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- Author or Editor: Qingfang Jiang x
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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, 
In addition, inversion splitting occurs downstream of a jump when 
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,
In addition, inversion splitting occurs downstream of a jump when
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Abstract
The impact of land surface snow processes on the Arctic stable boundary layer (ASBL) is investigated using the Navy’s Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) to reduce the cold bias caused by decoupling between the land surface and atmosphere. The Noah land surface model (LSM) with improved snow processes is examined using COAMPS forecast forcing in the one-dimensional mode for one month. The new snow physics shows that the snow properties, roughness length, and sensible heat flux are modified as expected to compensate for the old LSM deficiency. These new snow processes are incorporated into the COAMPS Noah LSM, and the 48-h forecasts using both old and new Noah LSMs are performed for January 2021 with an every-6-h data assimilation update cycle. Standard verifications of the 48-h forecasts have used all available observational datasets and the snow depth from the Land Information System (LIS) analyses. The statistics have shown reduced monthly mean cold biases ∼1°C by the new snow physics. The weaker strength of surface inversion and stronger turbulence kinetic energy (TKE) from the new snow physics provides a higher boundary layer due to significantly stronger eddy mixing. The simulations have also shown the insignificant impact of different lateral boundary conditions obtained from the global forecasts or analyses on the results of the new snow physics. This study highlights the importance of the revised snow physics in Noah LSM for reducing the decoupling problem, improving the forecasts, and studying ASBL physics over the Arctic region.
Abstract
The impact of land surface snow processes on the Arctic stable boundary layer (ASBL) is investigated using the Navy’s Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) to reduce the cold bias caused by decoupling between the land surface and atmosphere. The Noah land surface model (LSM) with improved snow processes is examined using COAMPS forecast forcing in the one-dimensional mode for one month. The new snow physics shows that the snow properties, roughness length, and sensible heat flux are modified as expected to compensate for the old LSM deficiency. These new snow processes are incorporated into the COAMPS Noah LSM, and the 48-h forecasts using both old and new Noah LSMs are performed for January 2021 with an every-6-h data assimilation update cycle. Standard verifications of the 48-h forecasts have used all available observational datasets and the snow depth from the Land Information System (LIS) analyses. The statistics have shown reduced monthly mean cold biases ∼1°C by the new snow physics. The weaker strength of surface inversion and stronger turbulence kinetic energy (TKE) from the new snow physics provides a higher boundary layer due to significantly stronger eddy mixing. The simulations have also shown the insignificant impact of different lateral boundary conditions obtained from the global forecasts or analyses on the results of the new snow physics. This study highlights the importance of the revised snow physics in Noah LSM for reducing the decoupling problem, improving the forecasts, and studying ASBL physics over the Arctic region.
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 
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
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.
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.
Abstract
The impact of moist processes on mountain waves over Sierra Nevada Mountain Range is investigated in this study. Aircraft measurements over Owens Valley obtained during the Terrain-induced Rotor Experiment (T-REX) indicate that mountain waves were generally weaker when the relative humidity maximum near the mountaintop level was above 70%. Four moist cases with a RH maximum near the mountaintop level greater than 90% have been further examined using a mesoscale model and a linear wave model. Two competing mechanisms governing the influence of moisture on mountain waves have been identified. The first mechanism involves low-level moisture that enhances flow–terrain interaction by reducing windward flow blocking. In the second mechanism, the moist airflow tends to damp mountain waves through destratifying the airflow and reducing the buoyancy frequency. The second mechanism dominates in the presence of a deep moist layer in the lower to middle troposphere, and the wave amplitude is significantly reduced associated with a smaller moist buoyancy frequency. With a shallow moist layer and strong low-level flow, the two mechanisms can become comparable in magnitude and largely offset each other.
Abstract
The impact of moist processes on mountain waves over Sierra Nevada Mountain Range is investigated in this study. Aircraft measurements over Owens Valley obtained during the Terrain-induced Rotor Experiment (T-REX) indicate that mountain waves were generally weaker when the relative humidity maximum near the mountaintop level was above 70%. Four moist cases with a RH maximum near the mountaintop level greater than 90% have been further examined using a mesoscale model and a linear wave model. Two competing mechanisms governing the influence of moisture on mountain waves have been identified. The first mechanism involves low-level moisture that enhances flow–terrain interaction by reducing windward flow blocking. In the second mechanism, the moist airflow tends to damp mountain waves through destratifying the airflow and reducing the buoyancy frequency. The second mechanism dominates in the presence of a deep moist layer in the lower to middle troposphere, and the wave amplitude is significantly reduced associated with a smaller moist buoyancy frequency. With a shallow moist layer and strong low-level flow, the two mechanisms can become comparable in magnitude and largely offset each other.
