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Peter H. Hildebrand

A width-height measure of cloud shape is used to investigate possible effects of seeding summer cumulus clouds with silver iodide. Evidence is presented to show that seeding may cause decreases in the width-height ratios of cloud echoes and that these effects may extend beyond the region of silver iodide transport.

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Peter H. Hildebrand

Abstract

The attenuation of 5 cm radar in rain is investigated theoretically for stratiform and thunderstorm drop size distributions. An iterative attenuation estimation scheme is presented. The effects of attenuation on radar precipitation measurements and the capabilities of the attenuation estimation technique are considered for a variety of hypothetical storm sizes and errors in radar calibration, assumed temperature and assumed drop size distribution.

This study indicates that 5 cm radar is an adequate precipitation measuring radar for storms under about 50 dBZ, and that if calibrated correctly and used with the iterative attenuation correction scheme, the 5 cm radar can function moderately well up to about 60 dBZ. Radar calibration accuracy is seen to be a limiting criterion for attenuation correction. The results of this study point out the need for raingages in most situations requiring accurate rainfall measurement.

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Peter H. Hildebrand

Abstract

A new technique for the experimental study of atmospheric turbulent diffusion is presented and applied to the study of diffusion in the lower 3 km of a cloudless summer atmosphere. This new technique makes use of Doppler radar and of radar reflecting chaff. The design and operation of a new chaff releasing device is presented. This device makes possible the creation of vertical columns of chaff through several kilometers of the atmosphere. The chaff is used to model atmospheric contaminants in diffusion measurements and as an air motion tracer in Doppler studies of turbulent air motions.

The shear is observed to have a strong effect on horizontal turbulent diffusion, with the diffusion rates frequently being an order of magnitude greater in the direction of the shear than across it. The strong shear enhancement of horizontal diffusion is interpreted to imply shear-generated eddies which are oriented by the shear. Empirical equations are presented which relate the apparent eddy diffusivity, the pollutant cloud size and the time of diffusion. Three distinct periods of chaff cloud growth are seen with the first period being due to small-scale turbulence, the second to energetic shear-induced turbulence of particular orientation, and the third to slower growth as the cloud size surpasses the maximum eddy size. The observed maximum growth rates agree well with the predictions of the shear diffusion theory.

The frequently used Gaussian model of turbulent diffusion is compared to the observations and is seen not to hold well in sheared atmospheric situations. The observed concentration distributions are more peaked than the Gaussian, particularly at times when the maximum turbulent eddy size would be expected to be large.

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Peter H. Hildebrand

Abstract

This paper describes a newly observed form of shear-parallel rainbands that were observed on 18 February 1993 in Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment using the NCAR ELDORA airborne Doppler radar. Interesting features of these rainbands included their rapidly changing and highly cellular nature, their complicated and layered kinematic structure, and the effects of the rainbands on the tropical environment. In their lower levels, the rainbands consisted of east–west elongated clusters of small convective cells, aligned with the z = 0–5 km shear vector. Strong low-level convergence, inflow from the north, and enhanced westerly winds extended from the surface up to 5 km. Much of this air was vented out of the storm at midlevels, exiting to the north in the 0°C stable layer at z = 5–7 km. This midlevel outflow fed an area of widespread, stratiform precipitation lying to the north of the rainband. The cold pool outflow emanating from this area of precipitation then fed southward, back into the north side of the rainband. This recycling of air spelled the eventual demise of this stationary rainband. Within the layer below 0°C, the westerly winds were strongly enhanced above ambient levels; this enhancement appears related to the combined effects of the low-level convergence, the release of latent heat in the rainbands, and the capping effects of the 0°C stable layer.

Over a 2-h period, the rainband was observed to remain fixed in location, expanding in north–south extent, and then to decay. Subsequently, a second rainband formed about 50 km to the south of rainband 1 and grew to exhibit features similar to rainband 1. The initial development of rainband 3 was later observed, about 50 km farther south of rainband 2. Radar and aircraft in situ data are used to investigate the structure and discrete propagation of these rainbands and to relate the speed of propagation to cold pool flow. The radar data also measure the increase in the westerlies and relate the observed increases in westerlies to the passage of the rainbands. These observations are in confirmatory agreement with the hypothesis, by others, that the origin of tropical westerly wind bursts is related to mesoscale convective processes in the tropical atmosphere.

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Peter H. Hildebrand and John McCarthy

A workshop was held at the National Center for Atmospheric Research (NCAR) during February of 1982 to consider the scientific needs for research aircraft in the next decade and the impacts of these needs upon the fleet of aircraft that the Research Aviation Facility (RAF) supports for the atmospheric sciences research community. The workshop was attended by a group of atmospheric scientists who represented the major research interests supported by RAF. The attendees discussed scientific priorities for research in the next 10 years, the types of new instrumentation expected within the next decade, and the operational requirements and aircraft fleet that would be required to serve these scientific goals.

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Peter H. Hildebrand and Bernice Ackerman

Abstract

The influence of an urban area on the turbulent convective boundary layer is studied using an instrumented aircraft. Data from three undisturbed days with low wind speeds are presented to describe the urban effects on turbulent profiles of heat, moisture, momentum and turbulent intensity. The profiles in the urban Area are observed to be considerably different than the rural. The urban area substantially enhances the surface heat flux, hence the vertical component of turbulence. The enhanced urban vertical turbulent intensifies extend through the urban boundary layer, resulting in stronger entrainment through the urban inversion than the rural. This, in turn, results in enhanced vertical fluxes of temperature, moisture and stress at the urban inversion. The effects of the urban roughness on boundary-layer structure are observed to be small in comparison with the effects of urban heating for the light wind cases studied.

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Peter H. Hildebrand and Cynthia K. Mueller

Abstract

This is the first of two papers which evaluate the capabilities of airborne Doppler radars for atmospheric sciences research. In Part I, we will evaluate the collection and analysis of dual-Doppler data using dual airborne Doppler data, and using combinations of airborne and ground-based Doppler data. In Part II, we will evaluate the capability of the airborne Doppler radar to collect predominantly vertically pointing data as a part of a triple-Doppler radar analysis.

This paper presents an evaluation of dual-Doppler radar observations of a convective storm. The storm was observed simultaneously by airborne and ground-based Doppler radars. Comparative analyses are performed which make use of dual airborne observations, dual ground-based observations and combined airborne and ground-based observations of the storm. Analysis of these observations shows that airborne Doppler radar produces observations with an accuracy similar to that provided by ground-based. It is suggested that the process of multiple-Doppler analysis produces effects that contribute strongly to the observed differences between the airborne and ground-based analyses. It is also noted that temporal effects, predominantly storm evolution, produce differences between the airborne and ground-based Doppler analyses. The airborne Doppler is seen to be a viable tool for atmospheric sciences research.

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Cynthia K. Mueller and Peter H. Hildebrand

Abstract

This is Part II of a paper dealing with the capabilities and use of airborne Doppler radar to observe motions within storms. Part I deals with dual-Doppler analyses of convective storm structure, using airborne and combinations of airborne and ground-based Doppler radars. Part I also discusses basics of airborne Doppler radar data processing. This paper discusses the capabilities of the airborne Doppler radar when used with ground-based radars in a triple-Doppler radar group. The data analyzed consist of ground-based dual-Doppler radar observations of a severe, microburst-producing storm, and airborne Doppler radar observations of the same storm. Comparisons of a ground-based dual-Doppler analysis with the triple-Doppler analysis support the validity of the airborne data. It is concluded that airborne Doppler radar data are likely to make major contributions to kinematic analysis of storm structure and related research topics. Guidelines are given that describe the area within which airborne Doppler data can reliably be added to the ground-based dual-Doppler data.

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Peter H. Hildebrand and R. S. Sekhon

Abstract

A method is described for the objective determination of the noise level in Doppler spectra. The method makes use of the physical properties of white noise and is suitable for automatic computation. It is shown that the method produces reliable results when used in conjunction with the lower-bound method of estimating vertical air velocities.

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Peter H. Hildebrand, Neil Towery, and Michael R. Snell

Abstract

Techniques of measuring area-mean convective rainfall over small areas (<2000 km2) are investigated using data from several gage-radar rainfall measurement studies. These data suggest that for low gage densities (≲1 gage per 250–300 km2) and for some climates (e.g., Illinois) that gage-radar area-mean convective rainfall measurements may be more accurate than gage-only measurements. The same result was not supported by data from another climate, suggesting that changes in raincell size and rain evaporation rate may affect the results. Further studies are needed to verify these suggestions. The data indicate that radar adds little to gage measurement of mean areal convective rainfall for gage densities of ≳1 gage per 100–200 km2.

Use of a space-variable gage adjustment of radar data is seen to be preferable to use of an averaged single adjustment factor, except in cases involving extrapolations of the space-variable adjustment factor. For such cases, e.g., over large lakes or outside of gage networks, use of the averaged single adjustment factor may be preferable.

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