The snowboarder is sitting down.
there is a snowboarder who is sitting
Who is sitting is a rephrasing of sitting down.
exists e1 s1 x1.(n_snowboarder(x1) & a_down(s1) & Manner(e1,s1) & Actor(e1,x1) & v_sit(e1))
exists p1 x1 e1 x2 x3.((x1 = x2) & Actor(e1,x3) & v_sit(e1) & (x2 = x3) & n_snowboarder(x2))