The structure and dynamics of atmospheric bores and solitons as determined from remote sensing and modeling experiments during IHOP

Solitary waves are a class of gravity waves consisting of a single elevation of finite amplitude that, owing to a balance between nonlinearity and dispersion, propagates without change of form. A family of solitary waves, which is termed a “soliton”, forms as the natural consequence of the evolution of a bore – a type of gravity wave (hydraulic jump) generated as a density current (such as cold air from a thunderstorm) intrudes into a fluid of lesser density, which in the case of the atmosphere, occurs beneath a low-level inversion. Observations and modeling of these phenomena during the International H20 Project (IHOP) will be presented. Highly detailed structure and evolution of these phenomena were produced from a wide variety of remote-sensing systems, including: reflectivity, radial velocity, and derived refractivity measurements from the S-POL radar at Homestead, Oklahoma; boundary layer height fluctuations from an FM-CW radar, the ground-based HARLIE aerosol backscatter and Raman lidars; moisture and temperature profiles from an Atmospheric Emitted Radiance Interferometer (AERI); horizontal and vertical winds from the NCAR Multiple Antenna Profiler (MAPR); moisture measurements from the Leandre-II DIAL (Differential Absorption Lidar) aboard the NRL P-3 aircraft; in situ meteorological measurements from the University of Wyoming King Air (UWKA); 3-hourly CLASS soundings; and high-frequency surface mesonetwork data. This unprecedented set of observations collected on the time-varying structure of bores and solitons catalogues 18 bore events during the six-week IHOP experiment, allowing for common aspects of their environment to be determined.

Three of these bore events were analyzed in detail in this study. Bores and solitons appeared as fine lines in S-POL reflectivity displays and their vertical structures were readily detected by the lidar and radar systems in IHOP, allowing direct comparison with hydraulic theory predictions. Refractivity, AERI, mesonet, and UWKA measurements indicated pronounced surface layer drying accompanied the passage of the bores, but cooling and moistening occurred aloft as the result of adiabatic lifting. Multiple solitary waves developed from the bores, which formed on a surface stable layer in each case. This inversion was lifted abruptly by the leading wave and further by each passing wave, in a stair step fashion, thereby destabilizing the atmosphere. The synthesis of these unprecedented observations with high-resolution MM5 modeling results (using nesting down to a horizontal resolution of 700m) indicate that solitary wave origin as explained by classical bore theory was not the general cause for the waves. Rather, “lee-wave” activity produced by strong jet-like flow over the bore head appeared to have generated the waves in the lee of the bores, suggesting a substantial modification to the existing theory is needed.