Editor Initial Decision: Publish subject to minor revisions (Editor review) (24 Jun 2010) by
Timothy J. Dunkerton
Comments to Preliminary review of "An aircraft case study of the spatial transition from closed
the Author: to open mesoscale cellular convection" by R. Wood, C.S. Bretherton, D. Leon,
A.D. Clarke, P. Zuidema, G. Allen and H. Coe
This observational study describes the macrostructure and microphysical properties
of a stratocumulus (Sc) cloud deck in the lower troposphere in the SE tropical
Pacific during the VOCALS-REx field campaign, using data from a pair of
consecutive flights. Flight patterns were designed to crisscross a boundary between
an overcast region of "closed" Sc and a nearby region of "open" cells bounded by
intermittent patches of low-level cumulus (Cu) and partly covered by a thin
"stratus" layer in the upper part of the MBL.
The paper is comprehensive, illustrating results from many of the onboard
instruments, and its liberal use of color exploits the excellent graphical capabilities
of ACP. It will be acceptable for publication in ACPD when the following
comments "FIXES & IMPROVEMENTS" are addressed. The remaining
comments "SCIENCE & PRESENTATION" are optional and are offered for
consideration by the authors at any stage of the review process.
Due to the length of the paper, there are several comments to address, enumerated
by page number. If needed, please request additional time from the Copernicus
staff for submission of the revised manuscript.
Dear Editor,
Thank you for your careful editing of our manuscript, and for taking the time to do
so a detailed review. This will undoubtedly improve the manuscript quality. We
respond to your specific comments below, and we’ve incorporated most of your
suggestions into a revised version which is uploaded along with these responses.
Some of the scientific changes we will leave until we see the reviewer comments.
Sincerely
Rob Wood
FIXES & IMPROVEMENTS
(page.comment)
1.1, title: May want to append "in the SE subtropical Pacific" for precision.
Although geographically in the tropics (18 S) the region is better described as
subtropical, in accord with prior usage (Wood et al, 2008 JGR D12207 page 1).
Added “over the southeast Pacific”
1.2: "closed" vs "open" needs clarification in the text. Is it a vertical opening (in the
sense of overcast vs ~clear sky), or a horizontal one (in the sense of hexagonal
cells vs semicircular arcs)? Prior usage suggests the former (the 2008 paper
provides the definition for reader's benefit).
This is well-enough established in the literature that I don’t think it necessary to
define open and closed cells.
---2.1, line 3: remote SE subtropical Pacific
Changed
2.2, line 6: "well-mixed" is ambiguous. On first reading it suggests that the Sc deck
is turbulent (at end of Abstract) but later on, vertical profiles show a well-mixed
MBL at all levels in the overcast region. In any case, you may want to delete "wellmixed" here anyway, since the sentence has a lot of adjectives already.
Changed to “....between a well-mixed MBL containing overcast....”
2.3, line 10: recommend that words be re-ordered as "...transition zone with
distinctly different structure from the two air masses on either side."
Done
2.4, line 12: My initial mental picture of the situation as described here, of a
shallow Cu reaching and penetrating a layer of stratus above, isn't borne out by the
observations presented in the paper. See comment 12.2 below.
The cells in the POC have strong updrafts and downdraft shells around them, grow
out of the subcloud layer, and have tops which are elevated above the surrounding
cloud. This seems consistent with penetrating cumulus.
2.5, line -10: can probably delete "clouds": In the POC and transition region,
although the majority...
Done
2.6, line -9: Here and later on (page 19, 2nd para) the meaning of "adiabatic" needs
clarification.
Replaced with “....although the majority of the condensate is in the form of drizzle,
the integrated liquid water path is remarkably close to that expected for a moist
adiabatic parcel rising from cloud base to top.”
---3.1, line 6: Regarding the apparent influence of POC (and its spatial transition
boundary) extending into the overcast region, there's an artificial aspect to this,
created by the compositing technique that uses the leading (POC) edge of the
transition zone as the reference line. The width of the transition zone varies from
one flight leg to the next, while a sharp gradient exists at the back edge of each
individual leg, regardless of its distance from the leading edge. Compositing with
respect to the leading edge smears out the gradient on the back side and creates the
appearance of an influence extending into the overcast region.
On the one hand, it is problematic to argue that the observations demonstrate
lateral mixing. On the other, it is plausible (because the back edge is ragged) that
lateral mixing could eventually lead to a horizontal exchange of properties with
respect to an Eulerian line (i.e., best fit line to trailing edge).
I agree with the potential artifact of choosing the leading edge to composite. I will
give this some more thought and see what the other reviewers have to say before
deciding how to proceed in terms of modification.
---5.1, line 3: MBL by suppressing
5.2, line 14: have advanced the assertion (Squires, 1958a,b)...
5.3, line 15: could limit precipitation, and have opened...
All done
---6.1: The sentiment of this paragraph, expressed clearly enough, is that while
aerosols are required for warm cloud formation, cloud **variability** is not
necessarily driven by aerosol variability.
Given that cloud variability creates aerosol variability, an important question that
remains whether meteorological factors, including externally imposed ones, are
primarily responsible for the cloud variability. More on this point later. Here I
recommend you create an "opening" for dynamical influences, to keep the idea
alive.
Changed to “Therefore, we would expect precipitation differentials, which might
be driven by dynamics, to drive CCN differentials”
6.2, line 15: only ~1.5 days
Added
6.3, line -3: SE subtropical Pacific
Added
---7.1, line 7, opening of 2.1: If you wish, some brief info on aircraft basing,
departure and recovery times would do justice to mission personnel, and reinforce
the notion of "remoteness" in this region of the world.
7.2, line -4: ...westwards of 79 deg W, as seen in the MODIS image (top panel).
Added
7.3, line -2: "evolution" suggests time transition, but the spatial transition is the
primary aspect (as in title). Could say "The juxtaposition of a narrow..." and so on.
The duration of the event (>40 h) is noteworthy too (Stevens et al 2005).
The suggestion of time evolution was intentional.
---8.1, line 13 & thereabouts: If I understand correctly, (i) the low-level clouds (e.g.,
Figure 4) drift to the NW with the low-level SE'ly flow. (ii) The sign convention of
u,v in Tables 3 & 4 is that wind components are positive eastward and northward,
respectively. (iii) The aircraft drifts a bit NW relative to a perfect 30/210 heading
in order to keep up with the low-level clouds & flow. However, (iv) at the end of
each leg the aircraft's turning maneuver creates a slight upstream (SE-ward)
displacement relative to the air mass. (v) In Figure 4, the back-and-forth flight legs
shown in yellow (superposed on the cloud imagery) and made to appear perfectly
parallel were made thus by backward advection (against the low-level flow) in
order to approximate the aircraft position **relative to the cloud pattern at the
particular instant the image which taken**. (This trick is justified because the
cloud pattern changes slowly with time in the flow-relative frame.) (vi) The upper
left panel shows a flight segment in the Earth-relative frame.
If this interpretation is correct, the heading is not exactly 30/210 in the Earthrelative frame.
I’m a little confused what the issue being raised is. The headings are approximately
30/210, but not exactly. I can give the exact values (slightly different for each leg)
but I don’t think these are particularly important.
Incidental comment: A brief word on the flow pattern would help, perhaps in
connection with the full-disk image of Figure 1. Is the large circular feature over
the SE Pacific cyclonic or anticyclonic (i.e., is the bright cloud at lower or upper
levels? ...visible is ambiguous). Interestingly (what is presumably) the high cloud
in Figure 4 drifts to the SE, opposite the low-level flow, suggesting a 1st-baroclinic
mode here.
The upper level flow is not strongly related to the low level flow on which we are
focusing. A discussion of the upper level flow is beyond the scope of this study.
---9.1, line 12: Are the first two bins large or small particles?
I removed the word “small”
9.2, line 14: "60 s" This is one of the few places (perhaps the only place) where we
gain a sense of sampling resolution. Horizontal resolution of the measurements can
be estimated from their time duration and the aircraft's speed. Later on it would be
help to know the horizontal resolution of sampled cloud features; e.g., underside of
overcast. If you have visual recollections of the scale of cloud features, that would
be helpful too.
The aerosols required the longest samples of all. Most other variables were
measured at 1 Hz or better, so we did not dwell on a discussion of these.
Added the following “All measurements were analyzed at~1~Hz time resolution
unless otherwise stated.”
---10.1, line 7: On the same point, can you estimate the width of a "column"?
Added info on horizontal resolution of the radar data
10.2, line 14: Recommend that WCL be mentioned explicitly in Figure 11 caption
also (as WCR was): "Cloud base data are from the cloud lidar on the two subcloud
legs." Otherwise the caption is rather cryptic regarding use or otherwise of WCL
data.
Done
10.3, bottom: Classification begs the question of what to do with drizzle in cloud...
a common experience landing at Sea-Tac in winter (:
----
11.1, line -9: CB
Done
11.2, line -2: location of the transition region inferred from WCR imagery
---12.1, line -11: Stronger reflectivity generally implies a deeper structure, certainly
with regard to drizzle fall. There's also a suggestion of higher cloud top height over
the core regions, which is presumably responsible for the appearance of threedimensional cloud top in visible imagery.
I’m not sure how to determine a three-dimensional structure from visible imagery,
other than shadows. Other than those cast from high clouds on the lower ones, I
don’t see clear shadows from the low clouds alone.
12.2, bottom: Since you mention WCL here, I'll make a couple of comments. There
are relatively few WCL measurements shown for reader's benefit (e.g., Figure 12)
but the paper frequently alludes to optically thin stratus in the POC. Presumably
the higher cloud fraction (60%) comes from the thin stratus contribution.
Yes
Regarding the thin stratus itself, as noted above, the MODIS image suggests wisps
of visibly thin cloud sometimes connected to the POC cumulus. The POC-like
region to the NW does not have such features. Moreover, some thin (presumably,
low) cloud is evident elsewhere in the image, NE of the overcast Sc deck and
perhaps over the Sc deck itself. These things are a bit hard to disambiguate, but all
in all, the schematic of thin St in Figure 20 is not supported by these observations,
as shown.
I disagree. Inside the POC the visible imagery (e.g. Fig. 4) is quite dark, but we
know from the WCL that the cloud cover is 60%. MODIS data in open cell regions
also show significant cloud cover from optically thin (tau<2) clouds. Thus much of
the cloud in the POC must be quite optically thin.
Perhaps you've examined more WCL data than shown here, and feel confident
about the thin St assertion. The reason I've included this as an "editorial" comment
(rather than purely scientific one) is that the reader has to affirm your conclusions
based on observations you show, and take un-illustrated points on faith. I'm happy
to be corrected on this point, if you feel that the existing figures are convincing
with regard to the thin St.
While we're on the subject, I'm curious to know whether aerosol sampling is
reliable in the thin St layer (generally the cloud and aerosol data, as shown, do not
intersect). It would be interesting to know of the thin St layer is also "ultra-clean".
The thin St layer has cloud droplet concentrations comparable to the concentration
of accumulation mode aerosols in the ultraclean layer, so I would probably
conclude that the cloud is also ultraclean. Thus, the ultraclean layer in the
schematic diagram transcends both the clear and cloudy regions.
---13.1, line -6: southeasterly
13.2, lines -2, -1: estimated sensible & latent heat fluxes
Both corrected
---14.1, line 8: The radiative flux divergences adjacent to nabla F in Table 3 are
obtained by differencing the net (up minus down) fluxes from different flight legs.
The divergence is meaningful only if the clouds encountered are the same.
True. Would need two aircraft to make this estimate.
14.2, bottom: Interesting remarks about deformation. You could actually evaluate
the deformation. Individual (normal & shear) components S_n, S_s are not
invariant under rotation, but the sum of their squares is.
Can you elaborate? I’m interested to hear this, but don’t quite follow your
argument here.
---15.1, line 6: "well-mixed" again. Presumably the entire MBL is well-mixed, noting
the constancy of theta_l w/ height.
Yes, and I make this clear now.
15.2, line 12: On the face of it, we would not consider 2K a "strong" cold pool.
Perhaps though, it appears such, in the local context.
Given that everything is scaled down in the MBL compared with the deep
convective case, 2K is quite a significant difference I think.
15.3, line -10: I can see the higher q_t but not the lower theta_l.
Averaging points in Fig. 7c shows this. Perhaps difficult to see in the plot.
15.4, Figure 7 & 8: Can't recall whether LCL was defined or discussed anywhere,
other than shown in the figures.
Mentioned now in caption of Fig. 7
---16.1, top: Nothing was shown or tabulated regarding trace gases.
This was intentional, since we do not particularly trust the absolute values
(especially for comparison between B409 and RF06).
16.2, top: What does "also" refer to?
Removed “also”.
16.3, line -13: "increases upwards/downwards" The observation may be
reasonable, but was derived from different flight legs.
This conclusion does assume a steady state.
16.4, line -8: In the POC and transition regions, as inferred from cloud-level flight
legs ?
Added
Presumably this statement refers to the upper band of symbols 1000-1400 m.
However, one of the triangles (the upper one, for transition region) has larger q_L.
The point is probably correct, but could be demonstrated more convincingly using
the underlying probability distributions, instead of their means only. As a side
benefit of PDF analysis you could demonstrate that the means differ significantly
in a statistical sense.
Would require more space to show pdfs of liquid water content, although I agree
that this could be done.
---17.1, line 6: Puzzling remark. Your results demonstrate clearly than POC Sc have
much higher precipitation (so why revise the thinking of Stevens et al?). The
problem here seems to be that the POC precipitation estimate has been diluted by
averaging over the whole POC area. Or am I missing the point?
Yes, the issue is that the Stevens paper suggests that POCs might delineate regions
where the mean precip rates are elevated. This work suggests that the surrounding
areas may also have significant precip.
17.2, 1st para: Please move this paragraph after the next one, to maintain proper
ordering of figure citations, and to finish the thoughts on Figure 9 before
introducing Figure 10.
Done
17.3, 2nd para: The in situ measurement of precipitation **rate** deserves
clarification (if not already done). One can measure the liquid water concentration,
but how is the downward flux of water through a horizontal surface measured?
E.g., are assumptions made regarding fall velocity?
Yes, we use a fall velocity-radius relationship (well understood for water drops). I
added a sentence to describe this in the “derived products” subsection.
---18.1, line -12: Was cloud top height from the WCR defined in terms of dBZ
somewhere in the text?
We now discuss this in the “derived products” section.
18.2, line -4: Here is an instance where visual recollections or photographs of
cloud base (e.g., "fractus") texture might help define actual horizontal scales, not
just the instrumentally resolved scales.
Good point. Added a few words.
---19.1, line 2: "stratiform clouds aloft" presumably does not refer to thin St, but to
features like that in Figure 5, left column (C2 leg, POC region, 60 km to left) and
Figure 21 (cell “C” at far right). In terms of reflectivity, this feature resembles
some moderately strong features in the right column (i) AC leg, overcast region, 50
km to right; (ii) C1 leg, overcast region, 85 km to right.
Both, since the lidar will detect both stratocumulus (e.g. as also detected by the
radar on the overcast side) and thin stratus not visible on the WCR cross section
composite. I now make this clearer.
19.2, bottom: with elevated (word missing).
Missed this initially. Have completed sentence.
---20.1, line 13: no overcast values above 500 g/m^2 are available.
But those from 400-500 g/m2 are not statistically different from those in the POC.
20.2, lines -9 to -10: thicker/thinner in LWC-speak.
Since the clouds appear to be largely adiabatic, these two are more or less
interchangeable.
---21.1, line 8: Recommend that r_e be defined in Table 5 caption also.
Do---22.1, line 13: Figure 16
Sorry, typo in .tex file.
---23.1, line 11: Cannot the CCN be replenished by evaporating droplets? Sorry to
ask such a dumb question.
Sure, but remember that any drizzle drop that is created by collision-coalescence
will remove thousands of CCN. Even if it evaporates it will leave a single CCN
behind. This is a significant loss term for CCN.
---24.1, line 1: Scavenging is irreversible to the extent that precipitation reaches the
surface.
Or even if it doesn’t (see previous comment).
---27.1, line 3: Sorry I can't recall, but are "the more widespread and quiescent
clouds" in the POC mentioned anywhere in the **prior** text? If not, the reader
may confuse with thin stratus in upper POC which has been mentioned several
times.
I have tried to clarify this.
27.2, 1st full para: Beware of comment 3.1 above.
27.3, line -7: Figure 23
Corrected
27.4, line -3: Yes, Figure 18a suggests largest N_d on the POC side of the
transition (perhaps another compositing artifact)... but the presence of ultra-clean
air immediately inside the POC is striking, and argues against lateral mixing on
this side of the reference (zero) line.
I am worrying more about the possible artifact now. Let me think about this.
27.5: On figure citation order: strongly discourage out-of-order figure citations.
Here and on following pages it would be easier to re-number Figures 20-23 in
accord with the text as written.
Done
---28.1, line 6: Andrew
Added proper citation now.
28.2, line 13: Table 4
Thanks. We added another table but I didn’t use the \ref command for this.
28.3, line 13: A close look at Figure 19 c suggests (i) a median value of w in
overcast region is about -0.2 m/s (100-200x the area-mean subsidence rate
estimated in the text); (ii) POC w has a fat tail on the positive side (consistent with
some strong isolated updrafts).
The aircraft cannot reliably give mean values to better than a few tens of cm, so I
do not trust that estimate. The fat tail certainly seems quite robust though.
---29.1, line -11: Figure 19 suggests that the overcast w distribution is ~twice as wide
(accounting for its negative mean offset) so perhaps the variance is four times
larger than POC? At least we're in the same ballpark.
Good to see some consistency.
---30.1, 1st para: Precipitation shafts generally curve inward while falling (with a hint
of flare-out at the surface) suggesting that the mid-level inflow is much larger than
the near-surface divergence.
It is also difficult to see the near-surface divergence from the radar since the
strongest winds are usually very near the surface where the radar has some
problems with clutter. Also difficult to get sense of mean cell conditions from one
snapshot.
30.2, bottom: The idea of bimodal distribution in the POC cloud base seems to
have gotten lost after Figure 11, so perhaps you could attempt to tie this
observation to the POC deep/active vs deep/quiescent vs half-height cloud
(comment 19.1) vs thin stratus, and (hopefully) explain the bimodality in terms of
some combination of these POC cloud types.
I changed to “Thick clouds in the POC consist of....” and added some words later
in the paragraph to discuss the thin stratus.
---37.1, Seinfeld and Pandis reference incomplete (current ACP begins 2001).
It is to their book of the same name as the journal. Reference now complete.
---40.1, Table 1, below top line: add [km] below the three L's.
Done
40.2, Table 2, Sorting by time would be clearer. Your call.
You’re right. I changed this now.
---41.1, next to MBL structure: Clarify whether the surface layer extends to 600 m or
(more likely) the stratified layer extends to 600 m above the surface, above the
surface layer.
Clarified
41.2, next to Cloud macrostructure: some words are garbled
Corrected
41.3, next to Cloud cover: recommend giving two types of data value, separating
the thin St contribution from the active Cu part. A single number doesn't do justice
to the POC heterogeneity.
This is actually not trivial to do, but let me think about this for the revisions.
---45.1, Table 5, r_e units = micron
Corrected
---46.1, line 2: Anticlockwise from bottom right
Good spot. Changed.
46.2, line 5: advection of oval or measurements? please clarify
Clarified.
---47.1, line 2: 18 hours before flight B409
Changed
47.2, caption: mention contours of SLP (this gives the uneducated reader their first
inference about low-level wind direction)
Added
---52.1, lines -2 & -3: define "boundary". Distances are larger than those shown in
Figure 5, etc... is the POC profile actually outside the POC?
Changed to distance from leading edge as defined in text. The POC profile is in the
open cell region, see Fig. 4, but only just.
---57.1, line 3: max reflectivity height shown with ovals
Added
----
58.1, Figure 13: define vertical error bars
Defined
---66.1, Figure 21: labels for cells A & B (and “C”, far right) aren’t obvious (probably
aren’t needed anyway).
Changed this.
---67.1, line 3: AC
Corrected
---68.1, line 3: equivalent
Corrected
SCIENCE & PRESENTATION
SP.1, on POC heterogeneity: There's a lot going on in the POC, different cloud
types and considerable variation of microphysical properties **within** the POC,
to say nothing of contrasts to the other regions. We've seen evidence in the paper
of active Cu, quiescent Cu, half-height cloud resembling moderate-strength cells in
the overcast region, and the "contentious" thin stratus. Because of the
heterogeneity, horizontal averaging must be performed with care. But there's
plenty of data, enough to construct conditional probabilities or joint PDFs using
two or more variables at once. If you prefer to leave further study of the POC to
another paper (and it's best that you do) then it would helpful to assemble the
information on different POC properties in relation to cloud structure in condensed
form (e.g., a table).
SP.2, on aerosol-cloud-precipitation interactions: A similar comment pertains to
inferences made, here and there, concerning a-c-p interactions: they are scattered
around, and won't benefit busy readers who lack to time to find them. A condensed
summary of inferences (and caveats attached to them) would help.
SP.3, on POC structure: Being unprejudiced on the subject of POCs, their opencell structure suggests to my naive mind a mirror image of the closed-cell structure.
The latter has air detraining and sliding down around the puffy cell; the former has
updrafts around the periphery of the cell, where the active Cu form and from which
the wispy thin St extend. Is this notion silly, or dead-on?
SP.4, (freebie) on oscillations of w and N_a in POC outside the active Cu: hints of
gravity waves on the strong vertical gradient of N_a (Figures 17, 18, 21). Perhaps
there’s another paper to be written on this subject.
However, before getting engaged in vertical velocity we need to understand its
measurement, and the reliability of the aircraft data, which in itself may be
contentious. The paper accepts these “standard” meteorological measurements
without comment.
SP.5, again on POC structure, in the horizontal plane: Flight measurements,
whether in time series or “curtain” form, dull our mind to the crucial fact that the
things seen are part of a horizontal matrix. So, what can be done to remind readers
of the mesoscale patterns being sampled **within** the POC and overcast
regions? Can data be analyzed with respect to individual cell boundaries? Would it
make sense to do so?
You make some good points here. I’d like to do more in the revisions to try to
quantify the roles of the different POC cloud types. Perhaps including a cloud
fraction not only from the lidar (all clouds) but also what fraction is seen by the
radar, what fraction has drizzle, etc. I’ll work on this in the meantime.
Your comment on gravity waves is also an interesting one. We next plan to do a
study with other POCs cases, and it will be a good time to look for waves. I’ll try
to see if we can find signatures of waves in quadrature between w and T. I do
expect to see them since the upper MBL is stable. In fact, they may be an important
part of the whole POC control process.
In summary: there's much pedagogical value to this paper; it’s practically a
textbook in itself. We appreciate your submission to ACP.
--Tim Dunkerton
(tim@nwra.com)