the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Diurnal evolution of non-precipitating marine stratocumuli in an LES ensemble
Abstract. We explore the impacts of the diurnal cycle, free-tropospheric (FT) humidity values, and interactive surface fluxes on the cloud system evolution of non-precipitating marine stratocumuli based on a large ensemble of large-eddy simulations. Cases are separated into three categories based on their degree of decoupling and cloud liquid water path (LWPc). A new budget analysis method is proposed to analyze the evolution of LWPc under both coupled and decoupled conditions. More coupled clouds start with relatively low LWPc and cloud fraction (fc) but experience the least decrease in LWPc and fc during the daytime. More decoupled clouds undergo greater daytime reduction in LWPc and fc, especially those with higher LWPc at sunrise because they suffer from faster weakening of a net radiative cooling. During the nighttime, a positive correlation between FT humidity and LWPc emerges, consistent with higher FT humidity reducing both radiative cooling and the humidity jump, both of which reduce entrainment and increase LWPc. The time rate of change in the LWPc is more likely to be negative for higher LWPc and greater inversion base height (zi), conditions under which entrainment dominates as turbulence develops. In the morning, the rate of the LWPc reduction depends on the LWPc at sunrise, zi, and the degree of decoupling, with distinct contributions from subsidence and radiation. Under well-mixed conditions, it takes about 10 h for the surface fluxes to offset 15 % of the changes in entrainment warming and drying, assuming no changes in transfer coefficients or surface wind speed.
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RC1: 'Comment on egusphere-2024-1033', Anonymous Referee #1, 04 Jun 2024
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Review of "Diurnal evolution of non-precipitating marine stratocumuli in an
LES ensemble" by Chen, Zhang, Hoffman, Yamaguchi, Glassmeier, Zhou and Feingold, Manuscript egusphere-2024-1033Summary:
An ensemble of large eddy simulations of marine stratocumulus clouds is run and analyzed for a variety of idealized summertime conditions over the Northeast Pacific Ocean. Building on earlier work by led by co-authors Hoffman and Glassmeier, the new simulations use a more realistic radiation parameterization, including a diurnal cycle, as well as interactive surface fluxes. The analysis seeks to understand how the diurnal cycle of cloud cover and LWPc (in-cloud liquid water path) are determined by the base state (sorting into bins with high/low decoupling and LWPc) and by different processes in understanding how different processes fix LWPc. Highlights include: daytime reductions in cloud fraction and LWPc are larger in more decoupled boundary layers. Larger free tropospheric humidity tends to support higher LWPc during nighttime.
Assessment:
The paper is well-written and nicely tells a story about how stratocumulus-capped boundary layers vary across the diurnal cycle. The manuscript is interesting and compelling, but I was struck that the paper focuses much more strongly on LWPc than on cloud fraction, which is presumably a stronger control on shortwave cloud radiative effects during the diurnal cycle in many of these simulations. However, I expect that another paper based on these simulations will tell that story later. I think the manuscript is a good fit for ACP and should be published, though I would ask the authors to consider the suggestions I make below along with those of the other reviewers.
Recommendation: Minor Revisions
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Major comment:
1. Regarding the LWPc budget:
1a. For the domain-mean budget, it is natural for the subsidence term to be based on the prescribed, large-scale subsidence. However, for the cloud-volume (CV) budget, I find it puzzling that the mean vertical velocity in cloudy columns was not used. I did not ponder this until the end of section 6.1 (on the uncertainty in the entrainment term). If the mean velocity profile in the cloudy columns was not saved, then keep the analysis as it is, but if not, wouldn't the analysis be more robust if the mean cloudy-column subsidence was used for the SUBS_CV term? Then, the ENTR_CV term would be based on the actual entrainment rate in the cloudy-columns, which might be more closely tied to the radiative cooling there. The authors are free to argue that the current setup is more compelling and/or that the differences are only important in the afternoon when the cloud fraction falls precipitously in some of the simulations, but I wanted to suggest this.
1b. The LWPc budget involves large terms with different signs (e.g., RAD, ENTR, BASE-n-LAT), with the important d(LWPc)/dt trend being the small residual after these large terms almost --- but not exactly --- offset each other. Since it's hard to visualize the result of such cancellation, I found myself seeking some way to understand the budget better. From this, I had two (very optional) suggestions:
- Instead of decomposing the dz_i/dt term into separate w_s and w_e as specified on line 272-273, include the whole d(z_i)/dt term in the budget. This is easier for the reader to understand and avoids another pair of opposing terms in the budget. Helpfully, this term switches sign from night to day in some of the simulations, so that changes in dz_i/dt --- driven here by changes in entrainment --- may play a role in the diurnal cycle of LWPc. Doing this would modify the ENTR term in the LWPc budget, since it would only represent the entrainment impacts on cloud base height through d<qt>/dt_CV and d<thetal>/dt_CV. Only make such a change if the authors find it clarifies the story about the budgets.
- (MORE SPECULATIVE) Re-group the terms, combining exchange within the boundary layer (BASE-n-LAT) with radiation (RAD) since both drive increases in LWPc. Both can also be seen as driving entrainment, both the familiar cloud top cooling through (RAD) and fluxes through cloud base (BASE). Write ENTR = - C_ENTR * (RAD + BASE-n-LAT), where C_ENTR is something like the entrainment drying efficiency of the (RAD + BASE-n-LAT) terms. Then, the budget for LWPc is dominated by SUBS and (1-C_ENTR)*(RAD + BASE-n-LAT), with variations in C_ENTR controlling whether LWPc grows or decays. Looking at the budgets in figure 8, C_ENTR seems roughly constant during the nighttime but varies across the different classes of simulations. I would hypothesize that the differences between C_ENTR across the simulations depends on things like the jumps across the inversion, free tropospheric humidity, decoupling and perhaps other quantities as well, so the differences in the outcomes, e.g., positive/negative d(LWPc)/dt, might be able to explained in terms of those quantities. As noted above, this is pretty speculative, but (if it works) could potentially make the storytelling a bit cleaner.
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Specific/minor comments (10/277 means p. 10, line 277):
1/3: My understanding is that the "cloud liquid water path (LWPc)" refers to the mean cloud liquid water path in cloudy columns. If so, could this be stated explicitly somewhere, even if it is broadly understood by others in the field. Also, please explain somewhere how a "cloudy column" is defined.
4/94: The broad range of initial boundary layer temperatures (10K) suggests that the initial boundary layer is relaxing towards quasi-equilibrium with the SST throughout the day, as suggested by the surface temperature jump in figure 4c. It's great to have such a variety of cases here, but I found myself wondering how such a transients might impact the results. Note also, that the mesoscale organization will also be developing through the nighttime and that this transient might have some impact on the results (e.g., the timing of precipitation development overnight).
10/277-278: The last subsection seemed to be foreshadowing a cloud-volume budget, so it's worth expanding BL and CV here for clarity.
Fig 6/bottom of page 10 (OPTIONAL): Given the degree of decoupling discussed previously, it's not surprising that the BL-budget predictions of LWPc are poor. If the authors agree, perhaps figure 6 could be removed from the paper, but a dashed or dash-dotted line could be added to the right column of figure 8 showing the d(LWPc)/dt prediction based on the BL-mean budget. That would make clear that the prediction is especially poor during the daytime and could be emphasized around the discussion of figure 8. However, if the authors believe that making this point clearly is important for the reader, please show the prediction of d(LWPc)/dt explicitly in the left and right panels of figure 6. Showing the actual and residual does not have the same impact as showing the actual prediction (for me at least).
Fig 8: My understanding is that the budgets in figures five and seven are closed by definition so that the sum of the process tendencies and the actual tendencies are identical. Since the LWPc budgets are not necessarily closed, it would be useful to plot the predicted tendency based on the sum of the individual processes instead of (or alongside) the actual tendency. If figure 6 is removed, both the BL-budget and CV-budget predictions of d(LWPc)/dt could be included in the right column of figure 8.
11/304: I don't understand what is meant by "The warming strengthens the stratification of the sub cloud layer". Is it a change in the surface temperature jump? Is the base of the cloud volume high enough that "sub cloud" includes the transition layer and part of the cloud layer? Does the sub cloud layer actually depart from being well-mixed?
14/401-407: Does decoupling explain part of the correlation of LWPc velocity and z_i, since deeper boundary layers are likely more decoupled?
14/z_i scaling: It's interesting that the z_i scaling works so well, but I found myself wishing it had been more clearly motivated. Why do we have so much faith in the z_i scaling of these budget terms when the BL-budgets did so poorly in predicting d(LWPc)/dt? As an aside, the suggestion above for computing C_ENTR resulted from efforts to understand the relationships between these budget terms better and seeking an alternative to the z_i scaling.
16/section 6.1: See major comment 1a above.
18/538-539: Glassmeier et al (2021, https://doi.org/10.1126/science.abd3980) seem to argue that long timescales are important for LWP adjustments. In such circumstances, the surface flux adjustment might play a role in modifying the steady-state LWP relative to one computed using fixed/prescribed surface fluxes. If it's feasible, the authors could make an estimate of how the inclusion of interactive surface fluxes might have impacted the slope of the predicted d(log LWP)/d(log N) in Glassmeier et al.
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Typographical/rephrasing suggestions (OPTIONAL):
14/404: "... sufficiently high to suppress _cloud base_ precipitation ..."
Citation: https://doi.org/10.5194/egusphere-2024-1033-RC1
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