A number of emerging technologies provide opportunities for advancement in BL observation.1 Ongoing dialogue between scientists and engineers can help identify opportunities to make improvements in both measurements and instrumentation. Specifically, instrumentation and device engineers can provide measurement innovation while metrology science and standards organizations provide evidence for measurement accuracy and precision. Bringing these communities together plays a key role in understanding the measurements that are crucial as well as the capabilities that meet existing needs. The panel discussions outlined in the following sections highlight new technologies as well as enhancements to existing measurement techniques with the goal of broadening scientific capabilities and understanding of the BL.
Advancements in optics, photonics, and sensors can expand current measurement systems, help fill gaps in observations, and enable new ways of understanding BL processes. In particular, enhancing capabilities to measure chemical components of the atmosphere could allow for improved understanding of BL characteristics (see Box 2 in the next chapter). New semiconductor laser technologies can provide opportunities for low cost, reliable, and compact instruments for GHG measurements in the BL. Leo Hollberg showed that there is a demonstrated capability to accurately detect atmospheric methane in the laboratory with high signal to noise ratios over path lengths as short as 1.5 m. The system uses an intraband semiconductor laser (3.27 µm) operating at room temperature and a PbSe detector, both of which could be inexpensive if they were manufactured in volume. Other new mid-IR sources can cover the complete molecular fingerprint region and have the potential to be used in instruments capable of measuring many of the GHGs and other atmospheric trace gases.
Advanced Cavity Ring Down Spectroscopy (CRDS) instruments allow detection of extremely rare carbon isotopes (14C) contained in GHGs and other carbon compounds at levels that are approaching those achieved by mass spectrometry. As noted by Adam Fleisher, this approach has the advantage of much faster analysis times (an hour for in-lab testing versus several weeks for offsite accelerator mass spectrometry) and much less expensive and simpler instrumentation. Improvements in sensitivity and precision are possible with a next generation instrument. New approaches applying coherent sources using electro-optic frequency combs and semiconductor lasers could provide compact instruments for GHG detection using frequency domain techniques. These spectroscopic techniques can generate data, which can be traceable to fundamental constants, providing reproducible and verifiable measurements.
Dual frequency comb spectroscopy (DCS) techniques using mid-infrared sources can provide wide spectroscopic bandwidths, allowing for the detection of multi-species atmospheric absorption features over long path lengths, and also allowing for regional monitoring. These techniques can provide path integrated absorption using fixed reflectors as well as horizontal and vertical profiles using a UAS mounted reflector. Kevin Cossel discussed portable systems that have been built and demonstrated for GHG measurements in Boulder, Colorado, by measuring CO2 and CH4 profiles continuously for two months. DCS combines the advantages of tunable laser spectroscopy, high resolution, and a spatially coherent beam with the advantages of Fourier Transform Spectroscopy for multi-species detection.
To understand the atmospheric thermodynamic structure and the core dynamics of the atmosphere, panelists noted that wind components, temperature, pressure, water vapor, and other air compositions are important measurements. A unique characteristic of the ABL is the turbulent transport of heat, momentum, kinetic energy, and materials (mass), which require measurements of the variables to be at the same time and location (for example, vertical velocity and temperature), as well as precision and response-time that are adequate to obtain the turbulence transports. While conventional meteorological measurements do not provide information on spatial variability, vertical structure, or measurements over difficult to sample environments, this information can be obtained from UAS. Emerging technologies for in situ measurements––such as Saildrone, UAV, tethered balloon systems, and altitude-controlled balloons––may have unique advantages (but also limitations) to address observational needs. Panelists Meghan Cronin, Gijs de Boer, and Paul Voss discussed the unique technical specifications (spatial ranges, payload, data collection, and sensor systems) of these in situ measurement instruments for BL research and provided examples of the use of these platforms from previous field studies. For example, Dr. Cronin discussed the use of Saildrone in the Salinity Processes in the Upper Ocean Regional Study-2 (SPURS-2) as part of the Tropical Pacific Observing System (TPOS) mission. Dr. de Boer highlighted work from the University of Colorado and the Department of Energy to deploy small UAS and tethered balloon systems on the North Slope of Alaska, at the Oliktok Point Observatory, and Dr. Voss noted that controlled meteorological balloons have completed more than 50 missions totaling more than 1,000 hours of flight time in environments ranging from the tropics to the poles.
Panelists discussed numerous advantages for these emerging technology systems. For example, measurements can be determined at specific locations and thus the measurement errors are relatively small (i.e., no spatial uncertainty). Increased mobility means and improved ability to take measurements in hard to reach areas such as the high latitudes (see Figure 6), providing supplementary measurements to current capacities and non-routine measurements either at the air-ocean interface or over other difficult to reach land and ice surfaces. These emerging technologies are relatively inexpensive and can cover time and space scales with high resolution, including vertical and horizontal measurements, and sometimes with long time series.
One of the more unique measurement aspects identified by the panel is that these types of measurement systems can provide mean wind speed (which can be difficult to measure on these moving platforms), along with temperature, pressure, and humidity. Researchers have also demonstrated the ability to measure these variables (including vertical velocity) at high enough frequencies to directly estimate the turbulent fluxes. These measurement systems are important assets for process studies and include reliable communication networks and lightweight power systems. This type of technology also provides opportunities to accomplish more than has been achievable in the past. Although extreme weather can sometimes pose challenges for certain types of
instrumentation, most often these kinds of platforms are the only tools that can be used in certain extreme conditions and their use reduces the risk to human life. In fact, some of the earliest uses of UAS were in hurricanes and hurricane force winds.
However, a number of limitations also exist, some of which apply to other platforms and instruments as well. Panelists discussed how extreme weather (e.g., high winds, snow, ice, rain) may affect data quality or platform operations and current measurements may be biased toward fair weather. As researchers increasingly try to understand clouds and air-sea interactions, this issue is particularly important and as increasingly complex measurements are used, it will be crucial to consider the kinds of technology that will be available. There are often hurdles associated with aviation safety and regulations. Many participants acknowledged that maintaining safe airspace is critical, and thus additional discussions and creative solutions may be needed. Finally, there are also considerations with payload limitation, although this may be less of a problem for platforms like Saildrone.
Airborne and ground-based mobile measurement platforms, such as active systems flown on National Center for Atmospheric Research aircrafts, come with a number of challenges and opportunities. Don Lenschow outlined some of these platforms including powered and unpowered aircraft, UAVs, cars, blimps, controlled towed vehicles (CTVs), a trolley suspended on a cable, and balloons. He also discussed profile technologies such as Water Vapor Differential Absorption Lidar (WV-DIAL) and opportunities to miniaturize and deploy on mobile platforms. Data and expanded platforms from air quality observing systems in the United States can also be leveraged, and Jim Szykman mentioned changes to the Environmental Protection Agency Photochemical Assessment Monitoring Stations (PAMS) Network to increase the value of the measurement suite at these sites to a larger community of interest. He discussed new trace gas measurements (column O3, NO2, and others) and profiles from low powered lidars.
Other existing and future in situ observational networks can be leveraged, such as the National Mesonet Program (NMP) by the National Weather Service, which supports the integration of non-federal sub-, near-, and above-surface observations collected by 40 commercial and academic institutions. John Horel discussed this public–private partnership concept and highlighted the importance of data management, data quality, and metadata.
Panelists noted that rapid developments are occurring in miniaturized Inertial Measurement Units (IMUs), and they are being combined with Global Positioning Systems (GPSs) for use on UAS and light aircrafts, although optimized systems for wind measurements are still desired. For application of these new devices to address gaps, measurement requirements and standardizations are needed (geophysical quantities, temporal and spatial scales, and precision). Emerging trace gas measurements will likely provide a different and useful perspective on BL dynamics. Effective coordination of ABL height and mixing layer height observations across operational networks could be helpful. Implementation of heterogeneous measurement
technology will require research to harmonize the meaning and use of the measurements.
Panelists also noted that metadata and data validity tools to manage the complex mix of in situ observations are available but additional improvements could be valuable. The NMP has proven that high quality regional and local scale networks developed through public–private partnerships can be built to address gaps, but business models (data rights and funding) could help keep these networks running, especially university run networks, while making data accessible to academic and operational communities. In areas where data is sparse, scientists rely on satellite data. However, in addition to satellites, it is important to think about how to transfer techniques from U.S. networks to other kinds of data that are available (e.g., pressure data from cell phones). This will help obtain surface information from data sparse regions. Although the workshop was specifically planned to focus on U.S. capabilities and gaps, it is also important to consider emerging technologies and partnerships with other countries.
Workshop participants considered synergies of remote sensing platforms and their role in the future of BL observations, specifically Observing System Experiments (OSE) and OSSEs. They also discussed the state of current measurements and forthcoming techniques to measure liquid water, water vapor, pressure, temperature, and winds in the lowest 3 km at high resolution and ways to reduce costs associated with existing techniques. Dave Emmitt discussed the versatility of Doppler wind lidar for marine and terrestrial BL studies (see Figure 7). He specifically looked at aerosol structure, BL depth, wind speed and direction, and turbulence; he also considered observations and model validation and verification as well as numerical weather prediction (including validation and assimilation). His talk included some examples of using Doppler wind lidar to better understand marine BL flux parametrization, Arctic circulations and land-sea-ice interactions, and tropical dynamics (coupled with radar). Part of the discussion also included a new set of global wind observations from satellite measurements.
Doppler cloud radars and a quantitative examination of turbulent kinetic energy (TKE) can aid in the understanding of entrainment processes by looking at spectral width and TKE dissipation rates. Bruce Albrecht showed how this could lead to a better understanding of cloud top entrainment processes. He also discussed an idea to obtain a better understanding of coupling through sensor synergy (Atmospheric Radiation Measurement [ARM] datasets) including ceilometer, radar, radiometer, radiation, and surface meteorological observations. Combining eddy dissipation rates with variance provides an estimate of vertical integral length scale. He suggested some future opportunities including coupling radar and lidar for turbulence characterization through the whole BL; decomposition of drizzle and shallow marine stratocumulus; and radar chaff for entrainment studies.
Radar wind profilers (RWPs) and weather radar (WR) instruments are accessible but expensive and there are atmospheric thermodynamic parameters associated with enhanced radar returns. Philip Chilson discussed RWP-improved resolution data that can be obtained with a range imaging (RIM) technique; this is comparable to S-band frequency modulated continuous wave radar (1-2 m). He also showed WR capability to operate S-band for clear-air turbulence and illustrated the synergy between RWP and WR (polarimetry) to get a better understanding of the BL. New NEXRAD profilers may have the potential for exploiting new retrieval techniques.
Workshop participants explored examples of current capabilities to observe the BL from space and considered if a more realistic BL characterization could be produced with current data and/or future instruments and missions. Mark Bourassa, Carol Anne Clayson, and Matthew Lebsock explored some options for remote sensing observations. For temperature and water vapor profiles, three key technologies are typically used: (1) IR sounders, which have horizontal resolution of around 10 km, vertical resolution of 1 to 2 km, and great spatial coverage (almost global coverage in one day), but are only effective in clear (or almost clear) regions; (2) microwave sounders (MW) that can operate in clear and cloudy conditions over the ocean with reduced horizontal (25-50 km) and vertical resolution compared to IR, while also providing near-surface temperature and humidity; and (3) GNSS-RO, which have horizontal resolutions on the order of 100 km; vertical resolution close to 200 m; and can be used in clear and cloudy regions, but have poor spatial coverage (currently about 1,000 profiles per day over the entire globe) and the temperature and water vapor observations are not independent. Also of note are MW imagers such as the Special Sensor Microwave Imager (SSM/I), which form the backbone of the climate data record over the ocean for near surface temperature and humidity. Ocean surface properties that are routinely measured from space include SST, surface winds, and salinity. Land surface properties include skin temperature, soil moisture, and emissivity; cloud properties include cloud cover, liquid water path, and cloud top height. Cloud liquid water profiles are also routinely observed from space. Other essential BL properties such as BL height are also produced routinely.
Based on this panel discussion, current satellite missions have not been optimized or specifically designed for BL science and applications, and current algorithms and products (such as surface fluxes) could be improved with focused sensors. Another important point made during the panel discussion was that more integration across different datasets, such as using data fusion methodologies, would likely lead to improved BL products. Continued improvements through better understanding of error sources and integrated products were also discussed. The panel noted the success of A-Train; however, the A-Train is aging and no clear replacement is planned.
In terms of suggestions for the future, a diversity of architectures for future missions could be considered and international collaborations and constellation flying could be pursued. Some participants suggested that it is not an adequate strategy to observe cloud properties independently from thermodynamic properties (such as temperature and water vapor) and that simultaneous observation of clouds and thermodynamics could be considered. In this context, blending different types of observations (e.g., IR and radar) to optimally and more completely characterize the BL thermodynamic structure is an approach that could be pursued. Observations with increased vertical and horizontal resolution could be useful, for example, to develop a more realistic characterization of BL structure and surface winds, and many of these improvements are technologically feasible (e.g., IR, lidar, radar). Active profiling methods for water vapor are within reach but require additional technology maturation. New measurements of surface winds, wind stress, and ocean properties (e.g., surface
currents) could also be considered. In addition to the more traditional large, single satellite missions, panelists also mentioned the possible development of smaller, cheaper sensors deployed from a constellation of smaller satellites to reduce sampling error.