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D. K. Lilly and P. J. Kennedy

Abstract

Analysis is presented of data obtained from instrumented aircraft flying in a mountain wave of moderate amplitude west of Denver, Colo., on 17 February 1970. Emphasis is placed on determination of the downward flux of westerly momentum generated by the wave, for which accurate measurements of vertical velocities on scales of order 50 km are essential. Three different methods are applied and compared: direct aircraft measurement, using vanes and an inertial platform; evaluation from the steady-state equation for conservation of potential temperature; and integration of the steady-state continuity equation. Each method produces errors, but by combining the results of the three methods a profile is obtained which agrees. fairly well with a steady-state theoretical prediction. An important side result is the discovery that gust-probe equipment is apparently not necessary for the direct aircraft measurement of wave momentum flux, but an inertial platform or similarly stable attitude reference level is essential.

A region of severe turbulence at 100 mb is found to he associated with the source of most of the downward wave momentum flux. Measurements of the loss of total energy along isentropes are found to he consistent with kinetic energy losses estimated from momentum flux divergence and with energy dissipation estimated from inertial-range aircraft measurements of the turbulent energy spectrum.

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M. A. Shapiro and P. J. Kennedy

Abstract

Radar altimeter and inertial wind velocity measurements from the NCAR Sabreliner research aircraft are combined with Defense Mapping Agency high-resolution (∼1 km) digital terrain height maps to obtain height analyses and geostrophic and ageostrophic winds for upper-level jet stream systems situated over land. Results show that given an accurate geographical location of the aircraft over an accurately mapped topography, it is possible to diagnose geostrophically and convectively forced ageostrophic motions over irregular topography.

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M. A. Shapiro and P. J. Kennedy

Abstract

Radar altimeter and inertial navigation wind velocity measurements from research aircraft are shown to give the ageostrophic wind velocity distribution within a jet stream system near the tropopause. Results from one case study reveal cross-height-contour ageostrophic winds exceeding 20 m s−1, and centripetal-acceleration-induced ageostrophic winds approaching 100 m s−1 in the exit region of a jet streak which has entered into the trough region of a synoptic wave. Diagnostic calculations are made of the horizontal velocity divergence and the geostrophic and ageostrophic components of the observed wind velocity acceleration. The results suggest caution in the application of near-straight jet-streak dynamics and the geostrophic momentum approximation to jet-streak systems embedded within large-amplitude synoptic wave regimes.

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P. J. Kennedy and M. A. Shapiro

Abstract

A series of aircraft penetrations into clear air turbulence zones within upper level fronts is presented. Heat and momentum diffusivities am estimated with a view toward obtaining more realistic parameterizations for numerical models. Two passes through severe turbulence are discussed in detail.

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P. J. Kennedy and M. A. Shapiro

Abstract

High sample rate aircraft data in a zone of moderate turbulence are analyzed to determine a turbulent energy budget. Combination of the Reynolds stress, buoyancy, and frictional dissipation terms produces a good balance. A further look at the Reynolds stress component shows the relation of the turbulent zone growth and decay with the synoptic situation. The need is demonstrated for including turbulent mixing processes in modeling fronts.

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M. A. Shapiro and P. J. Kennedy

Abstract

Two direct observations of wave motion associated with fronts by well instrumented meteorological research aircraft are presented. Wave cloud photographs corroborate the analysis of the temperature and three-dimensional air motion. Phase relationships among temperature and velocity components and a calculation of the momentum flux show, in one case, some evidence of trapped horizontally propagating waves.

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E. N. Brown, M. A. Shapiro, P. J. Kennedy, and C. A. Friehe

Abstract

The aeronautical use of electronic altimeters is to measure the absolute clearance of an aircraft above the earth's surface. In the support of atmospheric research, accurate high-range altimeters, in conjunction with accurate static pressure and navigation data, also can provide a means for measuring the heights of constant-pressure surfaces. From the derivatives of the measurements, surface slopes and dynamical quantities such as the geostrophic wind may be obtained. Although the technique is easiest over oceans or large bodies of water, it can be successfully used over land, if detailed terrain heights are known.

This paper describes the operational and research use of a high-altitude pulse-type radar altimeter system installed on the NCAR Sabreliner for jet stream research. An error analysis for “D-value”, derived from radar altitude and pressure measurements, gave an estimated error of ±6.0 m, which surpasses measurements from conventional balloon soundings or satellite-derived height analyses. For a case study of jet stream dynamics, the above error in D-value corresponded to an error of ±5% in the computed geostrophic wind.

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V. N. Bringi, P. C. Kennedy, G.-J. Huang, C. Kleinkort, M. Thurai, and B. M. Notaroš

Abstract

Comprehensive analysis of an unusual graupel-shower event recorded by an S-band polarimetric radar and two optical-imaging surface instruments is presented. The primary radar characteristic was negative differential reflectivity Z dr values along a vertical column. During the afternoon hours of 16 February 2015, a sequence of three showers that were composed primarily of small (8–15-mm diameter) graupel affected the ground instrumentation site that was established for the Multi-Angle Snowflake Camera and Radar (MASCRAD) experiment in the high plains of Colorado. While these showers passed the instrumentation site, the CSU–CHILL radar conducted high-time-resolution (~2.5-min cycle time) range–height indicator (RHI) scans from a range of 13 km. The RHI data show that the negative Z dr values extended vertically through much of the reflectivity cores, implying that the reflectivity-weighted mean axis ratios of the graupel particles in this event remained somewhat prolate throughout their lifetime. To be specific, the cores of the convective showers only extended to heights of ~3.5 km AGL and had fractionally negative (from ~−0.3 to −0.7 dB) Z dr levels in those cores. Particle-image data obtained by the MASC system and by a collocated 2D video disdrometer measured the diameters, shapes, and fall speeds of the graupel particles as they reached the surface. The graupel particles were found to be primarily of the lump type with a slightly prolate mean shape (especially for the larger-diameter particles). Microwave backscatter calculations confirm that the graupel-particle shape and orientation characteristics are consistent with the observed slightly, but consistently, negative Z dr values.

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M. F. Wilson, A. Henderson-Sellers, R. E. Dickinson, and P. J. Kennedy

Abstract

The soils data of Wilson and Henderson-Sellers have been incorporated into the land-surface parameterization scheme of the NCAR Community Climate Model after Dickinson. A stand-alone version of this land-surface scheme, termed the Biosphere-Atmosphere Transfer Scheme (BATS), has been tested in a series of sensitivity experiments designed to assess the sensitivity of the scheme to the inclusion of variable soil characteristics. The cases investigated were for conditions designed to represent a low-latitude, evergreen forest; a low-latitude sand desert; a high-latitude coniferous forest; high-latitude tundra; and prairie grasslands, each for a specified time of year. The tundra included spring snowmelt and the grassland incorporated snow accumulation. The sensitivity experiments included varying the soil texture from a coarse texture typical of sand through a medium texture typical of loam to a fine texture typical of clay. The sensitivity of the formulation to the specified total and upper soil column depth and the response to altering the parameterization of the soil albedo dependence upon soil wetness and snow-cover were also examined. The biosphere-atmosphere transfer scheme showed the greatest sensitivity to the soil texture variation, particularly to the associated variation in the hydraulic conductivity and diffusivity parameters. There was only a very small response to the change in the soil albedo dependence on wetness and, although the sensitivity to the snow-covered soil albedo via the response to roughness length/snow-masking depth was significant, the results were predictable. Changing the total depth of the active soil column produced a much smaller response than altering the depth of the upper soil layer, primarily because the degree of saturation of the upper layer plays an important role in the parameterized hydrology. Soil moisture responses can also be initiated by changes in vegetation characteristics such as the stomatal resistance through changed canopy interaction which modify the radiation and water budgets of the soil surface. Overall, this land-surface parameterization scheme shows considerable sensitivity to the choice of soil texture. This sensitivity seems to be at least comparable to that involving changes in vegetation characteristics and it may be more important because soil characteristics are very poorly known at a resolution appropriate for global climate models.

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Francesc Junyent, V. Chandrasekar, V. N. Bringi, S. A. Rutledge, P. C. Kennedy, D. Brunkow, J. George, and R. Bowie

Abstract

This paper describes the transformation of the Colorado State University–University of Chicago–Illinois State Water Survey (CSU–CHILL) National Radar Facility from a single-frequency (S band) dual-polarization Doppler weather radar system to a dual-frequency (S and X bands) dual-polarization Doppler system with coaxial beams. A brief history regarding the development of dual-wavelength radars is first presented. In the past, dual-wavelength measurements were used to detect hail using the dual-wavelength ratio defined as the ratio of intrinsic (or attenuation corrected) X-band reflectivity to the S-band reflectivity. Departures of this ratio from unity were taken to indicate the presence of hail, produced by Mie scattering at the shorter wavelength by hail. Most dual-wavelength radars were developed with attempts to match beams for S and X bands, which implies that the sample volumes for the two frequencies were essentially the same. The X-band channel of the CSU–CHILL radar takes a different approach, that of making use of the already existing dual-offset-fed antenna designed to give a 1° beamwidth at S band, resulting in an X-band beamwidth of approximately 0.3°, with very high gain. Thus, the X band provides about a factor of 3 more resolution than the S-band component while maintaining the same sensitivity as the S-band component. Examples of cold season and warm season data from the X-band and S-band radar components are presented, demonstrating the successful transformation of the CSU–CHILL radar into a unique multifrequency, multipolarization system. The new CSU–CHILL dual-wavelength, dual-polarization weather radar will serve as an important asset for the scientific community.

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