阅读下面这首诗，完成后面问题。
                                                                                      江亭夜月送别二首（其二）
                                                                                                    王勃
                                                                                               乱烟笼碧砌，
                                                                                               飞月向南端。
                                                                                               寂寞离亭掩，
                                                                                                江山此夜寒。

已知$\left|\overrightarrow{a}\right|=3,\left|\overrightarrow{b}\right|=4.$$\overrightarrow{a}$与$\overrightarrow{b}$的夹角为${120}^0$，则$\overrightarrow{a}$在$\overrightarrow{b}$方向上的投影为（  ）