Connecting an Offshore Dry Air Stratospheric Intrusion with the Outbreak of Soberanes Fire 2016

Abstract

This study investigates the connection between the arrival of dry stratospheric air with the Soberanes Fire (2016). The Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT) and Goddard Earth Observing System Forward Processing model (GEOS-FP) are used for back-trajectories and offshore deep stratospheric intrusion (SI) in conjunction with the ignition and outbreak of the fire. The back-trajectory analysis indicates most air reaching the vertical column was critically dry, exhibiting relative humidity values below 10%. As the fire ignited, dry air arrived from due west at heights of 1-3 km about 24 hours prior. During the overnight fire growth, dry air arrived from the northwest to north-northwest at heights of 3.5-5.5 km 48-72 hours prior. The synoptic and the GEOS-FP analysis demonstrate offshore mid-to-low stratospheric intrusion. On July 21, 2016, an enclosed upper-level low approached the California/Oregon border along the northwesterly subtropical jet stream hours before the fire outbreak. The GEOS-FP results of potential vorticity, specific humidity, and ozone along the back-trajectories to the west and northwest of the fire suggest a stratospheric intrusion event into the mid-to-low troposphere at the back-trajectory start points, and vertical velocity indicates sinking motion. The specific humidity analyzed at the arrival time shows the transport of the abnormally dry air to the Soberanes Fire. Results suggest a connection between dry stratospheric air transported to the Soberanes Fire at ignition and overnight accelerated growth, supported by a dark bank in satellite water vapor imagery. The prediction of low-level transport of dry stratospheric air to the coastal communities could help predict the occurrence of wildfire outbreaks, or periods of accelerated fire growth.