Vertical Velocity and Buoyancy Characteristics of Coherent Echo Plumes in the Convective Boundary Layer, Detected by a Profiling Airborne Radar

An airborne 95 GHz Doppler radar is used to examine the properties of radar echo plumes in the convective boundary layer mainly under synoptically quiescent conditions. The profiling airborne radar data were collected in the central Great Plains during IHOP_2002 (International H2O Project, May-June 2002). A conditional sampling technique based on close-range radar reflectivity is used to define echo plumes and background. The properties of defined echo plumes are investigated by using aircraft in situ measurements of gust probe vertical velocity, mixing ratio, potential temperature, and virtual potential temperature. The observed properties indicate that the echo plumes are dynamically equivalent to thermals since they are generally rising and buoyant. Echo plume buoyancy decreases with height and the rising speed of the plumes increases due to their high buoyancy at low level and slows down as their buoyancy fades. The distributions of plume width and spacing are approximately exponential with medians of 0.34 zi and 0.84 zi respectively. With the same algorithm of close-gate-reflectivity to define plumes, updrafts and downdrafts have been defined by using vertical velocity as the indicator variable. Very similar properties of the drafts are observed to those of close-gate-reflectivity-defined echo plumes. This confirms the conclusion that echo plumes are very closely connected to thermals. Also, 2-dimensional echo plumes are defined using radar reflectivity. The method captured the general shape and positions of the echo plumes. The number of plumes in general decreases with height and the plume width increases with height based on the 2D-reflectivity-defined echo plumes.