Airborne radar and lidar observations, dropsondes, and surface mesonet data are combined to document the initiation of convection on May 24 and June 19 during IHOP (International H20 Project). One of the primary objectives of IHOP was to focus all available platforms on convergence boundaries that were detectable by Doppler radars in the clear air. ELDORA (onboard a Navy P-3) flew elongated box-type patterns (75 – 80 km long) parallel to the boundaries in order to assess the along-frontal variability of the winds and the clear-air echoes. Rapid deployment of sondes (horizontal spacing approximately 20 km) from a LearJet flying perpendicular to the boundary provided kinematic and thermodynamic information on both sides of the boundary. In addition, vertical profiles of water vapor and aerosols were recorded on May 24 by the LASE flown onboard the NASA DC-8.

The case on May 24 is unique since it provided high-resolution data of the ‘triple point’ (the intersection between the a cold front and dryline). Kelvin-Helmholtz waves were resolved along the cold frontal boundary. The dryline was characterized by a cellular structure and there were suggestions that two drylines existed simultaneously. A clear-air mesocyclone with a diameter of 30-40 km was observed at the triple point and was associated with vertical vorticity greater than 2 x 10-3s-1. Interestingly, the severe convection initiated ~50 km east of the dryline. The LASE data combined with the thermodynamic information from the dropsondes supports the location where convection first develops. The region east of the dryline was characterized by the greatest potential instability.

Mobile IHOP platforms converged on a stationary dryline in northwest Kansas on June 19. Clear skies were quickly followed by cumulus congestus and thunderstorms along most of the line. Clear-air radar observations recorded by ELDORA revealed finescale variations along the dryline as the precipitation echoes formed. Misocyclones (small-scale cyclonic circulations less than 4 km) with horizontal spacing of ~10 km developed and tornadic-like vortices were visually observed by scientists from the ground and the aircraft. There were ‘breaks’ in the horizontal structure of the fine line and plumes of echoes (owing to insects) were carried aloft into the weak-echo vault of the storms. A vertical cross section of equivalent potential temperature perpendicular to the cold front revealed that the entire region was potentially unstable; however, the storms initiated at the leading edge of the dryline owing to forced uplift.