Current speed from velocity components — current

The magnitude of the horizontal velocity vector, sqrt(u^2 + v^2). Reproduces the uv covariate of Ross et al. (2023).

Usage

current_speed(env_dat, u = "UO", v = "VO", name = "speed")

Arguments

env_dat: an sf POINT object with one row per location and time step, as datamatch's access functions return
u: name of the eastward velocity column
v: name of the northward velocity column
name: name for the new column

Value

env_dat with a current speed column added, in the units of u/v

Details

Their uv_grad is the spatial derivative of that speed field, not of the velocity components, so it is two steps:

env <- current_speed(env)                    # uv
env <- horizontal_gradient(env, "speed")     # uv_grad

The order matters. Differentiating u and v separately and combining afterwards gives a different quantity: speed is a non-linear function of the components, so the gradient of the speed is not the speed of the gradients.

References

Ross C, Runge J, Roberts J, Brady D, Tupper B, Record N (2023). Estimating North Atlantic right whale prey based on Calanus finmarchicus thresholds. Marine Ecology Progress Series 703, 1-16. doi:10.3354/meps14204

Examples

if (FALSE) { # \dontrun{
# UO and VO are the defaults, so a dictionary fetch needs no column names
env <- current_speed(env) |> horizontal_gradient("speed")

# If the fetch used Copernicus codes instead
env <- current_speed(env, u = "uo", v = "vo")
} # }