NewTest

\begin{equation} \tag{154}

 \sigma_w = \left\{
 \begin{align}
  & 0.15 + 1.5 \sin^2 \left( \frac{\pi}{2}\frac{W_s}{u_{*w}} \right)
    \text{   if   }\frac{W_s}{u_{*w}}\le 1 \\
  & 1.0 + 0.65 \sin^2 \left( \frac{\pi}{2}\frac{u_{*w}}{W_s} \right)
    \text{     if   }\frac{W_s}{u_{*w}}>1 \\
 \end{align} \right.

\end{equation}