ALBERT

Description:    The effects of a non-uniform wind field along the path of a scintillometer are investigated. Theoretical spectra are calculated for a range of scenarios where the crosswind varies in space or time and compared to the ‘ideal’ spectrum based on a constant uniform crosswind. It is verified that the refractive-index structure parameter relation with the scintillometer signal remains valid and invariant for both spatially and temporally-varying crosswinds. However, the spectral shape may change significantly preventing accurate estimation of the crosswind speed from the peak of the frequency spectrum and retrieval of the structure parameter from the plateau of the power spectrum. On comparison with experimental data, non-uniform crosswind conditions could be responsible for previously unexplained features sometimes seen in observed spectra. By accounting for the distribution of crosswind, theoretical spectra can be generated that closely replicate the observations, leading to a better understanding of the measurements. Spatial variability of wind speeds should be expected for paths other than those that are parallel to the surface and over flat, homogenous areas, whilst fluctuations in time are important for all sites. Content Type Journal Article Pages 1-21 DOI 10.1007/s10546-011-9626-0 Authors H. C. Ward, Centre for Ecology and Hydrology, Wallingford, OX10 8BB UK J. G. Evans, Centre for Ecology and Hydrology, Wallingford, OX10 8BB UK C. S. B. Grimmond, Environmental Monitoring and Modelling Group, Department of Geography, King’s College London, London, WC2R 2LS UK Journal Boundary-Layer Meteorology Online ISSN 1573-1472 Print ISSN 0006-8314

Description:    One year of observations from a network of five 915-MHz boundary-layer radar wind profilers equipped with radio acoustic sounding systems located in California’s Central Valley are used to investigate the annual variability of convective boundary-layer depth and its correlation to meteorological parameters and conditions. Results from the analysis show that at four of the sites, the boundary-layer height reaches its maximum in the late-spring months then surprisingly decreases during the summer months, with mean July depths almost identical to those for December. The temporal decrease in boundary-layer depth, as well as its spatial variation, is found to be consistent with the nocturnal low-level lapse rate observed at each site. Multiple forcing mechanisms that could explain the unexpected seasonal behaviour of boundary-layer depth are investigated, including solar radiation, precipitation, boundary-layer mesoscale convergence, low-level cold-air advection, local surface characteristics and irrigation patterns and synoptic-scale subsidence. Variations in solar radiation, precipitation and synoptic-scale subsidence do not explain the shallow summertime convective boundary-layer depths observed. Topographically forced cold-air advection and local land-use characteristics can help explain the shallow CBL depths at the four sites, while topographically forced low-level convergence helps maintain larger CBL depths at the fifth site near the southern end of the valley. Content Type Journal Article Pages 1-21 DOI 10.1007/s10546-011-9622-4 Authors L. Bianco, University of Colorado Cooperative Institute for Research in the Environmental Sciences, Boulder, CO, USA I. V. Djalalova, University of Colorado Cooperative Institute for Research in the Environmental Sciences, Boulder, CO, USA C. W. King, National Oceanic and Atmospheric Administration/Earth Systems Research Laboratory, 325 Broadway, mailstop: PSD3, Boulder, CO 80305, USA J. M. Wilczak, National Oceanic and Atmospheric Administration/Earth Systems Research Laboratory, 325 Broadway, mailstop: PSD3, Boulder, CO 80305, USA Journal Boundary-Layer Meteorology Online ISSN 1573-1472 Print ISSN 0006-8314

Description:    On 15 January 2010, Thiruvananthapuram in India (8.5°N, 76.9°E) witnessed one of the longest possible noontime annular solar eclipses spanning a period of about 7 min centred at 1314 local time. Here, we present a case study on the behaviour of the atmospheric surface layer by comparing the eclipse-induced observations with similar measurements recorded on cloud-free/clear-sky days. During the peak period of the eclipse, the incoming solar irradiance was reduced by 87% of its normal values, resulting in an air-temperature decrease near the surface of 1.2°C in association with a significant reduction in turbulent kinetic energy, momentum flux and sensible heat flux. The rate of instantaneous decay in solar radiation and sensible heat flux from the first contact of the eclipse to its annularity was greater than that seen during normal evening hours. Content Type Journal Article Pages 1-8 DOI 10.1007/s10546-011-9627-z Authors D. Bala Subrahamanyam, Space Physics Laboratory, Vikram Sarabhai Space Centre, Department of Space, Government of India, Indian Space Research Organization, Thiruvananthapuram, 695 022 Kerala, India T. J. Anurose, Space Physics Laboratory, Vikram Sarabhai Space Centre, Department of Space, Government of India, Indian Space Research Organization, Thiruvananthapuram, 695 022 Kerala, India Mannil Mohan, Space Physics Laboratory, Vikram Sarabhai Space Centre, Department of Space, Government of India, Indian Space Research Organization, Thiruvananthapuram, 695 022 Kerala, India M. Santosh, Space Physics Laboratory, Vikram Sarabhai Space Centre, Department of Space, Government of India, Indian Space Research Organization, Thiruvananthapuram, 695 022 Kerala, India N. V. P. Kiran Kumar, Space Physics Laboratory, Vikram Sarabhai Space Centre, Department of Space, Government of India, Indian Space Research Organization, Thiruvananthapuram, 695 022 Kerala, India S. Sijikumar, Space Physics Laboratory, Vikram Sarabhai Space Centre, Department of Space, Government of India, Indian Space Research Organization, Thiruvananthapuram, 695 022 Kerala, India S. S. Prijith, Space Physics Laboratory, Vikram Sarabhai Space Centre, Department of Space, Government of India, Indian Space Research Organization, Thiruvananthapuram, 695 022 Kerala, India Marina Aloysius, Space Physics Laboratory, Vikram Sarabhai Space Centre, Department of Space, Government of India, Indian Space Research Organization, Thiruvananthapuram, 695 022 Kerala, India Journal Boundary-Layer Meteorology Online ISSN 1573-1472 Print ISSN 0006-8314

Description:    The impact of ocean spray on the dynamics of the marine near-surface air boundary layer (MABL) in conditions of very high (hurricane) wind speeds is investigated. Toward this end, a model of the MABL in the presence of sea-spume droplets is developed. The model is based on the classical theory of the motion of suspended particles in a turbulent flow, where the mass concentration of droplets is not mandatory small. Description of the spume-droplet generation assumes that they, being torn off from breaking waves, are injected in the form of a jet of spray into the airflow at the altitude of breaking wave crests. The droplets affect the boundary-layer dynamics in two ways: via the direct impact of droplets on the airflow momentum forming the so-called spray force, and via the impact of droplets on the turbulent mixing through stratification. The latter is parametrized applying the Monin–Obukhov similarity theory. It is found that the dominant impact of droplets on the MABL dynamics appears through the action of the ‘spray force’ originated from the interaction of the ‘rain of spray’ with the wind velocity shear, while the efficiency of the stratification mechanism is weaker. The effect of spray leads to an increase in the wind velocity and suppression of the turbulent wind stress in the MABL. The key issue of the model is a proper description of the spume-droplet generation. It is shown that, after the spume-droplet generation is fitted to the observations, the MABL model is capable of reproducing the fundamental experimental finding—the suppression of the surface drag at very high wind speeds. We found that, at very high wind speeds, a thin part of the surface layer adjacent to the surface turns into regime of limited saturation with the spume droplets, resulting in the levelling off of the friction velocity and decrease of the drag coefficient as U 10 - 2 , U 10 being the wind speed at 10-m height. Content Type Journal Article Pages 1-28 DOI 10.1007/s10546-011-9624-2 Authors V. N. Kudryavtsev, Nansen International Environmental and Remote Sensing Centre (NIERSC), Russian State Hydrometeorological University (RSHU), Saint Petersburg, Russia V. K. Makin, Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands Journal Boundary-Layer Meteorology Online ISSN 1573-1472 Print ISSN 0006-8314

Description:    Two mathematical models are proposed detailing the influence of ocean spray on vertical momentum transport under high-wind conditions associated with a hurricane or severe storm. The first model is based on a turbulent kinetic energy (TKE) equation and accounts for the so-called lubrication effect due to the reduction of turbulence intensity. The second model is based on Monin–Obukhov similarity (MOS) and uses available experimental data. It is demonstrated that the flow acceleration is negligible for wind speeds below a certain critical value due to the fact that the spray volume concentration is low for such speeds. For wind speeds higher than the critical value, the spray concentration rapidly increases, which results in significant flow acceleration. Both models produce qualitatively similar results for all turbulent flow parameters considered. It was found that the MOS-based model tends to predict a noticeably stronger lubrication effect than the TKE-based model, especially for lower wind speeds. The results of model calculations are in very good agreement with available experimental data for the spray production values near the upper bound. It is also shown that neither the value of the turbulent Schmidt number in the TKE-based model nor the choice of a stability profile function affects the spray-laden flow dynamics significantly. Content Type Journal Article Pages 1-20 DOI 10.1007/s10546-011-9625-1 Authors Yevgenii Rastigejev, Department of Mathematics, North Carolina A&T State University, Greensboro, NC, USA Sergey A. Suslov, Mathematics H38, FEIS, Swinburne University of Technology, Hawthorn, VIC 3156, Australia Yuh-Lang Lin, Department of Energy and Environmental Systems, North Carolina A&T State University, 302 Gibbs Hall, 1601 E. Market Street, Greensboro, NC 27411, USA Journal Boundary-Layer Meteorology Online ISSN 1573-1472 Print ISSN 0006-8314

Description:    The Gaussian distribution is a good approximation for transient (instantaneously released) puff concentration distributions within a short period of time after release. Artificial neural network (ANN) models for puff dispersion coefficients were developed, based on observations from field experiments covering a wide range of meteorological conditions (in March, May, August and November). Their average predictions were in very good agreement with measurements, having high correlation coefficients ( r  〉 0.99). A non-linear multi-variable regression model for dispersion coefficients was also developed, under the assumption that puff dispersion coefficients increase with time, and follow power laws. Both ANN-based and multi-regression non-linear models were able to use easily measured atmospheric parameters directly, without the necessity of predefining the Pasquill stability category. Predictions of ANN-based and multi-regression-based Gaussian puff models were compared with those of Gaussian puff models using Slade’s dispersion coefficients and COMBIC, a sophisticated model based on Gaussian distributions. Predictions from our two new models showed better agreement with concentration measurements than the other Gaussian puff models, by having a much higher fraction within a factor of two of measured values, and lower normalized mean square errors. Content Type Journal Article Pages 1-14 DOI 10.1007/s10546-011-9595-3 Authors Xiaoying Cao, Department of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, ON K7K 7B4, Canada Gilles Roy, Defence Research and Development Canada – Valcartier, Val-Belair, QC G3J 1X5, Canada William J. Hurley, Department of Business Administration, Royal Military College of Canada, Kingston, ON K7K 7B4, Canada William S. Andrews, Department of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, ON K7K 7B4, Canada Journal Boundary-Layer Meteorology Online ISSN 1573-1472 Print ISSN 0006-8314

Description:    An analytical model of atmospheric dispersion in urban areas in both daytime and nighttime conditions is presented. The model is based on a Gaussian formulation where the horizontal and vertical diffusion coefficients are determined according to analytical theories. The model is validated with dispersion measurements from field experiments conducted in Oklahoma City, Salt Lake City, St. Louis and London, U.K. The theory is in good agreement with the data for both daytime and nighttime conditions. The data support the conclusion that the magnitude of the nighttime stratification in the urban atmosphere is weak; however, its effects on dispersion are not negligible. The predicted existence of two distinct dispersion regimes, in the near and in the far field, is also confirmed by the data. The good collapse of the data suggests that urban dispersion is governed by the characteristic length scales of atmospheric boundary-layer turbulence, rather than urban canopy length scales that are more likely to affect dispersion only in the vicinity of the source. Content Type Journal Article Pages 1-15 DOI 10.1007/s10546-011-9593-5 Authors Pasquale Franzese, College of Science, George Mason University, Fairfax, VA 22030, USA Pablo Huq, College of Earth, Ocean and Environment, University of Delaware, Newark, DE 19716, USA Journal Boundary-Layer Meteorology Online ISSN 1573-1472 Print ISSN 0006-8314

Description:    In atmospheric dispersion, the “non-Gaussian” effects of gravitational settling, the vertical gradient in diffusivity and the surface deposition do not enter uniformly but rather break up parameter space into several discrete regimes. Here, we describe regime diagrams that are constructed for K -theory dispersion of effluent from a surface line source in unsheared inhomogeneous turbulence, using a previously derived Fourier–Hankel method. This K -theory formulation differs from the traditional one by keeping a non-zero diffusivity at the ground. This change allows for turbulent exchange between the canopy and the atmosphere and allows new natural length scales to emerge. The axes on the regime diagrams are non-dimensional distance defined as the ratio of downwind distance to the characteristic length scale for each effect. For each value of the ratio of settling speed to the K gradient, two to four regimes are found. Concentration formulae are given for each regime. The regime diagrams allow real dispersion problems to be categorized and the validity of end-state concentration formulae to be judged. Content Type Journal Article Pages 1-19 DOI 10.1007/s10546-011-9594-4 Authors Ronald B. Smith, Yale University, New Haven, CT 06520-8109, USA Journal Boundary-Layer Meteorology Online ISSN 1573-1472 Print ISSN 0006-8314

Description: “Surface-Based Remote Sensing of the Atmospheric Boundary Layer” by Stefan Emeis Content Type Journal Article Category Book Review Pages 1-3 DOI 10.1007/s10546-011-9659-4 Authors J. F. Barlow, University of Reading, Reading, UK Journal Boundary-Layer Meteorology Online ISSN 1573-1472 Print ISSN 0006-8314

Description:    Practically all extant work on flows over obstacle arrays, whether laboratory experiments or numerical modelling, is for cases where the oncoming wind is normal to salient faces of the obstacles. In the field, however, this is rarely the case. Here, simulations of flows at various directions over arrays of cubes representing typical urban canopy regions are presented and discussed. The computations are of both direct numerical simulation and large-eddy simulation type. Attention is concentrated on the differences in the mean flow within the canopy region arising from the different wind directions and the consequent effects on global properties such as the total surface drag, which can change very significantly—by up to a factor of three in some circumstances. It is shown that for a given Reynolds number the typical viscous forces are generally a rather larger fraction of the pressure forces (principally the drag) for non-normal than for normal wind directions and that, dependent on the surface morphology, the average flow direction deep within the canopy can be largely independent of the oncoming wind direction. Even for regular arrays of regular obstacles, a wind direction not normal to the obstacle faces can in general generate a lateral lift force (in the direction normal to the oncoming flow). The results demonstrate this and it is shown how computations in a finite domain with the oncoming flow generated by an appropriate forcing term (e.g. a pressure gradient) then lead inevitably to an oncoming wind direction aloft that is not aligned with the forcing term vector. Content Type Journal Article Category Article Pages 1-23 DOI 10.1007/s10546-011-9667-4 Authors Jean Claus, School of Engineering Sciences, University of Southampton, Southampton, UK O. Coceal, Department of Meteorology, University of Reading, Reading, UK T. Glyn Thomas, School of Engineering Sciences, University of Southampton, Southampton, UK S. Branford, Department of Meteorology, University of Reading, Reading, UK S. E. Belcher, Department of Meteorology, University of Reading, Reading, UK Ian P. Castro, School of Engineering Sciences, University of Southampton, Southampton, UK Journal Boundary-Layer Meteorology Online ISSN 1573-1472 Print ISSN 0006-8314