Seasonal and Annual Precipitation Efficiency in the Eastern United States·

PreCipitation efficiency (Pe) is the percentage of the average atmospheric water vapor above a station that is released and falls to earth as measurable preCipitation in a given time period. Pe is a single parameter describing the efficiency of atmospheric mechanisms in releasing moisture. Temporal and spatial variations in Pe are controlled primarily by synoptic atmospheric systems. The relationship between atmospheric systems and seasonal! annual variations in Pe is investigated for the United States east of the Rocky Mountains. Regional variations in seasonal and annual averages of Pe suggest the relative effectiveness of dynamic and thermodynamic preCipitation mechanisms. Basically, the pattern of Pe reveals the importance of the greater range of cyclonic activity and frontal zones. The presence of dynamic mechanisms is a prerequisite for the release of precipitation, but topographic enhancement of precipitation and readily available moisture (enhanced PWV supplemented via advection) can also intensify the Pe value.