Hye-Yeong Chun

An overview of the past, present and future of gravity‐wave drag parametrization for numerical climate and weather prediction models

Abstract An overview of the parametrization of gravity ‐wave drag in numerical ‐weather prediction and climate simulation models is presented. The focus is primarily on understanding the current status of gravity wave drag parametrization as a step towards the new parametrizations that will be needed for the next generation of atmospheric models. Both the early history and latest developments in the field are discussed. Parametrizations developed specifically for orographic and convective sources of gravity waves are described separately, as are newer parametrizations that collectively treat a spectrum of gravity wave motions. The differences in issues in and approaches for the parametrization of the lower and upper atmospheres are highlighted. Various emerging issues are also discussed, such as explicitly resolved gravity waves and gravity wave drag in models, and a range of unparametrized gravity wave processes that may need attention for the next generation of gravity wave drag parametrizations in models.

Momentum Flux by Thermally Induced Internal Gravity Waves and Its Approximation for Large-Scale Models

Abstract Gravity wave momentum flux induced by thermal forcing representing latent heating due to cumulus convection is investigated analytically from a viewpoint of a subgrid-scale drag for the large-scale flow, and a possible way to parameterize the momentum flux in large-scale models is proposed. For the formulations of the momentum flux and its vertical derivative, two-dimensional, steady-state, linear perturbations induced by thermal forcing in a uniform basic-state wind are considered. The calculated momentum flux is zero below the forcing bottom, varies with height in the forcing region, and remains constant above the forcing top with the forcing top value. The sign of the momentum flux at the forcing top depends on the basic-state wind according to the wave energy–momentum flux relationship. Inside the forcing region, there exists a vertical convergence or divergence of the momentum flux that can influence the zonal mean flow tendency. The maximum magnitude of the zonal mean flow tendency contribu...

A Laboratory Model of Urban Street-Canyon Flows

Abstract A circulating water channel is constructed to examine urban street-canyon flow. In the cases of an even-notch street canyon in which model buildings on both sides of the street have equal heights, one vortex is observed in model canyons with aspect ratios of 1 and 1.5, and two counterrotating vortices are observed in canyons with aspect ratios of 2, 2.4, and 3. In all of the even-notch cases, the center of the vortex (or the upper vortex) is located slightly downstream of the canyon center, and the downward motion downstream is stronger than the upward motion upstream. The magnitudes of the maximum updraft and downdraft are almost independent of the aspect ratio. In the case of a stepup notch, one vortex is observed in the canyon. In the case of a stepdown notch, two counterrotating vortices are observed. The upper vortex resembles to some extent an isolated roughness flow, and the lower vortex is characterized by a skimming flow. It is shown that the results of the water-channel experiments are ...

Generation Mechanisms of Convectively Forced Internal Gravity Waves and Their Propagation to the Stratosphere

Abstract Characteristics of gravity waves induced by mesoscale convective storms and the gravity wave sources are investigated using a two-dimensional cloud-resolving numerical model. In a nonlinear moist (control) simulation, the convective system reaches a quasi-steady state after 4 h in which convective cells are periodically regenerated from a gust front updraft. In the convective storms, there are two types of wave forcing: nonlinear forcing in the form of the divergences of momentum and heat flux, and diabatic forcing. The magnitude of the nonlinear source is 2 to 3 times larger than the diabatic source, especially in the upper troposphere. Three quasi-linear dry simulations forced by the wave sources obtained from the control (CTL) simulation are performed to investigate characteristics of gravity waves induced by the various wave source mechanisms. In the three dry simulations, the magnitudes of the perturbations produced in the stratosphere are comparable, yet much larger than those in the CTL si...

An Updated Parameterization of Convectively Forced Gravity Wave Drag for Use in Large-Scale Models

An updated parameterization of gravity wave drag forced by subgrid-scale cumulus convection (GWDC) in large-scale models is proposed. For an analytical formulation of the cloud-top wave stress, two-dimensional, steady-state, linear perturbations induced by diabatic heating are found in a two-layer structure with a piecewise constant shear with a critical level in the lower layer, a uniform flow in the upper layer, and piecewise constant buoyancy frequencies in each layer. The dynamical frame considered is relative to the diabatic forcing and the gravity waves obtained are stationary relative to the diabatic forcing, not necessarily stationary relative to the ground. The cloud-top wave momentum flux is proportional to the square of the magnitude of the convective heating, inversely proportional to the basic-state wind speed, and related to the buoyancy frequencies in each layer. The effect of wind shear in the convective region on the cloud-top momentum flux is negligible, while a difference in the stability between the two layers affects the momentum flux significantly. The cloud-top momentum flux increases as the stability in the convective region decreases and the stability above it increases. A global distribution of the 200-mb wave stress calculated using climatological data reveals that the wave stress in the present study is larger than that in a uniform wind and stability case. This is mainly due to the stability difference between the convective region and the region above it. A methodology of parameterizing GWDC in large-scale models using the wave saturation hypothesis is presented.

Effects of Diabatic Cooling in a Shear Flow with a Critical Level

Abstract The response of a two-dimensional, stably stratified shear flow to diabatic cooling, which represents the evaporative cooling of falling precipitation in the subcloud layer, is examined using both a linear analytical theory and a nonlinear numerical model. The ambient wind is allowed to reverse its direction at a certain height and the cooling is specified from the surface to a height below the wind reversal level. From a scale analysis of the governing equations a nonlinearity factor of the thermally induced finite-amplitude wave, gQ0l(cpT0U02N), is found. From a scale analysis of the linear system, it is shown that the wind shear can modify the condition in which the upstream propagation of the density current is opposed by the ambient wind. When the shear and the basic wind are of opposite sign, small basic wind is enough to prevent the upstream propagation of the density current. This is because part of the cooling is used to compensate the positive vorticity associated with the positive wind...

Impact of a Convectively Forced Gravity Wave Drag Parameterization in NCAR CCM3

A parameterization of gravity wave drag forced by subgrid-scale cumulus convection (GWDC) proposed by Chun and Baik is implemented into the National Center for Atmospheric Research Community Climate Model (NCAR CCM3) and its effect on perpetual January and July climate is investigated. The cloud-top gravity wave stress is concentrated in the intertropical convergence zone where persistent deep cumulus clouds exist. The resultant zonal wind acceleration due to the breaking of convectively forced gravity waves is predominantly found in the tropical lower stratosphere with westerly acceleration above cloud top and easterly acceleration just below it. Since the parameterized gravity waves are stationary relative to convective clouds, wave breaking occurs mainly in the tropical lower stratosphere where the zonal wind is weak enough for wave saturation. It is shown that the GWDC parameterization significantly alleviates the systematic model biases of zonal-mean zonal wind and temperature. In particular, excessive easterlies in the tropical stratosphere and excessive cold temperatures in the tropical lower stratosphere are reduced by more than 50% by including the GWDC parameterization. The horizontal wind divergence field in the tropical upper troposphere and lower stratosphere is also significantly improved with the GWDC parameterization. The impact of the GWDC parameterization extends to mid- to high latitudes through planetary wave activity in the winter hemisphere. The increased amplitude of zonal wavenumber 3 in the January Northern Hemisphere and the increased amplitude of zonal wavenumber 2 in the July Southern Hemisphere lead to significant improvements in model performance. The impact of the GWDC parameterization on Eliassen‐Palm (EP) flux divergence forcing by stationary waves is generally opposite to that by transient waves in the extratropics, especially in the Northern Hemisphere wintertime. Hence, the zonal-mean zonal wind change by the GWDC parameterization occurs mainly in the Tropics by direct gravity wave drag forcing.

Two-dimensional numerical modeling of flow and dispersion in the presence of hill and buildings

Abstract A two-dimensional numerical model with a k – e turbulence closure scheme and a non-uniform grid system is used to examine the effects of a single hill and/or two buildings on the flow and pollutant dispersion. In a single-hill configuration, the hill slope is an important factor determining the existence of a recirculation zone behind the hill. As the hill slope increases, the recirculation zone becomes wide. In the presence of a single street canyon formed by two isolated buildings, the ambient wind blows not parallel to the roof-level but passes above the canyon with a small curvature. This results in more momentum transfer into the street canyon by the ambient wind than that in a slot-flow case. When there exist both a single hill and two buildings, the vertical velocity at the top height of the canyon becomes negligible as the height of the downwind street canyon approaches to that of the upwind hill. A flow reattachment promoted by the upwind hill acts to restrict the vertical extent of vortex below the roof-level and to enlarge the size of recirculation zone behind the downwind building. The obstacle effects on pollutant plume dispersion are examined by analyzing the vertical standard deviation and average height of plume. The upward motion induced by flow impingement influences plume height in front of the obstacles, while behind them the downward motion related to flow reattachment to the ground largely affects the vertical spread and average height of plume. In the presence of both a single hill and two buildings, an accelerated reattachment at the top height of the canyon by the upwind hill plays an important role in the vertical spread and average height of plume.

A dynamical model for urban heat islands

Effects of nonlinearity on theairflow past an urban heat island and precipitationchange downwind, are investigated analytically in thecontext of the weakly nonlinear response of a stablystratified uniform flow to specified heating. Theheating structure is assumed to be bell-shaped in thehorizontal and exponentially decreasing with height.The forcing to the first-order equation exhibitscooling in the concentrated low-level heating region.The linear solution component shows upward motiondownstream as suggested by many previous studies. Theweakly nonlinear solution component shows downward orupward motion downstream depending on the heatingdepth. It is proposed that when the heating depth islarge, but still within a valid range of theperturbation expansion, the linear and weaklynonlinear effects constructively work together toproduce enhanced upward motion on the downstream side,not far from the heating centre. This explains toa greater extent the precipitation enhancement downstream ofthe heat island than is possible from the linear effect alone. Itis also proposed that when the heating depth is small,the linear and weakly nonlinear effects destructivelywork together to reduce upward motion on thedownstream side, not far from the heating centre. Thisexplains to a greater extent the lack of precipitation enhancementdownstream than is possible from the linear effect alone.

Transient, Linear Dynamics of a Stably Stratified Shear Flow with Thermal Forcing and a Critical Level

Abstract The transient, linear response of a stably stratified atmosphere to thermal forcing in the presence of a critical level is investigated analytically using the Green’s function method. The prescribed thermal forcing is located below or across a critical level. The target solution is for the finite-depth steady forcing, but intermediate solutions to the line-type pulse forcing, finite-depth pulse forcing, and line-type steady forcing are analyzed in some detail because these solutions give some insight into the basic dynamics of the response to the target forcing. The responses to the pulse forcings exhibit the moving mode whose center travels downstream with a speed of the basic-state wind. In the vicinity of the initial forcing, gravity waves are attenuated across the critical level. In response to the line-type pulse forcing, after some time the magnitude of the perturbation vertical velocity at the center of the moving mode remains almost unchanged with time. The transient critical level is a f...