Refer to figures.
The diagram depicts a situation where the cold front catches up with the warm front => OCCLUSION.
We can also observe that the cold front loses contact with the ground, the cold front becomes a front aloft =>

WARM OCCLUSION

DETAILED INFORMATION:
OCCLUDED FRONT. If a cold front catches up to and overtakes a warm front, the frontal boundary created between the two air masses is called and occluded front.

WARM OCCLUSION: the air ahead of the warm front is colder than the air behind the cold front.

• The cold air behind the cold front rises over colder air ahead of the warm front. The less cold air rises over the leading colder air => Cold front aloft. In a warm occlusion, the cold front loses contact with the ground.
• Warm, moist and unstable air is lifted.