Wynn L. Eberhard

A Method for Determining Cirrus Cloud Particle Sizes Using Lidar and Radar Backscatter Technique

Abstract A method to determine cirrus cloud effective radii remotely using lidar and radar backscatter data is presented. The difference in backscattered returns from instruments widely separated in wavelength holds information on the characteristic sizes of the scatterers. The method compares theoretically expected backscatter coefficients to observed backscatter returns from NOAAs 3.2-cm and 8.6-mm radars and the 10.6-µm lidar. Measurements were taken during a two-phase cloud experiment held in northeastern Colorado from 6 September to 5 October 1989 and 15 February to 31 March 1991. It was found that the particle sizes estimated from the lidar-radar method agree closely with in situ aircraft measurements. Case studies are presented to demonstrate the method and the potential for multiwavelength remote sensing of cirrus cloud radiative properties.

The 5–6 December 1991 FIRE IFO II Jet Stream Cirrus Case Study: Possible Influences of Volcanic Aerosols

Abstract In presenting an overview of the cirrus clouds comprehensively studied by ground-based and airborne sensors from Coffeyville, Kansas, during the 5–6 December 1992 Project FIRE IFO II case study period, evidence is provided that volcanic aerosols from the June 1991 Pinatubo eruptions may have significantly influenced the formation and maintenance of the cirrus. Following the local appearance of a spur of stratospheric volcanic debris from the subtropics, a series of jet streaks subsequently conditioned the troposphere through tropopause foldings with sulfur-based particles that became effective cloud-forming nuclei in cirrus clouds. Aerosol and ozone measurements suggest a complicated history of stratospheric-tropospheric exchanges embedded within the upper-level flow, and cirrus cloud formation was noted to occur locally at the boundaries of stratospheric aerosol-enriched layers that became humidified through diffusion, precipitation, or advective processes. Apparent cirrus cloud alterations incl...

Estimations of atmospheric boundary layer fluxes and other turbulence parameters from Doppler lidar data

Techniques for extraction of boundary layer parameters from measurements of a short pulse (≈0.4 μs) CO2 Doppler lidar (λ = 10.6 μm) are described. The lidar is operated by the National Oceanic and Atmospheric Administration (NOAA) Wave Propagation Laboratory (WPL). The measurements are those collected during the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE). The recorded radial velocity measurements have a range resolution of 150 m. With a pulse repetition rate of 20 Hz it is possible to perform scannings in two perpendicular vertical planes (x–z and y–z) in approximately 72 s. By continuously operating the lidar for about an hour, one can extract stable statistics of the radial velocities. Assuming that the turbulence is horizontally homogeneous, we have estimated the mean wind, its standard deviations, and the momentum fluxes. We have estimated the first, second, and, third moments of the vertical velocity from the vertically pointing beam. Spectral analysis of the radial velocities is also performed, from which (by examining the amplitude of the power spectrum at the inertial range) we have deduced the kinetic energy dissipation. Finally, using the statistical form of the Navier-Stokes equations, the surface heat flux is derived as the residual balance between the vertical gradient of the third moment of the vertical velocity and the kinetic energy dissipation. With the exception of the vertically pointing beam an individual radial velocity estimate is accurate only to ±0.7 m s−1. Combining many measurements would normally reduce the error, provided that it is unbiased and uncorrelated. The nature of some of the algorithms, however, is such that biased and correlated errors may be generated even though the “raw” measurements are not. We have developed data processing procedures that eliminate bias and minimize error correlation. Once bias and error correlations are accounted for, the large sample size is shown to reduce the errors substantially. We show, for instance, that a single momentum flux estimate has an accuracy of ±2m2s−2, but when combined with other measurements, the error can be reduced to ±0.10 m2 s−2. The principal features of the derived turbulence statistics for two case studies (July 11, 1987, 1611–1710 UTC and 1729–1810 UTC) are as follows: The mean surface wind is from the south and has a speed of approximately 15 m s−1. There is a southerly jet 1 km above the surface. The wind in the direction normal to the surface wind becomes more westerly with height. The derived momentum fluxes in the mixed layer agree with the unfiltered airplane measurements. They are of the order of approximately 0.5 m2 s−2 near the surface and are retarding the southerly wind. At the stable layer, the momentum fluxes are countergradient; they remain negative even though the southerlies are diminishing with height and are concentrated in thin layers. By spectrally analyzing the vertical beam, we have identified the Brunt-Vaisala frequency as the dominated frequency in the stable layer. The available evidence suggests that the large countergradient fluxes are caused by a critical layer singularity. Under these conditions, gravity wave flux divergence may be large. Current general circulation models (GCMs) generally do not parameterize countergradient momentum transport. If the phenomenon is prevalent, our study suggests that inclusion of such an effect may be important. The wind spectrum near the surface has a −5/3 power law at the inertial range. The heat flux estimate is 100 ± 20 W m−2, in apparent agreement with an average estimate of the surface stations but systematically larger than the filtered airplane data.

Doppler Lidar Measurement of Profiles of Turbulence and Momentum Flux

Abstract A short-pulse CO2 Doppler lidar with 150-m range resolution measured vertical profiles of turbulence and momentum flux. Example measurements are reported of a daytime mixed layer with strong mechanical mixing caused by a wind speed of 15 m s−1, which exceeded the speed above the capping inversion. The lidar adapted an azimuth scanning technique previously demonstrated by radar. Scans alternating between two elevation angles allow determination of 〈u2〉, 〈v2〉, and 〈w2〉. Expressions were derived to estimate the uncertainty in the turbulence parameters. A new processing method, partial Fourier decomposition, has less uncertainty than the filtering used earlier. Substantial improvements could be had with higher pulse rate, shorter pulses and wavelengths (to improve spatial resolution and minimum range by up to an order of magnitude), and operation from an aircraft.

Cloud Boundary Statistics during FIRE II

Abstract An 8-mm wavelength radar, 3-mm wavelength radar, and 10.6-µm wavelength lidar operated side by side in vertically pointing mode during the First ISCCP Regional Experiment (FIRE II). This data collection mode yielded detailed information on distribution of cloud and cloud boundaries as a function of altitude. Statistics on the location of cloud boundaries during the FIRE II experiment indicate that cloud bases tended to form at two discrete levels centered around 2.5 and 7.5 km, cirrus cloud tops formed most frequently at 9.5 km and cloud thickness were usually 2 km or less. The atmosphere had the highest incidence of cloudiness at 8.5 km AGL, with a secondary maximum at an altitude of 3.5 km AGL. The incidence of cloudiness fell off rapidly between 8 and 11 km; there was also a distinct minimum in cloudiness at 2 km AGL. The diurnal variation of upper-level cloud base and top heights was about 1.0 km AGL, with the highest bases and tops occurring at 0500 UTC and the lowest bases and tops occurrin...

Retrieval of aerosol properties from combined multiwavelength lidar and sunphotometer measurements

Simulation studies were carried out with regard to the feasibility of using combined observations from sunphotometer (SPM) and lidar for microphysical characterization of aerosol particles, i.e., the retrieval of effective radius, volume, and surface-area concentrations. It was shown that for single, homogeneous aerosol layers, the aerosol parameters can be retrieved with an average accuracy of 30% for a wide range of particle size distributions. Based on the simulations, an instrument combination consisting of a lidar that measures particle backscattering at 355 and 1574 nm, and a SPM that measures at three to four channels in the range from 340 to 1020 nm is a promising tool for aerosol characterization. The inversion algorithm has been tested for a set of experimental data. The comparison with the particle size distribution parameters, measured with in situ instrumentation at the lidar site, showed good agreement.

Multiwavelength Observations of a Developing Cloud System: The FIRE II 26 November 1991 Case Study

Simultaneous multiwavelength measurements of a developing cloud system were obtained by NOAA Doppler lidar, Doppler radar, Fourier transform infrared interferometer, and microwave and infrared radiometers on 26 November 1991. The evolution of the cloud system is described in terms of lidar backscatter, radar reflectivity and velocity, interferometer atmospheric spectra, and radiometer brightness temperature, integrated liquid water, and water vapor paths. Utilizing the difference in wavelength between the radar and lidar, and therefore their independent sensitivity to different regions of the same cloud, the cloud top, base, depth, and multiple layer heights can be determined with better accuracy than with either instrument alone. Combining the radar, lidar, and radiometer measurements using two different techniques allows an estimation of the vertical profile of cloud microphysical properties such as particle sizes. Enhancement of lidar backscatter near zenith revealed when highly oriented ice crystals were present. The authors demonstrate that no single instrument is sufficient to accurately describe cirrus clouds and that measurements in combination can provide important details on their geometric, radiative, and microphysical properties.

Ice-cloud depolarization of backscatter for CO 2 and other infrared lidars

The depolarization of backscatter from ice particles at the CO(2) lidar wavelength of 10.59 microm was investigated through field measurements, with simultanous depolarization measurements taken at 0.6943 microm for comparison. The depolarization ratio at the infrared wavelength was usually at or below the lidars sensitivity limit of 0.01, which is dramatically smaller than the typical 0.5 linear depolarization ratio for short-wave lidars. This behavior is explained by the strong absorption of ice at the infrared wavelength. Depolarization measurements at a 10.59-microm wavelength cannot discriminate between ice and water clouds in the manner of short-wave lidars. A possibility exists for more prominent depolarization at shorter CO(2) lidar wavelengths (e.g., 9.115 microm), but additional research is required. Depolarization at the 2.09-microm wavelength is predicted to be substantial and useful for hydrometeor observations.

Field studies of transport and dispersion of atmospheric tracers in nocturnal drainage flows

Abstract A series of tracer experiments were carried out as part of the Atmospheric Studies in Complex Terrain (ASCOT) program to evaluate pollutant transport and dispersion characteristics of nocturnal drainage flows within a valley in northern California. The results indicate that the degree of interaction of the drainage flows with the larger scale regional flows are strongly dependent on how well the shallow drainage flows are shielded by the surrounding topography from the external environment. For the valley under study, the drainage flows from about mid-slope elevations and below were generally decoupled from the externally generated flows; as evidenced by the similarity of the surface tracer distributions produced during widely varying regional flow conditions. However, tracers released immediately above the drainage flows near the ridge top did reveal considerable mixing between the transition layer flows and the underlying surface drainage flows. Likewise, the transport and dispersion of the tracers at elevated heights within the valley basin were extremely dependent on the influences of the regional scale flows on the valley circulations. The dispersion rates associated with the transition layer flows were dependent on topographic constraints but were appreciably higher than those reported for homogeneous flat terrain situations.

Correct equations and common approximations for calculating Rayleigh scatter in pure gases and mixtures and evaluation of differences.

Equations for Rayleigh scattering in a mixture of gases are derived and compared to frequent approximations in the literature. The traditional Rayleigh scattering equation as modified by King for scatter from a pure gas is correct, whereas another version sometimes appearing in modern literature is erroneous. Use of a mixtures refractive index, which is equivalent to assuming the isotropic molecular polarizabilities of the component gases are identical, is an approximation. Another common approximation is using only number-density weighting of the King factors. Approximation errors can be large when the major components of a mixture have disparate optical properties. Fortunately, the errors for Earths air are much smaller and comparable to errors from other sources.