如图所示的几何体中,\(ABC-A_{1}B_{1}C_{1}\)为三棱柱,且\(AA_{1}⊥\)平面\(ABC\),四边形\(ABCD\)为平行四边形,\(AD=2CD\),\(∠ADC=60^{\circ}\).
\((\)Ⅰ\()\)若\(AA_{1}=AC\),求证:\(AC_{1}⊥\)平面\(A_{1}B_{1}CD\);
\((\)Ⅱ\()\)若\(CD=2\),\(AA_{1}=λAC\),二面角\(C-A_{1}D-C_{1}\)的余弦值为\( \dfrac { \sqrt {2}}{4}\),求三棱锥\(C_{1}-A_{1}CD\)的体积.