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Wayne M. Angevine and W. L. Ecklund

Abstract

With the use of simultaneous correction for radial wind, the accuracy of radio acoustic sounding systems for the measurement of temperature has been substantially improved. The temperature accuracy can now be affected by a number of factors that have been considered negligible in previous work. This paper describes two types of errors, those due to atmospheric effects and those due to approximations in the temperature retrieval equation. The errors are examined in a set of convective boundary layer RASS and radiosonde data. In the category of atmospheric effects, two errors are computed. The first is caused by a range error due to the gradient of signal strength. This range error is newly proposed and is approximately 0.05°−0.1°C. The second is an error due to wind and turbulence of about 0.1°C. Commonly used approximations for factors in the retrieval equation contribute errors of a few tenths of a degree Celsius. A significant difference remains after these two corrections have been applied to the sample data.

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K. S. Gage, C. R. Williams, and W. L. Ecklund

Uncertainty in the magnitude and distribution of diabatic heating associated with precipitating cloud systems is one of the major factors giving rise to uncertainty in the simulation of large-scale atmospheric circulations in numerical models of the atmosphere. A major international effort is under way to develop an improved parameterization of the hydrological cycle within numerical models. Progress will require better observations of the distribution of the diabatic heating associated with cloud systems in the Tropics. In this paper new observations are presented demonstrating the potential of UHF profilers for diagnosing the vertical structure of convective systems in the Tropics. These preliminary results indicate that while mesoscale convective systems are prevalent in the Tropics there are important contributions to rainfall from smaller-scale warm rain systems that do not extend above the freezing level. They also show that extensive regions of upper-tropospheric precipitating clouds often exist at times when no rain is detected at the surface. These observations provide important information that should prove useful in developing improved methods for estimating precipitation from satellite observations.

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O. D. Nastrom, W. L. Ecklund, and K. S. Gage

Abstract

Radars that can make wind measurements in the clear air are expected to play an increasing role in meteorological observing systems in the future, especially for horizontal wind measurements. This paper considers the prospects for using these radars, which are sometimes called wind profilers, to also measure the large‐scale vertical velocity. Unfortunately, all radars for which vertical velocity data are available at this time are located in or near mountains, where standing lee‐wave effects often make the data representative of only small‐scale features. Confining attention to those times when lee wave effects are not expected, case‐study comparisons of the existing radar data with indirectly computed synoptic‐scale motions suggest that time averaged radar data are representative of large‐scale features smaller than the synoptic scale, perhaps more aptly termed subsynoptic‐scale features. Results from a three‐station radar network in France show that the time‐averaged vertical velocities are usually nearly the same at all stations, although there are some differences, and suggest that the spatial scale of the flow features they represent is greater than 50 km. Over a long-term average, the net influence of lee wave effects at mountain sites is small, and radar measurements appear to be useful for climatological studies of vertical velocity in large‐scale circulation systems.

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B. B. Balsley, W. L. Ecklund, and D. C. Fritts

Abstract

Recently published data (Ecklund and Balsley) describing VHF radar echo characteristics from the Arctic mesosphere and lower thermosphere show a remarkable seasonal dependence of both the echo height and echo intensity: during the three-month period around the summer solstice, intense and nearly continuous echoes are returned from a narrow (±2 km half-power) region centered at 86 km; during the remainder of the year, however, the echoes are much weaker, more sporadic and occur at a much lower altitude (70 km ± 9 km). In this paper, we present additional data that suggest that the summer echoes are primarily the result of shear instability of low-frequency (tidal) motions in the region of high stratification above the Arctic summer mesopause, while the winter echoes arise from the nonlinear breakup of upward-propagating gravity waves.

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W. L. Ecklund, D. A. Carter, and B. B. Balsley

Abstract

In this paper we describe a boundary layer radar recently developed at NOAA's Aeronomy Laboratory. This radar extends wind profiler technology by using a small, relatively inexpensive radar to provide continuous, high-resolution wind measurements in the first few kilometers of the atmosphere. Although the radar was developed for use in a “hybrid” mode with existing 50 MHz profilers in the tropical Pacific, the system can equally well be a stand-alone device to study boundary layer problems.

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W. L. Ecklund, K. S. Gage, B. B. Balsley, R. G. Strauch, and J. L. Green

Abstract

During March 1981 the Sunset and Platteville VHF clear-air radars located in Colorado to the east of the continental divide observed vertical winds continuously over a three-week period. The vertical winds at these locations contain fluctuations with periods from a few minutes to several hours and with magnitudes ranging up to a few meters per second. The Sunset radar, which is located in the foothills, observed systematically larger vertical velocities than the vertical velocities observed by the Platteville radar, which is located on the plains, some 60 km to the east. Although periods of enhanced vertical wind activity were observed to occur at the same time at both sites, attempts to correlate vertical wind structures over the two sites in detail were generally not successful.

The magnitude of vertical velocity fluctuations seen by both radars show large day-to-day variations with “active” periods alternating with “quiet” periods. An examination of upper level maps reveals that the occurrence of active and quiet periods are linked to the large-scale wind field. During the March experiment the magnitude of the vertical velocity variance was well correlated with the 500 mb zonal (west) wind.

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DA Carter, W.L. Ecklund, K.S. Gage, M. Spowart, H.L. Cole, W.F. Dabberdt, and J. Wilson

The Aeronomy Laboratory of the National Oceanic and Atmospheric Administration and the Atmospheric Technology Division of the National Center for Atmospheric Research are jointly developing Integrated Sounding Systems (ISS) for use in support of TOGA (Tropical Ocean Global Atmosphere) and TOGA COARE (Coupled Ocean–Atmosphere Response Experiment). Some of the ISS units will have to be operated on research ships during TOGA COARE's intensive observing period in late 1992 and early 1993. The greatest technical challenge in adapting the ISS to shipboard use is to stabilize the UHF wind profiler that is an integral part of the ISS. In June 1991 a UHF wind-profiling Doppler radar was installed on a stabilized platform aboard the NOAA research vessel Malcolm Baldrige on an eight-day cruise in the Atlantic Ocean. The wind profiler was gyrostabilized and profiler winds were corrected for ship motion utilizing the Global Positioning System. During the eight days at sea, CLASS (Cross-Chain LORAN Atmospheric Sounding System) and OMEGA Sounding System balloons were launched onboard ship for wind profile comparisons. Results of the comparisons show excellent agreement between wind profiles, with an rms difference of about 1 m s−1 in wind speed.

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K. S. Gage, W. L. Ecklund, A. C. Riddle, and B. B. Balsley

Abstract

The magnitude of backscattered power observed at vertical incidence by a VHF radar is related to atmospheric stability in accordance with the Fresnel scattering model. Utilizing a modified Fresnel scattering model, we can determine tropopause height objectively from the observed vertical profile of backscattered power. The method is tested with observations of the Alpine Experiment (ALPEX; France), Platteville, Colorado and Poker Mat, Alaska radars taken since 1979. Using 750 m resolution the tropopause is found to be within a few hundred meters of the tropopause determined from nearly simultaneous radiosonde observations and using 2.2 km resolution the tropopause is found to be within about 600 m. Furthermore, radar-determined tropopause heights can be automatically scaled from existing records, or even routinely determined on-line.

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R. R. Rogers, W. L. Ecklund, D. A. Carter, K. S. Gage, and S. A. Ethier

A small UHF radar wind profiler was operated over a 40-day period during the summer of 1990 at a site on the windward coast of the island of Hawaii. It provided continuous measurements of winds up to the height of the trade-wind inversion, which varied in altitude from about 2 to 4 km during the course of the experiments. The inversion was readily discernible in the data as an elevated layer of high reflectivity, caused by the sharp gradient of refractive index at that level. With a wavelength of 33 cm, the profiler has about the same sensitivity to light rain as to moderately reflective clear air. The data have provided unexpected information on rain development, wave motions on the inversion, sustained vertical air motions at low levels, and interactions between convection and the inversion echo. This paper gives examples of some of the observations, indicating the wide range of applications of boundary-layer profilers.

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Wayne M. Angevine, S. K. Avery, W. L. Ecklund, and D. A. Carter

Abstract

A 915-MHz boundary-layer wind profiler radar with radio acoustic sounding system (RASS) capability has been used to measure the turbulent fluxes of heat and momentum in the convective boundary layer by eddy correlation. The diurnal variation of the heat flux at several heights between 160 and 500 m above ground level and values of the momentum flux for 2-h periods in midday from 160 to 1000 m are presented, as well as wind and temperature data. The momentum flux is calculated both from the clear-air velocities and from the RASS velocities, and the two results are compared.

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