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R. A. Brown and Lixin Zeng

Abstract

A method of determining surface pressures in oceanic storm systems using ERS-1 scatterometer data is employed to determine the lowest pressures in 25 storms. This method uses the surface winds as a lower boundary condition on a planetary boundary layer model to determine gradient winds and, thereby, pressure gradients. An optimization scheme referenced to a pressure outside the storm provides a pressure field and an estimate of the low pressure. The values are compared to ECMWF analyses in each case; there is good agreement, with some expected differences.

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K. A. Browning and R. Wexler

Abstract

A technique is proposed for the measurement of kinematic properties of a wind field in situations of widespread precipitation, using a single Doppler radar to sense the motion of the precipitation particles. The technique is an extension of ideas put forward by Probert-Jones, Lhermitte, Atlas, Caton and Harrold, and is based upon the Velocity-Azimuth Display (VAD) obtained by scanning the radar beam about a vertical axis at a fixed elevation angle. Harmonic analysis of the VAD permits divergence to be obtained from the magnitude of the “zeroth” harmonic, wind speed and direction to be obtained from the amplitude and phase of the first harmonic, and resultant deformation and the axis of dilatation to be obtained from the amplitude and phase of the second harmonic. Although limitations to the accuracy of this technique are imposed by inhomogeneities in the horizontal distribution of precipitation fall speed and, in the presence of strong vertical wind shear, by elevation angle errors and reflectivity inhomogeneities, the errors resulting from these effects can be made acceptably small by scanning at appropriate elevation angles and ranges. An optimum scanning procedure is suggested. A short case study is also presented to support the view that meaningful estimates of mesoscale divergence and deformation can be obtained using this technique.

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R. A. Brown and W. Timothy Liu

Abstract

An operational planetary boundary layer model (Brown, 1974,1978, 1981) for determining surface winds and stress from free-stream flow has been modified for the marine layer by including surface roughness feedback, variable humidity and interfacial layer effects. The surface winds determined from synoptic-scale pressure and temperature fields are compared to surface measurements in GOASEX and JASIN.

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P. R. A. Brown, A. J. Illingworth, A. J. Heymsfield, G. M. McFarquhar, K. A. Browning, and M. Gosset

Abstract

The purpose of this paper is to assess the potential of a spaceborne 94-GHz radar for providing useful measurements of the vertical distribution and water content of ice clouds on a global scale.

Calculations of longwave (LW) fluxes for a number of model ice clouds are performed. These are used to determine the minimum cloud optical depth that will cause changes in the outgoing longwave radiation or flux divergence within a cloud layer greatear than 10 W m−2, and in surface downward LW flux greater than 5 W m−2, compared to the clear-sky value. These optical depth values are used as the definition of a “radiatively significant” cloud. Different “thresholds of radiative significance” are calculated for each of the three radiation parameters and also for tropical and midlatitude cirrus clouds. Extensive observational datasets of ice crystal size spectra from midlatitude and tropical cirrus are then used to assess the capability of a radar to meet these measurement requirements. A radar with a threshold of −30 dBZ should detect 99% (92%) of “radiatively significant” clouds in the midlatitudes (Tropics). This detection efficiency may be reduced significantly for tropical clouds at very low temperatures (−80°C).

The LW flux calculations are also used to establish the required accuracy within which the optical depth should be known in order to estimate LW fluxes or flux divergence to within specified limits of accuracy. Accuracy requirements are also expressed in terms of ice water content (IWC) because of the need to validate cloud parameterization schemes in general circulation models (GCMs). Estimates of IWC derived using radar alone and also using additional information to define the mean crystal size are considered. With crystal size information available, the IWC for samples with a horizontal scale of 1–2 km may be obtained with a bias of less than 8%. For IWC larger than 0.01 g m−3, the random error is in the range +50% to −35%, whereas for a value of 0.001 g m−3 the random error increases to between +80% and −45%. This level of accuracy also represents the best that may be achieved for estimates of the cloud optical depth and meets the requirements derived from LW flux calculations. In the absence of independent particle size information, the random error is within the range +85% to −55% for IWC greater than 0.01 g m−3. For the same IWC range, the estimated bias is few than ±15%. This accuracy is sufficient to provide useful constraints on GCM cloud parameteriation schemes.

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R. M. Brown, L. A. Cohen, and M. E. Smith

Abstract

Recent studies of particulate and gaseous materials in the atmosphere have raised important questions about diffusion at distances of 10–100 km. A photometric densitometer, initially developed for a quantitative study of oil-fog concentrations at ground level, has been adapted for use in an aircraft. Real-time measurements of ground-level and airborne particle concentrations are presented to distances of 120 km, and the implications of these data in terms of large-scale dispersion are discussed.

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N. E. Busch, R. M. Brown, and J. A. Frizzola

Abstract

The response characteristics obtained through wind tunnel tests of the Brookhaven National Laboratory bivane are presented and used in correcting the measured values of (ω′2/u*2), the variance of the vertical wind velocity fluctuations normalized by the square of the friction velocity. In neutral through unstable atmospheres the corrected values appear to be independent of thermal stability and height of observation, except for extreme instability at heights >46 m. The average value based on measurements at heights of 23, 46 and 92 m is 1.48 with a standard deviation of 0.38.

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A. A. N. Patrinos, M. J. Leach, R. M. Brown, R. L. Tanner, and F. S. Binkowski

Abstract

A field study in the Washington, D.C. area explored the impact of urban emissions and mesoscale meteorology on precipitation chemistry. The study was a follow-up to an earlier, considerably more industrialized, study in the Philadelphia area; emissions along the Delaware Valley were found to affect the deposition of nitrate and sulfate on the urban mesoscale. The Washington studies were designed to complement and enhance the earlier study with an expanded sampling domain, sequential precipitation sampling and airborne measurements. Four storms were sampled successfully between October 1986 and April 1987. Results appear to confirm the conclusions of the Philadelphia study, although the upwind-downwind contrast in nitrate and sulfate deposition is not as pronounced. This difference is attributed to the area's widely distributed emission patterns and to the prevailing theories regarding the production of nitric acid and sulfuric acid on the relevant time and space scales. The importance of mesoscale meteorology and hydrogen peroxide availability is highlighted in at least two of the sampled storms.

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A. K. Kochanski, E. R. Pardyjak, R. Stoll, A. Gowardhan, M. J. Brown, and W. J. Steenburgh

Abstract

Simulations of local weather and air quality in urban areas must account for processes spanning from meso- to microscales, including turbulence and transport within the urban canopy layer. Here, the authors investigate the performance of the building-resolving Quick Urban Industrial Complex (QUIC) Dispersion Modeling System driven with mean wind profiles from the mesoscale Weather Research and Forecasting (WRF) Model. Dispersion simulations are performed for intensive observation periods 2 and 8 of the Joint Urban 2003 field experiment conducted in Oklahoma City, Oklahoma, using an ensemble of expert-derived wind profiles from observational data as well as profiles derived from WRF runs. The results suggest that WRF can be used successfully as a source of inflow boundary conditions for urban simulations, without the collection and processing of intensive field observations needed to produce expert-derived wind profiles. Detailed statistical analysis of tracer concentration fields suggests that, for the purpose of the urban dispersion, WRF simulations provide wind forcing as good as individual or ensemble expert-derived profiles. Despite problems capturing the strength and the elevation of the Great Plains low-level jet, the WRF-simulated near-surface wind speed and direction were close to observations, thus assuring realistic forcing for urban dispersion estimates. Tests performed with multilayer and bulk urban parameterizations embedded in WRF did not provide any conclusive evidence of the superiority of one scheme over the other, although the dispersion simulations driven by the latter showed slightly better results.

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M. A. Nelson, E. R. Pardyjak, M. J. Brown, and J. C. Klewicki

Abstract

Velocity data were obtained within Park Avenue in Oklahoma City, Oklahoma, using three-dimensional sonic anemometers under unstable atmospheric conditions. These data are used to produce velocity spectra, cospectra, and weighted joint probability density functions at various heights and horizontal locations in the street canyon. This analysis has helped to describe a number of physically interesting urban flow phenomena. Previous research has shown that the ratio of Reynolds shear stresses to normal stresses is typically much smaller deep within the canopy than those ratios found at the top of canopy and in the roughness sublayer. The turbulence in this region exhibits significant contributions to all four quadrants of a weighted joint-probability density function of horizontal and vertical velocity fluctuations, yielding the characteristic small Reynolds shear stresses in the flow. The velocity cospectra measured at the base of the canopy show evidence of discrete frequency bands of both positive and negative correlation that yield a small correlation, as indicated by the Reynolds shear stresses. Two major peaks were often observed in the spectra and cospectra: a low-frequency peak that appears to be associated with vortex shedding off the buildings and a midfrequency peak generally associated with canyon geometry. The low-frequency peak was found to produce a countergradient contribution to the along-wind vertical velocity covariance. Standard spectral tests for local isotropy indicate that isotropic conditions occur at different frequencies depending on spatial location, demonstrating the need to be thorough when testing for local isotropy with the urban canopy.

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M. A. Nelson, E. R. Pardyjak, J. C. Klewicki, S. U. Pol, and M. J. Brown

Abstract

Velocity data were obtained from sonic anemometer measurements within an east–west-running street canyon located in the urban core of Oklahoma City, Oklahoma, during the Joint Urban 2003 field campaign. These data were used to explore the directional dependence of the mean flow and turbulence within a real-world street canyon. The along-canyon vortex that is a key characteristic of idealized street canyon studies was not evident in the mean wind data, although the sensor placement was not optimized for the detection of such structures. Instead, surface wind measurements imply that regions of horizontal convergence and divergence exist within the canopy, which are likely caused by taller buildings diverting the winds aloft down into the canopy. The details of these processes appear to be dependent on relatively small perturbations in the prevailing wind direction. Turbulence intensities within the canyon interior appeared to have more dependence on prevailing wind direction than they did in the intersections. Turbulence in the intersections tended to be higher than was observed in the canyon interior. This behavior implies that there are some fundamental differences between the flow structure found in North American–style cities where building heights are typically heterogeneous and that found in European-style cities, which generally have more homogeneous building heights. It is hypothesized that the greater three-dimensionality caused by the heterogeneous building heights increases the ventilation of the urban canopy through mean advective transport as well as enhanced turbulence.

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