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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?

Kay
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1 Answers1

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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.

BarocliniCplusplus
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  • Thank you very much for your answer. I have gridded data. I can't able to calculate. Can you please suggest how to calculate? – Kay Jul 21 '16 at 03:18
  • Which language are you using? In NCL it is very easy, in Python and MATLAB it is easy, but in a language like C++ or FORTRAN, it is hard. – BarocliniCplusplus Jul 21 '16 at 13:37
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    Please suggest in NCL or grads. Thank you – Kay Jul 22 '16 at 01:30
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    For horizontal water vapor flux, divergence try this NCL code: qfluxDiv=uv2dv_cfd(qu,qv,lat,lon,opt) – BarocliniCplusplus Jul 22 '16 at 03:55
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    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. – BarocliniCplusplus Jul 22 '16 at 13:43
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    @BarocliniCplusplus - can you add in your answer what q is ? –  Oct 31 '16 at 09:32
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    Done. Also, I made a correction to an admittedly silly math error regarding divergence. – BarocliniCplusplus Oct 31 '16 at 14:25
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    @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. –  Oct 31 '16 at 14:40
  • The method to get mixing ratio has been added, though it requires pressure. – BarocliniCplusplus Nov 01 '16 at 14:15
  • @BarocliniCplusplus - The mixing ratio is just the specific humidity. In which case you can just link to this answer - http://earthscience.stackexchange.com/questions/5076/how-to-calculate-specific-humidity-with-relative-humidity-temperature-and-pres –  Nov 01 '16 at 15:03
  • The nice thing about NCL @gansub, is that there is a built in function for that conversion. – BarocliniCplusplus Nov 01 '16 at 15:06
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    @BarocliniCplusplus Ok :-) Upvoted your answer. I am going to be using your code for a lecture class shortly. –  Nov 01 '16 at 15:08