Theoretical Framework for Symmetric Convergence Using Active and Passive Remote Sensing

The planetary boundary layer (PBL), the lowest and most dynamic region of the troposphere, is directly influenced by the earth’s surface by all its properties: topography, land cover, water, urbanization, and the many variations in between. Oftentimes nocturnal convection initiation (NCI) occurs in the presence of a stable surface layer when incoming solar radiation is not present and radiational cooling is occurring. Under these conditions, the organization of convection is typically triggered by more nuanced forces driven by outflows such as density currents, gravity waves, bore waves, or converging Planetary Boundary Layer (PBL) air masses. Most studies exploring NCI during convergence events only consider unbalanced convergence with the less dense air mass “riding” over the other. This study explores a convergence boundary where two balanced masses meet and lead to a nearly symmetric divergence of water vapor aloft. Henceforth this phenomenon will be referred to as “Symmetric Convergence”.

Vertical transects of an observed symmetric convergence event collected from sensors including Raman Lidar (RL), Atmospheric Emitted Radiance Interferometer (AERI), and 915 Mhz Radar Wind Profiler (RWP) provide information about the temperature, humidity, and wind magnitude and direction before, during, and after the overpassing of the symmetric convergence event. Moisture, temperature, and 3D wind vectors are observed by RL, AERI, and RWP respectively, and allow for CAPE, CIN, and Vorticity to be derived on time-height cross-sections. Based on these active and passive observations, a theory is constructed and presented for the phenomenon known as Symmetric Convergence. Previous theories of convergence-based NCI, many of which were developed using numerical simulations, are evaluated here under real, observed atmospheric conditions via the symmetric convergence case study. This case study and framework aim to highlight the importance of considering and further characterizing convergence events under balanced regimes, infrequently discussed in the current literature.