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Yong Han and Dennis W. Thomson

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.

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D. Keith Wilson and Dennis W. Thomson

Abstract

Acoustic tomography is proposed as a method for monitoring near-surface atmospheric temperature and wind velocity fields. Basic issues relating to the feasibility and implementation of atmospheric tomography are discussed. Among these issues are the causes of fluctuations in acoustic signals propagated through the atmosphere, appropriate spatial dimensions of an array, signal detection and processing techniques, mathematical inverse techniques and their numerical implementation, and whether or not tomography m provide measures of dynamical variables of interest to atmospheric scientists. Surface-layer, horizontal-slice tomography was implemented experimentally, with an array of three sources and seven receivers distributed over a region approximately 200 m square. Travel-time fluctuations at the receivers were used to reconstruct the temperature and wind fields with about 50-m resolution in the horizontal plane.

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Sven-Erik Gryning and Dennis W. Thomson

Abstract

For an ongoing elevated-source, urban-scale tracer experiment, an instrument system to measure the three-dimensional wind velocity and the turbulent sensible heat flux was developed. The wind velocity was measured with a combination of cup anemometer, propeller (vertical) and vane sensor. The temperature fluctuations were derived using a uniquely referenced thermocouple pair. The temperature sensor is described in detail. The wind sensors have already been described elsewhere.

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Allen B. White, C. W. Fairall, and Dennis W. Thomson

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 C q 2 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(C q 2) 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(C q 2) and log(q), which is maximized at an offset in height between the two instruments.

Closer analysis of a short-lived clearing event shows locally reduced values of C q 2 in a region of enhanced ∂q/∂z. This apparent paradox can be explained by noting the absence of enhanced entrainment associated with cloud-top radiative cooling. The combined wind profiler-rawinsonde datasets were also used to estimate the entrainment velocity w e for clear and cloudy conditions. An average value of w e equal to 0.38 cm s−1 was obtained for cloudy conditions; for the clear case a value of 0.13 cm s−1 was obtained.

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C. W. Fairall, A. B. White, and Dennis W. Thomson

Abstract

We examine the consequences of using a vertical wavenumber spectral model to describe variations of vertical profiles of atmospheric variables (horizontal and vertical wind, temperature, and other scalars) about a mean profile. At high wavenumbers the model exhibits a wavenumber to the -3 dependence, which is characteristic of a continuum of internal gravity waves whose amplitudes are controlled by a breaking process.

By employing a random phase between wavenumber amplitude components, a reverse fourier transform of the spectrum yields simulated profiles of velocity and thermal variability as well as shear and Brunt–Väisälä frequency variability. Individual components of the vertical shear of the horizontal wind and the Brunt–Väisälä frequency exhibit Gaussian distributions; the square of the magnitude of the shear exhibits a Rice–Nakagami distribution. Assuming regions with Ri < 0.25 are turbulent, we can examine a number of aspects of the occurrence of clear-air turbulent breakdown in the stratified free atmosphere. For a typical tropospheric condition, the average turbulent layer thickness turns out to be about 35 m and about 20% of the troposphere appears to be actively turbulent. The majority of the turbulent layers appear to be due to autoconvective overturning instead of Kelvin-Helmholtz dynamic instability. Computations of profiles of the refractive index structure function parameter, Cn2, and the rate of dissipation of turbulent kinetic energy, ε, are found to be quite sensitive to the assumptions of the relationship of turbulent length scale to layer thickness, the growth of turbulent layers after breakdown, and the threshold sensitivity and sampling strategy of measurement systems (e.g., clear-air radar).

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Gregory S. Forbes, Dennis W. Thomson, and Richard A. Anthes

Abstract

An interesting ice storm of moderate severity occurred along the east slopes of the Appalachians on 13–14 January 1980. Though surface temperatures were initially below freezing in most of this region, objective guidance indicated that large-scale warm would render the atmosphere conducive to rain. Warm advection did occur above about 900 mb, but below this level warm advection was prevented by a cold ~ shaped ridge of high pressure which became entrenched along the east slopes. Temperature in the lowest 0.5–1 km remained below freezing and an ice storm resulted.

This case study documents the evolution of the wedge ridge and the temperature and wind fields associated with it. Comparisons are made between the evolution of these fields within the quasi-stationary wedge ridge (a weather regime known as cold-air damming and their evolution during the preceding period, when the pressure ridge was progressing eastward across the Midwest The processes controlling the charges of temperature in these regimes are analyzed; cold advection and upslope flow maintain the cold dome. Cross sections are used to present detailed analyses of the vertical structure and evolution of the temperatures and winds within the damming region. Interesting features include the development of an “extended coastal front”—the sloping inversion separating the trapped cold dome from the warm onshore flow above, a jet parallel to the mountain at low levels, and an enhanced flow over the mountain near its crest. Apparently due to the lack of vertical resolution sufficient to capture such features operational numerical models exhibited substantial errors in this case.

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Brian D. Pollard, Samir Khanna, Stephen J. Frasier, John C. Wyngaard, Dennis W. Thomson, and Robert E. McIntosh

Abstract

The local structure and evolution of the convective boundary layer (CBL) are studied through measurements obtained with a volume-imaging radar, the turbulent eddy profiler (TEP). TEP has the unique ability to image the temporal and spatial evolution of both the velocity field and the local refractive index structure-function parameter, 2n. Volumetric images consisting of several thousand pixels are typically formed in as little as 1 s. Spatial resolutions are approximately 30 m by 30 m by 30 m.

CBL data obtained during an August 1996 deployment at Rocks Springs, Pennsylvania, are presented. Measurements of the vertical 2n profile are shown, exhibiting the well-known bright band near the capping inversion at z i, as well as intermittent plumes of high 2n. Horizontal profiles show coherent 100-m-scale 2n and vertical velocity (w) structures that correspond to converging horizontal velocity vectors. To quantify the scales of structures, the vertical and streamwise horizontal correlation distances are calculated within the TEP field of view.

To study the statistics and scales of larger structures, effective volumes larger than the TEP field of view are constructed through Taylor’s hypothesis. Statistics of 2n and w time series are compared to an appropriately scaled large eddy simulation (LES). While w time series comparisons agree very well, the LES 2n predictions agree only with some of the measured data. Finally, the scales of 2n structures in the TEP time series measurements are calculated and compared to the scales in the LES spatial domain. Good agreement is found only near the capping inversion layer, the area of largest structures. This study highlights the unique capabilities of the TEP instrument, and shows what are believed to be the first statistical comparisons of measured 2n data with LES derived results.

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