Maria Grazia Badas

Space‐time scaling in high‐intensity Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA‐COARE) storms

Received 8 August 2003; accepted 23 October 2003; published 17 February 2004. [1] A scale-invariance analysis of rainfall retrieved during the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE) campaign is discussed. As already found in the previous Global Atmospheric Research Program (GARP) Atlantic Tropical Experiment (GATE) rainfall data set, these new analyses of high-intensity storms confirm the evidence of scale invariance under selfsimilar space-time transformations. A simple interpretation of this space-time selfsimilarity accounting for the hierarchical organization of precipitation patterns is proposed. Finally, a downscaling model based on a log-Poisson generator is calibrated on the results of the multifractal analysis and applied to the generation of synthetic fields, reproducing observed statistical properties over a wide range of space scales and timescales. INDEX TERMS: 1854 Hydrology: Precipitation (3354); 3250 Mathematical Geophysics: Fractals and multifractals; 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854); 1869 Hydrology: Stochastic processes; KEYWORDS: rainfall, scaling processes Citation: Deidda, R., M. G. Badas, and E. Piga (2004), Space-time scaling in high-intensity Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE) storms, Water Resour. Res., 40, W02506,

Turbulence and Air Exchange in a Two-Dimensional Urban Street Canyon Between Gable Roof Buildings

We experimentally investigate the effect of a typical building covering: the gable roof, on the flow and air exchange in urban canyons. In general, the morphology of the urban canopy is very varied and complex, depending on a large number of factors, such as building arrangement, or the morphology of the terrain. Therefore we focus on a simple, prototypal shape, the two-dimensional canyon, with the aim of elucidating some fundamental phenomena driving the street-canyon ventilation. Experiments are performed in a water channel, over an array of identical prismatic obstacles representing an idealized urban canopy. The aspect ratio, i.e. canyon-width to building-height ratio, ranges from 1 to 6. Gable roof buildings with 1:1 pitch are compared with flat roofed buildings. Velocity is measured using a particle-image-velocimetry technique with flow dynamics discussed in terms of mean flow and second- and third-order statistical moments of the velocity. The ventilation is interpreted by means of a simple well-mixed box model and the outflow rate and mean residence time are computed. Results show that gable roofs tend to delay the transition from the skimming-flow to the wake-interference regime and promote the development of a deeper and more turbulent roughness layer. The presence of a gable roof significantly increases the momentum flux, especially for high packing density. The air exchange is improved compared to the flat roof buildings, and the beneficial effect is more significant for narrow canyons. Accordingly, for unit aspect ratio gable roofs reduce the mean residence time by a factor of 0.37 compared to flat roofs, whereas the decrease is only by a factor of 0.9 at the largest aspect ratio. Data analysis indicates that, for flat roof buildings, the mean residence time increases by 30% when the aspect ratio is decreased from 6 to 2, whereas this parameter is only weakly dependent on aspect ratio in the case of gable roofs.

Does the Bursa Pre-Macularis protect the fovea from shear stress? A possible mechanical role

Abstract Purpose of present study is to evaluate whether the Pre‐Macular Bursa (PMB) modifies Wall Shear Stress (WSS) at the retinal surface during saccadic movements. We created a mathematical model consisting of 25,000 grid cells and simulated a horizontal saccade spanning 50° in 0.17s, both in absence and in presence of the PMB. Wall Shear Stress SS was computed throughout the retinal surface and the posterior pole was divided into 3 Zones comprising 400 nodes each: Zone 1 (radius 3.5 mm; 0°–17°) corresponding to the PMB area; Zone 2 (concentric annular area 5 mm in radius; 22°) and Zone 3 (concentric annular area 5.5 mm; 28°). The PMB reduced WSS significantly at the macula and increased it in the immediate surroundings. Average WSS in Zone 1 was 1.53 ± 1.01 (max 4.23 Pa) with PMB Vs 6.94 ± 9.23 (max 35.83 Pa) without. Zone 2 WSS was 9.39 ± 10.33 (max 48.36 Pa) with PMB Vs 6.95 ± 9.40 (max 38.60 Pa) without Zone 3 WSS was 8.41 ± 10.03 (max 43.16 Pa) with PMB Vs 6.88 ± 9.42 (max 39.43 Pa) without (p < 0.001 in all cases). The PMB significantly reduces WSS over the retinal surface underlying the bursa region; conversely, WSS slightly increases it in the immediate neighboring areas. HighlightsThe Pre‐Macular Bursa (PMB) is a pocket of liquid within the vitreous gel structure and is placed anterior to the macula.The presence of the PMB significantly reduces Wall Shear Stress at the macula during saccadic motion by reducing friction.Shear Stress related gene expression has been largely reported in the literature.Shear stress modifications secondary to PMB disappearance with aging may trigger age‐related disease.