  We develop various lower bounds for the numerical radius $w(A)$ of a bounded
linear operator $A$ defined on a complex Hilbert space, which improve the
existing inequality $w^2(A)\geq \frac{1}{4}\|A^*A+AA^*\|$. In particular, for
$r\geq 1$, we show that \begin{eqnarray*}\frac{1}{4}\|A^*A+AA^*\|
  \leq\frac{1}{2} \left(
\frac{1}{2}\|\Re(A)+\Im(A)\|^{2r}+\frac{1}{2}\|\Re(A)-\Im(A)\|^{2r}\right)^{\frac{1}{r}}
  \leq w^{2}(A),\end{eqnarray*} where $\Re(A)$ and $\Im(A)$ are the real and
imaginary parts of $A$, respectively. Furthermore, we obtain upper bounds for
$w^2(A)$ refining the well-known upper bound $w^2(A)\leq \frac{1}{2}
\left(w(A^2)+\|A\|^2\right)$. Separate complete characterizations for
$w(A)=\frac{\|A\|}{2}$ and $w(A)=\frac{1}{2}\sqrt{\|A^*A+AA^*\|}$ are also
given.
