Preliminary Results from a Motion-Stabilized, Scanning, Doppler Lidar, Operated from the RV Revelle During DYNAMO

Introduction

NOAA's High Resolution Doppler Lidar (HRDL) was deployed on the RV Revelle during the Dynamics of the Madden Julian Oscillation (DYNAMO) experiment to characterize and monitor the dynamics of the marine atmospheric boundary layer from the surface of the ocean up through cloud base or the top of the aerosol layer. The lidar measurements fit into a set of complimentary observations on the ship that include underwater dynamics, surface flux, vertically-pointing cloud radar, scanning precipitation radar, wind-profiling radar, in-situ aerosol sampling and soundings. When combined, the ship-based measurements provide an opportunity to synergistically study the oceanic and atmospheric conditions experienced by the ship during the experiment. The lidar was mounted on a forward deck of the RV Revelle and operated continuously during the first three legs of the experiment (1 Sep – 24 Sep, 1 Oct – 2 Nov, 7 Nov – 10 Dec 2011). HRDL performed a repeating, 20 minute sequence of scans that were chosen to measure vertical profiles of the horizontal wind speed and direction, moments of the vertical wind speed, horizontal wind velocity variance, and vertical profiles of aerosol backscatter intensity (as indicated by the wideband signal to noise ratio, wbSnr). The scans included both high and low elevation angle azimuthal scans, elevation scans along two azimuths oriented orthogonally, and 50% of the time was spent staring vertically.

The System

HRDL operated with a motion stabilized, hemispheric scanner that allowed the system to maintain the pointing of the beam in the Earth's reference frame to within 0.5 degrees in moderate seas and to remove the effect of the ship's motion from the line-of-site LOS wind speed measurement to within 0.5 m/s. Operating at 2 microns, the transmitted laser light is invisible and eyesafe. It operated with a pulse repetition frequency of 200 Hz and averaged 100 pulses to form 2 Hz averaged beams of range-resolved, radial (LOS) wind speed and backscatter signal intensity. The along-beam resolution was matched to the length of the optical pulse which was 30m FWHM. The diameter of the beam was approximately 0.15m and was collimated at the output with a divergence angle of 20 µrad.

Real-time data products

At the end of every twenty minute scan sequence, basic data products (profiles of horizontal wind speed and direction, vertical velocity variance, and aerosol backscatter signal intensity) were automatically generated and posted to a ship-based web page for onboard usage. When the satellite internet connection was available, these products were also posted to a NOAA web page and were uploaded to the NCAR field catalog. When the connection was available, but limited, the results were uploaded every 4 hours. A complete catalog of images of all the profiles taken during the experiment in 12 hour periods is available at: http://esrl.noaa.gov/csd/lidar/dynamo.

Compiled Running Statistics

HRDL operated continuously during all three legs (up to the date of this abstract – 15 Nov) with no major outages - only going offline during brief periods of heavy rain. For legs 1 and 2, the system logged over 480 and 690 hours of operation respectively. The number of files (and hence scans) for the two legs are 7800 and 10700 scans. Generating typically about 1.25 Gigabytes per hour in raw data, HRDL has stored nearly 1800 Gigabytes of raw data during the first two legs of the experiment.

Preliminary Results

With the enormous amount of data taken during the experiment, every effort was made to analyze the data either in real time or within one day of operation, to ensure there were no subtle problems developing in the system, to monitor the conditions found on station, and to relate them to researchers on the ship and on shore. We will present results from data taken while on station during Legs 2&3, which include distributions as a function of height of the mean horizontal wind speed and direction, horizontal and vertical velocity variances, and composited results of the same quantities as a function of local solar time. We use the low-elevation-angle lidar azimuthal and elevation scans, together with results from the different observational systems on board, to study the temporal and spatial evolution of precipitation-driven outflows. The lidar data provides context for the other measurements and allows the strength and vertical extent of the outflows to be quantified.

Conclusion

The lidar provided measurements of the horizontal wind field (speed, direction, and variance) from within 5 meters of the ocean surface up through the base of the clouds or the top of the aerosol layer. Moments of the vertical velocity and aerosol backscatter signal strength were calculated from a minimum height of 200m to a maximum height similar to that of the horizontal measurements. These results are updated every 20 minutes with nearly continuous coverage in time. Using these results, we are able to build statistics of the dynamics of the atmospheric marine boundary layer near the ship during the experiment. Using the scanning data, we are able to study the evolution of complex flows near the surface associated with precipitation. Combining the lidar results with those taken by other complimentary obervations on the ship will provide a better understanding of the underlying processes than any single measurement by itself. We wish to acknowledge support from the NOAA Climate Program and the Office of Naval Research.