Lower Atmospheric Thermodynamics and Turbulence Experiment (LATTE): Overview and Initial Results

A multi-institutional and multi-sensor boundary-layer research experiment called LATTE (Lower Atmospheric Thermodynamics and Turbulence Experiment) was recently conducted in Colorado at the base of the Rocky Mountains. The aim of this presentation is to provide an overview of LATTE and to discuss some of the initial findings. LATTE was conducted at the National Oceanic and Atmospheric Observatory (NOAA) Boulder Atmospheric Observatory (BAO) during February 2014. The BAO provided an excellent venue for the experiment because of its 300-m meteorological tower; the infrastructure provided at the site to support the various instruments used during the experiment; and its proximity to the National Center for Atmospheric Research (NCAR), which assumed the lead role in organizing the experiment. Other institutions participating in LATTE included: the University of Oklahoma, NOAA, Lawrence Livermore National Laboratory, NorthWest Research Associates, Inc., the University of Colorado, Boulder, and Lockheed Martin Coherent Technologies. The three primary objectives of the experiment are to:

• Validate both wind and reflectivity measurements from the NCAR 449-MHz wind profiling radar (WPR) using 3-D sonic anemometers, Doppler lidars, and an instrumented unmanned aerial system (UAS): The NCAR WPR operates in a spaced antenna mode and recently has been upgraded to allow range imaging (RIM) operations as a means of improving its range resolution, thereby allowing wind measurements with improved spatial and temporal resolution as compared to conventional wind profilers using Doppler Beam Swinging (DBS) techniques. Observations from the scanning Doppler lidars are being used to validate the three-dimensional wind estimates produced by the radar. Additionally, wind observations from the tower-mounted sonic anemometers are being integrated into the comparison. Moreover, data from the UAS provide a means of generating estimates of C_n², which is related to the radar reflectivity produced by Bragg scatter. The UAS-derived winds can also be compared with the tower and wind profiler.

• Compare Bragg scatter from NCAR's S-band Polarimetric (S-Pol) weather radar with reflectivity data from the WPR: S-band weather radar can detect clear-air scatter produced by turbulent fluctuations of the clear-air refractive index. When the turbulence is homogeneous and isotropic, and if the radar's Bragg wave number (twice the radar wavelength) lies within the inertial subrange of the turbulence, then the backscattered power detected by radar at a given range is proportional to C_n² at that range. Here we are assuming the clear-air processes are the sole source of the radar backscatter. Range Height Indicator (RHI) scans using S-Pol have been made along a radial corresponding to the location of the BAO. Signals from S-Pol indicative of Bragg scatter, e.g., enhanced reflectivity, low differential reflectivity (ZDR), relatively high correlation coefficients, are being used to compare retrieved values of C_n² from S-Pol with those estimated using the 449-MHz WPR.

• Determine the extent to which Doppler lidars can be used to measure atmospheric turbulence: Doppler lidars not only provide measurements of the atmospheric mean wind, but they can also be used to study turbulent fluctuations of the wind. A variety of methods can be used to extract turbulence statistics from lidar measurements depending on the type of system and scanning strategy employed. Wind and turbulence estimates from the lidars are being compared against observations from tower-mounted sonic anemometers, optical image-motion anemometers, UAS, and radar. By examining data from LATTE we are exploring the strengths and weaknesses of several such techniques. Results can be used for such applications as wind energy, air traffic safety, and air quality monitoring.

Data collected during LATTE are providing a unique opportunity to cross-validate various sensor technologies developed for lower atmospheric observations and to investigate the structure and evolution of the planetary boundary layer.