The multiple uses of the Navy's Twin Otter Doppler Wind Lidar (TODWL) for atmospheric research

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1 The multiple uses of the Navy's Twin Otter Doppler Wind Lidar (TODWL) for atmospheric research G. D. Emmitt a, Steve Greco a, Chris O Handley a, Ralph Foster b, Dylan Reif c and Robert Bluth d a Simpson Weather Associates, 809 East Jefferson St., Charlottesville, VA b University of Washington, Applied Physics Laboratory, Seattle, WA c University of Oklahoma, School of Meteorology, Norman, OK d CIRPAS, 3200 Imjin Road, Marina, CA gde@swa.com The Twin Otter Doppler Wind Lidar (TODWL) has been used for more than 15 years in numerous research projects sponsored by NASA, DoD and NOAA. As an airborne instrument, TODWL has been used to conduct studies on Marine Boundary Layer jets and rolls, C17 aircraft wing tip vortices, complex terrain flows and numerical model validation. TODWL has also been used on the ground as part of mobile facility to look at low level winds near thunderstorms and supercells. We will describe the TODWL system and its latest companion, a HALO Photonics DWL. Keywords: Doppler wind lidar, airborne, boundary layer, winds 1. Introduction Over 15 years ago, the Integrated Program Office (IPO) of NPOESS, with an eye to eventual deployment in space, first funded the development of an airborne coherent Doppler Wind Lidar to be mounted in a Navy Twin Otter aircraft to conduct a variety of atmospheric boundary layer investigations [1] [2]. Subsequent funding for instrument development, data analysis and data processing has been provided by Office of Naval Research (), Army Research Office and the Army Research Laboratory. Since 2002, the Twin Otter Doppler Wind Lidar (TODWL) operated by s Center for Interdisciplinary Remotely- Piloted Aircraft Studies (CIRPAS) has flown more than 250 hours of atmospheric missions with most of that over the Pacific Ocean coastal waters and complex mountainous terrain within 50 km of the coast near Monterey, CA. In particular, there were seven dedicated TODWL wind lidar missions in 2002, 2003, 2004, 2006, and 2007 conducted by Simpson Weather Associates (SWA). A listing of these experiments is shown in Table 1. In addition, during September and October of 2012, the TODWL was flown over Monterey, CA as part of the Unified Physical Parameterizations for Season Prediction ( UPP) campaign to look at Organized Large Eddies (OLEs) [3] [4] and then, as part of the multi-agency MATERHORN project, over complex terrain near Dugway, UT [5], to help improve our understanding of the daytime and nocturnal evolution of the atmospheric boundary layer in the presence of complex topography. Most recently, the TODWL was flown just outside of the Yuma Proving Ground in Arizona with a goal to measure wing-tip vortices coming off a C-17 military aircraft. [6]. Scientific objectives of the airborne TODWL missions have included: boundary layer description and evolution [2[; 3-d wind flow over complex terrain [7]; validation of other observations and numerical models [2, 8, 9]; water surface returns; Organized Large Eddies [3, 4]; and surface flux studies [3]. In addition to the airborne missions, TODWL has also been used on the ground together with an X-band Doppler radar system in a mobile system called the Truck-mounted Wind Observing Lidar Facility (TWOLF) [10], where TODWL was used during several field campaigns (see Table 1) to measure boundary layer winds in the environment of severe storms and super cells. CLRC 2016, June 26 July 1 1

2 Table 1: List of TODWL field campaigns Dates Base Location Objectives Funding Agency Feb Mar Monterey, CA Water/Surface Returns; Atmospheric Wind NOAA 2002 and Boulder, CO Profiles February Monterey, CA Cross Mountain Wind Profiles NOAA;NASA 2003 NAST/TAMDAR Validation October 2006 Monterey, CA Atmospheric Wind Profiles NOAA Wild Files and Prescription Burns April 2007 Monterey, CA Atmospheric Wind Profiles NOAA;USARMY Ocean returns and OLEs Evening and Morning Transition BL November Monterey, CA Mountain-Valley circulations; OLEs and 2007 water surface returns Model and Profiler Validation May-June Oklahoma Boundary Layer Winds near Supercells and 2010 thunderstorms as part of 2010 VORTEX September Monterey, CA OLEs and Ocean returns 2012 Atmospheric Wind Profiles October 2012 Dugway, UT Circulations in Complex terrain Model validation April 2013 Yuma, AZ Wing tip vortices off C-17 USARMY Jun-Jul 2015 Kansas Low-level wind of the nocturnal boundary and severe storms as part of PECAN 2015 Figure 1 The Twin Otter Doppler Wind Lidar (TODWL) installed on the Twin Otter owned by the Navy s Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) in Monterey, CA. 2. Aircraft and Instrument The CIRPAS Twin Otter (Figure 1) is a non-pressurized turbo-prop, twin-engine aircraft that has a payload capacity of 1500 lbs. Meteorological sensors aboard the Twin Otter aircraft provide measurements of temperature, dew point, pressure (static and dynamic), humidity, horizontal wind speed and direction, and vertical wind speed. Additional aircraft sensors provide measurements of location (lat/long), altitude, ground speed, ground track and heading, pitch, roll, and true air speed. CLRC 2016, June 26 July 1 2

3 Table 2: Technical details of TODWL Wavelength (microns) 2.05 (eyesafe) Energy per pulse (mj) 2 mj Pulse repetition frequency (Hz) 500 Pulse length (m) 90 Scanner (side door mounted) 2 axis (+- 120; +- 30) Telescope diameter (cm) 10 Range resolution (meters) Total System Efficiency (%) 7-10 Power (KW) 1.0 Weight (lbs.) 750 including door mounted scanner LOS measurement accuracy (m/s) <.05 with.5 sec integration Wind component accuracy (m/s) u,v,w <.1 m/s nominal using a 30 degree 12 point step stare VAD and LADSA Aerosol backscatter threshold sensitivity Range dependent: ~ m sr-1 at 10km Nominal range to insensitivity (km) Aerosol dependent: nominal km in PBL and 2-5 km above PBL. The TODWL is a 2 micron coherent system built by Coherent Technologies, Inc. Table 2 summarizes many of the technical details of the lidar. A defining capability of the TODWL is the ability to profile above and below the flight level. This is possible because the lidar includes a bi-axis scanner mounted on the side door of the aircraft that allows vertical soundings of the wind profile above and below the aircraft as well as taking data with horizontal or vertical perspectives. With this side door mounted, bi-axis scanner (Figure 1) the beam can be directed in a variety of scan patterns including conical, raster, nadir stares and flight level stares. When not mounted on the aircraft, TODWL has been deployed together with an X-band radar on a mobile unit (truck) to measure boundary layer winds in severe weather environments. 3. TODWL Data Acquisition and Processing Although the primary product of the ADWL is a profile of the horizontal wind components between the aircrafts flight level and the surface, it can also measure line of sight winds, vertical motion, turbulence, aerosol structures, cloud tops/bottoms, ocean wave heights and ocean currents. Table 3 provides a listing of these data measurements along with the precision and both horizontal and vertical resolution. The maximum range of the TODWL will vary between 6 and 30 km, depending on the amount of aerosols. Table 3: TODWL data measurement and precision Product Precision Vertical Resolution Horizontal Resolution Comments.05 m/s 50m 50m Horizontal spacing between stares may vary depending upon aircraft cruise speed and DWL dwell times LOS resolution (also applies to vertical profiles of 3-D winds U, V, W component.10 m/s 50m 1.5 km Nominal spacing between soundings is 1.5km Aerosol N/A 5m <10m Inferred from SNR and using sliding signal processing Cloud top/bottoms 5m 5m N/A Ocean wave heights.5m estimated Ocean currents.10 m/s estimated N/A 1.5m to 20m Indirect measure of wave heights by measuring vertical motion of water surface. N/A 1-10km Derived from surface returns during conical scans CLRC 2016, June 26 July 1 3

4 The vertical wind profiles are determined by TODWL utilizing the bi-axial scanner. A vertical profile is derived in most instances from a 12 point step stare (30 degrees between stares) with a degree off nadir half angle. The dwells at each stare point vary from 1 2 seconds and the time to complete full step stare conical scan for wind profiles is about 25 seconds. Using this set-up, a 50 m/s ground speed for the aircraft produces a complete profile of u, v and w every km. From this conical scan, a single profile is constructed with 50 m resolution in the vertical. In addition to the off-nadir scans mentioned above, the scanner can also be pointed directly nadir (adjusted for aircraft pitch and roll). Using such a set-up, vertical motions of the near surface and lower atmosphere can be observed to within 10 cm/sec. 4. Future plans The has provided funds for the acquisition and installation of a HALO Photonics wind lidar on the CIRPAS Twin Otter. The installation is scheduled for late August The HALO will be flown in two modes: 1. In place of TODWL when other significant instruments will be flown and 2. Co flown with TODWL, sharing the scanner using a dichroic optical element (transmissive at 1.6um and reflective at 2.0 um). 5. References [1] Emmitt, G.D., C. O Handley, S. Greco, R. Foster and R. Brown, Airborne Doppler wind lidar investigations of OLEs over the eastern Pacific and the implications for flux parameterizations, Proc. of the Annual Amer. Met. Soc. Conference, Sixth Conference on Coastal Atmospheric and Oceanic Prediction and Processes, San Diego, CA, January, [2] Greco, S., G.D. Emmitt, S. Wood, C. O Handley and H. Jonsson, Synergisms and comparisons between airborne Doppler Wind Lidar observations and other remote and in-situ wind measurements and model forecasts, Proc. of the Annual Amer. Met. Soc. Conference, 12th Conference on IOAS-AOLS, New Orleans, LA, [3] Emmitt G.D., R.C. Foster, K. Godwin and S. Greco, Investigating the impacts of LLJs and OLEs on ABL exchanges and transports using an airborne Doppler wind lidar, AGU Fall meeting San Francisco. Atmospheric Boundary Layer Processes and Turbulence I Posters. A43A-0227, [4] Emmitt, G.D., R. C. Foster, S. F. J. De Wekker, and K. S. Godwin, Airborne Doppler Wind Lidar investigations of OLEs and LLJs in the marine boundary layer and their implications for flux parameterization, Session: Coastal and marine boundary layers in the atmosphere and ocean. AMS annual meeting, [5] Fernando, H. J. S., E. R. Pardyjak, S. Di Sabatino, F. K. Chow, S. F. J. De Wekker, S. W. Hoch, J. Hacker et al. "The MATERHORN: Unraveling the Intricacies of Mountain Weather." Bulletin of the American Meteorological Society 96, no. 11 (2015): , [6] Emmitt, G.D., K. Godwin and C. O Handley, Airborne DWL investigations of wing tip vortices and their dissipation, Annual Meeting of the Lidar Working Group for Space based Winds, Boulder, CO, [7] De Wekker, S.F.J., K.S. Godwin, G.D. Emmitt and S. Greco, Airborne Doppler lidar measurements of valley flows in complex terrain, Journal of Applied Meteorology and Climatology 51, no. 8 (2012): , [8] Greco, S. and G.D. Emmitt, Investigation of flows within complex terrain and along coastlines using an airborne Doppler wind lidar: Observations and model comparisons, Proc. of the Annual Amer. Met. Soc. Conference, Sixth Conference on Coastal Atmospheric and Oceanic Prediction and Processes, San Diego, CA, [9] Wang, Y., C. Williamson, G. Huynh, D. Emmitt and S. Greco, Diagnostic Wind Model Initialization over Complex Terrain Using the Airborne Doppler Wind Lidar Data, The Open Remote Sensing Journal, 3,17-27, [10] Bluestein, H.B., Houser, J.B., French, M.M., Snyder, J.C., Emmitt, G.D., PopStefanija, I., Baldi, C. and Bluth, R.T., Observations of the boundary layer near tornadoes and in supercells using a mobile, collocated, pulsed Doppler lidar and radar, Journal of Atmospheric and Oceanic Technology, 31(2), pp , CLRC 2016, June 26 July 1 4

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