Spatial and temporal distribution of stratospheric turbulence from global high-resolution radiosonde data - Preprint
Geoengineering News
Michael MacCracken
I would suggest that an important limitation of this and similar studies for application in planning small-scale field experiments of injecting aerosols into the stratosphere is that the results do not consider the gravitational, density driven spreading that was evident when a mass of well-mixed, polluted air was introduced into the inversion above Los Angeles and formed a thin, polluted layer across the whole basin. The particular situation of the initially mysterious layers was explained by UCLA meteorologist and professor James Edinger. His explanation was that afternoon sunlight striking the west and southwest facing hillsides surrounding the LA basin heated a thin layer of polluted air below the inversion enough to allow it to rise along the hillside surface without cooling by expansion until the air reached the tops of the hills (up to a few thousand meters high). Once there and not further heated, the polluted air could not rise further and so spread out at that altitude, not by advective flow or mixing by the type of turbulence being measured by fixed volume balloons that don't allow spreading. The spreading was because the air was of the same density as the air at just that level. The spreading flow led to thin polluted layers in the inversion covering much of the basin dependent on the particular heights of each of the hills.
Cal Tech did lab tank experiments verifying that this phenomenon could/would occur. The expert on all of this is atmospheric chemistry professor John Seinfeld of Cal Tech. As far as I know this phenomenon has not been considered in the planning of the injection experiments and I'd suggest it would make keeping track and measuring the injected material much more difficult (e.g., by a maneuvered balloon) and the concentrations considerably lower than has been planned for.
I think the rapid dispersal of contrails created by aircraft in relatively dry air is also an example of the spreading phenomenon. In relatively saturated air, such as where cirrus are already present, contrails tend to persist, but in dry air the contrails disappear rapidly (seconds to tens of seconds). In the proposed experiments, the air would be relatively clean and so spreading would quite likely quickly dilute the injected pollutants, reducing the likelihood (so rate) of the chemical reactions planned to be determined. Attempting the measurements in a volcanic plume might be possible, but then one does not have controlled conditions.
My personal view is that small-scale experiments are unlikely to provide useful results, and that the approach needed is going to be to start with small, initial deployment scale injections based on estimated rates of reactions from volcanic injections and to observe, adjust, and learn as SAI is slowly strengthened to exert a cooling influence.
Mike MacCracken
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Michael MacCracken
Hi Adrian--Just to note that it was boundary layer polluted air carried up well into the overlying inversion just the height of the surrounding hills/mountains. The layers were not a result of boundary layer turbulence, if that is what you meant.
When one flew into LAX (Los Angeles airport) one would descend slowly through thin, dark layers (going up, the transit was much faster). There were all sorts of attempts to explain them, including the reformation of a boundary layer deepened by afternoon heating, but they were too high for that. There were attempts with rawinsondes to detect an advective flow, but neither winds and the layers were too thin to really be measured.
There are actually likely some other instances of such flows up sunlit mountains. For example, my understanding is that there are some mountain ridge-top towns along the spine of mountains of Mexico where the overall weather is a large high pressure zone with very dry air, but the towns get evening moisture likely coming upslope and then the inserted air condensing as evening comes. The mountains run southeast toward northwest, so mountain faces face southwest.
And then, some more speculation, it is quite impressive that pollution south of the Himalayas can reach up to 30,000 feet or so. Normally, when moist polluted air rises, there is cooling, condensation, and rainout of pollution. Might it be that the southward facing Himalayas help loft air without precipitation and the polluted air spreads out? Just a thought.
Best wishes, Mike
Michael,
I agree completely with your general stance. However, I have reservations about using boundary layer turbulence in the LA basin as relevant to the lower stratosphere. I’m attaching some references, which contain several others references that are also relevant . I note that operational radiosondes take measurements on ascent, when the instrument package is in the turbulent wake of the balloon, unlike dropsondes as analysed in Hovde et al., INT. J. REMOTE SENSING, 32, 5891- 5918, (2011). dot:10.1080/01431161.602652
Best wishes,
Adrian
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