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1. Coupling Atmospheric and Biological Remote Sensing to Investigate Boundary-Layer Evolution and Animal Flight Behavior as Affected by the 2017 North American Solar Eclipse

   https://doi.org/10.3390/rs12040591  

   Stepanian, Phillip M.; Wainwright, Charlotte E. (February 2020, Remote Sensing)

   null (Ed.)

   The daytime atmospheric boundary layer is characterized by vertical convective motions that are driven by solar radiation. Lift provided by thermal updrafts is sufficiently ubiquitous that some diurnal birds and arthropods have evolved specialized flight behaviors to soar or embed in these atmospheric currents. While the diel periodicity of boundary-layer dynamics and animal flight has been characterized, rare disruptions to this cycle provide a chance to investigate animal behavioral responses to boundary layer motion and photoperiod that are disjointed from their expected circadian rhythm. To analyze these interactions, we couple radar-derived animal observations with co-located lidar measurements of the convective boundary layer over north-central Oklahoma, USA during the solar eclipse of 21 August 2017. Analysis of animal flight behavior confirmed that ascending and descending flight effort did change in the time period encompassing the solar eclipse, however, the response in behavior was coincident with proximate changes in boundary-layer turbulence. Both the animal behavioral response and decrease in atmospheric turbulence lagged changes in solar irradiance by approximately 30 min, suggesting that changes in flight activity were not cued by the eclipse directly, but rather by the modification of vertical air motions caused by the eclipse. 

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2. Statistically Resolved Planetary Boundary Layer Height Diurnal Variability Using Spaceborne Lidar Data

   https://doi.org/10.3390/rs16173252  

   Roldán-Henao, Natalia; Yorks, John E; Su, Tianning; Selmer, Patrick A; Li, Zhanqing (September 2024, Remote Sensing)

   The Planetary Boundary Layer Height (PBLH) significantly impacts weather, climate, and air quality. Understanding the global diurnal variation of the PBLH is particularly challenging due to the necessity of extensive observations and suitable retrieval algorithms that can adapt to diverse thermodynamic and dynamic conditions. This study utilized data from the Cloud-Aerosol Transport System (CATS) to analyze the diurnal variation of PBLH in both continental and marine regions. By leveraging CATS data and a modified version of the Different Thermo-Dynamics Stability (DTDS) algorithm, along with machine learning denoising, the study determined the diurnal variation of the PBLH in continental mid-latitude and marine regions. The CATS DTDS-PBLH closely matches ground-based lidar and radiosonde measurements at the continental sites, with correlation coefficients above 0.6 and well-aligned diurnal variability, although slightly overestimated at nighttime. In contrast, PBLH at the marine site was consistently overestimated due to the viewing geometry of CATS and complex cloud structures. The study emphasizes the importance of integrating meteorological data with lidar signals for accurate and robust PBLH estimations, which are essential for effective boundary layer assessment from satellite observations. 

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3. Investigation of Hydrostatic Imbalance with Field Observations

   https://doi.org/10.1175/JAMC-D-22-0206.1  

   Sun, Jielun; Wulfmeyer, Volker; Späth, Florian; Vömel, Holger; Brown, William; Oncley, Steven (January 2024, Journal of Applied Meteorology and Climatology)

   Abstract The hydrostatic equilibrium addresses the approximate balance between the positive force of the vertical pressure gradient and the negative gravity force and has been widely assumed for atmospheric applications. The hydrostatic imbalance of the mean atmospheric state for the acceleration of vertical motions in the vertical momentum balance is investigated using tower, the global positioning system radiosonde, and Doppler lidar and radar observations throughout the diurnally varying atmospheric boundary layer (ABL) under clear-sky conditions. Because of the negligibly small mean vertical velocity, the acceleration of vertical motions is dominated by vertical variations of vertical turbulent velocity variances. The imbalance is found to be mainly due to the vertical turbulent transport of changing air density as a result of thermal expansion/contraction in response to air temperature changes following surface temperature changes. In contrast, any pressure change associated with air temperature changes is small, and the positive vertical pressure-gradient force is strongly influenced by its background value. The vertical variation of the turbulent velocity variance from its vertical increase in the lower convective boundary layer (CBL) to its vertical decrease in the upper CBL is observed to be associated with the sign change of the imbalance from positive to negative due to the vertical decrease of the positive vertical pressure-gradient force and the relative increase of the negative gravity force as a result of the decreasing upward transport of the low-density air. The imbalance is reduced significantly at night but does not steadily approach zero. Understanding the development of hydrostatic imbalance has important implications for understanding large-scale atmosphere, especially for cloud development. Significance StatementIt is well known that the hydrostatic imbalance between the positive pressure-gradient force due to the vertical decrease of atmospheric pressure and the negative gravity forces in the vertical momentum balance equation has important impacts on the vertical acceleration of atmospheric vertical motions. Vertical motions for mass, momentum, and energy transfers contribute significantly to changing atmospheric dynamics and thermodynamics. This study investigates the often-assumed hydrostatic equilibrium and investigate how the hydrostatic imbalance is developed using field observations in the atmospheric boundary layer under clear-sky conditions. The results reveal that hydrostatic imbalance can develop from the large-eddy turbulent transfer of changing air density in response to the surface diabatic heating/cooling. The overwhelming turbulence in response to large-scale thermal forcing and mechanical work of the vast Earth surface contributes to the hydrostatic imbalance on large spatial and temporal scales in numerical weather forecast and climate models. 

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4. Circadian Rhythm of Dune‐Field Activity

   https://doi.org/10.1029/2020GL090924  

   Gunn, A.; Wanker, M.; Lancaster, N.; Edmonds, D. A.; Ewing, R. C.; Jerolmack, D. J. (March 2021, Geophysical Research Letters)

   Abstract Wind‐blown sand dunes are both a consequence and a driver of climate dynamics; they arise under persistently dry and windy conditions, and are sometimes a source for airborne dust. Dune fields experience extreme daily changes in temperature, yet the role of atmospheric stability in driving sand transport and dust emission has not been established. Here, we report on an unprecedented multiscale field experiment at the White Sands Dune Field (New Mexico, USA), where by measuring wind, humidity and temperature profiles in the atmosphere concurrently with sediment transport, we demonstrate that a daily rhythm of sand and dust transport arises from nonequilibrium atmospheric boundary layer convection. A global analysis of 45 dune fields confirms the connection found in situ between surface wind speed and diurnal temperature cycles, revealing an unrecognized climate feedback that may contribute to the growth of deserts on Earth and dune activity on Mars. 

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5. The Effect of Upstream Orography on the Onset of Sundowner Winds in Coastal Santa Barbara, CA

   https://doi.org/10.1029/2020JD033791  

   Duine, Gert‐Jan; Carvalho, Leila M. V.; Jones, Charles; Zigner, Katelyn (April 2021, Journal of Geophysical Research: Atmospheres)

   Abstract The impact of upstream terrain on the diurnal variability of downslope windstorms on the south‐facing slopes of the Santa Ynez Mountains (SYM) is investigated using numerical simulations. These windstorms, called Sundowners due to their typical onset around sunset, have intensified all major wildfires in the area. This study investigates the role of the orography upstream of the SYM in the diurnal behavior of Sundowners. Two types of Sundowners are examined: western sundowners (winds with dominant northwesterly direction) and eastern Sundowners (winds with dominant northeasterly direction). By using semi‐idealized simulations, in which we progressively reduce the upstream terrain, we show that the onset of the lee slope jet occurs in the late afternoon only when the flow approaches the SYM from the northeast, after interacting with a considerably higher mountain barrier. We demonstrate that during the eastern regime, the progressive reduction of the upstream terrain results in strong lee slope winds throughout the day. Conversely, the diurnal cycle of downslope winds during the western regime is less sensitive to the reduction of the upstream terrain. The Sundowner diurnal cycle during the eastern regime can be explained by boundary‐layer processes in the valley and the blocking effect of high mountains upstream of the SYM. These results contribute to a better understanding of the influence of upstream orography in the cycle and intensity of downslope windstorms in coastal mountains. 

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