the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Feb 2024
Microphysical view of development and ice production of mid-latitude stratocumulus during an extratropical cyclone
Abstract. The microphysical properties associated with the ice production importantly determine the precipitation rate of clouds. In this study, the microphysical properties including the size distribution and particle morphology of water and ice for stratocumulus during an extratropical cyclone over the northern China were in-situ characterized. Stages of cloud were investigated including young cells rich of liquid water, developing and mature stages with high number concentration of ice particles (Nice). The Nice could reach 300 L-1 at the mature stage, about two orders of magnitudes higher than the primary ice number concentration calculated from ice nucleation. This high Nice occurred at about −5 to −12 °C, spanning the temperature region of Hallett-Mossop process and possible other mechanisms for the secondary ice production (SIP). The Nice was positively associated with the number concentrations of large graupel with diameter (d) > 250 μm and large supercooled droplet (d > 50 μm). The SIP rate was 0.005-1.8 L-1s-1 derived from the measured Nice with known ice growth rate between two sizes. The SIP rate could be produced by a simplified collision-coalescence model within an uncertainty factor of 5, by considering the collection of large droplets by graupel. The collection efficiency between was found to increase when the size of droplet was closer to graupel which may improve the agreement between measurement and model. Importantly, the overall Nice was found to be highly related to the distance to cloud-top (DCT). The level with larger DCT had more abundant rimmed graupels falling from the above level, which promoted the coalescence processes between graupels and droplets, producing a higher fraction of smaller ice through SIP. This seeder-feeder process extended the avalanche SIP at lower temperature up to −14 °C beyond the temperature region of Hallett-Mossop process. The results illustrated the microphysical properties of clouds with convective cells under different stages, which will improve the understanding of the key processes in controlling the cloud glaciation and precipitation process.
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RC1: 'Comment on egusphere-2024-314', Anonymous Referee #1, 19 Mar 2024
It is good to see a set of microphysical measurements being reported.
They are difficult measurements to make and are valuable for checking
and improving models.Many assertions are made in this paper without evidence. For example it
is assumed that there is evolution in time of microphysical
characteristics from one cloud region to a different region studied.
There is no evidence presented to support this assumption.There are many errors in the written English which need to be corrected.
I felt unable to review pp 8-12 of the paper until the comments below are
addressed. It is important to state what has been changed on pp 8-12 in
response to the above comments if appropriate.
Detailed comments.Line 13. Are the clouds stratiform with embedded convection?
Line 18. Some definitions have graupel only forming when d > 250 um.
They are not large at that point.Line 21. Between what?
Lines 23-26. This is conjecture.
Line 80. As above, stratiform with embedded convection? They are not
stratocumulus clouds.Line 82. Does the temperature probe become wet in cloud? Most
temperature probes do suffer from this problem.Lines 98-99. What is the distance of the radar in Beijing to the cloud
system studied?Lines 114-116. What are the errors of the mass of ice determined in this
way?Line 136. It would be helpful to show the location of Beijing in Figure
S2. Although the text mentions that Figure S2 shows the movement of the surface cold front, the location of the surface cold and warm air is the
same over the 4-hour period for the two flight segments. Is this
relevant?Lines 137-140. Is Figure S3 essential? Perhaps add the location of Outer
Manchuria and the surface cold front if the figure is kept.Lines 143-144. The sentence is not clear. Where was the aircraft
observation area?Lines 151-153. It would be helpful to provide the times of the radar
plots and indicate key times of the aircraft tracks to show the
development of the system.It looks like the developing cells were observed in the southerly part
of the system, whereas the mature cells were observed in the northern
part. It is assumed, I think, that the clouds have the same dynamical
and microphysical properties in the two regions. It would be good to
discuss this point in the paper.Line 154. I don't think "evolved" is the correct word? Is it the case
that the aircraft passed from a region with dominant LWC to one with
dominant IWC? It is also curious why the LWC was so low in "developing
cells". Is the cloud base altitude known?Lines 154-155. There is no evidence given that the LWC was "consumed by
the growth of ice crystals". Is it the case, from the evidence in Fig 2,
that the aircraft made passes through a different region of cloud (at a
later time according to Figure 3), and measured higher values of IWC
than LWC?Lines 155-156. It isn't clear from Fig 2 that the radar reflectivity was
enhanced. Is this enhanced from the same region in Fig 2a1 and 2a2?
The values of radar reflectivity would be helpful.Line 156. How is it known, From Fig 2 that "the ice phase precipitation
process occurred"?Line 159. Figure 3 contains a lot of detailed information. It would be
helpful to include shorter time series with the details expanded. For
example, what is the structure of the region with a strong downdraft and
updraft. It isn't clear where the peaks in LWC and concentration of ice
particles occur relative to the updrafts and downdrafts. Also, is the
strength of the downdraft real? Have the vertical winds been filtered
for aircraft turns? It might be helpful to add vertical dotted lines in
Fig 3.Line 160. Is "Developing cells" a relative term since the same strength
of radar echoes appear as in the mature cells, and the clouds are
already quite deep with the top of the radar echoes at about 8 km. Are
there radar echoes from earlier and later times of the cells penetrated
in S1 just after 10 local?Line 170. Isn't it the case the S1 and S2 were in different cloud
regions? Is it correct to say "developed"?Line 172. There are still a few cores with reflectivity values close to
30 dBZ.Line 73. It is very surprising that the LWC values are so low with
vertical winds of +10 m/s. Is it actually the case that the cloud has
suffered significant entrainment and conversion to precipitation?Lines 174-175. There is no evidence that the liquid was consumed by
producing ice. Is it possible that the downdraft is a region
affected significantly by entrainment and the ice particles were
transported down from above?Line 177. There is no evidence that the drops in S1 grew to larger drops
and were consumed by ice in S2.Lines 178-179. Is the cloud measured in S4 really stratocumulus cloud?
Line 181. Some of the cells at the beginning of S3 do not appear to be
dissipating. Cloud tops are still above 8 km and the radar echoes are
approaching 30 dBZ (which is not a high value, but similar to the values
in S2).Line 181. S4 includes the first region of (weaker) radar echo with a top
above 6 km. Why is it treated as a young cell?Lines 183-184. The last sentence in this paragraph doesn't make sense.
Lines 188-190. Again, there is no evidence.
Line 195. No evidence of "consumed".
Lines 195-196. There is no evidence of "vigorous development of the
precipitating cloud".Lines 209-210. A more accurate statement is that there was an increase
in N_round at two levels. Fig 3 suggests one of those might be in a
region of low LWC and N_FCDP, and higher concentration of ice particles.Lines 221-222. What is the evidence for the statement.
Lines 222-225. It should be remembered that the pass through the cloud
regions are snapshots in time. There is no evidence of "... leading to
more small ice through the H-M process...". It is only a suggestion.
There is history to consider with vertical and horizontal transport.Citation: https://doi.org/10.5194/egusphere-2024-314-RC1 -
RC2: 'Comment on egusphere-2024-314', Anonymous Referee #2, 15 Apr 2024
Review of "Microphysical view of development and ice production of mid-latitude stratocumulus during an extratropical cyclone" by Du et al.
This manuscript presents airborne cloud microphysical measurements measured in a mid-latitude extratropical cyclone over China. The authors use the data to explore mechanisms responsible for ice production in different regions of the cloud field and make efforts to link the observed differences to the temporal evolution of the microphysical properties. They show compelling evidence of active secondary ice processes (SIP) in the cloud studied and I particularly liked the section on the production rate of secondary ice. That said, I do have some significant concerns about the analysis that I feel the authors need to address before this manuscript can be considered for publication.
Major comments
- The authors need to provide evidence that the observations from different regions of the cloud field are showing the temporal microphysical evolution of the cloud microphysics, rather than just presenting measurements that simply document the horizontal variability of cloud properties in the wider cloud field i.e. effectively measuring different clouds. This is key to how the discussion of the observations in the paper is structured, and I am not convinced that the data can be linked together in the way the authors propose. As a result, many of the discussion points made in the paper are speculative. I did wonder if using the ground-based radar measurements to track the temporal evolution of the clouds sampled by the aircraft (before and after the aircraft measurements) might at least enable the airborne data to be put into better context with the “local” cloud development.
- Are these clouds best described as stratocumulus as stated in the title and various other parts of the manuscript? There certainly seems to be convection embedded in the cloud field e.g. updrafts of 10m/s in Fig 3. Would convection embedded in widespread (post-frontal or frontal?) stratiform cloud be a better description? It might be useful to see some satellite imagery of the cloud field.
Additional comments
- Line 57: What is meant by “on top of the convective core”?
- Line 99: Is the spatial resolution of 1km in the horizontal? If yes, what is the vertical resolution at the typical aircraft location?
- Line 106: How good is the circularity threshold of 1.2 on removing out of focus drops i.e. as those show in the imagery in Fig 6? Have the authors performed any visual examination of particles classed as irregular for example?
- Line 116: Is a different M-D relation used to calculate IWC for the different habits?
- Line 119: Do the authors use the PCASP data for the calculation of INP? If so, where are these measurements located in relation to the cloud microphysics measurements?
- Fig 1: The caption refers to a blue line, but there is an orange line on the figure.
- The authors refer to both figures in the supplement as e.g. Fig. S1, S2,…etc and stages of the cloud development as S1, S2,….etc. I suggest that the authors differentiate these in any revision.
- Line 142-143: It is stated that “aircraft observation area was situated behind the cold front” and “aircraft sampled clouds formed….before the surface cold front”. These seem to say the opposite thing. Clarification is needed.
- Line 149: Give more detail on how the different stages are defined.
- Fig 2: Can you indicate the times of the radar data on the figure? And what altitude is the reflectivity data from? Is it at the height of the aircraft data in each stage or is it at a fixed altitude?
- Line 154: What does “evolved with almost opposite trend” mean?
- Line 160: What does “can tell the location of aircraft in cloud” mean?
- Fig 3: What is the uncertainty in the vertical velocity (w) data shown in Fig 3. When looking at the time-series, there seems to be a general negative bias in w. Were any level runs out of cloud performed to see if there was an offset? Also, the uncertainty in these types of measurements is often large when aircraft are not flying straight and level, and Fig 2 shows that there were several large turns and profiles made during the flight. Has this data been quality-checked?
- Line 168: How sensitive is the fraction of smaller ice to the 180 micron threshold?
- Line 171: States that S2 is the most “vigorously developed clouds”, yet the largest updrafts and downdrafts were in S1.
- Line 175: The statement of consumption of liquid water in producing ice in the downdraft region is speculative. Could this just be ice precipitation from above?
- Line 177: Statements such as “The droplets at S1 grew to large droplets and were consumed by ice at S2 during the development of cloud” are speculative. Unless it can be demonstrated that the clouds measured at S1 were advected into the region of the measurements at S2 using e.g. trajectories, then these measurements cannot be considered to have been made in the same cloud.
- Line 183: The measurements with the high drop concentration were also made at warmer temperatures ~ -3C and so it is perhaps not surprising that no ice was measured.
- Paragraph at line 185: Speculation in statements linking different clouds to stages of development.
- Line 192/Fig S5: The MODIS satellite imagery shows that there was large variability in cloud properties over the region sampled by the aircraft, which again highlights that it is not straightforward to link the observations in terms of stages of cloud development.
- Figure 6: The overlap between the FCDP and 2DS measurements is poor in the majority of example size distributions. Do the authors know why this is the case?
- Figure 6: There are many examples of out-of-focus drops (circles with holes in the centre). How were these handled in the processing of 2DS data?
- Line 217 and the INP spectra in Fig S6. Is this calculated from the Equation on page 4 using the PCASP aerosol concentration measured, and then increased by a factor of 10 to account for uncertainty in the measurements of Demott? And does it therefore represent a likely upper limit on primary INP concentrations?
- Line 233: but you do not know where this ice was generated and if it had been transported from other parts of the cloud e.g. that could have been in the H-M zone.
- Line 240: what upper layer?
- Line 260: Is DCT just a proxy for location with respect to convective cores? And if so, does it just illustrate the microphysical processes in the convection are different to the more widespread stratiform cloud? If so, I might expect a correlation between DCT and updraft strength or turbulence, but it is not obvious that is the case from Fig 3.
- Line 269: Again, speculation.
- Line 275: But the aircraft is measuring different clouds and so there could be many reasons why the ice concentration is different from penetrations made at the same height.
- Line 312: droplet > 25 microns?
- Line 322: it is assumed that all ice is graupel, but in Fig 6 the habit classification shows that plates are the dominant habit.
- Line 352: I think this is speculative.
- Line 364: The last sentence is rather generic. Can the authors provide some more information on how these measurements could be used to “improve the understanding of key processes” and “help find the region of supercooled water of clouds for the weather modification work”.
Finally, there are many instances where the English text could be improved on, and this is something that the reviewers should also try to address in any revision.
Citation: https://doi.org/10.5194/egusphere-2024-314-RC2
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