Explanation for growing circular echo on weather radar?
You are seeing the gust fronts or outflow boundaries of the thunderstorms around Denver.  The rainfall from the storm provides an evaporative flux that cools the air it falls into (preserving $\theta_e$ of air descending from aloft).  This cool air is more dense than the surrounding air at and near the surface and spreads out as a density current.  The cooler the air (relative to ambient air) the faster it will spread out.  You experience the passage of the boundary as the gusty wind conditions that often precede a storm.  

The movement of this density current causes the warmer air it displaces to rise (which can initiate further convection) and this concentrates and lofts insects, which is what you are seeing on radar.  In radar nomenclature you will hear these features referred to as "fine lines".