What is an ULL and how do they affect tropical systems?

[ticka1]

Just curious on what exactly an upper level low is and how they affect tropical systems?

What causes them?

[jfranklin]

Hurricanes, as you probably know, have their strongest circulations in the lower levels of the atmosphere and weaken with height. In fact, as you go up the size of the cyclonic circulation shrinks and is replaced by anticyclonic (clockwise) flow. This weakening with height is a consequence of the hurricane being a "warm-core" system.

Upper-level lows, on the other hand, are cold core, meaning the temperature near the center of an upper low is colder than its environment. Their structure is reversed from tropical cyclones, in that upper-level lows weaken with decreasing height.

There is no unique way in which ULLs affect tropical systems. They can help steer tropical cyclones just like any other environmental feature. They can hinder development if the flow around them causes shear over the tropical cyclone, or in some cases can aid development as they often are associated with regions of upper-level divergence, often just to their east.

James