the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Aug 2024
The impact of the rotation rate on an aquaplanet's radiant energy budget: Insights from experiments varying the Coriolis parameter
Abstract. We investigate the effect of changes in the Coriolis force caused by changes in the rotation rate on the top-of-atmosphere (TOA) radiant energy budget of an aquaplanet general circulation model with prescribed sea surface temperatures. We analyse the effective radiative forcing caused by changes from Earth-like rotation to values between 1/32 and 8 times the Earth's rotation rate. The forcing differs by about 60 Wm-2 between the fastest and slowest rotation cases, with a monotonically increasing positive forcing for faster than Earth-like rotations and a non-monotonically increasing negative forcing for slower rotations. The largest contributions to the forcing are, in that order, due to changes in the shortwave cloud radiative effect (SWCRE) and the clear-sky outgoing longwave radiation (OLR). From the fastest to the slowest rotation, the Hadley cell expands and the troposphere becomes drier, increasing the OLR. This contributes to negative forcing at slower and positive forcing at faster than Earth-like rotations. The SWCRE is influenced by changes in the low-level cloudiness within the Hadley cell and the baroclinic regime. With the expansion of the Hadley cell, the area of enhanced tropospheric stability increases, resulting in more low-level clouds, higher SWCRE, and increased negative forcing. The non-monotonicity results from an intermediate decrease in the SWCRE caused by the disappearance of baroclinic eddies as the Hadley cell reaches global extension. At rotations faster than Earth-like, the decrease in SWCRE, mainly due to the weakening of baroclinic eddies and storm systems, leads to an increase in positive forcing. In summary, changes in the SWCRE, driven by different circulation responses at slower and faster than Earth-like rotations, strongly influence the TOA radiant energy budget. These effects, along with a substantial contribution from the clear-sky OLR, could impact the habitability of Earth-like rotating planets.
- Preprint
(1482 KB) - Metadata XML
- BibTeX
- EndNote
Status: open (until 07 Oct 2024)
-
RC1: 'Comment on egusphere-2024-2473', Osamu Miyawaki, 26 Sep 2024
reply
General comments
The authors explores the response of top of atmosphere energy balance to varied rotation rates in an aquaplanet with prescribed SST. They find that radiative forcing behaves non-monotonically with rotation rate. The sensitivity of radiative forcing to rotation rate is dominated by the shortwave cloud radiative effect.
The paper sheds new insight into the top of atmosphere energy balance response to varied rotation rates as previous papers have not performed a detailed decomposition of radiative effects into clear vs cloudy sky and shortwave vs longwave components. However, the current version of the manuscript does not adequately acknowledge relevant literature on this topic. For example, Williams et al. (2024) found a similar non-monotonic dependence on cloudiness (as measured by albedo in their paper, see their Fig. 4). Top of atmosphere energy flux is considered to a limited degree (see their Fig. 5). Therefore, I believe it is not accurate to state that “the role of the Coriolis force for a planet’s radiant energy budget has not been explicitly addressed, so far” as the authors have in line 29-30.
Additionally, the authors’ claim that “the magnitude of such [the effect of humidity and consequently the clear-sky outgoing longwave radiation caused by changes in the Coriolis force] on the TOA radiant energy budget has not been studied before” seems to be inaccurate given discussions of similar concepts in Guzewich et al. (2020) and Haqq-Misra et al. (2018).
I recommend the authors review these papers and the cited literature therein to more clearly identify the unique contribution that their work adds to the existing literature on this topic. I believe their results are useful and interesting; my remark is simply on the matter of accurately acknowledging similar work in the literature and synthesizing how the authors’ findings fit with existing results.
Williams, D. A., X. Ji , P. Corlies, and J. M. Lora, 2024: Clouds and seasonality on terrestrial planets with varying rotation rates. Astrophys. J., 963, 36, https://doi.org/10.3847/1538-4357/ad192f.
Guzewich, S. D., J. Lustig-Yaeger, C. E. Davis, R. K. Kopparapu, M. J. Way, and V. S. Meadows, 2020: The Impact of Planetary Rotation Rate on the Reflectance and Thermal Emission Spectrum of Terrestrial Exoplanets around Sunlike Stars. ApJ, 893, 140, https://doi.org/10.3847/1538-4357/ab83ec.
Haqq-Misra, J., E. T. Wolf, M. Joshi, X. Zhang, and R. K. Kopparapu, 2018: Demarcating Circulation Regimes of Synchronously Rotating Terrestrial Planets within the Habitable Zone. ApJ, 852, 67, https://doi.org/10.3847/1538-4357/aa9f1f.
Specific comments
Line 85: “the model becomes unstable for even faster rotations”
Can the authors briefly explain the source of the instability? Does it arise from a specific parameterization scheme that breaks down at parameter spaces far away from where it was designed to be used? Any insight into this would be helpful for readers who are interested in pushing models to extreme limits.
Line 132, 161, 198, 204: “(not shown)”
What are the reasons for not showing these results? I understand if the authors prefer to not show the result if they plan on publishing these results in a separate paper. Otherwise, these should be provided in a supplement.
Line 145: “The cell becomes more slanted at \Omega / \Omega_e = 8”
Can the authors be more precise about how slantedness is measured? It is not clear to me from looking at the streamfunction contours in Fig. 1 that the Hadley cell is more slanted for \Omega / \Omega_e = 8 compared to 4.
Line 159-160: “This is similar to Wang et al. (2018) except that we use 500 hPa instead of 800 hPa.”
What is the motivation behind evaluating eddy heat flux at 500 hPa vs 800 hPa? Are the results consistent when evaluated at 800 hPa?
Line 182: “the meridional temperature gradient associated to the baroclinic instability is reduced”
Is this because as the Rossby deformation radius decreases with increasing rotation rates, the latitudinal extent of the baroclinic zone shrinks? Whether this is the correct interpretation or not it would be helpful to explain this explicitly. It would also be useful if the explanation is illustrated in Fig. 3a (e.g., the latitudinal extent of the baroclinic zones could be indicated as points on the line).
Line 183-184: “This leads to a weakening of the vertical wind shear with increasing rotation rate.”
Is this supported by any results or figures in the manuscript? If not I recommend including a plot supporting this in a supplement.
Fig. 3a: Why does temperature increase with latitude very close to the pole in the \Omega / \Omega_e = 8 experiment?
Line 194-195: “the lapse rate is close to a moist adiabat over a wider range of latitudes”
Is this supported by any results or figures in the manuscript? If not I recommend including a plot supporting this in a supplement.
Line 258-259: “the synoptic-scale storm systems in our simulations become smaller, but more numerous and frequent”
Can the authors show results from their simulation that support the claim that synoptic-scale storms become more numerous and frequent with increasing rotation rate?
Line 261-262: “the reduction in storm-track cloudiness corresponds well with the reduction in vertical wind variance”
Vertical velocity variance changes do not seem to fully explain cloudiness changes to my eyes. For example there is a monotonic decrease in vertical velocity variance from \Omega / \Omega_e = 2 to 8 but cloudiness changes are complex. There is a decrease in low clouds but an increase in clouds at 400 hPa around 45 deg N for \Omega / \Omega_e = 8. Is vertical velocity variance changes expected to be more consistent with extratropical low clouds more than high clouds? A more nuanced discussion seems warranted here.
Fig. 9 and 10: Would it not make more sense to move Fig. 10d as a new panel 9c? These panels are discussed together in line 276.
Line 293-294: “At faster than Earth-like rotations, the extent of high clouds increases, but the cloud fraction decreases.”
As I indicated in my comment above, there are some regions where high clouds increases with increasing rotation rate. It would be helpful to show a height and spatially averaged metric of high cloud fraction similar to Fig. 10c to support this statement.
Line 294-295: “This is due to weakening circulation and weakening convective updrafts at all latitudes”
This contradicts the statement made in line 198: “The larger instability for faster rotation leads to increased convective activity and increased convective precipitation at higher latitudes”. Can you reconcile this contradiction?
Technical comments
Line 107: “SSTs, enables” → “SSTs enables”
Line 130: “mid-latitute” → “mid-latitude”
Line 157: “wind, \theta” → “wind and \theta”
All figures: Can the authors make the scientific notation labels on colorbars and axes larger? They are currently very small and difficult to catch.
Line 209: “LW cloud radiative effect varies (LWCRE)” → “LW cloud radiative effect (LWCRE) varies”
Line 221: “regions, where” → “regions where”
Citation: https://doi.org/10.5194/egusphere-2024-2473-RC1
Model code and software
mpiesm-landveg.tar.qz Abisha Mary Gnanaraj https://doi.org/10.17617/3.G5CAJW
Interactive computing environment
analysis.tar.qz Abisha Mary Gnanaraj https://doi.org/10.17617/3.G5CAJW
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 97 | 37 | 7 | 141 | 2 | 3 |
- HTML: 97
- PDF: 37
- XML: 7
- Total: 141
- BibTeX: 2
- EndNote: 3
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1