Characterizing the Structure of Hurricane Karl (2010) Through Analysis of High-Resolution Doppler Radar Data and Numerical Simulations

Airborne radar data and a series of numerical simulations are employed to investigate the structure of Hurricane Karl (2010). Karl was a strong hurricane that reached its peak intensity in the Gulf of Mexico before making landfall on the mountainous coast of Veracruz, Mexico. Multiple aircraft extensively sampled Karl during the NASA GRIP campaign, including NASA's DC-8 aircraft instrumented with the Advanced Precipitation Radar 2 (APR-2). APR-2 is a high-resolution, dual-frequency Doppler radar capable of classifying hydrometeor types. Data from APR-2 provide a unique opportunity to characterize the precipitation structure of Hurricane Karl as it underwent orographic modification during landfall. After landfall in central Veracruz on 17 September 2010, the vertical structure of the precipitation echo varied spatially near the intersection of the Sierra Madre Oriental mountain range and the Trans-Mexican Volcanic Belt. Variation in the vertical structure of Karl was linked to several factors: movement over land, orientation of flow relative to the storm track and topographic features, and differing characteristics inherent to the eyewall and rainbands. Compared to the deep, continuous radar echo that surrounded the southern eyewall, the radar echo surrounding the northern eyewall was shallow and weakening. Despite the differences in the background reflectivity intensity across the storm, we show that low-level reflectivity enhancement occurred only around the northern eyewall where the flow was well-oriented to flow up the sloping terrain. The radar data indicate that the processes initially contributing to the reflectivity enhancement were warm-cloud processes, either through collection of orographically-generated cloud water or shallow convection. As Karl continued to weaken, the low-level enhancement processes were overshadowed by deep convection that developed along the terrain. Rain gauge data and thermodynamic information from nearby dropsondes provide context for the precipitation structures seen by the radar.

Analysis of the radar data is complemented by a series of numerical simulations. The simulated microphysical and kinematic structures are compared to the patterns derived from the APR-2 reflectivity, velocity and hydrometeor snapshots. These comparisons allow for an assessment of the ability of the simulations to reproduce the observed structure and processes. The resulting thermodynamic structures and flow patterns are examined to provide information missing from the radar data. Finally, we use terrain modification experiments to examine the manner in which Karl responds to the mountainous terrain and land surface characteristics.