The impact of heat and inflow wind variations on vertical transport around a supertall building - The One Vanderbilt field experiment.

The impact skyscrapers have on wind flow remains poorly characterized, thus affecting atmospheric dispersion predictions in dense urban centers. A new mobile observatory equipped with remote sensors controlled by a smart sampling protocol was developed to collect high-resolution (18 m, 15 s) observations throughout the atmospheric layer below 1.5 km. A series of four deployments was performed around the One Vanderbilt skyscraper (H1 = 427 m) located in Manhattan, NY to document wind flow and temperature in canyons with relatively high width-to-depth ratios (H2/W ~ 1.2-7.5; H2 being the height of the adjacent building) and steepness (H1/H2= 2.1-11.2) and that under a range of inflow wind and solar heating conditions. A series of flow features were common to all case studies with head-on winds. A stagnation point was observed 2/3 of the way up the impeded portion of the One Vanderbilt, pointing to the importance of the upwind building height in controlling vertical air flow. In the canyons parallel to the flow, three sets of mirroring counterrotating vortices were detected pointing to the fact that H2 is not as important a parameter in controlling flow in canyons parallel to the inflow wind. Plumes of rapidly rising air were detected near building heat vents under both 10 m s-1 and 3 m s-1 inflow wind conditions, at night and in the morning respectively. This suggests that anthropogenic heat may be an important energy source especially in the absence of solar heating. In the presence of solar heating, a systematic tendency for upward flow was observed above H1. We associate this pattern to the presence of rising thermals, a common mechanism for planetary boundary layer growth. Below H2, complete flow reversal (relative to mechanically driven circulations) was detected ~20 % of the time, showing evidence of dominant thermal effects even under 7 m s-1 inflow wind conditions.

New insights into ice multiplication using remote-sensing observations of slightly supercooled mixed-phase clouds in the Arctic.

Secondary ice production (SIP) can significantly enhance ice particle number concentrations in mixed-phase clouds, resulting in a substantial impact on ice mass flux and evolution of cold cloud systems. SIP is especially important at temperatures warmer than -[Formula: see text]C, for which primary ice nucleation lacks a significant number of efficient ice nucleating particles. However, determining the climatological significance of SIP has proved difficult using existing observational methods. Here we quantify the long-term occurrence of secondary ice events and their multiplication factors in slightly supercooled clouds using a multisensor, remote-sensing technique applied to 6 y of ground-based radar measurements in the Arctic. Further, we assess the potential contribution of the underlying mechanisms of rime splintering and freezing fragmentation. Our results show that the occurrence frequency of secondary ice events averages to <10% over the entire period. Although infrequent, the events can have a significant impact in a local region when they do occur, with up to a 1,000-fold enhancement in ice number concentration. We show that freezing fragmentation, which appears to be enhanced by updrafts, is more efficient for SIP than the better-known rime-splintering process. Our field observations are consistent with laboratory findings while shedding light on the phenomenon and its contributing factors in a natural environment. This study provides critical insights needed to advance parameterization of SIP in numerical simulations and to design future laboratory experiments.