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The Canopy Horizontal Array Turbulence Study (CHATS) experimental study was conducted at a Cilker Orchards's walnut blocks in Dixon, California, before and after leaves emerged, to help improve understanding, simulation capabilities, and modeling of coupled vegetation-atmosphere-land surface interactions. The campaign took place over 12 weeks in the spring of 2007 and was broken into three 4-week phases. Canopy-induced mechanical and thermodynamical vertical variation of turbulence transport characteristics and the linkages between canopy-scale motions and the larger scale planetary boundary layer (PBL) turbulence were measured. The observations focused on measurements characterizing stratification influences on the spatial structure of canopy-induced turbulence, the trace gas source/sink distribution associated with vegetation, and the overall impact of canopy-induced processes on trace gas transport.

High-resolution observations from scanning Doppler and aerosol lidars, wind profiler radars, as well as surface and aircraft measurements during the Terrain-induced Rotor Experiment (T-REX) provide the first comprehensive documentation of small-scale intense vortices associated with atmospheric rotors that form in the lee of mountainous terrain. Although rotors are already recognized as potential hazards for aircraft, it is proposed that these small-scale vortices, or subrotors, are the most dangerous features because of strong wind shear and the transient nature of the vortices. A life cycle of a subrotor event is captured by scanning Doppler and aerosol lidars over a 5-rain period. The lidars depict an amplifying vortex, with a characteristic length scale of ~500-1000 m, that overturns and intensifies to a maximum spanwise vorticity greater than 0.2 [s.sup.-1]. Radar wind profiler observations document a series of vortices, characterized by updraft/downdraft couplets and regions of enhanced reversed flow, that are generated in a layer of strong vertical wind shear and subcritical Richardson number. The observations and numerical simulations reveal that turbulent subrotors occur most frequently along the leading edge of an elevated sheet of horizontal vorticity that is a manifestation of boundary layer shear and separation along the lee slopes. As the subrotors break from the vortex sheet, intensification occurs through vortex stretching and in some cases tilting processes related to three-dimensional turbulent mixing. The subrotors and ambient vortex sheet are shown to intensify through a modest increase in the upstream inversion strength, which illustrates the predictability challenges for the turbulent characterization of rotors.

Backscatter lidar detection systems have been designed and integrated at NASA Langley Research Center using IR heterojunction phototransistors. The design focused on maximizing the system signal-to-noise ratio rather than noise minimization. The detection systems have been validated using the Raman-shifted eye-safe aerosol lidar (REAL) at the National Center for Atmospheric Research. Incorporating such devices introduces some systematic effects in the form of blurring to the backscattered signals. Characterization of the detection system transfer function aided in recovering such effects by deconvolution. The transfer function was obtained by measuring and fitting the system impulse response using single-pole approximation. An iterative deconvolution algorithm was implemented in order to recover the system resolution, while maintaining high signal-to-noise ratio. Results indicated a full recovery of the lidar signal, with resolution matching avalanche photodiodes. Application of such a technique to atmospheric boundary and cloud layers data restores the range resolution, up to 60 m, and overcomes the blurring effects. OCIS codes: 280.3640, 010.0280, 290.1090, 250.0040, 100.1830.

A Raman shifter is optimized for generating high-energy laser pulses at a 1.54 [micro]m wavelength. A forward-scattering design is described, including details of the multiple pass and nonfocused optical design, Stokes injection seeding, and internal gas recirculation. First-Stokes conversion efficiencies up to 43%--equivalent to 62% photon conversion efficiency--were measured. Experimental results show output average power in excess of 17.5 W, pulse energies of 350 mJ at 50 Hz, with good beam quality ([M.sup.2] < 6). Narrow bandwidth and tunable output is produced when pumping with a single longitudinal mode Nd:YAG laser and seeding the process with a Stokes wavelength narrowband laser diode. OCIS codes: 010.3640, 280.3640, 140.3550, 280.1100, 280.1120.

The design features of, and first observations from, a new elastic backscatter lidar system at a wavelength of 1543 nm are presented. The transmitter utilizes stimulated Raman scattering in high-pressure methane to convert fundamental Nd:YAG radiation by means of the 1st Stokes shift. The wavelength-converting gas cell features multipass operation and internal fans. Unlike previous lidar developments that used Raman scattering in methane, the pump beam is not focused in the present configuration. This feature prevents optical breakdown of the gas inside the cell. Additionally, the gas cell is injection seeded by a diode to improve conversion efficiency and beam quality. The receiver uses a 40.6-cm-diameter telescope and a 200-[micro].m InGaAs avalanche photodiode. The system is capable of operating in a dual-wavelength mode (1064 and 1543 nm simultaneously) for comparison or in a completely eye-safe mode. The system is capable of transmitting an energy of more than 200 mJ/pulse at 10 Hz. Aerosol backscatter data from vertical and horizontal pointing periods are shown. OCIS codes: 010.3640, 280.3640, 140.3550, 280.1100, 280.1120.

An arrangement of boundary conditions is described and demonstrated that facilitates the large-eddy simulation (LES) of inhomogeneous boundary layers such as internal boundary layers. In addition to the domain where the internal boundary layer develops, the method requires a section of domain over the upwind surface that is of the order of 10 boundary layer thicknesses and thus similar in size to that needed for the LES of the upwind boundary layer. In addition to periodic lateral, closed upstream, and open downstream boundary conditions, a simple and efficient perturbation recycling technique, which follows from one of Lund, Wu, and Squires, is used to generate a steady supply of fully developed turbulence from the inflow boundary. The arrangement is used to simulate a convective internal boundary layer during a cold-air outbreak over water. Results show the method consistently produces a solution that is homogeneous over the upwind surface and inhomogeneous over the downwind surface, and that the statistics are stationary after spinup. The sensitivity to the placement of the outflow boundary is tested, and examples of instantaneous and mean fields generated by the simulations are shown.

An airborne differential absorption lidar (DIAL) system has been developed at the NASA Langley Research Center for remote measurements of atmospheric water vapor ([H.sub.2]O) and aerosols. Asolid-state alexandrite laser with a 1-pm linewidth and >99.85% spectral purity was used as the on-line transmitter. Solid-state avalanche photodiode detector technology has replaced photomultiplier tubes in the receiver system, providing an average increase by a factor of 1.5-2.5 in the signal-to-noise ratio of the [H.sub.2]O measurement. By incorporating advanced diagnostic and data-acquisition instrumentation into other subsystems, we achieved additional improvements in system operational reliability and measurement accuracy. Laboratory spectroscopic measurements of [H.sub.2]O absorption-line parameters were performed to reduce the uncertainties in our knowledge of the absorption cross sections. Line-center [H.sub.2]O absorption cross sections were determined, with errors of 3-6%, for more than 120 lines in the 720-nm region. Flight tests of the system were conducted during 1989-1991 on the NASA Wallops Flight Facility Electra aircraft, and extensive intercomparison measurements were performed with dew-point hygrometers and [H.sub.2]O radiosondes. The [H.sub.2]O distributions measured with the DIAL system differed by [less than or equal to] 10% from the profiles determined with the in situ probes in a variety of atmospheric conditions. Key words: Differential absorption lidar, alexandrite laser, water vapor, aerosols.

The authors use large-eddy simulation (LES) to investigate entrainment and structure of the inversion layer of a clear convectively driven planetary boundary layer (PBL) over a range of bulk Richardson numbers, Ri. The LES code uses a nested grid technique to achieve fine resolution in all three directions in the inversion layer. Extensive flow visualization is used to examine the structure of the inversion layer and to illustrate the temporal and spatial interaction of a thermal plume and the overlying inversion. It is found that coherent structures in the convective PBL, that is, thermal plumes, are primary instigators of entrainment in the Ri range 13.6 [less than or equal to] Ri [less than or equal to] 43.8. At Ri = 13.6, strong horizontal and downward velocities are generated near the inversion layer because of the plume-interface interaction. This leads to folding of the interface and hence entrainment of warm inversion air at the plume's edge. At Ri = 34.5, the inversion's strong stability prevents folding of the interface but strong horizontal and downward motions near the plume's edge pull down pockets of warm air below the nominal inversion height. These pockets of warm air are then scoured off by turbulent motions and entrained into the PBL. The structure of the inversion interface from LES is in good visual agreement with lidar measurements in the PBL obtained during the Lidars in Flat Terrain field experiment. A quadrant analysis of the buoyancy flux shows that net entrainment flux (or average minimum buoyancy flux [Mathematical Expression Omitted]) is identified with quadrant [Mathematical Expression Omitted] motions, that is, warm air moving downward. Plumes generate both large negative quadrant [Mathematical Expression Omitted] and positive quadrant [Mathematical Expression Omitted] buoyancy fluxes that tend to cancel. The maximum vertical gradient in potential temperature at every (x, y) grid point is used to define a local PBL height, [z.sub.i](x, y). A statistical analysis of z[z.sub.i] shows that skewness of [z.sub.i] depends on the inversion strength. Spectra of [z.sub.i] exhibit a sensitivity to grid resolution. The normalized entrainment rate [w.sub.e]/[w.sub.*], where [w.sub.e] and [w.sub.*] are entrainment and convective velocities, varies as [ARi.sup.-1] with A [approximately equal to] 0.2 in the range 13.6 [less than or equal to] Ri [less than or equal to] 43.8 and is in good agreement with convection tank measurements. For a clear convective PBL, the authors found that the finite thickness of the inversion layer needs to be considered in an entrainment rate parameterization derived from a jump condition.

Numerical and field experiments were conducted to test an optimized cross-correlation algorithm (CCA) for the remote sensing of two-component wind vectors from horizontally scanning elastic backscatter lidar data. Each vector is the result of applying the algorithm to a square and contiguous subset of pixels (an interrogation window) in the lidar scan area. Synthetic aerosol distributions and flow fields were used to investigate the accuracy and precision of the technique. Results indicate that in neutral static stability, when the mean flow direction over the interrogation window is relatively uniform, the random error of the estimates increases as the mean wind speed and turbulence intensity increases. In convective conditions, larger errors may occur as a result of the cellular nature of convection and the dramatic changes in wind direction that may span the interrogation window. Synthetic fields were also used to determine the significance of various image processing and numerical steps used in the CCA. Results show that an iterative approach that dynamically reduces the block size provides the largest performance gains. Finally, data from a field experiment conducted in 2013 in Chico, California, are presented. Comparisons with Doppler lidar data indicate excellent agreement for the 10-min mean wind velocity computed over a set of 150 h: the root-mean-square deviations (and slopes) for the u and υ components are 0.36 m s−1 (0.974) and 0.37 m s−1 (0.991), respectively, with correlation coefficients > 0.99. [ABSTRACT FROM AUTHOR]

A motion estimation algorithm was applied to image sequences produced by a horizontally scanning elastic backscatter lidar. The algorithm, a wavelet-based optical flow estimator named Typhoon, produces dense two-component vector flow fields that correspond to the apparent motion of microscale aerosol features. To validate the efficacy of this approach for the remote measurement of wind fields in the lower atmosphere, an experiment was conducted in Chico, California, in 2013 and 2014. The flow fields, estimated every 17 s, were compared with measurements from an independent Doppler lidar. Time series of wind speed and direction, statistical assessment of the 10-min averages, and examples of wind fields are presented. The comparison of 10-min averages at 100 m AGL reveals excellent correlations between estimates from the Typhoon algorithm and measurements from the Doppler lidar. Power spectra and spectral transfer functions are computed to estimate the filtering effects of the algorithm in the spatial domain. [ABSTRACT FROM AUTHOR]

Two-component horizontal motion vectors of aerosol features were calculated by applying a cross-correlation algorithm to square image blocks extracted from consecutive pairs of elastic backscatter lidar scans. The resulting vector components were compared with corresponding horizontal wind components from tower-mounted sonic anemometers located at the center of the image blocks. In the analysis 180 245 pairs of vectors derived from 75 days of field data collected between 19 March and 11 June 2007 were used. Examples of time series comparisons from 4-h periods during light, strong, and changing wind conditions are presented. Mean signal-to-noise ratios (SNRs) of the block backscatter data, maxima of the cross-correlation functions (CCFs), observed wind speed, and turbulent kinetic energy (TKE) were also calculated for each velocity component comparison. The correlation between the lidar-derived motion components and sonic anemometer wind components tends to be highest during light wind conditions with low TKE. An empirical relationship is presented that enables the elimination of vectors that are likely to be significantly different than the anemometer measurement. When applied to the entire set of scans available, this quality control (QC) method increases the correlation between the two forms of measurements. Finally, the cross-correlation algorithm and QC method are applied to a mesh of locations over pairs of scans. Two examples of two-dimensional and two-component vector flow fields are shown. In one case, the flow field reveals a rotational circulation associated with a vortex and, in the other case, convergence and transport near the leading edge of a density current front. [ABSTRACT FROM AUTHOR]

Seven atmospheric density current fronts are identified in sequences of ground-based, scanning aerosol backscatter lidar images and in situ micrometeorological time series data that were collected simultaneously and nearly continuously between 15 March and 11 June of 2007 in Dixon, California. The fronts, observed on different days, had the following features in common: 1) an increase in aerosol backscatter intensity, 2) a decrease in air temperature, 3) an increase in water vapor mixing ratio, 4) movement toward the north, 5) airflow from the south in the denser air mass, and 6) occurrence within a 3.5-h time span in the afternoon. The observations support the hypothesis that the fronts are the leading edges of shallow marine air masses advancing northward from the Sacramento--San Joaquin River Delta. The observations are used to test an empirical relationship between front speed, airmass density difference, depth of the dense air mass, and speed of the opposing flow. Prominent features of the fronts such as lobe and cleft structure, billows, and nose and head structure are described. Time-lapse animations of the lidar scans are available in the online version of this article. [ABSTRACT FROM AUTHOR]

Seven atmospheric density current fronts are identified in sequences of ground-based, scanning aerosol backscatter lidar images and in situ micrometeorological time series data that were collected simultaneously and nearly continuously between 15 March and 11 June of 2007 in Dixon, California. The fronts, observed on different days, had the following features in common: 1) an increase in aerosol backscatter intensity, 2) a decrease in air temperature, 3) an increase in water vapor mixing ratio, 4) movement toward the north, 5) airflow from the south in the denser air mass, and 6) occurrence within a 3.5-h time span in the afternoon. The observations support the hypothesis that the fronts are the leading edges of shallow marine air masses advancing northward from the Sacramento--San Joaquin River Delta. The observations are used to test an empirical relationship between front speed, airmass density difference, depth of the dense air mass, and speed of the opposing flow. Prominent features of the fronts such as lobe and cleft structure, billows, and nose and head structure are described. Time-lapse animations of the lidar scans are available in the online version of this article. [ABSTRACT FROM AUTHOR]

A field-deployable scanning direct-detection elastic backscatter lidar system that is eye safe at all ranges is presented. The first two-dimensional spatial images created by scanning this new 1.54-μm wavelength system, and time-lapse animations (viewable in the online version of this article) of those spatial images, are shown. The system has a useful range from approximately 500 m to several kilometers or more (depending on weather conditions) with 3-m range resolution. The time-lapse animations reveal the advection of the macroscopic structure of aerosol scattering due to atmospheric motion. The images and animations are ideal for the location of sources of pollutants and their dispersal and the elucidation of structure in the cloud-free atmosphere. Most importantly, the data presented were collected in a populated area, which would have not been possible with a non-eye-safe system. [ABSTRACT FROM AUTHOR]

The authors apply data analysis techniques that demonstrate the power of using volume imaging lidar observations to evaluate several aspects of large-eddy simulations (LESs). They present observations and simulations of an intense and spatially evolving convective boundary layer on 13 January 1998 during the Lake-Induced Convection Experiment (Lake-ICE). To enable comparison of observed and simulated eddy structure, aerosol scattering was estimated from LES output of relative humidity, a passive tracer, and liquid water. Spatial and temporal correlation functions of aerosol structure from horizontal planes reveal the mean and turbulent convective structure. The correlation functions of the observed and simulated aerosol backscatter are presented as a function of altitude and offshore distance. Best-fit ellipses to the closed contours encircling the origin of the correlation functions are used to obtain the mean ellipticity and orientation of the structures. The authors demonstrate that these two parameters are not sensitive to minor changes in the functional relationship between humidity and optical scattering. The lidar-derived mean wind field is used as a reference for evaluating the LES mean flow. The ellipses from lidar data indicate that structures near the surface tend to be aligned with the mean wind direction, while in the entrainment zone they are aligned perpendicular to the mean wind direction. In the middle of the mixed layer, convective plumes tended to be circular and, therefore, had no preferred orientation at small lags. At longer lags, however, the correlation functions from the middle of the mixed layer show that the observed convective plumes were organized into linear bands oriented perpendicular to the mean wind direction. The perpendicular bands suggest the important role of gravity waves in organizing convective structures. The study shows that the model generates reasonable coherent structures (open cells) where the LES technique is... [ABSTRACT FROM AUTHOR]

Presents a letter to the editor about physical scientists and how they are perceived by high school students in response to James F. Jackson's article in the January issue on page 112.

Presents several letters to the editor on science developments in the January 1989 issue of the journal 'The New York Academy of Sciences.' Reason on studying science; Characterization of language as creationism; Impact of science illiteracy on governance.

A biological aerosol attack in a city could infect tens of thousands of people. In the absence of announcements by the attacker or detection by present point detection systems, victims would be unaware of their exposure prior to developing symptoms. Since infections are most effectively countered before the onset of symptoms, detection technologies that provide early awareness of an attack should be given high priority. Current biological point detection (BPD) systems collect environmental air samples and then analyze them in laboratories so as to permit detection within 12-36 hours of an attack. Improvements in the pipeline may reduce this lag time to as little as a few hours. However, BPD systems have inherent weaknesses when used to detect and respond to an aerosol attack. The likelihood of a limited number of BPD systems intercepting an aerosol plume in a vast attack space may be low. Moreover, BPD systems do not provide critical information needed for response, such as the source location, precise time, and geographic reach of an attack. The missing information would invaluably guide prophylaxis distribution, identification of contaminated areas, and criminal apprehension. This article describes how lidars used for real-time observation of aerosol plumes could complement BPD systems by providing fine-scale spatial and temporal information. A lidar system also could be used to corroborate positive BPD system results, to improve reaction to positive results, and/or to provide an independent basis for low-regret protective steps. Lidar systems can resolve key biodefense challenges, and this article describes three compatible concepts of operations. Leveraging lessons from a lidar system now operating at the Pentagon, a test of an expanded lidar network would provide immediate protection for key Washington, DC, assets, demonstrate the synergy of BPD systems and lidars, and provide a test bed for research to improve lidar's shortcomings.