Demetrius Venable

Airborne and Ground-Based Measurements Using a High-Performance Raman Lidar

Abstract A high-performance Raman lidar operating in the UV portion of the spectrum has been used to acquire, for the first time using a single lidar, simultaneous airborne profiles of the water vapor mixing ratio, aerosol backscatter, aerosol extinction, aerosol depolarization and research mode measurements of cloud liquid water, cloud droplet radius, and number density. The Raman Airborne Spectroscopic Lidar (RASL) system was installed in a Beechcraft King Air B200 aircraft and was flown over the mid-Atlantic United States during July–August 2007 at altitudes ranging between 5 and 8 km. During these flights, despite suboptimal laser performance and subaperture use of the telescope, all RASL measurement expectations were met, except that of aerosol extinction. Following the Water Vapor Validation Experiment—Satellite/Sondes (WAVES_2007) field campaign in the summer of 2007, RASL was installed in a mobile trailer for ground-based use during the Measurements of Humidity and Validation Experiment (MOHAVE-II...

Lamp mapping technique for independent determination of the water vapor mixing ratio calibration factor for a Raman lidar system

We have investigated a technique that allows for the independent determination of the water vapor mixing ratio calibration factor for a Raman lidar system. This technique utilizes a procedure whereby a light source of known spectral characteristics is scanned across the aperture of the lidar systems telescope and the overall optical efficiency of the system is determined. Direct analysis of the temperature-dependent differential scattering cross sections for vibration and vibration-rotation transitions (convolved with narrowband filters) along with the measured efficiency of the system, leads to a theoretical determination of the water vapor mixing ratio calibration factor. A calibration factor was also obtained experimentally from lidar measurements and radiosonde data. A comparison of the theoretical and experimentally determined values agrees within 5%. We report on the sensitivity of the water vapor mixing ratio calibration factor to uncertainties in parameters that characterize the narrowband transmission filters, the temperature-dependent differential scattering cross section, and the variability of the system efficiency ratios as the lamp is scanned across the aperture of the telescope used in the Howard University Raman Lidar system.

Raman water vapor lidar calibration

We show here new results of a Raman LIDAR calibration methodology effort putting emphasis in the assessment of the cross-section ratio between water vapor and nitrogen by the use of a calibrated NIST traceable tungsten lamp. Therein we give a step by step procedure of how to employ such equipment by means of a mapping/scanning procedure over the receiving optics of a water vapor Raman LIDAR. This methodology has been independently used at Howard University Raman LIDAR and at IPEN Raman LIDAR what strongly supports its reproducibility and points towards an independently calibration methodology to be carried on within an experiment routine.

Systematic distortions in water vapor mixing ratio and aerosol scattering ratio from a Raman lidar

The purpose of this study is to estimate the errors due to several systematic distortions which occur in the procedure of computing water vapor mixing ratio (WVMR) and aerosol scattering ratio (ASR). For WVMR, the systematic distortions are encountered during the following processes: gluing, omission of temperature dependent molecular backscatter coefficient (temperature dependent lidar equation), incomplete overlap correction, and calibration while for ASR during the gluing and omission of temperature dependent molecular backscatter coefficient. The data analyzed are taken with Howard University Raman Lidar (HURL) during WAVES 2006 field campaign. The HURL system operates at the third harmonic of a Nd:YAG laser and acquires data within three channels (354.7 nm, 386.7 nm and 407.5 nm). The study shows the relative errors in computing WVMR and ASR when considering different combinations of the systematic distortions (15 cases for WVMR and 3 cases for ASR) with respect to the case as considered true. The analyses were performed over a time series of WVMR and ASR of about 6 hours and over the altitude range up to 5 km. The range of the relative errors is between ~ 1 % and ~ 18 % for WVMR and between ~ 1 % and ~ 8 % for ASR.

Gluing for Raman lidar systems using the lamp mapping technique

In the context of combined analog and photon counting (PC) data acquisition in a Lidar system, glue coefficients are defined as constants used for converting an analog signal into a virtual PC signal. The coefficients are typically calculated using Lidar profile data taken under clear, nighttime conditions since, in the presence of clouds or high solar background, it is difficult to obtain accurate glue coefficients from Lidar backscattered data. Here we introduce a new method in which we use the lamp mapping technique (LMT) to determine glue coefficients in a manner that does not require atmospheric profiles to be acquired and permits accurate glue coefficients to be calculated when adequate Lidar profile data are not available. The LMT involves scanning a halogen lamp over the aperture of a Lidar receiver telescope such that the optical efficiency of the entire detection system is characterized. The studies shown here involve two Raman lidar systems; the first from Howard University and the second from NASA/Goddard Space Flight Center. The glue coefficients determined using the LMT and the Lidar backscattered method agreed within 1.2% for the water vapor channel and within 2.5% for the nitrogen channel for both Lidar systems. We believe this to be the first instance of the use of laboratory techniques for determining the glue coefficients for Lidar data analysis.

Assessing the temperature dependence of narrow-band Raman water vapor lidar measurements: a practical approach.

Narrow-band detection of the Raman water vapor spectrum using the lidar technique introduces a concern over the temperature dependence of the Raman spectrum. Various groups have addressed this issue either by trying to minimize the temperature dependence to the point where it can be ignored or by correcting for whatever degree of temperature dependence exists. The traditional technique for performing either of these entails accurately measuring both the laser output wavelength and the water vapor spectral passband with combined uncertainty of approximately 0.01 nm. However, uncertainty in interference filter center wavelengths and laser output wavelengths can be this large or larger. These combined uncertainties translate into uncertainties in the magnitude of the temperature dependence of the Raman lidar water vapor measurement of 3% or more. We present here an alternate approach for accurately determining the temperature dependence of the Raman lidar water vapor measurement. This alternate approach entails acquiring sequential atmospheric profiles using the lidar while scanning the channel passband across portions of the Raman water vapor Q-branch. This scanning is accomplished either by tilt-tuning an interference filter or by scanning the output of a spectrometer. Through this process a peak in the transmitted intensity can be discerned in a manner that defines the spectral location of the channel passband with respect to the laser output wavelength to much higher accuracy than that achieved with standard laboratory techniques. Given the peak of the water vapor signal intensity curve, determined using the techniques described here, and an approximate knowledge of atmospheric temperature, the temperature dependence of a given Raman lidar profile can be determined with accuracy of 0.5% or better. A Mathematica notebook that demonstrates the calculations used here is available from the lead author.

A Numerical Model of the Performance of the Howard University Raman Lidar System

At the Howard University Atmospheric Observatory in Beltsville, MD, a Raman Lidar system was developed to provide both daytime and nighttime measurements of water vapor, aerosols, and cirrus clouds with 1 min temporal and 7.5 m spatial resolution in the lower troposphere. Signals at three wavelengths associated with Rayleigh/Mie scattering for aerosols and cirrus clouds at 354.7 nm, Raman scattering for nitrogen at 386.7 nm, and water vapor at 407.5 nm are analyzed. The transmitter is a triple harmonic Nd: YAG solid state laser. The receiver is a 40 cm Cassegrain telescope. Our detector system consists of a multi‐channel wavelength separator unit and data acquisition system. We are developing a numerical model to provide a realistic representation of the system behavior. The variants of the lidar equation in the model use system parameters and are solved to determine the return signals for our lidar system. In this paper, we report on two of the five case studies being investigated: clear sky and cirrus c...

Application of the lamp mapping technique for overlap function for Raman lidar systems.

Traditionally, the lidar water vapor mixing ratio (WVMR) is corrected for overlap using data from another instrument, such as a radiosonde. Here we introduce a new experimental method to determine the overlap function using the lamp mapping technique (LMT), which relies on the lidar optics and detection system. The LMT discussed here involves a standard halogen lamp being scanned over the aperture of a Raman lidar telescope in synchronization with the lidar detection system [Appl. Opt.50, 4622 (2011)APOPAI0003-693510.1364/AO.50.004622, Appl. Opt.53, 8538 (2014)APOPAI0003-693510.1364/AO.53.008535]. In this paper, we show results for a LMT-determined overlap function for individual channels, as well as a WVMR overlap function. We found that the LMT-determined WVMR overlap functions deviate within 5% of the traditional radiosonde-determined overlap.

Independent measurements of Raman LIDAR water vapor calibration factor

One of the goals of LIDAR scientists is to obtain long term monitoring of water vapor using Raman LIDAR [1]. Previous LIDAR research suggests that the measurement of water vapor can be improved by better analysis of the LIDAR systems calibration factor. Currently LIDAR scientists generally use radiosonde data to calibrate LIDAR data. We are using a standard lamp calibration technique to calibrate the LIDAR data to compare with the radiosonde technique in efforts to independently calibrate the LIDAR system. The lamp calibration technique we implement here involves two motion controllers scanning a halogen lamp over the aperture of a LIDAR telescope. When we compared the calibration factor of the lamp mapping technique to the radiosonde technique we found that they agreed within 5%. Using this method, we have determined a calibration of a Raman LIDAR system with accuracy in the range of 5% [2]. Future work involves obtaining temperature measurements directly from the HURL system to improve water vapor measurements. We plan to obtain the temperature directly from the LIDAR system by extracting rotational Raman cross sections using two narrow band-pass filters and taking the ratio of the two measurements. Using the lamp calibration and the temperature measurements found directly from the LIDAR we can calculate a water vapor mixing ratio that is less dependent on radiosonde data [3].