Backscatter Lidar Observations of Lower Tropospheric Dynamics During Southern California Wildfires

Abstract

Wavelike features suggesting gravity waves were revealed by lidar observations (from El Segundo, California) of smoke layers produced by large wildfires in the Southern California region during a Santa Ana event. Unique features of the observations were multiple thin layers that enabled precise determinations of wave frequency, amplitude, and vertical structure. The data revealed persistent wavelike oscillations that showed no phase variation with altitude, an amplitude of 20 m, and a period near 12 min. The observations were averaged over 1.5 min with a vertical resolution of 3 m and were obtained over a period of 5 h on 25 October and 8 h on 26 October 2007. Vertical profiles of temperature and winds from the Aircraft Communication Addressing and Reporting System (ACARS) of commercial aircraft departing Los Angeles International Airport (LAX) were selected for temporal and spatial coincidence with the lidar observations. In addition, satellite images of the smoke distribution over the Los Angeles Basin and the coastal areas (including coastal waters) to the south were obtained from Moderate Resolution Imaging Spectroradiometer (MODIS) data from overflights 5.5 h prior to the lidar observation window. The images show wavelike features with horizontal wavelengths of ∼10 km or less. The temperature data showed an inversion layer topped at 500-m altitude. The wind data were consistent with a residual sea breeze near the surface and Santa Ana easterlies above. A simple model of wave ducting showed that the observed features were the evanescent extension of resonant waves ducted in the lower stable inversion layer. These are the first detailed observations, including vertical structure, of ducted waves associated with Santa Ana conditions. It is suggested that such waves should be a common feature in conditions occurring during Santa Ana events.

Keywords: Lidar observations; Troposphere; North America; Forest fires; Waves, atmospheric