Operational recognition of high precipitation efficiency and low echo centroid convection

Detection of flash flood producing rainfall remains one of the most difficult challenges facing National Weather Service warning operations today. Although the nationwide deployment of the WSR-88D with precipitation estimation algorithms had led to substantial improvements in real-time monitoring of heavy rainfall, these algorithms assume that convection over an entire radar domain has identical cloud physics properties. This assumption may contribute to poor precipitation estimation and becomes especially dangerous when rogue convection or unique rainfall events have higher precipitation efficiency than assumed.

Research on several devastating flash flood events over the last several decades often cites abnormally high precipitation efficiency as a contributing factor. The literature suggests convection with high precipitation efficiency often has a low-echo centroid, meaning most of the cell's reflectivity is located below the freezing level. Low-echo centroid storms are more characteristic of tropical environments and are less common across the central United States where forecaster recognition may be impeded by their infrequency. Additionally, real-time identification of low-echo centroids can be difficult because of a vaguely defined recognition guideline and seemingly benign reflectivity characteristics associated with such convection.

The objective of this study was to develop a conceptual model of the synoptic conditions favorable for the formation of high precipitation efficiency convection and its operational identification through the use of real-time data sets. The model was developed from an analysis of five abnormally heavy rainfall events that occurred across the Central and Southern Plains of the United States. A variety of parameters were examined to identify common convective cell traits. Familiarity with this model should increase forecaster recognition of the potential for high efficiency precipitation so that better flash flood warning decisions can be made.