Scientists at the Center for Clouds, Chemistry and Climate (C4) at the Scripps Institution of Oceanography have successfully tested Unmanned Aircraft Systems (UASs) for measurements of radiation fluxes, aerosol concentrations and cloud microphysical properties [Corrigan et al., 2008; Ramana et al., 2007; Ramana et al., 2010; Ramanathan et al., 2007; Roberts et al., 2008]. Building on this success, a payload to measure water vapor fluxes using the eddy covariance (EC) technique has been recently developed and tested. Progress has been made in UAS turbulence payloads previously [Spiess et al., 2007; Van den Kroonenberg et al., 2008], but to our knowledge this is the first UAS system to incorporate water vapor measurements.
The driving aim of the water vapor flux system's development is to investigate atmospheric rivers' in the north-western Pacific Ocean, these can lead to sporadic yet extreme rainfall and flooding events upon landfall in California. Such a flux system may also be used to investigate other weather events (e.g. the formation of hurricanes) and can be combined with the existing aerosol/radiation and cloud microphysics UAS payloads; to offer an unprecedented method for the simultaneous measurement of linkages within the cloud-dynamics-radiative forcing-aerosol system (CDRFA). The atmospheric vertical wind component (w) is derived by this system at up to 100Hz according to equations found in Lenschow  using data from a GPS/Inertial Measurement Unit (GPS/IMU) combined with a fast-response gust probe mounted on the UAV. Measurements of w are then combined with equally high frequency water vapor data (collected using a Campbell Scientific Krypton Hygrometer) to calculate latent heat fluxes (λE).
Two test flights were conducted at the NASA Dryden test facility on 27th May 2010, located in the Mojave Desert. Horizontal flight legs were recorded at four altitudes between 1000-2500 masl within the convective boundary layer. Preliminary data analysis indicates averaged spectral data follow the theoretical -5/3 slope (Figure 1a), and altitude-averaged λE fluxes have mean values close to zero at all altitudes (Figure 1b). This absence of flux divergence with altitude is indicative of similar λE fluxes at the surface and in the above-cloud entrainment zone, as seen in some manned aircraft observations [Gioli et al., 2004; Mahrt et al., 2001]. Extrapolation of the flux profile to the surface resulted in λE of 1.6 W m-2; in good agreement with 1.0 W m-2 λE measured by NOAA from a surface tower using standard flux techniques. Further test flights are planned for September 2010 at a marine location, and the potential for the system to participate in CDRFA experiments is also discussed.
Acknowledgments We would like to acknowledge the significant contributions to this system made by the late Katrin Lehmann whose life was tragically cut short by a hiking accident. Katrin was responsible for the initial design, construction and programming of the UAS elements, and in doing so laid solid foundations for the system. We are indebted to NOAA, for funding this project through the research grant NOAA NA17RJ1231. Thank you also to Mike Marston of NASA, the BAE systems crew Phillip Corcoran and Rafael Gaytan, and Mike Rizen of UCSD Physics workshop for their mission roles. We would also like to thank NSF for long term support of the C4 UAS Program.
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