12 March 2010 [Friday] @ March 12, 2010 edit
Negatively tilted Troughs
  • Hi Folks,
    a subject I have wanted to understand better ....

    Below ... A definition taken from a NWS site

    NEGATIVELY TILTED - Usually used in referring to an upper level trough. The base of the trough moves out ahead
    of the rest of the trough. This is a good pattern for severe thunderstorms.

    question time ...

    1) are there particular circumstances that cause the base to move out ahead ?

    2) can this be recognised, a) ... visually in the field ?, b) ... on any of the weather data sets that can be accessed eg GFS etc?

    3) why is it more likely to produce more intense storms ?

    ok that will do for a start
    hopefully some wx guru's can shed some light on this for me, am just willing to learn :)

    cheers


  • ok Andrew and co,

    I'm gonna get to understand this even if it kills me hahaha
    please bear with me on this. all in the name of education :)
    the pic's above I was trying to visualise looking along the axis of the low
    pressure trough.

    ok here's a portion of a current synoptic map of estrn Australia, (thanks Weatherzone.com)
    showing a trough extending down from the NW over the top of my home and towards the SE.

    http://www.sydneystormcity.com/map33.jpg

    now appreciating your comment about changing the y - axis to latitude,
    and this pic shows the low pressure region extending SE'wards in latitude.
    But surely there must be a vertical component to this low press. region as well ?
    ie. ... ummmm ... lets say we have a container full of air in which the pressure is contant
    throughout. Into this we place a hollow ball in which the inside pressure is lower. This 'pocket'
    of low press. has shape and size in 3 dimensions.

    http://www.sydneystormcity.com/lowpress.bmp

    Now making the container huge, our atmosphere,
    we can still have a 'pocket' of low press. which may or may not be symetrical, .. yes ? I realise that now,
    on this scale, we have a measurable variation in air pressure that drops as we go upwards in elevation. I
    assume that that 3 dimensional pocket still exists as a defined region and isnt something that goes from
    ground level to whatever altitude as a column ? ... or maybe some times it does.
    but since I hear of upper or lower level lows, this assumes that its a pocket of low press. within the
    surrounding air column. ?

    Then we should be able to draw a basic cross-section looking through this region.

    Just trying to get my thoughts in to some semblance of order....
    The trough in that synoptic pic above, can you define it as positive, negative or something else and why ?
    I had the idea that the positive or negative tilt had to do with what the 'toe' of the low pressure region
    was doing. But thinking more maybe its purely how that low pressure region is intruding (overall) into
    the surrounding region as viewed from above ?

    cheers


  • Thanks Jon,

    that answered some Q's but opened others :)

    I need to get a handle on what this looks like in cross-section through the atmosphere.
    From the map on that reference page you gave, I couldnt really see what the author was referring to.

    am I on the right path in the pic below or totally screwed up ?

    http://www.sydneystormcity.com/troughs.bmp


    so (A) would be neutral (B) negative tilt and (C) positive tilt ?


    cheers


  • hahha looks more like a wallcloud but you hopefully get what I mean :)


  • Jeff Haby's site has some info on negatively tilted troughs:

    http://www.theweatherprediction.com/habyhints/127/

    Hope this helps a little
    Jon Miller
    KT8NDO


  • Dave, if you replaced elevation on your y-axis with latitude you would be on to something. Basically as the surface low deepens and wraps cold air around the southern side of the storm, and warmer air wraps around the northern side of the storm you can start developing a negative tilt. The 850 temperatures dictate the geopotential height fields, therefore you will have height falls wrapping around south of the surface low, and higher height values on the eastern/Northeastern side of the low. This will cause the upper level trough to tilt westward with increased latitude. This negative tilt lends itself to strong difluence in the upper level flow and thus increased divergence, vertical motions, etc etc. Its quite a bit more complex than that, but its a start for you.

    Note: I just realized your from the Southern hemisphere, so invert all that for your neighborhood







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