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William H. Jasperson and Gregory D. Nastrom

Abstract

Comparison between in situ aircraft observations of temperature and National Meteorological Center and Global Weather Central analysis fields of temperature is presented for a continental and oceanic flight route. The standard deviations of the temperature differences over several hundred flights are found to be 2.5 and 3.5°C for the continental and oceanic route, respectively. A bias towards warm temperatures of about 0.85°C for the analysis fields was found for the oceanic route. Only small differences are found between the NMC and GWC analysis field temperatures.

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Gregory D. Nastrom and David C. Fritts

Abstract

Aircraft measurements of winds and temperatures collected during the GASP program are used to study the effects of topography as a source of mesoscale variability. Variances of fluctuations at the mesoscale over rough terrain are enhanced up to nearly two orders of magnitude compared to nonsource regions in some cases and are frequently enhanced by an order of magnitude. The implications of these episodic enhancements of variances for the vertical transports of energy and momentum are considered in the framework of gravity wave theory. The observed flight data are used to estimate the momentum flux uw on several flight segments. Results show that the flux is generally negative with mean value −0.26 m2 s−2 and with magnitudes ranging up to −1.5 m2 s−2. Spectral analysis shows that the largest contributions to the net flux come from horizontal scales of ∼25 < λ x <60 km. Finally, the observed momentum fluxes are used to infer the anisotropy factor of gravity waves over rough terrain, which is found to be about 0.45.

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David C. Fritts and Gregory D. Nastrom

Abstract

We present studies of four cases of mesoscale variance enhancements of horizontal velocity and temperature due to frontal activity, nonfrontal convection, and wind shear. These data were obtained aboard commercial aircraft during the Global Atmospheric Sampling Program (GASP) in 1978 and 1979 and from the corresponding meteorological analyses and satellite imagery. Additional GASP data were used to permit a statistical assessment of the importance of various sources of enhanced variances. Our results, and those in a companion paper addressing the variance enhancements associated with topography, represent refinements of previous source analyses using the GASP dataset. Significant findings include mean variance enhancements of velocity and temperature due to convection and jet-stream flow ranging from ∼2 to 8 for 64-km and 256-km data segments, and enhancements for individual segments as high as ∼20 to 100. The mean 64-km variance enhancement for all variables and source types, relative to a quiescent background, was estimated to be 6.1. These results suggest a major role for localized sources in energizing the mesoscale motion spectrum at horizontal scales < ∼100 km, and correspondingly greater influences for such motions at greater heights.

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Anthony R. Hansen, Gregory D. Nastrom, Jason A. Otkin, and Frank D. Eaton

Abstract

The effect of deep convection on the intensities of gravity waves and turbulence during the summer at White Sands, New Mexico, is investigated using 50-MHz mesosphere–stratosphere–troposphere (MST) radar observations and surface weather reports. Radar data taken at 3-min intervals from the summers of 1991 through 1996 (with occasional gaps of varying length) are used to construct hourly means, medians, and standard deviations of wind speed, spectral width (σ2turb), and backscattered power calibrated as the refractivity turbulence structure constant (C2n). The hourly variance of the vertical velocity σ2w is used as an indicator of high-frequency gravity wave intensity. Surface observations taken near the radar site are used to identify periods marked by convection at or near the radar. During cases in which no convection is reported, the median hourly σ2w is nearly constant with altitude (about 0.04 m2 s−2 below and 0.03 m2 s−2 above the tropopause). Values of σ2w, C2n, and σ2turb are significantly enhanced from no-convection cases to thunderstorm cases. Largest increases are about 12 dB relative to the no-convection cases at about 11 km for σ2w, about 9.5 km for σ2turb, and about 7.5 km for C2n. The relatively lower height for the maximum of C2n is likely due to the influence of humidity advected upward during convection on the mean gradient of the refractive index. The probability density distributions of C2n and σ2turb near their levels of maximum enhancement are unimodal, with the modes steadily increasing with increasing proximity of convection. However, the probability density distribution of σ2w is bimodal in all instances, suggesting that there can be enhanced wave activity even when visible convection is not present and that the presence of a thunderstorm at the station does not necessarily indicate greatly enhanced wave activity.

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William H. Jasperson, Gregory D. Nastrom, Richard E. Davis, and James D. Holdeman

Abstract

A climatology of high-altitude cloud encounters using data obtained between 1975 and 1979 from commercial airliners participating in the Global Atmospheric Sampling Program (GASP) is presented. The statistics are based on three different measures of cloudiness derived from the GASP data set. This climatology depicts the seasonal, latitudinal and altitudinal variation in the cloudiness parameters, as well as differences in the high-altitude cloud structure attributed to cyclone- and convective cloud-generation mechanisms. A qualitative agreement was found between the latitudinal distribution of cloud cover derived from the GASP data and satellite-derived high-altitude cloud statistics available in the literature. Relationships between the three different measures of cloudiness and the relative vorticity at high altitudes, stratified by season, latitude and distance from the tropopause are also presented. In midlatitudes, for example, the average cloudiness, when stratified by the sign of the relative vorticity, exhibits a seasonal cycle with the 1argest differences occurring in the layer 0–1.5 km below the tropopause. Seasonal and latitudinal patterns can also be seen in the other cloudiness parameters.

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Patricia M. Pauley, Robert L. Creasey, Wallace L. Clark, and Gregory D. Nastrom

Abstract

This study examines the consistency between VHF horizontal wind measurements and those interpolated from routine objective analyses. First, the agreement between the two U components and between the two V components measured on opposing beams (here referred to as the beam-to-beam intercomparison) by the Flatland 49.8-MHz wind profiler is examined to determine the beam-to-beam consistency and relative precision of this radar. This part of the study demonstrates the ability of this technique to detect system problems affecting only one radar beam and provides a benchmark for comparison with radar systems operating near the Front Range of the Rockies and for the comparison in the second part of this study. This second comparison is between the Flatland observations and the spatially smooth winds from the National Meteorological Center's (NMC) regional objective analysis for July through November 1990. The location of the Flatland profiler near Champaign-Urbana, Illinois, is free of significant orographic features, in contrast to the proximity to the Colorado Rockies of many of the radars employed in earlier studies.

The beam-to-beam intercomparison is presented in terms of the mean and standard deviation of the differences between the measurements made on opposing beams. The Flatland difference standard deviations of about 0.8 m s−1 are roughly one-third of those for radars in the lee of the Rocky Mountains, reflecting reduced vertical velocities. However, the mean differences are approximately −0.25 m s−1 for both the U and V components, consistent with the tropospheric monthly mean downward motion of 2–6 cm s−1 indicated in the Flatland vertical beam measurements since its construction, including the period of this study. In fact, when the data were stratified into periods with and without precipitation based on estimates of latent heating from the NMC data, the precipitation periods showed standard deviations of about 1.3 m s−1, with mean differences two to three times that for nonprecipitation cases. This behavior is consistent with larger downward velocities during precipitation, whether from clear-air or hydrometeor scatterers. Thus, these vertical-motion biases, which the authors believe are of atmospheric origin (whether bulk motion or reflectivity effects), must be accounted for in long-term climatological studies.

Finally, for the Flatland–NMC comparison, 4-h averages of Flatland winds were chosen to better correspond to the spatially smooth NMC winds. The correlation coefficients, larger than 0.95, indicate very good agreement, but not as good as the 0.99 found in the beam-to-beam intercomparison. The larger 2.3 m s−1 difference standard deviations are similar to those found in studies comparing profiler and rawinsonde winds near the Front Range of the Rockies, indicating the applicability of the Taylor hypothesis implicit in this comparison of the 4-h temporally averaged Flatland winds and the spatially avenged NMC-analyzed winds. The consistency between these two datasets implies that increases in accuracy of objective analyses may result more from the increased time resolution of the profiler data rather than from an inherent increase in accuracy of the observations.

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