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Qingfang Jiang

Abstract

Applicability of the reduced-gravity shallow-water (RGSW) theory to a shallow atmospheric layer capped by an inversion underneath a deep stratified atmosphere over a two-dimensional ridge has been investigated using linear analysis and nonlinear numerical simulations. Two key nondimensional parameters are identified: namely, and , where g′ is the reduced-gravity acceleration; H 0 is the RGSW layer depth; and N and U are the buoyancy frequency and wind speed, respectively, in the layer above the inversion. If J and γ are around unity or larger, the response of the RGSW flow over the ridge can be significantly modified by pressure perturbations aloft. Any jumplike perturbations in the RGSW layer rapidly decay while propagating away from the ridge as the perturbation energy radiates into the upper layer. With J and γ much less than unity, RGSW theory is more adequate for describing RGSW flows.

In addition, inversion splitting occurs downstream of a jump when , where N i is the buoyancy frequency in the inversion and h m stands for the ridge height. A less stratified upper layer with slower winds in general has less influence on the RGSW flow below and favors the application of the RGSW theory. For a thick inversion (d), the equivalent RGSW flow depth is approximately given by H + d/2, where H is the depth of the neutral layer below the inversion.

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Qingfang Jiang

Abstract

Land–sea breezes (LSBs) induced by diurnal differential heating are examined using a three-dimensional linear model employing fast Fourier transform with emphasis on the complex coastline shape and geometry, the earth’s rotation, and background wind effects. It has been demonstrated that the low-level vertical motion associated with LSB can be significantly enhanced over a bay (peninsula) because of convergence of perturbations induced by differential heating along a seaward concave (convex) coastline. The dependence of surface winds and vertical motion patterns and their evolutions on the coastline geometries such as the width and the aspect ratio of the bay, the earth’s rotation, and the background winds are investigated.

The LSB induced by an isolated tropical island is characterized by onshore flow and ascent over the island in the afternoon to early evening, with a reversal of direction from midnight to early morning. The diurnal heating–induced vertical motion is greatly enhanced over the island and weakened offshore because of the convergence and divergence of perturbations. In the presence of background flow, stronger diurnal perturbations are found at the downwind side of the island, which can extend far downstream associated with inertia–gravity waves.

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Qingfang Jiang

Abstract

Characteristics and dynamics of offshore diurnal waves induced by land–sea differential heating are examined using linear theory. Two types of heating profiles are investigated, namely a shallow heating source confined within an atmospheric boundary layer (BL) and a deep heating source located above the boundary layer.

It is demonstrated that a boundary layer top inversion or a more stable layer aloft tends to partially trap diurnal waves in the BL and consequently extend perturbations well offshore. The wave amplitude decays with offshore distance due to BL friction and leakage of energy into the free atmosphere. The dependence of trapped waves on the inversion height and strength, atmosphere stratification, latitude, BL friction, and background winds is investigated. Diurnal waves generated by a deep heating source extending well above the BL are characterized by longer wavelengths, faster propagation, and substantially longer e-folding decay distances than waves induced by a BL source. For the latter, BL friction has little impact on the e-folding decay distance, as waves are mostly located in the free atmosphere rather than in a frictional BL.

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Qingfang Jiang

Abstract

Many topographic barriers are comprised of a series of concave or convex ridges that modulate the intensity and distribution of precipitation over mountainous areas. In this model-based idealized study, stratiform precipitation associated with stratified moist airflow past idealized concave ridges is investigated with a focus on windward blocking, flow confluence, and the associated precipitation enhancement.

It is found that flow confluence and precipitation enhancement by a concave ridge are controlled by the nondimensional ridge height M (M = Nmhm /U, where Nm is the moist buoyancy frequency, hm is the maximum ridge height, and U is the wind speed), based on which three dynamical regimes can be defined. In the linear regime (M < 0.4), a flow confluence zone is present over the upwind slope of the ridge vertex, where precipitation is significantly enhanced. The precipitation enhancement is due to the additional updraft driven by the horizontal flow convergence with a considerable contribution from lateral confluence. In the blocking regime (0.4 < M < Mc ), the area and intensity of the flow confluence zone decrease with increasing mountain height due to low-level blocking. The critical nondimensional ridge height (Mc ) for windward flow stagnation decreases with increasing concave angle. In the two regimes, flow confluence and precipitation enhancement are more pronounced for concave ridges with a longer cross-stream dimension or a larger concave angle. In the flow reversal regime (M > Mc ), no steady state can be achieved and the precipitation enhancement at the vertex is absent.

In addition, the flow confluence and precipitation enhancement upstream of a concave ridge are sensitive to the presence of a relative gap or peak at the vertex, the earth’s rotation, and the incident wind. The relevant dynamics has been examined.

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Qingfang Jiang

Abstract

The impact of Kelvin–Helmholtz billows (KHBs) in an elevated shear layer (ESL) on the underlying atmospheric boundary layer (BL) is examined utilizing a group of large-eddy simulations. In these simulations, KHBs develop in the ESL and experience exponential growth, saturation, and exponential decay stages. In response, strong wavy motion occurs in the BL, inducing rotor circulations near the surface when the BL is stable. During the saturation stage, secondary instability develops in the ESL and the wavy BL almost simultaneously, followed by the breakdown of the quasi-two-dimensional KH billows and BL waves into three-dimensional turbulence. Consequently, during and after a KH event, the underlying BL becomes more turbulent with its depth increased and stratification weakened substantially, suggestive of significant lasting impact of elevated KH billows on the atmospheric BL. The eventual impact of KHBs on the BL is found to be sensitive to both the ESL and BL characteristics.

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Qingfang Jiang

Abstract

The influence of swell on turbulence and scalar profiles in a marine surface layer and underlying physics is examined in this study through diagnosis of large-eddy simulations (LES) that explicitly resolve the surface layer and underlying swell. In general, under stable conditions, the mean wind and scalar profiles can be significantly modified by swell. The influence of swell on wind shear, turbulence structure, scalar profiles, and evaporation duct (ED) characteristics becomes less pronounced in a more convective boundary layer, where the buoyancy production of turbulence is significant. Dynamically, swell has little direct impact on scalar profiles. Instead it modifies the vertical wind shear by exerting pressure drag on the wave boundary layer. The resulting redistribution of vertical wind shear leads to changes in turbulence production and therefore turbulence mixing of scalars. Over swell, the eddy diffusivities from LES systematically deviate from the Monin–Obukhov similarity theory (MOST) prediction, implying that MOST becomes invalid over a swell-dominated sea. The deviations from MOST are more pronounced in a neutral or stable boundary layer under relatively low winds and less so in a convective boundary layer.

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Qingfang Jiang
and
Shouping Wang

Abstract

The impact of gravity waves on marine stratocumulus is investigated using a large-eddy simulation model initialized with sounding profiles composited from the Variability of American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Regional Experiment (VOCALS-Rex) aircraft measurements and forced by convergence or divergence that mimics mesoscale diurnal, semidiurnal, and quarter-diurnal waves. These simulations suggest that wave-induced vertical motion can dramatically modify the cloud albedo and morphology through nonlinear cloud–aerosol–precipitation–circulation–turbulence feedback.

In general, wave-induced ascent tends to increase the liquid water path (LWP) and the cloud albedo. With a proper aerosol number concentration, the increase in the LWP leads to enhanced precipitation, which triggers or strengthens mesoscale circulations in the boundary layer and accelerates cloud cellularization. Precipitation also tends to create a decoupling structure by weakening the turbulence in the subcloud layer. Wave-induced descent decreases the cloud albedo by dissipating clouds and forcing a transition from overcast to scattered clouds or from closed to open cells. The overall effect of gravity waves on the cloud variability and morphology depends on the cloud property, aerosol concentration, and wave characteristics. In several simulations, a transition from closed to open cells occurs under the influence of gravity waves, implying that some of the pockets of clouds (POCs) observed over open oceans may be related to gravity wave activities.

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Qingfang Jiang
and
Shouping Wang

Abstract

The impact of aerosol replenishment in a stratocumulus-topped boundary layer (STBL) on cloud morphology and albedo is examined using a large-eddy simulation (LES) model in conjunction with a prey–predator-type dynamical model following Koren and Feingold. In both the LES and the prey–predator models, the aerosol replenishment is represented as a relaxation term toward an ambient aerosol concentration with a time scale . The LESs suggest the existence of three distinct cloud regimes corresponding to different aerosol relaxation times. Specifically, for a small , the simulations are characterized by a large aerosol concentration, weak precipitation, relatively thick cloud depth, and closed cells. For a moderate , the simulated clouds exhibit open cellular patterns, in accordance with low aerosol concentration and moderate precipitation that oscillates in time. For a large , the aerosol may be depleted by intense drizzling and accordingly the cloud disappears. The critical aerosol relaxation times that separate these regimes vary with the ambient aerosol number concentration and cloud depth. Solutions from the low-order dynamical model with parameters relevant to the LESs are in general consistent with the LES results and provide further insight into the interplay among clouds, aerosol, and precipitation.

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Qingfang Jiang
and
James D. Doyle

Abstract

The characteristics of gravity waves excited by the complex terrain of the central Alps during the intensive observational period (IOP) 8 of the Mesoscale Alpine Programme (MAP) is studied through the analysis of aircraft in situ measurements, GPS dropsondes, radiosondes, airborne lidar data, and numerical simulations.

Mountain wave breaking occurred over the central Alps on 21 October 1999, associated with wind shear, wind turning, and a critical level with Richardson number less than unity just above the flight level (∼5.7 km) of the research aircraft NCAR Electra. The Electra flew two repeated transverses across the Ötztaler Alpen, during which localized turbulence was sampled. The observed maximum vertical motion was 9 m s−1, corresponding to a turbulent kinetic energy (TKE) maximum of 10.5 m2 s−2. Spectrum analysis indicates an inertia subrange up to 5-km wavelength and multiple energy-containing spikes corresponding to a wide range of wavelengths.

Manual analysis of GPS dropsonde data indicates the presence of strong flow descent and a downslope windstorm over the lee slope of the Ötztaler Alpen. Farther downstream, a transition occurs across a deep hydraulic jump associated with the ascent of isentropes and local wind reversal. During the first transverse, the turbulent region is convectively unstable as indicated by a positive sensible heat flux within the turbulent portion of the segment. The TKE derived from the flight-level data indicates multiple narrow spikes, which match the patterns shown in the diagnosed buoyancy production rate of TKE. The turbulence is nonisotropic with the major TKE contribution from the υ-wind component. The convectively unstable zone is advected downstream during the second transverse and the turbulence becomes much stronger and more isotropic.

The downslope windstorm, flow descent, and transition to turbulence through a hydraulic jump are captured by a real-data Coupled Ocean–Atmosphere Mesoscale Predition System (COAMPS) simulation. Several idealized simulations are performed motivated by the observations of multiscale waves forced by the complex terrain underneath. The simulations indicate that multiscale terrain promotes wave breaking, increases mountain drag, and enhances the downslope winds and TKE generation.

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Qingfang Jiang
and
Ronald B. Smith

Abstract

To better understand mountain-induced gravity wave breaking and potential vorticity generation in the troposphere, a two-layer hydrostatic flow over a three-dimensional Witch-of-Agnesi type of mountain is investigated. It is suggested that a two-layer model is the simplest model in which the partitioning of upper- and lower-level wave breaking and dissipation can be studied.

High-resolution shallow water model runs are carried out with unsheared upstream flow and a wide variety of mountain heights. A regime diagram is constructed, in which gravity wave breaking is classified based on shock number, location, and type. It is demonstrated that different types of shocks identified in the numerical simulations can be consistently described using a shock regime diagram, derived from viscous shock theory. Four curious shock properties are shown to influence orographic flow: the steepening requirement, the tendency for external jumps to amplify shear, the bifurcation in external jumps, and the “double shock.” Some results are compared with continuously stratified flow simulations by a nonhydrostatic mesoscale model.

It is demonstrated that vertical wind shear controls the vertical distribution of wave breaking and potential vorticity generation.

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