Shipborne Dual-Doppler Operations during TOGA COARE: Integrated Observations of Storm Kinematics and Electrification

Abstract

Shipborne Doppler radar operations were conducted over the western Pacific warm pool during TOGA COARE using the Massachusetts Institute of Technology and NOAA TOGA C-band Doppler radars. Occasionally the ships carrying these radars were brought to within 50 km of each other to conduct coordinated dual-Doppler scanning. The dual-Doppler operations were considered a test of the logistical and engineering constraints associated with establishing a seagoing dual-Doppler configuration. A very successful dual-Doppler data collection period took place on 9 February 1993 when an oceanic squall line developed, intensified, and propagated through the shipborne dual-Doppler lobes. Later on the same day, NOAA P-3 aircraft sampled a more intense squall line located approximately 400 km to the south-east of the shipborne operations. This study provides an overview of the shipborne dual-Doppler operations, followed by a comparison of the kinematic and precipitation structures of the convective systems sampled by the ships and aircraft. Special emphasis is placed on interpretation of the results relative to the electrical characteristics of each system. Soundings taken in the vicinity of the ship and aircraft cases exhibited similar thermodynamic instability and shear. Yet Doppler radar analyses suggest that the aircraft case exhibited a larger degree of low-level forcing, stronger updrafts, more precipitation mass in the mixed-phase region of the clouds, and a relatively higher degree of electrification as evidenced by lightning observations. Conversely, convection in the ship case, while producing maximum cloud-top heights of 16 km, was associated with relatively weaker low-level forcing, weaker vertical development above the -5°C level, moderate electric fields at the surface, and little detectable lightning. Differences in the kinematic and precipitation structures were further manifested in composite vertical profiles of mean convective precipitation and vertical motion. When considered relative to the electrical properties of the two systems, the results provide further circumstantial evidence to support previously hypothesized vertical velocity and radar reflectivity thresholds that must be exceeded in the 0° to -20°C regions of tropical cumulonimbi prior to the occurrence cf lightning.