Dennis W. Thomson

Radar Observations of Humidity Variability in and above the Marine Atmospheric Boundary Layer

Abstract Humidity variability at the top of the marine atmospheric boundary layer and in the overlying free troposphere was examined using data collected during the marine stratocumulus phase of the First Regional Experiment (FIRE) of the International Satellite Cloud Climatology Program. A time series of the humidity structure-function parameter Cq2 derived from Doppler wind profiler reflectivity data is compared to a concurrent time series of specific humidity q. Both q and its vertical gradient were calculated from rawinsonde data obtained from sondes launched within 500 m of the profiler. Time-height correlation analysis between log(Cq2) and log(∂q/∂z)2 shows that the two time series are highly correlated at and just above the inversion base, with r approximately equal to 0.7. The correlation is slightly lower in the free troposphere where r is about 0.5 (a value of r greater than 0.2 is significant at the 95% confidence level). There is also correlation between log(Cq2) and log(q), which is maximized...

Indirect sensing of plant canopy structure with simple radiation measurements

Abstract Easy and reliable estimates of plant canopy structure are needed for many applications but direct measurements are laborious and often destructive. A technique for indirectly sensing canopy structure from simple, manageable measurements of sunlight transmitted through the canopy is examined. The integral relationship between sunlight transmission and leaf area index and leaf angle distribution is discussed and a numerical inversion technique is described. An analysis of the kernels of the integral equation (relationship) reveals the relationship between the transmission measurement errors and the amount of canopy structural information contained in those measurements. Results of tests of the inversion technique with simulated transmission data characterizing that from a wide variety of plant canopies and with actual measurements in corn support the conclusions drawn from the information analysis. In all tests, with relative errors ranging from 0.5 to 4%, the true canopy type (angle distribution) is identified and the leaf area index is estimated to within 2% of the simulated inputs and to within 9% of the direct estimates in corn.

Acoustic propagation through anisotropic, surface‐layer turbulence

The effects of atmospheric wind and temperature fluctuations on acoustic signal variability is discussed, with emphasis on the effects of large‐scale turbulence (motions having size larger than or comparable to the integral length scale). Such large‐scale turbulence is anisotropic, is generated by both shear and buoyancy instabilities, and has structure that depends strongly on the meteorological conditions as well as the distance from the ground. Previous research in the atmospheric sciences literature regarding length scales and anisotropy is reviewed and incorporated into an acoustic propagation model. The model is based on a multiply scaled, six‐termed, sound‐speed correlation function. A second, simpler model, based on fluctuating curvature of the vertical wind profile, is also proposed. Both models are compared with experimental measurements of amplitude and travel‐time fluctuations obtained during the Rock Springs Tomography Experiment, which involved concurrent monitoring of acoustic fluctuations ...

Sound propagation in the atmospheric surface layer: Comparison of experiment with FFP predictions

Abstract This paper presents a set of acoustical and meteorological data from an outdoor sound propagation experiment. This experiment was done in a farm field near Rock Springs, Pennsylvania, on 7 July 1990. Meteorological and acoustical measurements were recorded simultaneously during six different times in the day. The meteorological measurements permitted determination of the sound speed profiles during each of the measurement sessions, using a method based on surface-layer similarity scaling. The acoustical measurements allowed precise determination of the relative sound pressure levels for a frequency range up to 3150 Hz at six different distances (66, 88, 125, 175, 250 and 350 m). The results show atmospheric conditions have an important effect on sound propagation. At medium and high frequencies, variations of the relative SPL have been measured at distances as short as 62 m. These effects increased with the distances so that variations as great as 30 dB have been measured during that day. Comparisons with the fast field program predictions are also presented, and amply demonstrate the accuracies of this model, especially for the downward refraction cases.

Two-point velocity statistics over Lake Ontario

Cross-spectra between wind speeds on several masts in Lake Ontario have been analyzed. As previously predicted, coherence over water (small intensity of turbulence) between wind speeds measured on masts lined up with the wind appears to be larger than over land, and increases with decreasing Richardson numbers. As a result, in cold air over warm water, wind speed fluctuations are well predictable from upstream measurements. For large angles between the anemometer line and wind, the difference between coherence over land and water disappears. Furthermore, there is no significant difference in vertical coherence between water and land. When the wind is parallel to the anemometer line, small ‘eddies’ travel, in agreement with Taylors hypothesis, with the local mean wind speed. Large eddies travel significantly faster. Vertical phase delay increases with increasing Richardson number.

Atmospheric scattering for varying degrees of saturation and turbulent intermittency

Atmospheric turbulence is inherently inhomogeneous and intermittent. Short periods of high activity are embedded in longer periods of relative calm. Local spatial and temporal changes in sound speed associated with this intermittency increase the likelihood of measuring large values of scattered acoustic signals. Previous work successfully predicted the probability density functions (pdfs) of fully saturated, scattered signals measured within an acoustic shadow zone [Wilson et al., J. Acoust. Soc. Am. 99, 3393-3400 (1996)]. The more general case of incompletely saturated scattering is considered in this paper; using the Rice-Nagakami distribution a theory is developed. The predicted intensity pdf has two free parameters: one to describe the degree of intermittency and a second for the degree of saturation. For validation purposes, outdoor propagation measurements were made over a flat, hard ground at ranges of 146-283 m and at frequencies of 50-540 Hz. The saturation parameter was determined from the acoustic data and also estimated from the turbulence conditions. The degree of saturation increased with frequency, and measured intensity pdfs were found to be in excellent agreement with the theory.

Multichannel Microwave Radiometric Observations at Saipan during the 1990 Tropical Cyclone Motion Experiment

Abstract To estimate mesoscale variations in integrated water vapor, cloud liquid water, and temperature in a tropical region, multiwavelength microwave radiometric observations were carried out over a seven-week period on the island of Saipan during the 1990 Tropical Cyclone Motion Experiment. Methods to combine radiometric and ceilometer measurements were developed to improve both the retrieval accuracies of integrated water vapor and liquid water. The rms difference between the retrieved and radiosonde-measured integrated water vapor was 6% relative to the mean. Compared to radiosondes the temperature profiles retrieved using a linear statistical inversion technique were accurate to 1.28°C. However, since the radiometric temperature profiles were no more accurate than profiles obtained from climatology, the surface-based radiometer provided essentially no new information regarding the environmental temperature profiles.

The ‘MESOGERS 84’ experiment: A report

AbstractDuring the MESOGERS 84 experiment in SW France from September 10 to October 5, 1984, different aspects of the Atmospheric Boundary Layer over complex terrain were investigated using a regional meteorological network, two instrumented aircraft and a Doppler Sodar network.The main objective has been to understand:flow properties over inhomogeneous terrain;fluxes and inversion height behaviour from a regional point of view. A description of the experiment and of the different studies undertaken is given.

Natural temporal variability of atmospheric acoustic absorption coefficients

Abstract Since acoustic absorption coefficients depend upon environmental temperature and humidity, they vary in response to chaning atmospheric conditions on a variety of time scales. This natural temporal variability is often neglected in outdoor noise assessment work, where averaged meteorological data are used to estimate absorption. In this study the authors assess the validity of calculating absorption coefficients from meteorological data which have been averaged on various time scales. The results indicate that the use of annual mean meteorological data can result in frequency-dependent r.m.s. errors ranging from 20 to 50%. Seasonal averaging can result in r.m.s. errors of 20 to 30%. Even daily averages may be unsatisfactory: for some of the cases presented absorption changes by 200% during the course of a single day.

John C. Wyngaard: His Career in Boundary-Layer Meteorology

To celebrate the distinguished scientific career of John C. Wyngaard, a symposium (http:// ploneprod.met.psu.edu/news-events/john-c.-wyngaard-symposium) was held at the Pennsylvania State University, State College, during 24–25 June 2010. From the original 78 oral presentations, this special issue contains 13 articles on topics that are closely related to John’s main interests in atmospheric turbulence and boundary-layer processes. Below we introduce the articles included in the Special Issue, followed by a brief description of John’s professional career. In the atmospheric surface layer, abrupt changes in the fluctuations of scalars in particular are ubiquitous. Such changes are evident as ramp structures occurring at frequencies that increase with wind shear. Shapland et al. (Parts I and II) report on a new approach to systematically identify ramp structures ordinarily observed as part of plant canopy-atmosphere scalar exchange. Since ramp structures meaningfully influence this exchange, the investigators propose approaches that identify the turbulence intermittency, duration, and their associated flux contribution using spectral analysis. The proposed method is ideally suited to investigate the surface-atmosphere scalar exchange over bare ground and short plant canopies. Improved Monin-Obukhov similarity functions are still required to derive surface sensible heat and water vapour fluxes from scintillometer measurements. Since most existing similarity relationships fail during extreme unstable and stable conditions, Li et al. propose new similarity relationships to determine the turbulent transport of sensible and latent heat over homogeneous surfaces. The relationships do not require the use of the friction velocity and provide reliable flux estimates for moderate to strong unstable conditions. Then, Salesky and Chamecki outline and test a scale-similarity model to estimate the sub-filter scale energy in large-eddy simulations (LES) of the atmospheric boundary layer (ABL). The model is derived from a stability-dependent relationship of the energy spectrum in the ABL, and accounts for the effects of buoyancy and mean wind shear as a function of the Monin-Obukhov stability parameter.