Search

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.