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>Characterization of the Afternoon to Evening Transition of the Planetary Boundary Layer Over North Alabama and Implication for Convective Maintenance, Enhancement, and Initiation.
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Characterization of the Afternoon to Evening Transition of the Planetary Boundary Layer Over North Alabama and Implication for Convective Maintenance, Enhancement, and Initiation.
Investigation of 143 cases of the north Alabama planetary boundary layer's afternoon to evening transition (AET) using surface, profiling, and radar observations contributes to the relative scarcity of datasets for this period. Results indicate several characteristics previously deemed important for convective initiation and/or maintenance, namely a rise in boundary layer water vapor and convergence. Waning surface heating causes turbulent eddies to decay, decreasing vertical heat and momentum fluxes, evident in declining surface wind speeds as horizontal flow above the surface layer increases. After surface temperature variances decline, the rate at which vertical velocity fluctuations decay increases as vigorous thermals diminish, and the fastest decline of surface horizontal wind variance proceeds an accelerated decrease in the vertical wind variance. Steady increases in horizontal wind speed and radar-derived convergence above the surface layer span the entire AET.;Detailed case studies evaluate structural changes along convergent boundaries propagating during the AET. Overall, analyses reveal a relative steadiness, or even increase, in convergence above the surface layer at the boundaries' leading edge, supporting the hypothesis that characteristic AET processes can contribute to convective maintenance or enhancement, and potentially aid in convective initiation. Dual-Doppler derived kinematic fields and the visual cloud field character generally exhibit a qualitative slabularity increase (decreased along-line variability) as boundary layer eddies diminish. Results also indicate that thermodynamic changes during the AET can promote substantial vertical cloud development, though this visual signal is often unobserved by radars. Further, a key finding among these studies is the evolution of gust fronts to wave-like entities near sunset, indicating the AET can be far more complex than initially perceived.;In a simplified, 2-D modeling experiment, completed by altering the input in a gravity current simulation with profiles observed across two AETs, the evolution of the idealized boundaries expresses a similar behavior as the observed case studies: convergence at the leading edge of the density currents shows a steadiness and/or increase during the transition period, followed by a dramatic decline after 2 h post-sunset. Altogether, results portray the transitional boundary layer as a potentially favorable habitat for maintaining, enhancing, and even initiating, convective elements.
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