Lidar-radar Analyses of Convection Initiation By Gravity Currents, Bores, and Solitons

Abstract

Gravity currents, bores, and solitary wave trains (solitons) observed in several cases by a large variety of ground-based and airborne profiling systems and also numerically simulated with an ultra-high resolution numerical model are collectively used to understand the convection initiation process. Lidar instrumentation includes moisture measurements from the Leandre-II Differential Absorption Lidar (DIAL) on the NRL P-3 aircraft; and ground-based measurements from the NASA lidars – GLOW Doppler lidar, HARLIE aerosol backscatter lidar, and water vapor from the Scanning Raman Lidar. Radar data includes: reflectivity and radial velocities from 10-cm Doppler weather radars; boundary layer height fluctuations from a Frequency Modulation-Continuous Wave (FM-CW) radar; three-dimensional winds from a 915-MHz boundary layer wind profiler and the National Center for Atmospheric Research (NCAR) Multiple Antenna Profiler (MAPR); and refractivity measurements from the NCAR S-POL radar. In addition, profiles of temperature and moisture retrieved by an Atmospheric Emitted Radiance Interferometer (AERI), observed by the NCAR Integrated Sounding System were used, and measured by a Radio Acoustic Sounding System (RASS) were used. Also, in one case, microwave radiometer measurements of path-integrated total water vapor, cloud liquid water, and cloud-base temperature were available. The bores were generated as a density current, associated either with a cold thunderstorm outflow boundary or a cold front, intruded into a stably stratified boundary layer of sufficient depth near the ground. A train of amplitude-ordered solitary waves evolved in the wake of the bore head as a jet-like current of air flowed over the bore. Bores and solitons appeared as fine lines in radar reflectivity displays. Their vertical structures and circulations were readily detected by the lidar and radar systems, allowing direct comparison with theory and numerical models. The remote sensing data and model simulations both showed that the bores wafted moist air up to the middle troposphere and weakened the capping inversion, thus reducing inhibition to deep convection development. Application of parcel displacement profiles derived from the GLOW and wind profiler analyses to representative pre-bore soundings permitted assessments to be made of the effects of the bore and gravity current passage on the atmosphere. These calculations suggest that the strong bore-induced lifting was insufficient to trigger the storms, but in the one cold frontal case, the dual lifting provided by the bore and the subsequent gravity current that made it possible for low-level parcels to reach their level of free convection. In a more general sense, we find that even though bores can produce strong lifting, this may not be sufficient to trigger deep convection if the lifting is confined to too shallow a layer and/or is of insufficient duration, or the pre-bore environment contains a very strong capping inversion with limited moisture.