sensors Article On the Use of Rotary-Wing Aircraft to Sample Near-Surface Thermodynamic Fields: Results from Recent Field Campaigns Temple R. Lee 1,2, * , Michael Buban 1,2 , Edward Dumas 2,3 and C. Bruce Baker 2 1 Cooperative Institute for Mesoscale Meteorological Studies, Norman, OK 73072, USA; michael.buban@noaa.gov 2 NOAA ARL Atmospheric Turbulence and Diffusion Division, Oak Ridge, TN 37830, USA; ed.dumas@noaa.gov (E.D.); bruce.baker@noaa.gov (C.B.B.) 3 Oak Ridge Associated Universities, Oak Ridge, TN 37830, USA * Correspondence: temple.lee@noaa.gov; Tel.: +1-865-454-6515; Fax: +1-865-220-1733 Received: 4 October 2018; Accepted: 17 December 2018; Published: 20 December 2018   Abstract: Rotary-wing small unmanned aircraft systems (sUAS) are increasingly being used for sampling thermodynamic and chemical properties of the Earth’s atmospheric boundary layer (ABL) because of their ability to measure at high spatial and temporal resolutions. Therefore, they have the potential to be used for long-term quasi-continuous monitoring of the ABL, which is critical for improving ABL parameterizations and improving numerical weather prediction (NWP) models through data assimilation. Before rotary-wing aircraft can be used for these purposes, however, their performance and the sensors used therein must be adequately characterized. In the present study, we describe recent calibration and validation procedures for thermodynamic sensors used on two rotary-wing aircraft: A DJI S-1000 and MD4-1000. These evaluations indicated a high level of confidence in the on-board measurements. We then used these measurements to characterize the spatiotemporal variability of near-surface (up to 300-m AGL) temperature and moisture fields as a component of two recent field campaigns: The Verification of the Origins of Rotation in Tornadoes Experiment in the Southeast U.S. (VORTEX-SE) in Alabama, and the Land Atmosphere Feedback Experiment (LAFE) in northern Oklahoma. Keywords: sUAS; atmospheric boundary layer; sensors 1. Introduction Earth’s atmospheric boundary layer (ABL) has traditionally been difficult to sample, yet adequately representing the physical processes occurring within it is essential to weather forecasting. Surface-based observational platforms, e.g., weather stations and flux towers, only penetrate at most a few tens of meters into the ABL. Rawinsondes provide a snapshot of the ABL, as they are released only twice daily from locations that are unevenly distributed across the world [1–3]. Like rawinsondes, surface-based remote sensing instruments, including microwave radiometers (MWR) and Atmospheric Emitted Radiance Interferometers (AERI), for measuring atmospheric thermodynamic quantities, lidars, and sodars for deriving winds, provide profiles at only one point in space and have limited resolution near the ground [4,5]. Whereas radars provide better horizontal coverage than rawinsondes and remote sensing instruments, oftentimes radars overshoot the ABL and thereby do not sample it well. Thus, despite its proximity to the ground, the ABL still presents a significant observation gap. Closing this gap is essential in many weather forecasting applications. For example, knowledge of the lower atmospheric stability and strength of the elevated inversion atop the ABL is critical for severe weather development [6]. Additionally, knowing the depth of the freezing layer is critical for forecasting Sensors 2019, 19, 10; doi:10.3390/s19010010 www.mdpi.com/journal/sensors