这可能比必要的更笨重,但它有效:
\documentclass{article}
\usepackage{amsmath,graphicx} % amsmath for \text, graphicx for \scalebox
\newcommand{\bignwarrow}{\text{\scalebox{2}{\ensuremath{\nwarrow}}}}
\begin{document}
\begin{math}
\textsf{\textbf{T}} \underbrace{(a)}_\bignwarrow \\
\end{math}
\end{document}