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I have temperature, u wind, v wind and relative humidity. I am wondering to calculate water vapor flux divergence and convergence.

Would anybody kindly help to find our a solution for this?

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The formula for moisture flux is $$q\vec{V} $$ where $q$ is the water vapor mixing ratio, which can be found using mixhum_ptr and $\vec{V}$ is the velocity.

Therefore the divergence of the moisture flux must be $$\nabla{\dot{}q\vec{V}}=\frac{\partial(qu)}{\partial x}+\frac{\partial(qv)}{\partial y}+\frac{\partial(qw)}{\partial z} $$ $$\approx\frac{\delta(qu)}{\delta x}+\frac{\delta(qv)}{\delta y}+\frac{\delta(qw)}{\delta z}$$

which is computable if you have gridded data.

For example, you could compute this by setting $$qu=q\ast u$$ $$qv=q\ast v$$ $$qfluxDiv=uv2dv\_cfd(qu,qv,lat,lon,opt) $$ Use uv2dv_cfd per http://www.ncl.ucar.edu/Document/Functions/Built-in/uv2dv_cfd.shtml

Note: If you wanted the turbulent moisture flux, you would need to subtract $\nabla{\dot{}\bar{\vec{V}}\bar{q}}$ from your answer.

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    $\begingroup$ Please suggest in NCL or grads. Thank you $\endgroup$
    – Kay
    Jul 22, 2016 at 1:30
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    $\begingroup$ For horizontal water vapor flux, divergence try this NCL code: qfluxDiv=uv2dv_cfd(qu,qv,lat,lon,opt) $\endgroup$ Jul 22, 2016 at 3:55
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    $\begingroup$ I added that to the answer, but without data and code, it is hard to give an example. I provided a link to the NCL function, so that you may know how to use the function. There is an example there on how to use the function. $\endgroup$ Jul 22, 2016 at 13:43
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    $\begingroup$ @BarocliniCplusplus - Thank you. The only other suggestion is that his input is relative humidity. You may want to link how to convert water vapor mixing ratio to relative humidity. So this becomes a complete answer. $\endgroup$
    – user1066
    Oct 31, 2016 at 14:40
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    $\begingroup$ @BarocliniCplusplus Ok :-) Upvoted your answer. I am going to be using your code for a lecture class shortly. $\endgroup$
    – user1066
    Nov 1, 2016 at 15:08

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