# -*- coding: utf8 -*-
# Copyright or © or Copr. Actimar/IFREMER (2010-2015)
#
# This software is a computer program whose purpose is to provide
# utilities for handling oceanographic and atmospheric data,
# with the ultimate goal of validating the MARS model from IFREMER.
#
# This software is governed by the CeCILL license under French law and
# abiding by the rules of distribution of free software. You can use,
# modify and/ or redistribute the software under the terms of the CeCILL
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# "http://www.cecill.info".
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import numpy as N, MV2
from vacumm.data.cf import format_var
[docs]def wind_stress(u, v, rhoa=1.25, cd=0.016, format_axes=False, alongxy=None):
"""Compute the sea surface zonal and meridional wind stress from 10m wind components
Output variables are formatted using :func:`~vacumm.data.cf.format_var`.
:Formula:
.. math::
\\tau_x = \\rho_a C_d (U_{10m}^2+V_{10m}^2)^{\\frac{1}{2}}U_{10m}
\\tau_y = \\rho_a C_d (U_{10m}^2+V_{10m}^2)^{\\frac{1}{2}}V_{10m}
:Params:
- **u/v**: Wind at 10m above sea surface.
- **rhoa**, optional: Air density (in kg.m-3).
- **cd**, optional: Drag coefficient.
- **format_axes**, optional: Also format axes using
:func:`~vacumm.data.cf.format_axis`.
- **alongxy**, optional: Format variables considering components are along X/Y
direction and not along zonal/meridional direction.
:Return: ``us, vs``
"""
# Init and format variables
us = MV2.asarray(u).clone()
vs = MV2.asarray(v).clone()
if alongxy is None:
alongxy = hasattr(u, 'long_name') and 'x' in u.long_name.lower()
if alongxy:
usname, vsname = 'taux', 'tauy'
else:
usname, vsname = 'tauu', 'tauv'
format_var(us, usname, format_axes=format_axes)
format_var(vs, vsname, format_axes=format_axes)
# Compute
uvmod = N.ma.sqrt(u**2+v**2)
us.assignValue(cd*rhoa*uvmod*u)
vs.assignValue(cd*rhoa*uvmod*v)
del uvmod
return us, vs
[docs]def ws2w(us, vs, rhoa=1.25, cd=0.016, format_axes=False, alongxy=None):
"""Convert from wind stress to 10m wind components
This function is the reverse one of :func:`wind_stress`.
Output variables are formatted using :func:`~vacumm.data.cf.format_var`.
:Formula:
.. math::
U_{10m} = (\\rho_a C_d)^{-\\frac{1}{2}}
(\\tau_x^2+\\tau_y^2)^{-\\frac{1}{4}} \\tau_x
V_{10m} = (\\rho_a C_d)^{-\\frac{1}{2}}
(\\tau_x^2+\\tau_y^2)^{-\\frac{1}{4}} \\tau_y
:Params:
- **us/vs**: Wind stress components.
- **rhoa**, optional: Air density (in kg.m-3).
- **cd**, optional: Drag coefficient.
- **format_axes**, optional: Also format axes using
:func:`~vacumm.data.cf.format_axis`.
- **alongxy**, optional: Format variables considering components are along X/Y
direction and not along zonal/meridional direction.
:Return: ``u, v``
"""
# Init and format variables
u = MV2.asarray(us).clone()
v = MV2.asarray(vs).clone()
if alongxy is None:
alongxy = hasattr(u, 'long_name') and 'x' in us.long_name.lower()
if alongxy:
uname, vname = 'ux10m', 'vy10m'
else:
uname, vname = 'u10m', 'v10m'
format_var(u, uname, format_axes=format_axes)
format_var(v, vname, format_axes=format_axes)
# Compute
zero = us.filled(1)==0.
zero &= vs.filled(1)==0.
uvsmod = (us**2+vs**2)**-0.25
uvsmod /= N.sqrt(rhoa*cd)
u.assignValue(uvsmod*us)
v.assignValue(uvsmod*vs)
u.assignValue(MV2.where(zero, 0., u))
v.assignValue(MV2.where(zero, 0., v))
del uvsmod
return u, v
if __name__=='__main__':
import MV2
u = MV2.arange(3.)
v = 2*u
us, vs = wind_stress(u, v)
U, V = ws2w(us, vs)
print u, v
print U, V
