LSF Tides

Least Squares Fit for ROMS Tides

<wikitex>A ROMS state variable, $\phi$, can be represented in terms of its time mean, $\bar\phi$, plus a set of $N$-tidal harmonics of frequency, $\omega_k$.

$$\phi(t) = \bar\phi + \sum_{k=1}^N A_k \sin\omega_k t + \sum_{k=1}^N B_k \cos\omega_k t$$

The unknown tidal amplitude $A_k$, and $B_k$ and unknown state $\phi$ coefficients are evaluated by minimizing the least-squares error function defined by:

$$\varepsilon^2 = \frac{1}{T} \int_{t_1}^{t_2} \left[\phi - \left(\bar\phi + \sum_{k=1}^N (A_k \sin\omega_k t) + \sum_{k=1}^N (B_k \cos\omega_k t)\right)\right]^2dt$$

In discrete space:

$$\varepsilon^2 = \frac{1}{M} \sum_{i=1}^M \left[\phi_i - \left(\bar\phi + \sum_{k=1}^N (A_k \sin\omega_k t_i) + \sum_{k=1}^N (B_k \cos\omega_k t_i)\right)\right]^2$$

Minimization subject to the additional constraints $\frac{\partial \varepsilon^2}{\partial \bar\phi} = 0$, $\frac{\partial \varepsilon^2}{\partial A_k} = 0$, $\frac{\partial \varepsilon^2}{\partial B_k} = 0$ result in a linear set of equations:

$$\sum_{i=1}^M \left[-2\phi_i + 2\bar\phi + 2 \sum_{k=1}^N (A_k \sin\omega_k t_i) + 2 \sum_{k=1}^N (B_k \sin\omega_k t_i)\right] = 0$$

$$\sum_{i=1}^M \Bigg[ -2 \phi_i \sin\omega_k t_i + 2 \bar\phi \sin\omega_k t_i + 2\sum_{p=1}^N (A_p \sin\omega_p t_i \sin\omega_k t_i) + 2\sum_{p=1}^N (B_p\cos\omega_p t_i \sin\omega_k t_i)\Bigg] = 0$$

$$\sum_{i=1}^M \Bigg[ -2 \phi_i \cos\omega_k t_i + 2 \bar\phi \cos\omega_k t_i + 2\sum_{p=1}^N (A_p \sin\omega_p t_i \cos\omega_k t_i) + 2\sum_{p=1}^N (B_p\cos\omega_p t_i \cos\omega_k t_i) \Bigg] = 0$$

in matrix form ($N$ harmonics). Note: all instances of $\sum$ are actually $\sum_{i=1}^M$ where $M$ is the number of time-steps in the time-averaging window.

$$\matrix{\left[ \matrix{ \cr M & \sum \sin\omega_1 t_i & \sum \sin\omega_2 t_i & \cdots & \sum \cos\omega_1 t_i & \cdots \cr \cr \sum \sin\omega_1 t_i & \sum \sin^2 \omega_1 t_i & \sum \sin\omega_2 t_i \sin\omega_1 t_i & \cdots & \sum \cos\omega_1 t_i \sin\omega_1 t_i & \cdots \cr \cr \sum \sin\omega_2 t_i & \sum \sin\omega_1 t_i \sin\omega_2 t_i & \sum \sin^2 \omega_2 t_i & \cdots & \sum \cos\omega_1 t_i \sin\omega_2 t_i & \cdots \cr \cr \vdots & \vdots & \cdots & \cdots & \cdots & \cdots \cr \cr \sum \sin\omega_7 t_i & \sum \sin\omega_1 t_i \sin\omega_7 t_i & \sum \sin\omega_2 t_i \sin\omega_7 t_i & \cdots & \sum \cos\omega_1 t_i \sin\omega_7 t_i & \cdots \cr \cr \sum \sin\omega_1 t_i & \sum \sin\omega_1 t_i \cos\omega_1 t_i & \sum \sin\omega_2 t_i \cos\omega_1 t_i & \cdots & \sum \cos^2 \omega_1 t_i & \cdots \cr \cr \sum \cos\omega_2 t_i & \sum \sin\omega_1 t_i \cos\omega_2 t_i & \sum \sin\omega_2 t_i \cos\omega_2 t_i & \cdots & \sum \cos\omega_1 t_i \cos\omega_2 t_i & \cdots \cr \cr \vdots & \vdots & \vdots & \vdots & \vdots & \vdots \cr \cr \sum \cos\omega_7 t_i & \sum \sin\omega_1 t_i \cos\omega_7 t_i & \sum \sin\omega_2 t_2 \cos\omega_7 t_i & \cdots & \sum \cos\omega_1 t_i \cos\omega_7 t_i & \cdots \cr \cr } \right] & \left[ \matrix{ \cr \bar\phi \cr \cr A_1 \cr \cr A_2 \cr \cr \vdots \cr \cr A_7 \cr \cr B_1 \cr \cr B_2 \cr \cr \vdots \cr \cr B_7 \cr \cr } \right] & = & \left[ \matrix{ \cr \sum\phi_i \cr \cr \sum\phi_i \sin\omega_1 ti \cr \cr \sum\phi_i \sin\omega_2 ti \cr \cr \vdots \cr \cr \sum\phi_i \sin\omega_7 ti \cr \cr \sum\phi_i \cos\omega_1 ti \cr \cr \sum\phi_i \cos\omega_2 ti \cr \cr \vdots \cr \cr \sum\phi_i \cos\omega_7 ti \cr \cr } \right] \cr A & x & & b}$$ </wikitex>