Dissolution hole over London
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(ii) Changes forced by natural features such as solar variability or volcanic eruptions. (iii) Changes forced by human activity. Investigation of the roles of these different fac- tors is carried out in two different ways. In the first it is assumed that all the contributing factors are known, and their physical and chemical prop- erties understood; and these are then represented, as fully as possible, in a time-dependent simula- tion of the global system. The results of such mod- elling efforts may then be compared with the observations. In the second the observational record is analysed statistically in order to detect the contributions of different factors. These approaches are, of course, complementary and both are presented in the TAR. The model simulations reviewed by the IPCC are coupled atmosphere± ocean general circulation models and, as such, are about as realistic and complex as can currently be managed given limita- tions in computing power, although of course great advances in computing power are being made constantly. It is interesting that attempts to simulate the temperature record of the past 150 years show much better agreement with the sur- face temperature record, at least between about 1910 and the 1970s, when solar and volcanic influences are included (see Allen 2002). How- ever, the TAR ``Summary for Policymakers’’ notes that this does not exclude the possibility that other (cancelling) factors may have contributed. Furthermore, Chapter 8 of the TAR presents a careful assessment of the models’ capabilities and limitations and notes that the models appear to underestimate natural internal climate variability. This is a very important caveat to the results and it is only by further model development and, as Mr Morgan suggests, understanding the modes of variability that our current state of understanding will be advanced. I have a particular interest in how solar variability affects climate but I am not aware of the ``modellers and their supporters’’ who are deriding the ``evidence and its proponents’’ ± perhaps Mr Morgan could provide a reference? Furthermore, recent work by Thomas Crowley (2000) has shown how a simple energy balance model using only solar and volcanic indices can be used to simulate quite well temperature variability over the past 1000 years ± except the twentieth-century increases. In this context it is worth noting that if the sun were the source of the recent warming then, using reconstructions of past solar activity produced by solar physicists, it is difficult to explain why its previous influence was so small. As climate scientists we are involved in a fasci- nating, but enormously complex, subject. We rea- lise that there is much we do not understand but we also know that our work has huge relevance for society in the future and we are therefore obliged to assess, as best we can, what is the cur- rent status of our knowledge. The IPCC process is a good-faith attempt to do that and the consensus view it reached in the TAR was that human activity is currently making the global average climate warmer than it would be otherwise. Note: The views expressed above are personal and do not represent those of any other body. References Allen, M. (2002) Climate of the twentieth century: Detection of change and attribution of causes. Weather, 57, pp. 296-303 Crowley, T. (2000) Causes of climate change over the past 1000 years. Science, 289, pp. 270± 277 Folland, C. K., Karl, T. R. and Salinger, M. J. (2002) Observed climate variability and change. Weather, 57, pp. 269± 278 doi: 10.1256/wea.16.03 Dissolution hole over London On 28 October 2002, a series of holes, aligned north± south, appeared in the cloud sheet over London and lasted for almost an hour. They were fairly small, especially to the north, and there were five or six of them at one time. Some never seemed to enlarge much and subsequently filled in. Virga were observed falling from some of the larger holes. Figure 1 shows some of these holes developing. Met Office, London Colin McAllister Jim Galvin (Photo. Editor) replies: As Colin noted, the cloud sheet was forming ice crystals which then fell to produce the hole, but these are not clearly visible in the photographs. It is probable, since the whole of the cloud sheet was not dissolving, that there was some trigger that started the process of dissolution. This is supported by the radiosonde observations from Herstmonceux, East Sussex, and Watnall, Nottinghamshire, at midday on 28 October. Inter- polation of these profiles suggests a cloud base a little above 5200 m; at this height, the temperature was ± 218C. Below ± 208C, supercooled water readily turns to ice and the process of glaciation becomes dominant. 313 Weather Vol. 58 August 2003
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(ii) Changes forced by natural featuressuch as solar variability or volcaniceruptions.
(iii) Changes forced by human activity.
Investigation of the roles of these different fac-tors is carried out in two different ways. In thefirst it is assumed that all the contributing factorsare known, and their physical and chemical prop-erties understood; and these are then represented,as fully as possible, in a time-dependent simula-tion of the global system. The results of such mod-elling efforts may then be compared with theobservations. In the second the observationalrecord is analysed statistically in order to detectthe contributions of different factors. Theseapproaches are, of course, complementary andboth are presented in the TAR.
The model simulations reviewed by the IPCC arecoupled atmosphere± ocean general circulationmodels and, as such, are about as realistic andcomplex as can currently be managed given limita-tions in computing power, although of coursegreat advances in computing power are beingmade constantly. It is interesting that attempts tosimulate the temperature record of the past 150years show much better agreement with the sur-face temperature record, at least between about1910 and the 1970s, when solar and volcanicinfluences are included (see Allen 2002). How-ever, the TAR `̀ Summary for Policymakers’ ’ notesthat this does not exclude the possibility thatother (cancelling) factors may have contributed.Furthermore, Chapter 8 of the TAR presents acareful assessment of the models’ capabilities andlimitations and notes that the models appear tounderestimate natural internal climate variability.This is a very important caveat to the results andit is only by further model development and, asMr Morgan suggests, understanding the modes ofvariability that our current state of understandingwill be advanced.
I have a particular interest in how solarvariability affects climate but I am not aware of the`̀ modellers and their supporters’’ who are deridingthe `̀ evidence and its proponents’’ ± perhaps MrMorgan could provide a reference? Furthermore,recent work by Thomas Crowley (2000) hasshown how a simple energy balance model usingonly solar and volcanic indices can be used tosimulate quite well temperature variability overthe past 1000 years ± except the twentieth-centuryincreases. In this context it is worth noting that ifthe sun were the source of the recent warmingthen, using reconstructions of past solar activityproduced by solar physicists, it is difficult toexplain why its previous influence was so small.
As climate scientists we are involved in a fasci-nating, but enormously complex, subject. We rea-lise that there is much we do not understand butwe also know that our work has huge relevancefor society in the future and we are thereforeobliged to assess, as best we can, what is the cur-rent status of our knowledge. The IPCC process isa good-faith attempt to do that and the consensusview it reached in the TAR was that human activityis currently making the global average climatewarmer than it would be otherwise.
Note: The views expressed above are personaland do not represent those of any other body.
References
Allen, M. (2002) Climate of the twentieth century:Detection of change and attribution of causes.Weather, 57, pp.296-303
Crowley, T. (2000) Causes of climate change overthe past 1000 years. Science, 289, pp.270± 277
Folland, C. K., Karl, T. R. and Salinger, M. J.(2002) Observed climate variability and change.Weather, 57, pp.269± 278
doi: 10.1256/wea.16.03
Dissolution hole over London
On 28 October 2002, a series of holes, alignednorth± south, appeared in the cloud sheet overLondon and lasted for almost an hour. They werefairly small, especially to the north, and therewere five or six of them at one time. Some neverseemed to enlarge much and subsequently filledin. Virga were observed falling from some of thelarger holes. Figure 1 shows some of these holesdeveloping.
Met Office, London Colin McAllister
Jim Galvin (Photo. Editor) replies:
As Colin noted, the cloud sheet was forming icecrystals which then fell to produce the hole, butthese are not clearly visible in the photographs. Itis probable, since the whole of the cloud sheetwas not dissolving, that there was some triggerthat started the process of dissolution. This issupported by the radiosonde observations fromHerstmonceux, East Sussex, and Watnall,Nottinghamshire, at midday on 28 October. Inter-polation of these profiles suggests a cloud base alittle above 5200 m; at this height, the temperaturewas ± 218C. Below ± 208C, supercooled waterreadily turns to ice and the process of glaciationbecomes dominant.
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Weather Vol. 58 August 2003
However, it is of interest to see a hole formingfrom a series of points. The initial glaciation islikely to have spread as ice crystals formed onefrom another, each freezing as a neighbouring icecrystal came into contact. This suggests that a trig-ger mechanism was required in this case. On thisoccasion, there was a well marked jet stream overLondon, reaching speeds over 50ms± 1 (100kn)above the cloud layer. An explanation was givenin the Meteorological Service of Canada onlinepublication Zephyr, Spring 2001:
`̀ Typically, when this cloud formation occurs, ajet stream moves rapidly above a cloud deck,which is made up of layers of ice crystals andsuper-cooled water droplets. Interactionbetween the jet stream and the cloud deck pro-
duces a wave action (possibly gravity wavesbetween the stable layers) that causes the icecrystals in the descending portion of the waveto fall into the super-cooled liquid layer. Thisaction enhances ice crystal growth at theexpense of the super-cooled water droplets. Pre-cipitation then falls from the cloud, eroding ahole in the deck. The dry atmosphere in themid-layers below the cloud deck helps to subli-mate the precipitation, resulting in virga(precipitation not reaching the ground).’’(`̀ Hole-punch cloud moves across Halifaxskies, December 21, 2000’’, sourced throughhttp:// cdebsjournal.topcities.com /CarolinasJan171802.htm.)
doi:10.1256/wea.32.03
Fig. 1 Two photographs taken from the roof of the Met Office, London, at around 1300 GMTon 28 October 2002. Theyshow several holes gradually opening in a layer of altocumulus clouds.
Weather Vol. 58 August 2003
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