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Harindra J. S. Fernando

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Harindra J. S. Fernando

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This paper contains a summary of the results from some laboratory and theoretical studies on the diffusive interface in double diffusive convection, paying particular attention to the recent work of Fernando. A simple model is developed which predicts the thickness of the convecting layers in a thermohaline staircase structure. The laboratory buoyancy-flux measurements and the model results are extrapolated for oceanic situations and comparisons are made with field measurements.

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Sang-Mi Lee and Harindra J. S. Fernando

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Two meteorological models, the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) and the hydrostatic version of the Higher-Order Turbulence Model for Atmospheric Circulation (HOTMAC), were employed to simulate circulation and surface temperature in the Phoenix, Arizona, valley under weak synoptic forcing. The performances of these models were evaluated using field data collected during the first Phoenix Air Flow Experiment (PAFEX-I). MM5 showed a reasonable agreement with observations of the surface energy budget and surface temperature. The local flow, which was largely governed by thermodynamics, was also simulated well by MM5. In HOTMAC, a relatively uniform wind field was attributed to hydrostatic dynamics, active vertical mixing, and the zero-gradient lateral boundary condition used. The cold bias observed in HOTMAC results appears to be caused by the attenuation of shortwave irradiance within the canopy layer and the assumption of horizontal homogeneity in initialization. Differences in the formulation of surface energetics of the two models were examined and compared quantitatively. Statistical analysis of model performance showed that MM5 results are the closest to the observations.

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Barbara Anne Ayotte and Harindra J. S. Fernando

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Daniel Vassallo, Raghavendra Krishnamurthy, Robert Menke, and Harindra J. S. Fernando

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This paper reports the findings of a comprehensive field investigation on flow through a mountain gap subject to a range of stably stratified environmental conditions. This study was embedded within the Perdigão field campaign, which was conducted in a region of parallel double-ridge topography with ridge-normal wind climatology. One of the ridges has a well-defined gap (col) at the top, and an array of in situ and remote sensors, including a novel triple Doppler lidar system, was deployed around it. The experimental design was mostly guided by previous numerical and theoretical studies conducted with an idealized configuration where a flow (with characteristic velocity U 0 and buoyancy frequency N) approaches normal to a mountain of height h with a gap at its crest, for which the governing parameters are the dimensionless mountain height G = Nh/U 0 and various gap aspect ratios. Modified forms of G were proposed to account for real-world atmospheric variability, and the results are discussed in terms of a gap-averaged value G c. The nature of gap flow was highly dependent on G c, wherein a nearly neutral flow regime (G c < 1), a transitional mountain wave regime [G c ~ O(1)], and a gap-jetting regime [G c > O(1)] were identified. The measurements were in broad agreement with previous numerical and theoretical studies on a single ridge with a gap or double-ridge topography, although details vary. This is the first-ever detailed field study reported on microscale [O(100) m] gap flows, and it provides useful data and insights for future theoretical and numerical studies.

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Jong-Jin Baik, Jae-Jin Kim, and Harindra J. S. Fernando

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A three-dimensional computational fluid dynamics (CFD) model is developed to simulate urban flow and dispersion, to understand fluid dynamical processes therein, and to provide practical solutions to some emerging problems of urban air pollution. The governing equations are the Reynolds-averaged equations of momentum, mass continuity, heat, and other scalar (here, passive pollutant) under the Boussinesq approximation. The Reynolds stresses and turbulent fluxes are parameterized using the eddy diffusivity approach. The turbulent diffusivities of momentum, heat, and pollutant concentration are calculated using the prognostic equations of turbulent kinetic energy and its dissipation rate. The set of governing equations is solved numerically on a staggered, nonuniform grid system using a finite-volume method with the semi-implicit method for pressure-linked equation (SIMPLE) algorithm. The CFD model is tested for three different building configurations: infinitely long canyon, long canyon of finite length, and orthogonally intersecting canyons. In each case, the CFD model is shown to simulate urban street-canyon flow and pollutant dispersion well.

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Rui-Rong Chen, Neil S. Berman, Don L. Boyer, and Harindra J. S. Fernando

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Laboratory experiments were conducted to simulate the diurnal heating-cooling cycle in the vicinity of a ridge of constant cross section. In the model the fluid is a water solution stratified with salt to simulate the background stratification of the atmosphere. The flow is driven by recirculating water of a controlled temperature beneath the model; the model surface temperature is thus varied in a specified way to simulate the surface heating by solar insolation during the daytime hours and surface cooling by radiation during the nighttime.

The pertinent similarity parameters are shown to be G c, for daytime convective flow and G d for nocturnal flow; here G c = H b/H c, G d = H b/H d, where H b, is the mountain height, H c the neutral buoyancy height of free convection. and H d the characteristic thickness of the nighttime drainage flow. The model demonstrates some of the principal features of thermally driven mountain circulations, including daytime upslope winds and nocturnal downslope drainage flows. The spatial and temporal structures of these motion fields are delineated, with the following being among the most important observations: (i) during the daytime, the upslope convective flow in the vicinity of the mountain tends to suppress convective turbulence over the horizontal plains; (ii) during the early evening, horizontal jets, with the principal one directed toward the mountain, develop above the mountain surface, and vortices in the vertical cross section develop both above and below the jets, following the collapse of the convective motion over the mountain; and (iii) in the evening, a downslope drainage flow is initiated following the establishment of a vertical vortex on the mountain slope and under the jet.

Quantitative experimental observations are made, which demonstrate the variation of various flow observables with the pertinent similarity parameters. These results are applied to the atmosphere following similarity relations between the physical model and the atmosphere. The predicted characteristic speeds and length scales of the daytime upslope flow and the nocturnal drainage flow for typical atmospheric parameters are in reasonable agreement with limited field observations.

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Sergey I. Voropayev, Grady B. McEachern, Don L. Boyer, and Harindra J. S. Fernando

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The aim of this contribution is to present the results of laboratory experiments on the dynamics of basic self-propagating vortices generated in a large volume of fluid when a linear (P) and an angular (M) momentum are applied locally to a fluid. Using the method proposed, it is possible to generate a whole family of isolated (net vorticity is equal to zero) vortices with different values of the nondimensional parameter ε, which is proportional to the ratio of linear to angular momentum (ε ∝ RP/M, R is the eddy size). Typical examples include monopole (ε = 0), quasi monopole (ε = 0.1–0.3), quasi dipole (ε ≈ 1), and dipole (ε = ∞).

One of the possible applications is the dynamics of oceanic eddies. Recently, Stern and Radko considered theoretically and numerically a symmetric barotropic eddy, which is subject to a relatively small amplitude disturbance of unit azimuthal wavenumber on an f plane. This case corresponds to a self-propagating quasi monopole. They analyzed the structure of the eddy and predicted that such an eddy remains stable and could propagate a significant distance away from its origin. This effect may be of importance for oceanographic applications and such an eddy was reproduced in laboratory experiments with the purpose of verifying these theoretical predictions.

Another possible application may include large eddies behind maneuvering bodies. Recent experiments by Voropayev et al. show that, when a submerged self-propelled body accelerates, significant linear momentum is transported to the fluid and unusually large dipoles are formed in a late stratified wake. When such a body changes its direction of motion, an angular momentum is also transported to the fluid and the resulting structure will depend on the value of ε.

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Luigi Brogno, Francesco Barbano, Laura Sandra Leo, Harindra J. S. Fernando, and Silvana Di Sabatino

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In the realm of boundary layer flows in complex terrain, low-level jets (LLJs) have received considerable attention, although little literature is available for double-nosed LLJs that remain not well understood. To this end, we use the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) dataset to demonstrate that double-nosed LLJs developing within the planetary boundary layer (PBL) are common during stable nocturnal conditions and present two possible mechanisms responsible for their formation. It is observed that the onset of a double-nosed LLJ is associated with a temporary shape modification of an already-established LLJ. The characteristics of these double-nosed LLJs are described using a refined version of identification criteria proposed in the literature, and their formation is classified in terms of two driving mechanisms. The wind-driven mechanism encompasses cases where the two noses are associated with different air masses flowing one on top of the other. The wave-driven mechanism involves the vertical momentum transport by an inertial–gravity wave to generate the second nose. The wave-driven mechanism is corroborated by the analysis of nocturnal double-nosed LLJs, where inertial–gravity waves are generated close to the ground by a sudden flow perturbation.

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Peter P. Sullivan, James C. McWilliams, Jeffrey C. Weil, Edward G. Patton, and Harindra J. S. Fernando

Abstract

Turbulent flow in a weakly convective marine atmospheric boundary layer (MABL) driven by geostrophic winds V g = 10 m s−1 and heterogeneous sea surface temperature (SST) is examined using fine-mesh large-eddy simulation (LES). The imposed SST heterogeneity is a single-sided warm or cold front with jumps Δθ = (2, −1.5) K varying over a horizontal x distance of 1 km characteristic of an upper-ocean mesoscale or submesoscale front. The geostrophic winds are oriented parallel to the SST isotherms (i.e., the winds are alongfront). Previously, Sullivan et al. examined a similar flow configuration but with geostrophic winds oriented perpendicular to the imposed SST isotherms (i.e., the winds were across-front). Results with alongfront and across-front winds differ in important ways. With alongfront winds, the ageostrophic surface wind is weak, about 5 times smaller than the geostrophic wind, and horizontal pressure gradients couple the SST front and the atmosphere in the momentum budget. With across-front winds, horizontal pressure gradients are weak and mean horizontal advection primarily balances vertical flux divergence. Alongfront winds generate persistent secondary circulations (SC) that modify the surface fluxes as well as turbulent fluxes in the MABL interior depending on the sign of Δθ. Warm and cold filaments develop opposing pairs of SC with a central upwelling or downwelling region between the cells. Cold filaments reduce the entrainment near the boundary layer top that can potentially impact cloud initiation. The surface-wind–SST-isotherm orientation is an important component of atmosphere–ocean coupling. The results also show frontogenetic tendencies in the MABL.

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