Model simulation and remote sensing of bore and solitary wave mixing processes

Solitary wave trains associated with two bores were observed by a very large number of ground-based and airborne profiling systems during the International H2O Project (IHOP) and also numerically simulated with a multiply-nested numerical model down to a resolution of 0.7 km. This study marks the first successful attempt to use such a high-resolution numerical weather prediction (NWP) model initialized with real data to simulate an observed bore and solitary waves. The observations consisted of the National Center for Atmospheric Research (NCAR) S-POL radar reflectivity data, three-dimensional winds from the NCAR Multiple Antenna Profiler (MAPR), boundary layer height fluctuations from a Frequency Modulation-Continuous Wave (FM-CW) radar, profiles of temperature and moisture retrieved by an Atmospheric Emitted Radiance Interferometer (AERI), and the NCAR Integrated Sounding System, all at the Homestead, Oklahoma observing site, plus surface mesonetwork data, and in situ measurements taken by the University of Wyoming King Air.

The model produced a bore that corresponded well in timing and location with the second of the two bores, but the first bore was not successfully simulated due to problems with forecasting convective precipitation correctly. Although both bores were in their dissipating stage when they passed over the Homestead observing site, the remote sensing systems all showed that the antecedent nocturnal inversion depth was nearly doubled by the passage of the bore. The observational and model data 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. Near-surface observations showed pronounced decreases in water vapor mixing ratio accompanied the passage of the bores during their active phase, but during bore collapse, moistening appeared in the lowest 0.5 km of the atmosphere.

The results indicate that low-level drying near the surface was due to vigorous downward turbulent mixing of air by the wave circulations. Turbulent kinetic energy was generated immediately behind the bore head, then advected rearward and downward by the solitary waves. During the dissipation stage, the lifting by the bore head produced adiabatic cooling aloft and distributed the very moist air near the surface upwards through the bore depth, but without any drying due to the absence of vigorous mixing.

This study shows that it is possible for NWP models to predict bores and solitons, and to be used as research tools, in combination with remote sensing systems, to understand the dynamics of these phenomena. However, the value of the model depends upon whether it can skillfully forecast observed precipitation patterns, given the sensitivity of the density current and bore occurrence to this factor. Other necessary ingredients for successful numerical simulation include the proper simulation of the waveguide, such as a frontal system acting as a horizontal delimiter, and the strength of the low-level jet, which acts as an important mechanism for trapping vertical wave energy propagation.