Wind rotation question

Datasets and Usage C3S - Datasets and Usage
1

We are interested in converting the grid-relative wind components provided in the CARRA dataset into earth-relative (true north/east) wind components, u_earth and v_earth. For the CARRA West domain we are using this code:

!projection parameter for CARRA west
real, parameter :: LoV = 324.0 ! GRIB_LoVInDegrees
real, parameter :: Latin1 = 72.0 ! GRIB_Latin1InDegrees
real, parameter :: n_cone = sin(Latin1 * pi/180.0)

do j = 1, ny
do i = 1, nx
dlon = lon(i,j) - LoV
if (dlon > 180.0) dlon = dlon - 360.0
if (dlon < -180.0) dlon = dlon + 360.0

! local rotation of the model grid
alpha(i,j) = n_cone \* dlon

! rotate grid-relative wind components to earth-relative
angle = -alpha(i,j)\*pi/180.0
u_earth(i,j) = u(i,j)\*cos(angle) - v(i,j)\*sin(angle)
v_earth(i,j) = u(i,j)\*sin(angle) +&nbsp;v(i,j)\*cos(angle)

end do
end do

We assume the wind components archived in CARRA are grid-relative (i.e., aligned with the LCC grid rather than true north/east), so we compute the grid rotation angle at each point and apply the corresponding rotation to recover the earth-relative components.

Is this assumption correct? Does this approach, including the sign convention used in the rotation, correctly convert the CARRA grid-relative winds to earth-relative wind components?

2

Dear Eva,

since no one replied until now, I try an answer. However, I am a user like you. I have done that kind of calculation several years ago and I am not comparing those calculations with yours. In the mean time the CARRA1 and the CARRA2 data set provide wind speed and wind direction (relative to North). I recommend to download these two parameters instead and calculate uv-wind easily from that and avoiding all these projection issues. At least, I have done that for the CARRA2 data, now.

Best wishes

Peter