  We report an out-of-plane magnetic field induced large photoluminescence
enhancement in WS${}_2$ flakes at $4$ K, in contrast to the photoluminescence
enhancement provided by in-plane field in general. Two mechanisms for the
enhancement are proposed. One is a larger overlap of electron and hole caused
by the magnetic field induced confinement. The other is that the energy
difference between $\Lambda$ and K valleys is reduced by magnetic field, and
thus enhancing the corresponding indirect-transition trions. Meanwhile, the
Land\'e g factor of the trion is measured as $-0.8$, whose absolute value is
much smaller than normal exciton, which is around $|-4|$. A model for the trion
g factor is presented, confirming that the smaller absolute value of Land\'e g
factor is a behavior of this $\Lambda$-K trion. By extending the valley space,
we believe this work provides a further understanding of the valleytronics in
monolayer transition metal dichalcogenides.
