William M. Porch

Wave optics simulation of atmospheric turbulence and reflective speckle effects in CO 2 lidar

Laser speckle can influence lidar measurements from a diffuse hard target. Atmospheric optical turbulence will also affect the lidar return signal. We present a numerical simulation that models the propagation of a lidar beam and accounts for both reflective speckle and atmospheric turbulence effects. Our simulation is based on implementing a Huygens-Fresnel approximation to laser propagation. A series of phase screens, with the appropriate atmospheric statistical characteristics, are used to simulate the effect of atmospheric turbulence. A single random phase screen is used to simulate scattering of the entire beam from a rough surface. We compare the output of our numerical model with separate CO(2) lidar measurements of atmospheric turbulence and reflective speckle. We also compare the output of our model with separate analytical predictions for atmospheric turbulence and reflective speckle. Good agreement was found between the model and the experimental data. Good agreement was also found with analytical predictions. Finally, we present results of a simulation of the combined effects on a finite-aperture lidar system that are qualitatively consistent with previous experimental observations of increasing rms noise with increasing turbulence level.

Ship-produced cloud lines of 13 July 1991

Abstract On 13 July 1991, a well-defined cloud line produced by an unidentified steaming ship was detected in satellite imagery and was simultaneously photographed from the R/V EGABRAG III . The EGABRAG produced a much less well-defined cloud line. Measurements made from the EGABRAG revealed that the cloud lines formed in a shallow boundary layer which was nearly saturated, unstable, drizzling and nearly free of cloud condensation nuclei (CCN). The EGABRAG passed through the plume of the ship as indicated by elevated CCN concentrations coincident with the cloud line. Thereafter, both ships passed under a shallow stratus layer where background CCN concentrations increased significantly. Only the cloud line produced by the ship extended into the stratus layer, the EGABRAG did not affect the layer. The CCN and updraft from the ship were involved in the formation of the cloud line. In contrast, the CCN and updraft from the EGABRAG were insufficient to produce a well-defined cloud line. Production of the cloud lines appeared dependent on a combination of environmental conditions and ship-produced CCN and updrafts.

Ventilation of liquefied petroleum gas components from the Valley of Mexico

The saturated hydrocarbons propane and the butane isomers are both indirect greenhouse gases and key species in liquefied petroleum gas (LPG). Leakage of LPG and its component alkanes/alkenes is now thought to explain a significant fraction of the volatile organic burden and oxidative potential in the basin which confines Mexico City. Propane and the butanes, however, are stable enough to escape from the basin. The gas Chromatographie measurements which have drawn attention to their sources within the urban area are used here to estimate rates of ventilation into the free troposphere. The calculations are centered on several well studied February/March pollution episodes. Carbon monoxide observations and emissions data are first exploited to provide a rough time constant for the removal of typical inert pollutant species from the valley. The timescale obtained is validated through an examination of meteorological simulations of three-dimensional flow. Heuristic arguments and transport modeling establish that propane and the butanes are distributed through the basin in a manner analogous to CO despite differing emissions functions. Ventilation rates and mass loadings yield outbound fluxes in a box model type computation. Estimated in this fashion, escape from the Valley of Mexico constitutes of the order of half of 1% of the northern hemispheric inputs for both propane and n-butane. Uncertainties in the calculations are detailed and include factors such as flow into the basin via surface winds and the size of the polluted regime. General quantification of the global propane and butane emissions from large cities will entail studies of this type in a variety of locales.

Ship trails and ship induced cloud dynamics

Abstract A combination of satellite image analysis and numerical cloud modeling has provided new insight into how ship trails are formed and why they occur where they do. The cold ocean waters and shallow marine boundary layers in the East Pacific, and in the North Atlantic make these regions particularly sensitive to cloud formation triggered by heat releases over the ocean. This heat may be released directly from the ships engines and by latent heat released from the cloud condensation nuclei (CCN) to cloud droplet formation process. Detailed image analysis of an exceptionally high resolution photograph of ship trails observed by the Apollo-Soyuz mission in July 1975 shows that these ship trails began as a brightening of background marine stratocumulus and eventually grew to a point where the ship trails generated a plume like structure with clear regions on both sides. A numerical cloud model was constructed with initial meteorological conditions similar to those observed by ships in a nearby region of the ocean. A heat input of 30 MW produced a similar ship-trail cloud behavior to that observed in the photograph after a 1-h simulation time.

Observations of ship tracks from ship-based platforms

Abstract Ship-based measurements in June 1994 provided information about ship-track clouds and associated atmospheric environment observed from below cloud levels that provide a perspective different from satellite and aircraft measurements. Surface measurements of latent and sensible heat fluxes, sea surface temperatures, and meteorological profiles with free and tethered balloons provided necessary input conditions for models of ship-track formation and maintenance. Remote sensing measurements showed a coupling of ship plume dynamics and entrainment into overlaying clouds. Morphological and dynamic effects were observed on clouds unique to the ship tracks. These morphological changes included lower cloud bases early in the ship-track formation, evidence of raised cloud bases in more mature tracks, sometimes higher cloud tops, thin cloud-free regions paralleling the tracks, and often stronger radar returns. The ship-based lidar aerosol measurements revealed that ship plumes often interacted with the over...

Blue moons and large fires.

Theoretical analysis of simulations of optical effects from the 1950 Canadian forest fires has revealed what conditions are necessary for large fires to cause blue moons and suns. This study shows how large fires can be used to improve our understanding of long range pollution transport on a global scale as well as the evolution of aerosol radiative effects so important to global climate studies. The most important aerosol characteristics are the initial submicron smoke particle concentration and areal extent of the fire and its effect on fire plume dispersion. Capping clouds above the fire and near saturation humidity effects are simulated and found to help establish anomalous optical effects. Data are included showing probable anomalous extinction events associated with concentrated fire plumes.

Comparisons of meteorological structure parameters in complex terrain using optical and acoustical techniques.

Remotely sensed wind, temperature changes with height, and turbulence have been compared with in situ instrumentation using optical and acoustic techniques across and within valleys. These comparisons have provided data useful in the interpretation of spatial averages inherent in remote sensing with spatial and temporal representativity of point meteorological measurements in complex terrain. Of particular interest in this study was comparison of the optical turbulence derived index-of-refraction structure function C(n) with the acoustically derived temperature structure function C(T) over divergent scales across and within a valley.

Smoke flow visualization in a tributary of a deep valley

Abstract Smoke pot and oil fog smoke tracers have been used to plan meteorological instrument placement and quantitatively estimate air volume flow from a tributary during nocturnal drainage wind conditions. The estimated volume flow agrees well with estimates of the flow using tethered-balloon and remotely obtained wind velocity measurements. The smoke visualization shows a very complex flow structure caused by tributary flow interactions with the flow down the main valley. The magnitude of the outflow volume from the tributary was greater than expected. If the tributary studied is representative of the other tributaries in the valley, most of the volume flow in the main valley may enter through the tributaries.

Huygens-Fresnel wave-optics simulation of atmospheric optical turbulence and reflective speckle in CO2 differential absorption lidar (DIAL)

The measurement sensitivity of CO2 differential absorption LIDAR (DIAL) can be affected by a number of different processes. Two of these processes are atmospheric optical turbulence and reflective speckle. Atmospheric optical turbulence affects the beam distribution of energy and phase on target. The effects of this phenomenon include beam spreading, beam wander and scintillation which can result in increased shot-to-shot signal noise. In addition, reflective speckle alone has been shown to have a major impact on the sensitivity of CO2 DIAL. We have previously developed a Huygens-Fresnel wave optics propagation code to separately simulate the effects of these two processes. However, in real DIAL systems it is a combination of these phenomena, the interaction of atmospheric optical turbulence and reflective speckle, that influences the results. In this work, we briefly review a description of our model including the limitations along with a brief summary of previous simulations of individual effects. The performance of our modified code with respect to experimental measurements affected by atmospheric optical turbulence and reflective speckle is examined. The results of computer simulations are directly compared with lidar measurements and show good agreement. In addition, simulation studies have been performed to demonstrate the utility and limitations of our model. Examples presented include assessing the effects for different array sizes on model limitations and effects of varying propagation step sizes on intensity enhancements and intensity probability distributions in the receiver plane.