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D. H. Lenschow and W. T. Pennell

Abstract

Measurements of air temperature with an immersion thermometer from an aircraft are invariably affected by the increased temperature of the decelerated air in the vicinity of the element. For dry air and a dry thermometer this effect is well known and usually taken into account. However, the evaporation of water from an element which has been wetted either intentionally (as in a wet-bulb thermometer) or unintentionally (by cloud or rain droplets) reduces this temperature increase. The psychrometric equation generalized for high-speed flow is used to calculate the aerodynamic correction factor for a wet temperature sensor. As an example of the magnitude of the evaporation effect, the temperature difference between a wet and a dry thermometer in a saturated airstream moving at 70 m sec−1>1C.

Aircraft measurements in clouds from 3 different temperature sensors are discussed. The temperature differences between an exposed and a protected thermometer are found to be as large as 1C in conditions where the exposed thermometer is wet and the protected thermometer is dry. More importantly, the outputs of the two sensors are well correlated in clear air but are uncorrelated in cloud. Humidity measured with a wet-bulb depression sensor is found to compare very well with the output of a dewpoint hygrometer in clear air. This sensor is also a good cloud indicator since the wet-bulb depression is ∼0 only when the dry-bulb thermometer is completely wet.

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W. T. Pennell and M. A. LeMone

Abstract

Measurements of air velocity, temperature and humidity were made from an aircraft in the fair-weather trade wind boundary layer. On the day of the experiment, the region studied was characterized by north-south bands of cloud-free and moderately clouded areas roughly 40 km in width. Mean winds, turbulence quantities, and thermodynamic variables were measured in both a clear and a partly cloudy region. Production of turbulence energy in the subcloud layer of both regions was mainly from wind shear. In the cloud-free region, the turbulence energy and momentum flux budgets were measured. One of the most striking features was the large region (covering almost two-thirds of the depth of the mixed layer) of negative production of turbulence energy by wind shear. Notwithstanding this unusual feature, the terms of the turbulence energy budget agree quite well with a model developed by Lenschow.

In the cloudy area, a layer of strong wind shear was observed near cloud base. This, coupled with corresponding minima in the turbulence quantities, suggests a weak coupling, on the turbulence scale, between the cloud and subcloud layers.

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D. H. Lenschow, J. C. Wyngaard, and W. T. Pennell

Abstract

Terms in the budgets of turbulence kinetic energy, temperature and humidity variances, and temperature and humidity fluxes have been evaluated for a baroclinic, convective boundary layer using data obtained from the NCAR Electra aircraft during the Air Mass Transformation Experiment (AMTEX). Although the mean temperature and momentum budgets, which were also evaluated, are strongly influenced by the horizontal temperature gradient, the second-moment budgets are little affected. The mean momentum budget is not well balanced, probably due to a combination of neglect of horizontal advection (aircraft advection measurements are shown to be statistically unreliable) and error in the surface geostrophic wind. For the most part, the measured terms in the second-moment budgets agree with previous estimates. Turbulence dissipation, however, was systematically less than that found in previous tower-based experiments. We find that over most of the mixed layer the temperature variance is maintained by turbulent transport and the temperature flux by buoyant production while, in contrast, the humidity variance and flux are maintained primarily by gradient production. Near the top of the mixed layer both temperature and humidity statistics are strongly affected by entrainment processes.

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J. C. Wyngaard, W. T. Pennell, D. H. Lenschow, and M. A. LeMone

Abstract

The behavior of the temperature-humidity covariance (θq) budget in the convectively driven boundary layer is determined through analysis of data from AMTEX and (to a lesser extent) Kansas and Minnesota. In the near-neutral surface layer a balance is found between production and molecular destruction; in the mixed layer, transport is also important. We extend the Corrsin theory for inertial subrange scalar spectral behavior to the temperature-humidity cospectrum, and thus relate the molecular destruction rate of θq to its inertial range level. Destruction rates inferred from AMTEX cospectra agree with those found from the imbalance of production and transport terms. The budgets within the surface layer and the mixed layer are parameterized separately with appropriate scales.

Both temperature and humidity fluctuations contribute to the small-scale refractive index variations which affect acoustic and electromagnetic wave propagation in the atmosphere. Our results indicate that their joint contribution CTq to the refractive index structure parameter is directly related to the molecular destruction rate of θq. The results provide a basis for understanding and predicting the behavior of CTq in the convective boundary layer.

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D. J. McNaughton, N. E. Bowne, R. L. Dennis, R. R. Draxler, S. R. Hanna, T. Palma, S. L. Marsh, W. T. Pennell, R. L. Peterson, J. V. Ramsdell, S. T. Rao, and R. J. Yamartino

The Eighth Joint Conference on Applications of Air Pollution Meteorology with the Air and Waste Management Association was held in conjunction with the AMS 74th Annual Meeting in Nashville, Tennessee, on 23–28 January 1994. Sessions at the meeting covered a broad range of topics including the dispersion environment, meteorology in emissions determination, long-range and mesoscale pollutant transport and fate, meteorology and photochemistry, advanced dispersion models and modeling systems, model evaluation, complex flows affecting dispersion near structures, and coastal and complex terrain issues. Papers followed some recurrent themes but many reported applications of new technology that provide new opportunities to see atmospheric characteristics and complexities for the first time. Innovative techniques were described in data analysis and presentation and modeling.

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