As a free action, you can hold the sunbrella above you and say your decided command word transforming it into a window to a sunny blue sky. The umbrella creates a 5ft diameter and up to 15ft tall cylinder of daylight in your space and it shines dim light on a 15ft radius, the air in the cylinder is warm and negates cold weather.
While in the cylinder, creatures with sunlight sensitivity have disadvantage attacking you, and creatures that take damage from sunlight receive 10 radiant damage each time they make a melee attack toward you.
If you don't hold it vertically the window only shines dim light.