C. Russell Philbrick

Numerical Investigation of Boundary-Layer Evolution and Nocturnal Low-Level Jets: Local versus Non-Local PBL Schemes

Numerical simulations of the evolution of the planetary boundary layer (PBL) and nocturnal low-level jets (LLJ) have been carried out using MM5 (version 3.3) with four-dimensional data assimilation (FDDA) for a high pollution episode in the northeastern United States during July 15–20, 1999. In this paper, we assess the impact of different parameterizations on the PBL evolution with two schemes: the Blackadar PBL, a hybrid local (stable regime) and non-local (convective regime) mixing scheme; and the Gayno–Seaman PBL, a turbulent kinetic energy (TKE)-based eddy diffusion scheme. No FDDA was applied within the PBL to evaluate the ability of the two schemes to reproduce the PBL structure and its temporal variation. The restriction of the application of FDDA to the atmosphere above the PBL or the lowest 8 model levels, whichever is higher, has significantly improved the predicted strength and timing of the LLJ during the night. A systematic analysis of the PBL evolution has been performed for the primary meteorological fields (temperature, specific humidity, horizontal winds) and for the derived parameters such as the PBL height, virtual potential temperature, relative humidity, and cloud cover fraction. There are substantial differences between the PBL structures and evolutions simulated by these two different schemes. The model results were compared with independent observations (that were not used in FDDA) measured by aircraft, RASS and wind profiler, lidar, and tethered balloon platforms during the summer of 1999 as part of the NorthEast Oxidant and Particle Study (NE-OPS). The observations tend to support the non-local mixing mechanism better than the layer-to-layer eddy diffusion in the convective PBL.

Design of a lidar receiver with fiber-optic output

Design considerations for a coaxial lidar receiver are examined, including details of coupling to an optical fiber for transfer of return light to a remote detector box. Attention is concentrated on the influence of fiber position on return-light capture efficiency and dynamic range of the return signal. The effect of a central obstruction on short-range signals is included. The analysis is augmented with simulations of lidar receiver performance.

Resonance enhanced Raman scatter in liquid benzene at vapor-phase absorption peaks

The resonance enhanced Raman spectra in the 1B2u mode of the forbidden benzene electronic transition band, ~230-270 nm, has been investigated. Resonance enhanced Raman scattering in both liquid benzene and liquid toluene exhibit the greatest enhancement when the wavelength of excitation is tuned to the vapor-phase absorption peaks; even though the sample volume is in a liquid state. Raman signals for the symmetric breathing mode of the carbon ring are found to be resonantly enhanced by several orders of magnitude (>500X) with deep UV excitation compared to non-resonant visible excitation. Since the benzene absorbs near this resonant wavelength, its effect on the sampled volume cannot be neglected in determining the resonance gain, as we discuss in detail. Large resonant gains correspond with excitation at the 247, 253, and 259 nm absorption peaks in the benzene vapor spectrum. The narrow region of resonance gain is investigated in detail around the absorption peak located at 259 nm using 0.25 nm steps in the excitation wavelength. We observe the resonance gain tracking the vapor phase absorption peaks and valleys within this narrow range. Results are interpreted in terms of the coherence forced by the use of a forbidden transition for resonance excitation.

Measurement of atmospheric oxygen using long-path supercontinuum absorption spectroscopy

Abstract The concentration of atmospheric oxygen is measured over a 540-m path using supercontinuum absorption spectroscopy. The absorption data compared favorably with MODTRAN™ 5 simulations of the spectra after adjusting for the differences of index of refraction of air and matching the instrument spectral resolution, as described by the effective slit width. Good agreement with the expected atmospheric oxygen concentration is obtained using a previously developed multiwavelength maximum likelihood estimation inversion algorithm. This study demonstrates the use of the SAS technique for measuring concentrations of chemical species with fine absorption structure on long-atmospheric paths.

Lower tropospheric temperature measurement using a rotational Raman lidar

The performance of the lidar atmospheric profile sensor (LAPS) instrument for temperature measurements in the lower troposphere has been investigated. LAPS is an automated lidar system that measures temperature from the rotational Raman return of atmospheric nitrogen and oxygen. We present the measurement technique, the data analysis and the performance of the LAPS instrument. Comparisons to radiosonde profiles are discussed.

Comparison of lidar water vapor measurements using Raman scatter at 266 nm and 532 nm

The performance of the Lidar Atmospheric Profile Sensor (LAPS) instrument for measurements of water vapor in the lower troposphere has been investigated. LAPS is an automated lidar system that measures water vapor from the vibrational Raman backscatter in the visible and in the ultraviolet wavelength range. We present a comparison of water vapor profiles measured with the lidar and balloon sondes as well as measured with the two lidar channels. With the UV channels it is possible to infer ozone profiles in the boundary layer. Data are presented that reveal the high variability of the water vapor in the boundary layer.

ERRATUM: “LIGHT SCATTERING FROM EXOPLANET OCEANS AND ATMOSPHERES” (2010, ApJ, 723, 1168)

Orbital variation in reflected starlight from exoplanets could eventually be used to detect surface oceans. Exoplanets with rough surfaces, or dominated by atmospheric Rayleigh scattering, should reach peak brightness in full phase, orbital longitude (OL) = 180◦, whereas ocean planets with transparent atmospheres should reach peak brightness in crescent phase near OL = 30◦. Application of Fresnel theory to a planet with no atmosphere covered by a calm ocean predicts a peak polarization fraction of 1 at OL = 74◦; however, our model shows that clouds, wind-driven waves, aerosols, absorption, and Rayleigh scattering in the atmosphere and within the water column dilute the polarization fraction and shift the peak to other OLs. Observing at longer wavelengths reduces the obfuscation of the water polarization signature by Rayleigh scattering but does not mitigate the other effects. Planets with thick Rayleigh scattering atmospheres reach peak polarization near OL = 90◦, but clouds and Lambertian surface scattering dilute and shift this peak to smaller OL. A shifted Rayleigh peak might be mistaken for a water signature unless data from multiple wavelength bands are available. Our calculations suggest that polarization alone may not positively identify the presence of an ocean under an Earth-like atmosphere; however, polarization adds another dimension which can be used, in combination with unpolarized orbital light curves and contrast ratios, to detect extrasolar oceans, atmospheric water aerosols, and water clouds. Additionally, the presence and direction of the polarization vector could be used to determine planet association with the star, and constrain orbit inclination.

Supercontinuum lidar applications for measurements of atmospheric constituents

Recent advances in the field of supercontinuum lasers have provided a unique opportunity for developing lidar instruments that cover a wide spectral range. These instruments permit many simultaneous measurements of differential absorption spectra (DIAL and DAS techniques) to determine species density. Application of MODTRANTM 5 and other simulation software has allowed us to design and validate the findings of supercontinuum lidar systems developed at Penn State Lidar Laboratory. The multiple line differential absorption concepts have been demonstrated with various system topologies for a host of atmospheric windows in the visible to near infrared regions. During the past three years, we have developed and demonstrated several systems that are capable of measuring concentrations of various atmospheric constituents at background or elevated levels through long path absorption by transmitting only milliwatts of optical power. Our most recent supercontinuum lidar system utilizes a nanosecond supercontinuum laser fiber optically coupled to a transceiver system for remote sensing of atmospheric species concentrations. Due to the flexibility of the design, the operational prototype is currently being used to demonstrate the capability for accurately measuring real world open path atmospheric concentrations across the Penn State campus. The purpose of this study is to develop the technology and to demonstrate the capability for accurately measuring species concentrations without the complexities and uncertainties inherent in hyper-spectral remote sensing using the sun as a source, or the limitations and errors associated with using pairs of laser lines for DIAL measurements of each species. Initial simulations and measurements using this approach are presented.

Application of Raman lidar to air quality measurements

Raman lidar techniques have been demonstrated which provide most valuable descriptions of the evolution of air pollution events. The vibrational and rotational Raman lidar signals provide simultaneous profiles of meteorological data, ozone and measurements of airborne particulate matter. An operational prototype Raman lidar instrument was prepared and demonstrated for the US Navy and is now used for scientific investigations. It makes use of second and fourth harmonic generated laser beams of a Nd:YAG laser to provide both daytime and nighttime measurements. The Raman scatter signals from vibrational states of water vapor and nitrogen provide robust profiles of the specific humidity in the lower atmosphere. The temperature profiles are measured using the ratio of rotational Raman signals at 530 and 528 nm from the 532 nm (second harmonic) beam of the Nd:YAG laser. In addition, the optical extinction profiles are determined from the measured gradients in each of several molecular profiles compared to the molecular scale height. We currently use the wavelengths at 284 nm (nitrogen vibrational Raman), 530 nm (rotational Raman) and 607 nm (nitrogen vibrational Raman) to determine profiles of optical extinction. The ozone profiles in the lower troposphere are measured using a DIAL analysis of the ratio of the vibrational Raman signals for nitrogen (284 nm) and oxygen (278 nm), which are on the steep side of the Hartley band of ozone. Several data sets have been obtained during air pollution events and the results from these events have been the subject of recent studies. The examples presented in this paper have been selected to show the new level of understanding of air pollution events that is being gained from applications of lidar techniques.

Multiple‐wavelength Raman lidar measurements of atmospheric water vapor

Height profiles of atmospheric water vapor obtained using a multiple-wavelength Raman lidar are examined. The water vapor profiles exhibit vertical structure with scales on the order of the resolution of the lidar (75 m). To determine whether such structure is atmospheric in origin, measurements obtained simultaneously in a common volume at two independent wavelengths were compared. Correlation of the gradients of the water vapor profiles obtained from these two wavelengths yielded an average correlation factor of 0.88. It was also observed that for the given meteorological conditions, the vertical structure decorrelated with a time constant of approximately three hours.