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Margaret A. LeMone and Lesley F. Tarleton

Abstract

Pressure perturbations are measured from an aircraft by subtracting its pressure altitude from its actual altitude. The pressure perturbation is equal to the resulting “D-value” multiplied by the acceleration of gravity and density of air. Normally, the actual altitude is measured using a radar altimeter, but this becomes increasingly difficult over increasingly complex terrain.

Here, we document a technique in which inertial altitude is used instead of radar altitude, eliminating the need for extremely accurate navigation or simple terrain, and apply it to document the pressure field at the base of an evolving cumulus congestus in CCOPE. Analysis of both this case study and aircraft self-calibration maneuvers in clear, undisturbed air suggests that a D-value (pressure) accuracy of 2 m (20 Pa) is achievable at cumulus-congestus scales. This accuracy is degraded, however, if the phenomenon of interest is large compared to the flight track.

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Lesley F. Tarleton and Richard W. Katz

Abstract

A reanalysis of the same Phoenix daily minimum and maximum temperature data examined by Balling et al. has been performed. As evidenced by substantial increasing trends in both the mean minimum and maximum temperatures, this area has experienced a marked heat island effect in recent decades. Balling et al. found that a statistical model for climate change in which simply a trend in the mean is permitted is inadequate to explain the observed trend in occurrence of extreme maximum temperatures. The present reanalysis establishes that by allowing for the observed decrease in the standard deviation, the tendency to overestimate the frequency of extreme high-temperature events is reduced. Thus, the urban heat island provides a real-world application in which trends in variability need to be taken into account to anticipate changes in the frequency of extreme events.

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Margaret A. Lemone, Lesley F. Tarleton, and Gary M. Barnes

Abstract

We examine the pressure fields wound the cloud-base updraft of three cumulus clouds observed in environments with low vertical shear of the horizontal wind near cloud base. These fields are compared to the corresponding pressure fields beneath convective clouds embedded in moderate to large shear. All of the pressure fields are derived from aircraft measurements taken during the 1981 Cooperative Convective Experiment, CCOPE.

The pressure fields associated with these low-shear clouds are weaker than those for the clouds in higher shear. Furthermore, the low-shear fields are not consistently dominated by the dynamic pressure created by the interaction of the cloud-base updraft with the vertical shear of the horizontal wind. The weaker dynamic pressure is due to the smaller size and intensity of the cloud-base updraft as well as the smaller vertical shear of the horizontal wind. The reduction of the dynamic Pressure allows buoyancy effects on the pressure field to become more apparent.

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Margaret A. LeMone, Gary M. Barnes, James C. Fankhauser, and Lesley F. Tarleton

Abstract

Perturbation pressure fields are measured by aircraft around the cloud base updrafts of seven clouds ranging in size from weak cumulus congestus to intense cumulonimbus during CCOPE (1981). The fields are characterized by a high-low pressure couplet of similar size to the updraft, but a quarter-wavelength out of Phase, with the minimum pressure downshear of the updraft maximum. An estimate of the terms in the Poisson equation for pressure show that the pressure perturbation results chiefly from the interaction of the updraft with the vertical shear of the environmental horizontal wind. The behavior of the pressure oscillation is well predicted by inserting sinusoidal functions in the corresponding terms in the Poisson equation. The amplitude of the pressure oscillation is proportional to the wavelengths of the pressure and vertical-velocity fields, the amplitude of the vertical-velocity oscillation, and the vertical shear of the horizontal environmental wind through cloud base, measured in the direction of the maximum pressure gradient.

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