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Thread: Please explain it

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    Question Please explain it

    I was analyzing SST fields during a La-nina event. My friend asked me a question that is,whether it is SST or the gradient which is prominent? I am really confused about the question itself. He smiled and ran a way.Can anybody explain me regarding the same?

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    gradient and advection.

    Hi Vishal,

    The question you asked is interesting. I think more answers will be
    adding to this.

    I think:
    First of all, Temperature (T) gradient is purely a local measurement.
    It tells "how much of T will change with unit distance (unit is
    degree-C/meter)
    . The following figure shows an example field of
    Temperature and its gradient in x-direction. Look at the phase of the
    T and dT/dx. They are different. (axis units are in meters).



    The term dT/dx gives the regions where maximum/minimum change in
    temperature with distance occur. Physically, a gradient plot may be
    meaningful if the analysis is interested on terms involving dT/dx.

    Gradient and Advection

    If we multiply the dT/dx with velocity (U), then U.dT/dx is called
    the temperature advection flux. Virtually, any dT/dx can be
    interpreted in the following way,

    dT/dx = 1.dT/dx, suppose U = 1 m/s.

    Thus a gradient plot can tell (at least), the amount of temperature change, in
    unit time and in unit distance, which might occur if there exist a flow
    of magnitude 1 m/s.
    This is equivalent to the amount of temperature
    advected to that region. Since the time component involves in the
    flux expression, the pattern begins to advect from one region to
    another. The more complete form is, dT/dt + U.dT/dx = 0, is
    called the advection-equation.

    However, the gradient-plots are meaningful in some analysis. For
    example, Liu et al. (2000) suggested the phase relation with SST
    gradient
    and surface winds, namely, the surface wind convergence
    is in phase with the SST gradient and a 90-degree phase shift with
    the SST itself. However please note that the physical reason for this
    phase relationship is nothing to explain with the gradient of SST (refer
    the paper).

    In conclusion, in your analysis, if you are concerned with terms
    involving dT/dx, then a gradient-plot will be meaningful. Also, from
    the above example figure, you can find the phase of a "field" and
    its "gradient" are different. Therefore care must be taken while
    interpreting the gradient fields.

    Liu W. et al., 2000: Atmospheric manifestation of Tropical instability
    wave observed by QuikSCAT and Tropical Rain Measuring Mission,
    Geophysical Research Letters, Vol.27, No.16, 2545-2548.


    -Thanks

    -

  3. #3
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    Thanks vinu for your explanation. To compute the gradient of sst, I should measure dt/dx or dt/dy or ?

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