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Any tropical weather aficionado feel like explaining 200mb Velocity Potential charts?

  • 11-07-2020 11:50am
    Registered Users Posts: 17,796 ✭✭✭✭

    One aspect of annual tropical forecasting which has always flown over my head entirely and which I've decided to try to get a handle on this year is the 200mb Velocity Potential chart. Full disclosure - I have almost no idea how these work and base my reading of them on stitched-together assumptions based on others' commentary of different scenarios over the years. It seems to be impossible to find any sort of FAQ or weather tutorial which specifically covers the meaning of these charts.

    At a very, very, very basic level, I've gleaned that "velocity potential" refers to the vertical movement of large scale airmasses from 200mb downwards, and that green areas represent upward momentum whilst red areas represent downward momentum, such that regions underneath red areas are likely to experience sinking air and thus suppressed convection whilst regions underneath green areas are likely to experience rising air and thus enhanced convection. This activity also seems to be independent of and to a certain extent "superior" to factors such as warm SSTs, wind shear, etc - in other words, even if all of those factors are favourable for convection, a positive VP200 anomaly will likely suppress it regardless.

    On the other hand, a recent exchange with M.J Ventrice has muddied the waters somewhat - he stated that "The area to the west of the minima VP200 anomaly and to the east of the maxima of the positive VP200 is the area where large scale conditions are often favorable for TCs". This would imply that my simplistic "green areas are favourable, red areas are unfavourable" is a gross misrepresentation of what's really going on.

    For example: Many charts for this year's peak hurricane season pointed to what several commentators referred to as an "Indian Ocean Phase 2 Standing Wave". This involved large scale deep green (negative VP200 anomalies) centred just off the Southwest coast of India, and large scale deep red over the Central Pacific stretching all the way Eastward to the mid-Atlantic. This was implied to be a heavily favourable pattern for Atlantic cyclogenesis and has been one of the reasons so many have called for a potentially hyperactive season in their forecasts, but to my rudimentary understanding, the fact that the positive anomaly stretches far enough East to cover the Gulf of Mexico and Caribbean would imply that the Atlantic will be very much suppressed by this pattern, regardless of how a more favourable Indian Ocean pattern would increase the generation of tropical waves - that such waves would choke and die under sinking air as soon as they reached the Atlantic.

    An example of the kind of chart which is being touted as a favourable pattern, but which due to my sketchy understanding of these charts, I would have assumed is in fact an unfavourable pattern for the Atlantic:


    (NOTE: This chart is out of date by now, I'm using it merely because it perfectly illustrates the discrepancy between what I regard as an unfavourable pattern but which others have looked and said "wow, seriously busy Atlantic Hurricane Season lies ahead")

    Does anyone feel like doing an ELI5 of sorts? Would be massively appreciated if so! I'd like to know not just how to read these charts, but what they actually mean - as in, what does "velocity potential" actually describe as a term, why is it also sometimes called a CHI Anomaly, what is meant by Tropical Tidbits' inclusion of the mathematical figures m^2s^-1x10^-7 in their charts, etc. But mostly, I'd like to know how to read them accurately in terms of predicting which regions will experience enhanced tropical convection and which regions will be suppressed, as these charts seem to be a gigantic indicator for predicting the future formation areas of hurricanes and typhoons.

    Thanks in advance for anyone who indulges! :D


  • Registered Users Posts: 8,219 ✭✭✭Gaoth Laidir

    I would imagine the reason is to do with windshear. The 200 hPa velocity potential is a measure of upper divergence. You can imagine that to the west of that (green) zone the upper and lower winds will both be easterly and if of similar speeds then this would lead to low windshear and hence a more conducive environment for tropical cyclones.

    East of the (red) upper convergence zone would similarly have low shear as the induced anomalous westerly surface flow slows down and at some point becomes easterly again.

    I'm only theorising here. I checked my copy of Holton's book but, although Chapter 11 does have a good bit on equatorial waves, etc., there's not much reference to velocity potential in it.