Bruce M. Gentry

Wind measurements with 355-nm molecular Doppler lidar

A Doppler lidar system based on the molecular double-edge technique is described. The system is mounted in a modified van to permit deployment in field operations. The lidar operates with a tripled Nd:YAG laser at 355 nm, a 45-cm-aperture telescope, and a matching azimuth-over-elevation scanner to permit full sky access. Validated atmospheric wind profiles were measured from 1.8 to 35 km with a 178-m vertical resolution. The range-dependent rms deviation of the horizontal wind speed is 0.4-6 m/s. The measured wind speed and direction are in good agreement with the rawinsonde wind measurements made simultaneously from the same location.

Line mixing in a N2‐broadened CO2 Q branch observed with a tunable diode laser

Line‐mixing effects have been observed in the infrared Q branch of the (1110,0310)I–0000 band of CO2 at 2076 cm−1. A tunable diode laser spectrometer was used to record spectra of CO2 broadened by N2 and O2 at total pressures ranging from 100 to 720 Torr. The observed absorption coefficients are up to 65% lower than those calculated using an isolated Lorentzian line approximation. A simple energy gap scaling law is used to determine the off‐diagonal relaxation matrix elements from the known pressure‐broadening coefficients. The spectra calculated using these matrix elements reproduces the observed absorption coefficients to within several percent.

Rotational collisional narrowing in an infrared CO2 Q branch studied with a tunable‐diode laser

Rotational collisional narrowing, or line mixing, has been observed in the infrared absorption spectrum of the Q branch of the (1110,0310)II – 0000 band of CO2 at 1932.47 cm−1 using a tunable diode laser. Spectra were recorded at pressures ranging from 100 to 745 Torr both inside the Q branch and below the band origin. The observed absorption coefficients below the band origin were up to 80% less than those calculated using an isolated Lorentz line approximation. A simple fitting law for the state‐to‐state rotationally inelastic rate constants is determined using the known line broadening coefficients. A calculation of line mixing using these rates agrees with the observed absorption coefficients to within several percent.

Theory of the double-edge technique for Doppler lidar wind measurement

The theory of the double-edge technique is described by a generalized formulation that substantially extends the capabilities of the edge technique. It uses two edges with opposite slopes located about the laser frequency. This doubles the signal change for a given Doppler shift and yields a factor of 1.6 improvement in the measurement accuracy compared with the single-edge technique. Use of two high-resolution edge filters reduces the effects of Rayleigh scattering on the measurement by as much as an order of magnitude and allows the signal-to-noise ratio to be substantially improved in areas of low aerosol backscatter. We describe a method that allows the Rayleigh and aerosol components of the signal to be independently determined. The effects of Rayleigh scattering are then subtracted from the measurement, and we show that the correction process does not significantly increase the measurement noise for Rayleigh-to-aerosol ratios as high as 10. We show that for small Doppler shifts a measurement accuracy of 0.4 m/s can be obtained for 5000 detected photons, 1.2 m/s for 1000 detected photons, and 3.7 m/s for 50 detected photons for a Rayleigh-to-aerosol ratio of 5. Methods for increasing the dynamic range to more than +/-100 m/s are given.

Raman Lidar Measurements during the International H2O Project. Part I: Instrumentation and Analysis Techniques

The amount of water vapor in the atmosphere helps to determine the likelihood that severe storms may develop. The concentration of water vapor, though, is highly variable in space and time. And yet small changes in water vapor concentration over a short period of time or over a short spatial distance can determine whether a storm may or may not develop. Therefore, in order to improve the ability to forecast severe weather such as thunderstorms it is important to measure water vapor in the atmosphere with high spatial and temporal resolution. One of the most attractive research tools for measuring water vapor in the atmosphere with high spatial and temporal resolution is a Raman lidar. A Raman lidar consists of a laser transmitter, a telescope receiver and optics and electronics for processing opticand electronic signals. A laser pulse is emitted into the atmosphere and it interacts with molecules in the atmosphere causing them to become excited and to emit, through the Raman process, photons of different wavelength than emitted by the laser. The molecule that emitted these emitted. This is the way that a Raman lidar identifies water vapor molecules in the atmosphere. can be identified based on the wavelength of the photons One of the great challenges in Raman lidar measurements has been to make useful daytime measurements of the water vapor profile under bright daytime conditions. In this first of two papers, we describe the instrumentation and analysis of the first documented Raman lidar that is able to measure water vapor in the daytime with sufficient quality to permit the study of developing storm systems.

LIDAR-MEASURED WIND PROFILES The Missing Link in the Global Observing System

The three-dimensional global wind field is the most important remaining measurement needed to accurately assess the dynamics of the atmosphere. Wind information in the tropics, high latitudes, and stratosphere is particularly deficient. Furthermore, only a small fraction of the atmosphere is sampled in terms of wind profiles. This limits our ability to optimally specify initial conditions for numerical weather prediction (NWP) models and our understanding of several key climate change issues. Because of its extensive wind measurement heritage (since 1968) and especially the rapid recent technology advances, Doppler lidar has reached a level of maturity required for a space-based mission. The European Space Agency (ESA)s Atmospheric Dynamics Mission Aeolus (ADM-Aeolus) Doppler wind lidar (DWL), now scheduled for launch in 2015, will be a major milestone. This paper reviews the expected impact of DWL measurements on NWP and climate research, measurement concepts, and the recent advances in technology that ...

Edge technique Doppler lidar wind measurements with high vertical resolution

We have developed a Doppler lidar system using the edge technique and have made atmospheric lidar wind measurements. Line-of-sight wind profiles with a vertical resolution of 22 m have a standard deviation of 0.40 m /s for a ten-shot average. Day and night lidar measurements of the vector wind have been made for altitudes from 200 to 2000 m. We validated the lidar measurements by comparing them with independent rawinsonde and pilot balloon measurements of wind speed and direction. Good agreement was obtained. The instrumental noise for these data is 0.11 m /s for a 500-shot average, which is in good agreement with the observed minimum value of the standard deviation for the atmospheric measurements. The average standard deviation over 30 mins varied from 1.16 to 0.25 m /s for day and night, respectively. High spatial and temporal resolution lidar profiles of line-of-sight winds clearly show wind shear and turbulent features at the 1 -2-m /s level with a high signal-to-noise ratio and demonstrate the potential of the edge-technique lidar for studying turbulent processes and atmospheric dynamics.

Edge technique for high-accuracy Doppler velocimetry

The edge technique has been used in simple laboratory experiments to demonstrate velocity measurements with an experimental error, standard deviation, as small as 12 cm/s, which represents a Doppler-shift measurement accuracy of 8 parts in 10(10) of the laser frequency. An edge filter with a spectral width 140 times larger than the measurement accuracy achieved is used. The measurements are made in the presence of short-term frequency drifts equivalent to velocities of 5 to 10 m/s, which are eliminated by the differential frequency measurement used in the edge technique. Long-term frequency drifts are compensated for by servo locking the edge to the laser frequency. High accuracy is achieved for a range of locations on the edge from 0.33 to 4.5 fringe half-widths (half-width at half-maximum), a dynamic range greater than 500 times the measurement accuracy.

The dryline on 22 May 2002 during IHOP_2002: Convective-scale measurements at the profiling site

Abstract A detailed analysis of the structure of a double dryline observed over the Oklahoma panhandle during the first International H2O Project (IHOP_2002) convective initiation (CI) mission on 22 May 2002 is presented. A unique and unprecedented set of high temporal and spatial resolution measurements of water vapor mixing ratio, wind, and boundary layer structure parameters were acquired using the National Aeronautics and Space Administration (NASA) scanning Raman lidar (SRL), the Goddard Lidar Observatory for Winds (GLOW), and the Holographic Airborne Rotating Lidar Instrument Experiment (HARLIE), respectively. These measurements are combined with the vertical velocity measurements derived from the National Center for Atmospheric Research (NCAR) Multiple Antenna Profiler Radar (MAPR) and radar structure function from the high-resolution University of Massachusetts frequency-modulated continuous-wave (FMCW) radar to reveal the evolution and structure of the late afternoon double-dryline boundary layer...

Raman Lidar Measurements during the International H2O Project. Part II: Case Studies

Abstract The NASA GSFC Scanning Raman Lidar (SRL) participated in the International H2O Project (IHOP) that occurred in May and June 2002 in the midwestern part of the United States. The SRL system configuration and methods of data analysis were described in Part I of this paper. In this second part, comparisons of SRL water vapor measurements and those of Lidar Atmospheric Sensing Experiment (LASE) airborne water vapor lidar and chilled-mirror radiosonde are performed. Two case studies are then presented: one for daytime and one for nighttime. The daytime case study is of a convectively driven boundary layer event and is used to characterize the daytime SRL water vapor random error characteristics. The nighttime case study is of a thunderstorm-generated cirrus cloud case that is studied in its meteorological context. Upper-tropospheric humidification due to precipitation from the cirrus cloud is quantified as is the cirrus cloud optical depth, extinction-to-backscatter ratio, ice water content, cirrus pa...