The knob is the graft area (weeping cherry trees are grafted)
These trees need good drainage and compacted soil is not good- you can
plunge a spading fork into the ground and pry back and forth to help
ease compaction.
And you could add sifted compost or used coffee grounds to the areas
you disturb. Grounds are a nice mild fertilizer and source of organic
material for the soil.
Sealants are not usually reccomended but you may want to clean any
debris out of the crown as the branches may form a bowl that never
quite dries out.
An arborist might insert a drain tube drilled into this area but it is
generally beyond home gardener skills.

Go online and read up on weeping cherries - look for websites ending
in .edu for answers that have been researched.