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Kenneth R. Hardy and Hans Ottersten

Abstract

Two types of convective patterns are observed with radar in the clear atmosphere. One pattern consistsof small thermal-like cells (Type I) which are in the order of 1-3 km in diameter and several hundredmeters in height. The cells may persist for 20-30 min. In plan view, the Type I cell echoes are doughnut-shaped; that is, they typically appear circular or elliptical and have echo-free centers. The structure ofthe cell and its time history are consistent with the view that the relative flow within the cell is upward inits center, and outward and possibly downward around its periphery. These clear-air cells are detectedregularly with ultra-sensitive radars at wavelengths >10 cm. The echoes are caused by scattering fromfluctuations in refractive index which are particularly marked at the cell boundary.

The other pattern is made up of clear air Be'nard-like convection cells (Type II) which are 5-10 km indiameter and 1-2 km in height. The centers of these cells are also echo-free. The overall pattern may persistfor up to 4 hr and individual Benard-like cells may have lifetimes of at least 30 min. The Benard-like patternis composed of several small cells organized around the circumference of the larger Type II cells. Thesesmall cells are probably established through the same air flow that generates Type I cells. Thus, it is expected that the Type II cell is characterized by cores of updrafts around its periphery and by downwardflow in its center. Echoes from the Type II Benard-like cells were observed on three different clear dayswith a 3.2-cm radar of moderate sensitivity. These echoes were caused by the scattering from an unusuallylarge number of insects. Benard-like clear air patterns have also been observed with sensitive 10-cm radarsby virtue of the scattering from refractive index fluctuations. The 10-cm radar observations confirm thatthe Type II pattern is composed of Type I convection cells organized around the periphery of the largerBenard-like cells.

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William H. Hooke and Kenneth R. Hardy

Abstract

Washington, D. C., microbarograph records for 18 March 1969 reveal gravity-wave-associated pressure oscillations which appear to be directly related to upper tropospheric wave structure observed at the same time with a Wallops Island 10-cm wavelength radar. The consistency between the two sets of data provides new observational support for a hypothesis of long standing in the microbarograph community; namely, that shear instability in the upper tropospheric flow is indeed the mechanism responsible for the generation of such waves. The comparison presented here suggests that microbarograph arrays might be useful adjuncts to future radar studies of upper tropospheric wave dynamics, supplying such wave parameters as phase velocity and wavelength in favorable cases. A closer examination of the radar data pertinent to this event reveals an apparent vertical wave phase variation, permitting a very approximate and somewhat uncertain estimate of the wave-associated vertical flux of horizontal momentum, which is found to be ∼4 dyn cm−2. While approximate, this illustrative calculation yields a value several times greater than the annual average flux at temperate latitudes, and since microbarograph data show such events to be fairly common wintertime phenomena, we are tempted to infer that wave generation by shear instability in the upper tropospheric air flow and the resulting vertical momentum transport may be an important element of the global atmospheric momentum budget. More extensive and conclusive studies are obviously indicated.

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Richard J. Reed and Kenneth R. Hardy

Abstract

Widespread and persistent clear air turbulence (CAT) occurred over the Eastern Seaboard of the United States between New York and South Carolina on 18 March 1969. The major synoptic features and a qualitative discussion of the factors contributing to the development of the large vertical wind shears associated with the turbulence are presented. The turbulent region in the vicinity of Wallops Island, Va., was probed with a NASA T-33 research aircraft and with sensitive radars. The clear air radar echoes and the most intense turbulence occurred principally within an upper level frontal zone of about 2 km depth which was produced by the confluence of two currents of widely different origin. The smoothed Richardson number was less than 1.0 throughout the zone and reached its lowest value of ∼0.25 in the region of strongest turbulence. Three distinct types of wave structures were evident in the clear air radar echoes. These were: 1) long sinusoidal arches moving at approximately the wind speed which were oriented in the direction of the wind and wind shear and which had wavelengths of 15–30 km and crest-to-trough amplitudes of nearly 2 km; 2) unstable waves or billows of about 1.6 km wavelength which were superposed on a portion of the long arches and were also oriented in the shear direction; and 3) braided wave-like patterns having a wave-length of ∼5 km and a crest-to-trough amplitude of more than 1 km which were oriented in the cross-wind (and cross-shear) direction.

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David Atlas, Kenneth R. Hardy, and Keikichi Naito

Abstract

A general analysis is made of the turbulent refractivity spectrum in and beyond the limiting microscale and a relation derived for its scattering reflectivity in either the back or bistatic directions. Radar reflectivity is computed as a function of wavelength for regions of CAT. The results are compared to the minimum detectable reflectivity of airborne radars having optimum state of the art characteristics at each wavelength. It is shown that the best radars now feasible can barely detect the most reflective CAT at 10 n mi (i.e., 1 minute warning). A 20-db improvement in sensitivity is required for detection of most CAT, which appears to be just attainable by pre-detection integration. The optimum wavelength to implement is 5–6 cm. The best radar at this wavelength will also detect circus clouds reliably. Whether detecting clouds or chaff a measure of the echo fluctuation (or Doppler) spectrum is required to identify the intensity of CAT. However, in the case of high altitude clear air echoes, there is an indication that the reflectivity in excess of some minimum threshold value is a sign of some degree of mechanical turbulence. It is also demonstrated that a ground-based forward-scatter link holds great promise for reliable CAT detection. A tentative quantitative classification of CAT severity is also proposed.

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Keith D. Hutchison, Kenneth R. Hardy, and Bo-Cai Gao

Abstract

The accurate identification of optically thin cirrus clouds in global meteorological satellite imagery by automated cloud analysis algorithms is critical to environmental remote sensing studies, such as those related to climate change. While significant improvements have been realized with the arrival of multispectral, meteorological satellite imagery, collected by NOAA's Advanced Very High Resolution Radiometer (AVHRR), difficulties can be encountered when attempting to make pixel-level cloud decisions over large and diverse geographic areas found around the globe. These problems are due, in part, to the effects of atmospheric attenuation on cloud spectral signatures, caused primarily by variations in water vapor, since the signatures of water vapor and optically thin cirrus are similar in the nighttime AVHRR infrared channels. In this paper, the authors describe an improved thin-cirrus detection technique that uses the brightness temperature differences between AVHRR channel 3 and channel 5 along with total integrated water vapor information. It is concluded that algorithms must accurately compensate for the impact of water vapor on cloud spectral signatures for enhanced detection of optically thin cirrus clouds in nighttime AVHRR imagery.

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Kenneth M. Glover, Roland J. Boucher, Hans Ottersten, and Kenneth R. Hardy

Abstract

The results of simultaneous studies of clear air turbulence (CAT) in the lower 15 km of the atmosphere by multi-wavelength radar, jet aircraft and special rawinsondes at the JAFNA radar facility at Wallops Island, Va., are reported. The most important finding is that for the particular aircraft and velocity used in these experiments, every clear air radar echo above 3 km is associated with aircraft reports of at least some perceptible degree of turbulence. Between the altitudes of 3 and 6 km, all CAT is detected by the radars; however, the ability of the radars to detect weak CAT decreases with increasing altitude and only the more intense turbulence is detected above 12 km. The indications are that strong CAT at high altitudes in the free atmosphere is generally associated with zones of increased refractive index variability and enhanced radar backscattering. Therefore, if radars of extreme sensitivity are employed, the useful range for CAT detection may be extended considerably and may possibly satisfy the requirements of an operational ground-based CAT detecting radar system. The vertical vector wind shear appears to be the most significant meteorological factor in specifying turbulent regions. A wind shear criterion ≥ 0.8 × 10−2 sec−1 applied to rawinsonde data specifies the presence or absence of turbulence correctly in 77% of all cases, including 100% of the cases involving CAT greater than light.

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