\begin{table}%T2
\caption{\label{tab:domains}The simulation parameters with the size of computational
domain and vertical ($\delta{z}$) and horizontal ($\delta{x}$)
grid spacing.} 
\small %\centering
\par
\begin{tabular}{l c c c}
\hline \hline\noalign{\smallskip}
Run & Height/width  & $\delta{z}/\delta{x}$ & $R_{\rm m}$ \\
&  [Mm] & [km] & \\
\hline
mixed polarity & 1.4/6.0 & 14/20.8 & $\sim$$300$\\
dynamo C& 1.4/4.86 & 10/7.5 & $\sim$$2600$\\
dynamo G & 1.4/4.86 & 7/5 & $\sim$$5200$\\
\hline
\end{tabular}
\end{table}