Conversion from mathematical to meteorological convention

[skg]

Hello,
I have the u and v components of wind field from a model output. I calculated wind speed(sp) and direction(theta) from that. I took into consideration the signs of u and v while calculating direction. Then the angle (theta) I got must be in mathematical convention. I want to convert it into meteorological convention. Does anyone know the formulas to convert angles from mathematical to meteorological and oceanographic conventions?
It will be of great help if someone can give me any link/material regarding this.
Regards,
skg.

[MoHDa]

Hi,

Probably u ask something like this: http://www.mers.byu.edu/docs/reports/MERS9401.pdf

but I am wondering what do u mean by saying, "mathematical convection".

Usually, i choose the display of my data according to the audience/readers or the journal's rules.

Have in ur mind that:
1. Display the data the way that it is obvious what u want to show (or hide :D)
2. Define clearly in ur mind and at the caption what the image presents.

Hope that this post helps u.

[skg]

hello MoHDa,
Thankyou for your reply.
First of all let me explain what I meant by mathematical convention.
In mathematical convention, 0° is along positive X axis, 90° is positive Y axis, 180° is negative X axis, 270° is negative Y axis.It is a right handed system of coordinates.
In meteorological convention, 0° is coming from north to south, 90° is coming from east to west, 180° is coming from south to north and 270° is coming from west to east.

You are right about the link sent. I am having that document already. My problem is not displaying the data!
I am having a data set of wind speed components u and v. I want to get a data set of (not to plot) wind speed and direction. The direction I calculate using
theta=atan(v/u)
will be in mathematical convention. I want to convert theta to meteorological convention.
I think it is clear to you now.
I had tried the formula given in the document you cited. It will give negative values for theta(meteorological) for some values of angles. Also they haven't mentioned how they got the formula, the logic behind it.
Hope my doubt is clear to you now.
Regards,
skg.

[MoHDa]

Hello skg,

Let's c ur queries 1 by 1.

1. The logic behind the formulas: The explanation is at the chapter 2, if u feel like studying about the mathematical background is "Rotation of Coordinate system", the easiest way to prove it is presented at the first paragraphs of chapter 2 of the above publication.
What u need more is a table with the properties of the sin and cos, check the site: http://www.gomath.com/formulatogo.html at the

Trigonometric Properties (I) --> # Odd and Even Function Properties
Trigonometric Properties (II) --># Cofunction Properties for Circular Functions
(Better to print them)

And finally make a detail schema of the angles phi_u, phi_m (for u, phi is theta)etc.

2. The negative values of the theta is bcoz of the confuction properties of circular fuction, u ll realise it at the above link (formulatogo).

3. If u cannot extract the results by following the above but u can plot them, probably the problem is at the "output routines" and not at the calculation.

That's all, if u still need help, let me know. I promise that I ll write a little bit earlier :) (local time 02.00)

[mkm_navy]

hi Skg
you may find the solution in
"Introduction to Dynamical Meteorology by HOLTON"
BYE.

[yaswant]

hello MoHDa,
Thankyou for your reply.
First of all let me explain what I meant by mathematical convention.
In mathematical convention, 0° is along positive X axis, 90° is positive Y axis, 180° is negative X axis, 270° is negative Y axis.It is a right handed system of coordinates.
In meteorological convention, 0° is coming from north to south, 90° is coming from east to west, 180° is coming from south to north and 270° is coming from west to east.

You are right about the link sent. I am having that document already. My problem is not displaying the data!
I am having a data set of wind speed components u and v. I want to get a data set of (not to plot) wind speed and direction. The direction I calculate using
theta=atan(v/u)
will be in mathematical convention. I want to convert theta to meteorological convention.
I think it is clear to you now.
I had tried the formula given in the document you cited. It will give negative values for theta(meteorological) for some values of angles. Also they haven't mentioned how they got the formula, the logic behind it.
Hope my doubt is clear to you now.
Regards,
skg.skg, if you dont want to dig into the coordinate system, a quick patch like this should work well for your purpose:

theta = atan(v/u)*180./PI

if (theta > 90. OR theta <=180.) then theta_met = 450.-theta
else theta_met = 90.-theta

theta_met is your desired angle.

[yaswant]

skg, if you dont want to dig into the coordinate system, a quick patch like this should work well for your purpose:

theta = atan(v/u)*180./PI

if (theta > 90. OR theta <=180.) then theta_met = 450.-theta
else theta_met = 90.-theta

theta_met is your desired angle.Sorry it should be if (theta > 90. AND theta <=180.)

[skg]

Hello friends,
I was out of station and couldn't check the solutions suggested by you. I will look into it and get back to you if my problem persists.
skg.