An Extended Dual-Doppler Radar Study of the Houston Rainfall and Lightning Anomalies

Rainfall and cloud-to-ground (CG) lightning anomalies over and downwind of the Houston urban area have been well documented. The placement of a Shared Mobile Atmospheric Research and Teaching Radar (SMART-R) near the KHGX Weather Surveillance Radar – 1988 Doppler (WSR-88D) during the 2005 summer season presents a unique opportunity to investigate the role of boundary-layer convergence in modulating convective frequency and intensity and thereby likely causing the rainfall and CG lightning anomalies. The relative roles of the urban heat island (UHI) and the sea-breeze as sources of low-level convergence leading to enhanced convection over Houston were also examined. Hourly average dual-Doppler winds and convergence were synthesized on 1 km2 horizontal grids for a nearly continuous eleven-week period. By using these analyses along with average CG lightning, rainfall, and reflectivity for a large Houston centered domain, it was possible to discern a correlation between low-level convergence and convection. Enhancements in CG lightning and rainfall over and downwind of Houston were consistent with past studies and were associated with a large increase in the frequency of deep convective events as compared to the surrounding domain. During the period studied, it was found that UHI, rather than sea-breeze, was likely the primary causative mechanism in the development of these anomalies. An area of weaker enhancement south of Houston, not discussed in previous studies, was possibly caused by interactions between the bay-breeze off of the Galveston Bay and the sea-breeze. The influence of mesoscale forcing on convective intensity and frequency appeared to be sensitive to local changes in land use. Maxima in deep convection and CG lightning were found over areas designated as medium-to-high density urban. Minima of deep convective activity and CG lightning were found over Memorial Park, which encompasses 1500 wooded acres in west Houston. While forested and non-cropland areas were generally associated with suppressed deep convective activity, enhanced areas of deep convection and CG lightning were present over cultivated herbaceous land. A gridded database of seven (1997-2003) years of summer (JJA) season KHGX WSR-88D reflectivity and National Lightning Detection Network (NLDN) CG lightning data will be used to investigate these correlations further.