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Harindra J. S. Fernando

Abstract

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

Abstract

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

Abstract

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

Abstract

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

Abstract

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

Abstract

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.

Open access
Neil S. Berman
,
Don L. Boyer
,
Anthony J. Brazel
,
Sandra W. Brazel
,
Rui-Rong Chen
,
Harindra J. S. Fernando
, and
Mark J. Fitch

Abstract

Synoptic classification is used to identify meteorological conditions characteristic of high-pollution periods at Nogales, Arizona. Low surface winds determined by local surface cooling at night with little vertical mixing were found to be most important. This condition was simulated in a 0.79-m-square box filled with water with the lower surface made to model a 12-km-square region of the surface topography of the United States-Mexico border at Nogales. The aluminum base was cooled to induce the downslope flows. Photographs of dye initially placed on the surface at many locations were used to obtain a set of surface velocities that formed the input to the Diagnostic Wind Model (DWM). The DWM provided hourly velocity data with grids of 500- and 250-m spacings.

The similarity arguments used to analyze the relationship of the physical model to the atmosphere are discussed. Although the magnitude of the wind vectors in the physical model cannot be matched to the atmosphere, the directions can be used to assess the accuracy of the wind field obtained from a sparse set of field sites. A range of locations of these sites is analyzed to determine a strategy for obtaining sufficient wind data to depict satisfactory wind fields in this complex terrain.

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Patrick Conry
,
Ashish Sharma
,
Mark J. Potosnak
,
Laura S. Leo
,
Edward Bensman
,
Jessica J. Hellmann
, and
Harindra J. S. Fernando

Abstract

The interaction of global climate change and urban heat islands (UHI) is expected to have far-reaching impacts on the sustainability of the world’s rapidly growing urban population centers. Given that a wide range of spatiotemporal scales contributed by meteorological forcing and complex surface heterogeneity complicates UHI, a multimodel nested approach is used in this paper to study climate-change impacts on the Chicago, Illinois, UHI, covering a range of relevant scales. One-way dynamical downscaling is used with a model chain consisting of global climate (Community Atmosphere Model), regional climate (Weather Research and Forecasting Model), and microscale (“ENVI-met”) models. Nested mesoscale and microscale models are evaluated against the present-day observations (including a dedicated urban miniature field study), and the results favorably demonstrate the fidelity of the downscaling techniques that were used. A simple building-energy model is developed and used in conjunction with microscale-model output to calculate future energy demands for a building, and a substantial increase (as much as 26% during daytime) is noted for future (~2080) climate. Although winds and lake-breeze circulation for future climate are favorable for reducing energy usage by 7%, the benefits are outweighed by such factors as exacerbated UHI and air temperature. An adverse change in human-comfort indicators is also noted in the future climate, with 92% of the population experiencing thermal discomfort. The model chain that was used has general applicability for evaluating climate-change impacts on city centers and, hence, for urban-sustainability studies.

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Manuela Lehner
,
C. David Whiteman
,
Sebastian W. Hoch
,
Derek Jensen
,
Eric R. Pardyjak
,
Laura S. Leo
,
Silvana Di Sabatino
, and
Harindra J. S. Fernando

Abstract

Observations were taken on an east-facing sidewall at the foot of a desert mountain that borders a large valley, as part of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) field program at Dugway Proving Ground in Utah. A case study of nocturnal boundary layer development is presented for a night in mid-May when tethered-balloon measurements were taken to supplement other MATERHORN field measurements. The boundary layer development over the slope could be divided into three distinct phases during this night: 1) The evening transition from daytime upslope/up-valley winds to nighttime downslope winds was governed by the propagation of the shadow front. Because of the combination of complex topography at the site and the solar angle at this time of year, the shadow moved down the sidewall from approximately northwest to southeast, with the flow transition closely following the shadow front. 2) The flow transition was followed by a 3–4-h period of almost steady-state boundary layer conditions, with a shallow slope-parallel surface inversion and a pronounced downslope flow with a jet maximum located within the surface-based inversion. The shallow slope boundary layer was very sensitive to ambient flows, resulting in several small disturbances. 3) After approximately 2300 mountain standard time, the inversion that had formed over the adjacent valley repeatedly sloshed up the mountain sidewall, disturbing local downslope flows and causing rapid temperature drops.

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Jaynise M. Pérez Valentín
,
Harindra J. S. Fernando
,
G. S. Bhat
,
Hemantha W. Wijesekera
,
Jayesh Phadtare
, and
Edgar Gonzalez

Abstract

The relationship between eastward-propagating convective equatorial signals (CES) along the equatorial Indian Ocean (EIO) and the northward-propagating monsoon intraseasonal oscillations (MISOs) in the Bay of Bengal (BOB) was studied using observational datasets acquired during the 2018 and 2019 MISO-BOB field campaigns. Convective envelopes of MISOs originating from just south of the BOB were associated with both strong and weak eastward CES (average speed ∼6.4 m s−1). Strong CES contributed to ∼20% of the precipitation budget of BOB, and they spurred northward-propagating convective signals that matched the canonical speed of MISOs (1–2 m s−1). In contrast, weak CES contributed to ∼14% of the BOB precipitation budget, and they dissipated without significant northward propagation. Eastward-propagating intraseasonal oscillations (ISOs; period 30–60 days) and convectively coupled Kelvin waves (CCKWs; period 4–15 days) accounted for most precipitation variability across the EIO during the 2019 boreal summer as compared with that of 2018. An agreement could be noted between high moisture content in the midtroposphere and the active phases of CCKWs and ISOs for two observational locations in the BOB. Basin-scale thermodynamic conditions prior to the arrival of strong or weak CES revealed warmer or cooler sea surface temperatures, respectively. Flux measurements aboard a research vessel suggest that the evolution of MISOs associated with strong CES are signified by local enhanced air–sea interactions, in particular the supply of local moisture and sensible heat, which could enhance deep convection and further moisten the upper troposphere.

Significance Statement

Eastward-propagating convective signals along the equatorial Indian Ocean and their relationship to the northward-propagating spells of rainfall that lead to moisture variability in the Bay of Bengal are studied for the 2018 and 2019 southwest monsoon seasons using observational datasets acquired during field campaigns. Strong convective equatorial signals spurred northward-propagating convection, as compared with weak signals that dissipated without significant northward propagation. Wave spectral analysis showed CCKWs (period 4–15 days), and eastward ISOs (period 30–60 days) accounted for most of the precipitation variability, with the former dominating during the 2018 boreal summer. High moisture periods observed from radiosonde measurements show agreement with the active phases of CCKWs and ISOs.

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