mound, grave, rice field dike
壟 centers on a raised earthen structure: a mound, a burial mound (grave), or a raised ridge between rice fields (dike).
壟 combines 土 (earth) with 龍 (dragon), which likely provides the sound. The character originally depicted a raised earthen mound, perhaps shaped like a dragon's back.
A dragon (龍) made of earth (土) rises from the ground, forming a long mound like a grave or a rice field dike.
For ロウ, imagine a low mound: low -> ロウ, and the dragon-shaped earth rises just a little.