a bit out of place. They are used to show that ecCKD is an accurate radiative transfer model, but that doesn’t really have any baring on the main focus of the paper: the impact of continuum model on LW feedback.
Line 351: I’m not a fan of using the term “error” relative to 4.1.1, implying 4.1.1 is truth. The authors could use “difference” like fig 4 does
Line 354-356: This qualitative explanation of why SW feedback is stronger at warmer temperatures isn’t clear. This same argument – increased moisture reduces upwelling radiation – could be said for lower temperatures. A rewrite with a little more detail would be helpful here.
Citation: https://doi.org/10.5194/egusphere-2024-3051-RC1
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