Under what circumstances did you create the Song of Snow in the poem "Farewell to Tian Shuji Wu's Homecoming"?

The writing background of "Bai Xuege Farewell Tian Shuji Wu Guijia": In the summer and autumn of Tianbao 13th year of Tang Xuanzong (754), he turned to the Northern Dynasty, and in the second year of Tang Xuanzong (757), he returned to the East at the turn of spring and summer. This poem counts as this issue. At that time, there were frequent wars in the northwest frontier. With the ambition to contribute to the Great Wall, Cen Can went to the frontier twice and worked in the border guards for six years. Therefore, he has long observed and experienced the life of pommel horse and the ice and snow scenery outside the Great Wall. In the thirteenth year of Tianbao, this is the second time to go to the frontier fortress. I served as the judge of Feng Changqing in the Northwest Courtyard (the subordinate of my envoy), and Wu's judge was his predecessor. The poet wrote this poem to send him back to Beijing (Chang 'an) on the podium.