FROM SEA TO CITY: OBSERVING THE COMPLEX EVOLUTION OF THE COASTAL-URBAN PLANETARY BOUNDARY LAYER, SEA BREEZE MODIFICATIONS TO THE BOUNDARY LAYER, AND INVESTIGATING MESOSCALE PROCESSES NECESSARY FOR CONVECTION INITIATION IN HOUSTON, TEXAS DURING THE CUBIC FIELD CAMPAIGN

Abstract

Coastal-urban environments are characterized by complex interactions between mesoscale processes and abrupt surface characteristic changes that influence the planetary boundary layer (PBL) evolution, convection potential, and air quality conditions. One of the most prevalent and influential mesoscale circulations that affects coastal-urban regions is the sea breeze (SB), driven by daytime thermal contrasts between the land and sea. The SB is responsible for advecting cooler, more moist air from over the water inland, which can result in thermodynamic and kinematic changes to PBL properties, lead to convection initiation (CI), and affect local air quality. The overarching goal of this dissertation is to further our understanding of the complex coastal-urban environment and interactions between mesoscale processes with a particular focus on SB effects on the PBL. The Coastal-Urban Boundary Layer Interactions with Convection (CUBIC) field campaign utilized an array of PBL profiling instruments to gather observations across the coastal-urban region of Houston, Texas, with the objective to characterize Houston’s PBL and examine the role of the SB and urban heat island (UHI) effects on the evolution of the PBL. Each of the three sites collected continuous, high-resolution kinematic and thermodynamic data for a four-month period to culminate in a rich dataset suitable for spatial and temporal analyses of the PBL and SB. First, a classification method for handling the large number of CUBIC cases was developed. Initially, a self-organizing map (SOM) technique was applied to objectively sort each CUBIC day into one of four synoptic regimes based on 700 hPa geopotential heights following the techniques applied by Wang et al. (2022). However, verification of the SOM’s ability to represent the regional flow over the Houston area showed an inability for the SOM to capture the local conditions. Thus, a regional flow classification method based on sounding analyses (700 hPa flow) over the Houston area was adapted to ensure connections between the upper-level flow and PBL responses to the SB were local and consisted of four regimes: onshore, offshore, northeasterly, and southwesterly. Next, the SB was identified using a multivariate approach in which 4 out of 5 specified criteria were needed for a case to be flagged as a SB. A total of 52 days, or 41% of CUBIC dates, developed a SB. Of the SB cases, 98.2% occur when land-sea temperature differences are at least 3°C, with signs of seasonal changes to the SB, with later onset times and faster propagation speeds in September SB cases. Investigations in regional flow regime effects on SB timing show no significant differences amongst regimes, suggesting that onset timing and propagation depend on additional mesoscale and seasonal forcings beyond ΔT or regional regime. Examination of the PBL height (PBLH), estimated via a fuzzy logic algorithm combining data from multiple instruments, showed substantial spatial heterogeneity across the coastal-urban interface. This spatial heterogeneity was attributed to mesoscale influences, such as the SB and UHI effects. The urban site exhibited PBLH values ~480 m higher than at the coastal location, as well as a two-hour temporal difference in when the maximum PBLH is reached. In the overnight period, the nocturnal PBLH at the urban site remains elevated by up to 250 m, further indicating that sharp contrasts in surface characteristics can lead to meaningful differences in PBL structure. Regional-scale flow regimes revealed limited influence on the maximum PBLH, but some regime-dependent differences during the morning and evening transition periods. A larger role in observed PBLH variability was found to be the SB, with SB events shown to take place on days with deeper PBLs across all sites when compared to non-SB cases. To better understand the array of PBL responses to the SB and possible effects from regional flow, two detailed case studies, compared with composite analyses, revealed that the depth, structure, and turbulence signatures of the PBL response to the SB evolve as the circulation moves inland. The urban site consistently exhibited stronger and deeper wind profiles, with offshore cases producing greater post-SB wind speeds and a more pronounced jet-like structure. In contrast, thermodynamic responses varied across space but showed no systematic differences between regional flow regimes, suggesting that pre-existing thermodynamic structure is a more important determinant of SB modification than regional flow. These results point to local-scale and surface processes, such as surface fluxes, cloud cover, and spatial heterogeneity in land cover, as key drivers of the observed variability in PBL responses to the SB across Houston. Finally, a case that was characterized by exceptionally strong thermodynamic and kinematic SB responses and CI along the SB front (SBF), offered an ideal opportunity to integrate high-resolution observations with a numerical weather prediction (NWP) ensemble to explore the processes important to SB-forced CI. The analysis indicated that persistent ascent along or ahead of the inland-propagating SBF, in combination with pre-existing moisture, plays a critical role in successful CI. Ensemble simulations further revealed forced ascent across much of the domain prior to CI, filling gaps between the coastal and urban observations and providing a more complete depiction of the spatial evolution of the SB.