Spatially distributed atmospheric boundary layer properties in Houston - A value-added observational dataset.

In 2022, Houston, TX became a nexus for field campaigns aiming to further our understanding of the feedbacks between convective clouds, aerosols and atmospheric boundary layer (ABL) properties. Houston's proximity to the Gulf of Mexico and Galveston Bay motivated the collection of spatially distributed observations to disentangle coastal and urban processes. This paper presents a value-added ABL dataset derived from observations collected by eight research teams over 46 days between 2 June - 18 September 2022. The dataset spans 14 sites distributed within a ~80-km radius around Houston. Measurements from three types of instruments are analyzed to objectively provide estimates of nine ABL parameters, both thermodynamic (potential temperature, and relative humidity profiles and thermodynamic ABL depth) and dynamic (horizontal wind speed and direction, mean vertical velocity, updraft and downdraft speed profiles, and dynamical ABL depth). Contextual information about cloud occurrence is also provided. The dataset is prepared on a uniform time-height grid of 1 h and 30 m resolution to facilitate its use as a benchmark for forthcoming numerical simulations and the fundamental study of atmospheric processes.

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.

Spaceborne Cloud and Precipitation Radars: Status, Challenges, and Ways Forward.

Spaceborne radars offer a unique three-dimensional view of the atmospheric components of the Earth's hydrological cycle. Existing and planned spaceborne radar missions provide cloud and precipitation information over the oceans and land difficult to access in remote areas. A careful look into their measurement capabilities indicates considerable gaps that hinder our ability to detect and probe key cloud and precipitation processes. The international community is currently debating how the next generation of spaceborne radars shall enhance current capabilities and address remaining gaps. Part of the discussion is focused on how to best take advantage of recent advancements in radar and space platform technologies while addressing outstanding limitations. First, the observing capabilities and measurement highlights of existing and planned spaceborne radar missions including TRMM, CloudSat, GPM, RainCube, and EarthCARE are reviewed. Then, the limitations of current spaceborne observing systems, with respect to observations of low-level clouds, midlatitude and high-latitude precipitation, and convective motions, are thoroughly analyzed. Finally, the review proposes potential solutions and future research avenues to be explored. Promising paths forward include collecting observations across a gamut of frequency bands tailored to specific scientific objectives, collecting observations using mixtures of pulse lengths to overcome trade-offs in sensitivity and resolution, and flying constellations of miniaturized radars to capture rapidly evolving weather phenomena. This work aims to increase the awareness about existing limitations and gaps in spaceborne radar measurements and to increase the level of engagement of the international community in the discussions for the next generation of spaceborne radar systems.

Relationships Between Precipitation Properties and Large-Scale Conditions During Subsidence at the Eastern North Atlantic Observatory.

Three years of reanalysis and ground-based observations collected at the Eastern North Atlantic (ENA) observatory are analyzed to document the properties of rain and boundary layer clouds and their relationship with the large-scale environment during general subsidence conditions and following cold front passages. Clouds in the wake of cold fronts exhibit on average a 10% higher propensity to precipitate and higher rain-to-cloud fraction than cloud found in general subsidence conditions. Similarities in the seasonal cycle of rain and of large-scale properties suggest that the large-scale conditions created by the cold front passage are responsible for the unique properties of the rain forming in its wake. The identification of monotonic relationships between rain-to-cloud fraction and rain rate with surface forcing and boundary layer stability parameters as well as between virga base height with stability and humidity measures further supports that large-scale conditions impact precipitation variability. That being said, these relationships between the large-scale and rain properties are less clear than those established between cloud and rain properties, suggesting that cloud macrophysics have a more direct impact on the properties of rain than the large-scale environment. The applicability of previously documented relationships between cloud thickness and rain properties is tested and the relationships adjusted to accommodate the complex shallow clouds and melting precipitation observed to occur in the ENA region. Establishing these relationships opens up opportunities for parametrization development and suggests that a realistic representation of precipitation properties in models relies on the accurate representation of both clouds and the large-scale environment.