Doppler Lidar Retrievals in LUMEX and PECAN: Dual Doppler and Vertical Wind Analyses

The Lidar Uncertainty Measurement Experiment (LUMEX) was a cooperative experiment performed during the period of June 23rd to July 13th, 2014. Participants included NOAA, UMBC, CU Boulder, and NWRA/CoRA. The primary focus of LUMEX was to bring several different lidars together for inter-comparison, allowing for the determination of measurement uncertainties and relative sensitivity. Turbulence and wind vector retrieval techniques were also tested. LUMEX featured five Doppler lidars of both commercial and research grade. The lidars were brought together at the Boulder Atmospheric Observatory (BAO) tower in Colorado, a three hundred meter tall tower fitted with sonic anemometers at 6 levels and other in situ instruments. This provided many “truth” values for the experiment.

One phase of LUMEX was dedicated to dual Doppler measurements. Dual Doppler is a method of remotely measuring wind speed and direction that utilizes two Doppler lidars, not co-located, simultaneously looking at the same point in space. By applying geometric formulas to the two measured line-of-sight winds, dual Doppler scans can yield the two-dimensional wind vector at a location. By scanning intersecting vertical slices, one can produce a virtual tower of wind speeds at the intersection point. This method, when implemented correctly, can quickly and accurately measure wind speeds in a column over a point in space.

NOAA's High Resolution Doppler Lidar (HRDL) and UMBC's commercially manufactured Leosphere WINDCUBE(TM) 200S performed dual Doppler scans for several days during LUMEX, with RHI scans intersecting near the BAO tower as well as directly over a WINDCUBE(TM) V1 wind profiling lidar, which uses the Doppler Beam Swinging (DBS) method to estimate wind profiles. The tower and profiling lidar provided accurate measurements against which to compare the dual Doppler results. The different beam intersection angles of these virtual towers show a clear affect on retrieval accuracy. Two different scan styles were run to observe any measurement improvement with increased time spent at a single virtual tower location.

The data gathered during the dual Doppler phase of LUMEX will be used to test and improve dual Doppler processing code. Also, the strengths and weaknesses of these dual Doppler scanning techniques can be evaluated from this experience, which is valuable for future endeavors. These results and findings as they apply to dual Doppler scanning, wind energy surveying, and the lidar community at large will be discussed in this presentation.

The presentation will also address vertical wind data taken from both LUMEX and PECAN (Plains Elevated Convection At Night). PECAN was a very large cooperative experiment conducted on the flatlands of the central United States in June-July 2015. The main focus was to study nocturnal convection and other atmospheric events. UMBC's Leosphere WINDCUBE(TM) 200S operated almost continuously throughout the duration of PECAN at a fixed site in Greensburg, Kansas.

The lidar vertical wind data from LUMEX and PECAN will be analyzed to address the common assumption that average vertical wind speed is zero in the first couple kilometers above ground level, especially if averaging over longer time and/or space intervals. This assumption is assumed to hold true for vertical wind integrated over the entire boundary layer in calm, stable conditions. Conversely, time periods with low level jets, daytime convection, or other common atmospheric events may not yield an integrated vertical velocity of zero. Such cases will be examined for comparison and validity of the zero assumption.