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Boundary Layer Structure and Dynamics

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Boundary Layer Structure and Dynamics

The portion of our atmosphere that directly interacts with the Earth’s surface and is directly influenced by such effects as surface heating, frictional drag, and turbulence is called the atmospheric boundary layer (ABL) or the planetary boundary layer (PBL). This atmospheric layer is typically about 1 km deep, is characterized by complex spatial and temporal structure, and can vary dramatically over a single diurnal cycle. The figure below depicts some of the many factors that affect the behavior of the ABL.

Depiction of some process typically active within the atmospheric boundary layer.
Depiction of some process typically active within the atmospheric boundary layer.

Overall, processes in the ABL can vary dramatically over a single diurnal cycle, as depicted in the figure below. Although this conceptual model of the ABL is idealized, it helps to illuminate several common features of the ABL structure: mixed layer (ML), capping inversion (CI), the stable boundary layer (SBL), entrainment zone (EZ), the residual layer (RL), and so forth. Above the ABL is the free atmosphere (FA). The temperature profile corresponding to five particular “snapshots” of this idealized ABL diurnal cycle are depicted and labeled as A--E. Time (A) corresponds to nocturnal conditions characterized by a stable boundary layer near the surface with a well-mixed residual layer above. Shortly after sunrise, the ABL begins to transition as depicted at times (B) and (C). The mixed layer is forming near the surface and as it continues to grow, the stable boundary layer is lifted and compressed until it later forms the entrainment zone. During mid-day, the ABL is largely characterized as a mixed layer as shown at time (D). At sunset, surface cooling begins to occur, which in turn sets up the development of the stable boundary layer again, as shown at time (E).

 Schematic depicting the idealized structure of the ABL. The left side shows one diurnal cycle under quiescent conditions. Vertical profiles of the temperature at five particular times (denoted as A-E) are presented to the right. In this cloud-free example, the structure of the ABL is primarily driven by thermal forcing produced by insolation.
Schematic depicting the idealized structure of the ABL. The left side shows one diurnal cycle under quiescent conditions. Vertical profiles of the temperature at five particular times (denoted as A-E) are presented to the right. In this cloud-free example, the structure of the ABL is primarily driven by thermal forcing produced by insolation.

Observations of the Lower Atmosphere

To fully characterize the structural evolution of the ABL, we need measurements with “adequate” temporal and spatial resolution of the state (pressure, temperature, and humidity) and dynamic (wind speed, wind direction, and turbulence) parameters. This requires novel technologies paired with conventional sampling strategies. Addressing this need has become one of the defining goals of CASS. The CASS team is deploying small unmanned aircraft systems (UAS) to collect routine in situ measurements of the thermodynamic and kinematic state of the atmosphere in conjunction with other weather observations, which could significantly improve weather forecasting skill and resolution. 

One example of a weather-monitoring UAS (WxUAS) being deployed by CASS is the CopterSonde. The CopterSonde was developed in house by CASS engineers and meteorologists to address the challenge of filling the observational gap present in the ABL. It is typically flown in vertically profiling mode and provides high-resolution measurements of pressure, temperature, humidity, and wind speed, wind direction, which are critical to the understanding of atmospheric boundary layer processes that are tied to air-surface (land, ocean, and sea ice) exchanges of energy, momentum, and moisture and advection of the same parameters.

The CopterSonde 2 being flown next to the CASS meteorological tower at the OU Kessler Atmospheric and Ecological Field Station (KAEFS) The CopterSonde 2 being flown next to the CASS meteorological tower at the OU Kessler Atmospheric and Ecological Field Station (KAEFS)

The OU CASS team has participated in numerous field campaigns as outlined below. Here we provide example data collected with the CopterSonde during a local field deployment at the place at the Kessler Atmospheric and Ecological Field Station (KAEFS), located just south of the OU campus. These data correspond to a morning ABL transition from nighttime (stable) to day time (convective) and reveal many interesting features regarding the process. Flights started at civil twilight and went through the full morning transition. The temperature plot shows a strong nocturnal inversion creating a stable boundary layer from the surface to 800 m and a residual layer above that (see profile A in the depiction of the ABL presented above). As the sun continued to rise, the surface warmed rapidly between 14:00 UTC and 15:00 UTC, creating a multi-layered structure (profile B in the figure above). At 14:30 UTC, a moisture surge was observed that was coincident with observed evaporation of dew from the grass. After this surge, the moisture mixed throughout the profile and became well mixed in height. This example highlights the value of WxUAS data for ABL observations.

Time-height profiles of temperature data collected on 18 October 2018 at KAEFS in Oklahoma using the CopterSonde. The data correspond to a boundary layer transition from stable to well mixed. There was a vertical surge of moisture at the time of the transition. Time-height profiles of temperature data collected on 18 October 2018 at KAEFS in Oklahoma using the CopterSonde. The data correspond to a boundary layer transition from stable to well mixed. There was a vertical surge of moisture at the time of the transition.
Time-height profiles of humidity (mixing ratio) data collected on 18 October 2018 at KAEFS in Oklahoma using the CopterSonde. The data correspond to a boundary layer transition from stable to well mixed. There was a vertical surge of moisture at the time of the transition. Time-height profiles of humidity (mixing ratio) data collected on 18 October 2018 at KAEFS in Oklahoma using the CopterSonde. The data correspond to a boundary layer transition from stable to well mixed. There was a vertical surge of moisture at the time of the transition.

Field Campaigns

The CASS team has experience deploying in a variety of weather conditions and geographical locations, from hot summers in the US South Central Plains to winter sea ice in the Artic. Not only does this provide us with a wide range of conditions to test and harden our platforms like the CopterSonde in, it allows us to explore the natural dynamics and variations of the PBL.

CLOUD-MAP (Collaboration Leading Operational UAS Development for Meteorology and Atmospheric Physics) 2016: The National Science Foundation has provided $6M to Oklahoma State University, University of Nebraska, University of Kentucky, and University of Nebraska - Lincoln, to investigate the utility of using WxUAS for atmospheric research. In the summer of 2016, these four institutions assembled in Stillwater, OK to participate in joint flight operations with the intention of demonstrating the simultaneous flights across the different research teams could be conducted safely and successfully while collecting atmospheric observations. The primary focus of the OU team was measuring the vertical structure of the thermodynamic and kinematic fields of the atmospheric boundary layer.

During the summer of 2017, a repeat of CLOUD-MAP 2016 was conducted in Stillwater, OK. Whereas CLOUD-MAP 2016 focused more on the logistics of operations and testing the performance of the various WxUAS, in 2017 there was more of an emphasis on harmonizing data collection, intercomparison across the different WxUAS, and targeted science objectives. Again, the OU team focused on measuring the vertical structure of the thermodynamic and kinematic fields of the atmospheric boundary layer.

EPIC (Environmental Profiling and Initiation of Convection): With funding from the NOAA UAS Program Office, the National Severe Storms Laboratory (NSSL) led a project to investigate how WxUAS can be used to investigate convection initiation. WxUAS observations were conducted by OU and Meteomatics (rotary-wing aircraft) and the University of Colorado (fixed-wing aircraft). Flights were conducted in fall 2016 and spring 2017. Data collected in the field were made available to personnel in the Norman Forecast Office in real-time. This was the CASS team’s first “on- demand” field deployment.

ISOBAR (Innovative Strategies for Observations of the Arctic Atmospheric Boundary LAyeR): This project aimed to study the superstable arctic boundary layer using unmanned systems and remote sensing instrument. The CASS team took four WxUAS: two rotary-wing WxUAS for thermodynamic and kinematic profiling, a fixed-wing WxUAS for photogrammetry, and a fixed-wing WxUAS to examine carbon dioxide profiles. The data from this campaign currently still being analyzed and quality controlled, but will soon be available as a publicly available dataset. This deployment also provided a great experience for the team on how to operate in extreme conditions, which has been extremely beneficial for the work led by CASS affiliate faculty member Dr. Martin that aims to improve winter weather precipitation forecasting. 

LAPSE-RATE (Lower Atmospheric Process Studies at Elevation - a Remotely-piloted Aircraft Team Experiment): This field campaign was held in conjunction with the 2018 ISARRA annual conference in Boulder, CO and consisted of representatives from fourteen different institutions. The main thrust of scientific questions focused on understanding pre-convective environments, cold flow drainage, and boundary layer evolution in the San Luis Valley, CO, which is historically undersampled by radars and other traditional methods. Additionally, this event had a strong community outreach component where locals were invited to come and learn more about the platforms and research being conducted. Two data papers publishing the datasets from WxUAS and ground-based observations are being prepared as well as a sensor intercomparison paper that is characterizing the accuracy of the measurements.

Flux Capacitor: This field campaign was borne out of discussions from two separate proposals (NSF Convergence and NSF MRI), which were in development by teams at OU at the time. The NSF MRI proposal has since been submitted. The primary motivation was to examine how to best combine remote sensing elements with WxUAS technologies to improve flux measurements on a local and regional scale. The initial planning of this event resulted in our first 24-hour deployment and erection of an instrumented tower at KAEFS. It was conducted during September and October of 2019.

OUTFLOW (Oklahoma UAS Targeted Flights for Low-level Observations of Weather): We are teaming with CIMMS, NSSL, and OSU to organize a field campaign to study conditions leading up to convection and how storms mature in Oklahoma and parts of Texas. The field campaign is being supported in part by CIMMS to support the research into NOAA’s objective to advance Warn on Forecast. We will deploy WxUAS as a means of observing the vertical and horizontal structure of the thermodynamic and kinematic properties of the lower atmosphere.

3D Mesonet

We are exploring the potential value of extending atmospheric observations collected at networked fixed-location surface-observing sites through the operation of instrumented WxUAS during designated times. We refer to such a network of autonomous weather WxUAS designed for atmospheric profiling and capable of operating in most weather conditions as a 3D Mesonet. 

Our concept of a 3D Mesonet consists of several fundamental components: (1) a network of tower-based surface observing stations distributed over a designated surface area; (2) a potentially less spatially dense yet complementary network of ground stations from which small WxUAS can be launched, recovered, and reused; (3) the ability to operate unattended and with minimal human interaction in diverse weather conditions; (4) a method of monitoring the airspace in which the WxUAS are operating and mitigating the risk of potential air collisions between WxUAS and manned aircraft and between WxUAS and other UAVs (deconfliction); (5) a robust and reliable methodology for communicating operational critical commands between the WxUAS and the ground station; and (6) a robust and reliable method of communicating data from all sensors to a physical and/or cloud-based command center. Since the focus is on profile data, the WxUAS would execute a vertical ascent and descent. The proposed modular system will allow for customization, upgrade, and in-field replacement of sensor packages as desired. Such a system would facilitate off-site maintenance and calibration and would provide the ability to add new sensors as they are developed, or as new requirements are identified. The small WxUAS must be capable of handling the weight of all sensor packages and have lighting, communication, and aircraft avoidance systems necessary to meet existing or future FAA regulations. The system must be able to operate unattended or with remote pilots at such time that FAA regulations allow.

Illustration of how a 3D Mesonet station could be configured. The extra dGPS (Differential Global Positioning System) antenna is used to improve the accuracy of the estimated position of the WxUAS during flight. Illustration of how a 3D Mesonet station could be configured. The extra dGPS (Differential Global Positioning System) antenna is used to improve the accuracy of the estimated position of the WxUAS during flight.

Some specific current and future projects related to  studies of the structure and evolution of the ABL include:

  • The application of scaling laws to better understand processes in stable atmospheric boundary layers

  • Characterization of atmospheric turbulence in the atmospheric boundary layer

  • Exploring how the atmospheric boundary layers transition from night time to daytime and daytime to night time.

  • Investigating how better ABL observations contribute to an understanding of convection initiation

  • Merging different sampling techniques such as WxUAS and ground-based remote senors for ABL studies

  • Research into storm processes

  • Exploring methods of WxUAS data assimilation in the numerical weather prediction models