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Jong-Jin Baik

Abstract

Two-dimensional airflow characteristics past a heat island are investigated using both a linear analytic model and a numerical model in the context of the response of a stably stratified atmosphere to specified low-level heating in a constant shear flow. Results from the steady-state, linear, analytic solutions exhibit typical flow response fields that gravity waves produce in response to the local heat source in the presence of environmental flow. The magnitude of the perturbation vertical velocity is shown to be much larger in the shear-flow case than in the uniform-flow case. This is because the basic-state wind shear is a source of the perturbation wave energy.

Nonlinear numerical model experiments over a wide range of heating amplitudes are performed to examine nonlinear effects on the simulated flow field. For smaller heating amplitude (hence, smaller nonlinearity factor), the flow response field is similar to that produced by the linear gravity waves. On the other hand, two distinct flow features are observed for larger heating amplitude (hence, larger nonlinearity factor): the gravity-wave-type response field on the upstream side of the heat island and the strong updraft circulation cell located on the downstream side. As the heating amplitude increases, the updraft circulation cell strengthens and shifts farther downwind. The strong updraft cell is believed to be partly responsible for precipitation enhancement observed on the downstream side of the heat island. It is found that the continuing downwind propagation of the updraft circulation cell is related to basic-state wind speed.

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Steven Businger and Jong-Jin Baik

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A mesoscale “arctic hurricane” developed over the western Bering Sea on 7 March 1977 and traveled eastward parallel to the ice edge along a zone of large sea surface temperature gradient. Satellite imagery reveals spiral cloud bands of unusual symmetry and mesoscale dimension associated with the mature stage of the low. The track of the low pressure center passed over the rawinsonde station at St. Paul Island where time series of surface data show a pronounced maximum in equivalent potential temperature at the core of the low. The storm made landfall with surface winds >30 m s−1; at Cape Newenham, Alaska, on 9 March and rapidly dissipated thereafter.

Synoptic analyses show that the arctic hurricane formed at the leading edge of an outflow of arctic air that originated over the ice and passed over the open water of the western Bering 5u. In the mid- and upper troposphere a large cold-core low dominated the Bering Sea region. Quasi-geostrophic analysis at 0000 UTC 7 March 1977 reveals conditions conducive to synoptic-scale ascent over the region of the incipient low, as a sharp upper-level short wave crosses the Siberian coast. Conversely, during its mature stage little quasi-geostrophic forcing is seen over the low.

In order to investigate the ability of sea surface heat fluxes to develop and maintain the arctic hurricane, an analytical model based on the Carnot cycle, and an axisymmetric numerical model with the Kuo cumulus parameterization scheme are applied. The analytical calculation of the pressure drop from the outermost closed isobar to the storm center results in a central pressure of 973 mb, which agrees well with observation. When the initial environment of the numerical model is set to be similar to that observed with the arctic hurricane, the model correctly predicts the minimum sea-level pressure, strength of the wind circulation, and the magnitude of sensible beat fluxes observed with the storm. The dynamic and thermodynamic structures of the simulated storm are similar to those of tropical cyclones. The predicted development time of the storm is longer than observations suggest, and the diameter of the simulated anvil outflow is somewhat larger, pointing to the likely importance of baroclinic processes in the evolution of the disturbance, and the need for further numerical studies with mesoscale models that employ full three-dimensional primitive equations.

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Hyunho Lee and Jong-Jin Baik
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Hyunho Lee and Jong-Jin Baik

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A physically based parameterization for the autoconversion is derived by solving the stochastic collection equation (SCE) with an approximated collection kernel. The collection kernel is constructed using the terminal velocity of cloud droplets and the collision efficiency between cloud droplets that is obtained using a particle trajectory model. The new parameterization proposed in this study is validated through comparison with results obtained by a bin-based direct SCE solver and other autoconversion parameterizations using a box model. The autoconversion-related time scale and drop number concentration are employed for the validation. The results of the new parameterization are shown to most closely match those of the direct SCE solver. It is also shown that the dependency of the autoconversion rate on drop number concentration in the new parameterization is similar to that in the direct SCE solver, which is partially caused by the shape of drop size distribution. The new parameterization and other parameterizations are implemented into a cloud-resolving model, and idealized shallow warm clouds are simulated. The autoconversion parameterizations that yield the small (large) autoconversion rate tend to predict large (small) cloud optical thickness, small (large) cloud fraction, and small (large) surface precipitation amount. Cloud optical thickness and cloud fraction are changed by up to ~45% and ~20% by autoconversion parameterizations, respectively. The new parameterization tends to yield the moderate autoconversion rate among the autoconversion parameterizations. Moreover, it predicts cloud optical thickness, cloud fraction, and surface precipitation amount that are generally the closest to those of the bin microphysics scheme.

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Jae-Jin Kim and Jong-Jin Baik

Abstract

This study investigates thermal effects on the flow and pollutant dispersion in urban street canyons. A two-dimensional numerical model with a kϵ turbulent closure scheme is developed, and the heat transfer between the air and the building wall or street-canyon bottom is effectively represented by a wall function. For each of seven cases with different aspect ratios (building height/width between buildings = 0.5, 1, 1.5, 2, 2.5, 3, and 3.5), four thermal situations (no heating, upwind building-wall heating, street-canyon bottom heating, and downwind building-wall heating) are considered.

In the cases of upwind building-wall heating, one vortex appears regardless of aspect ratio. When the aspect ratio is greater than or equal to 1.5, the upward motion forced by upwind building-wall heating overcomes the downward motion that appears in the cases of no heating. In the cases of street-canyon bottom heating, when the aspect ratio is less than 3, flow patterns are similar to those in the cases of upwind building-wall heating. This similarity is because the maximum temperature axis is shifted toward the upwind side by the horizontal motion. However, when the aspect ratio is 3 or 3.5, the horizontal velocity is not strong enough to shift the maximum temperature axis toward the upwind side. When the maximum temperature axis is located near the center of the street canyon, two counterrotating vortices appear side by side in the lower layer due to the thermal upward motion around the axis, while the vortex in the upper layer is little influenced by bottom heating. With downwind building-wall heating, two counterrotating vortices appear except in the 0.5 aspect ratio case. To a large extent, the vortex in the upper layer is mechanically induced by the ambient wind, while the vortex in the lower layer is thermally induced by downwind building-wall heating.

The dispersion of pollutants released at the street level is shown to be quite dependent upon aspect ratio and heat source location. The vortex number and intensity greatly influence the residue concentration ratio (ratio of the total pollutant amount remaining in the street canyon to the total amount of pollutants emitted) by controlling the travel pathway and escape time of pollutants.

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Jong-Jin Baik and Jae-Jin Kim

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The flow and pollutant dispersion in urban street canyons are investigated using a two-dimensional numerical model with the k–ε turbulent closure scheme. It is shown that the flow field is characterized mainly by the number and intensity of vortices produced in the street canyon. As the street aspect ratio (ratio of the building height to the width between buildings) increases, the number of vortices increases. In the upper-canyon region, the downward motion near the downwind building is stronger than the upward motion near the upwind building, and the turbulent kinetic energy (TKE) is higher near the downwind building than near the upwind building because of stronger wind shears near the downwind building. The TKE budget analysis shows that the shear production is high near the interface between the ambient flow and the street canyon flow and that the turbulent dissipation is also high where the shear production is high. The horizontal advection and diffusion are found to play a crucial role in splitting the vortex into two or more. There is a critical value of the ambient wind speed above which the number and distribution pattern of vortices remain the same regardless of the ambient wind speed. For the given flow fields, two different emission sources (street-level source and advected source) are considered to examine pollutant dispersion in the street canyons that have a one-vortex flow regime and a two-vortex flow regime. Results indicate that the distribution of pollutant concentration in the street canyons during each period of continuous emission and nonemission can be largely explained in terms of the vortex circulation.

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Han-Gyul Jin and Jong-Jin Baik

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A new parameterization of the accretion of cloud water by snow for use in bulk microphysics schemes is derived as an analytic approximation of the stochastic collection equation (SCE), where the theoretical collision efficiency for individual snowflake–cloud droplet pairs is applied. The snowflake shape is assumed to be nonspherical with the mass–size and area–size relations suggested by an observational study. The performance of the new parameterization is compared to two parameterizations based on the continuous collection equation, one with the spherical shape assumption for snowflakes (SPH-CON), and the other with the nonspherical shape assumption employed in the new parameterization (NSP-CON). In box model simulations, only the new parameterization reproduces a relatively slow decrease in the cloud droplet number concentration, which is predicted by the direct SCE solver. This results from considering the preferential collection of cloud droplets depending on their sizes in the new parameterization based on the SCE. In idealized squall-line simulations using a cloud-resolving model, the new parameterization predicts heavier precipitation in the convective core region compared to SPH-CON, and a broader area of the trailing stratiform rain compared to NSP-CON due to the horizontal advection of greater amount of snow in the upper layer. In the real-case simulations of a line-shaped mesoscale convective system that passed over the central Korean Peninsula, the new parameterization predicts higher frequencies of light precipitation rates and lower frequencies of heavy precipitation rates. The relatively large amount of upper-level snow in the new parameterization contributes to a broadening of the area with significant snow water path.

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Hye-Yeong Chun and Jong-Jin Baik

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The weakly nonlinear response of a two-dimensional stably stratified atmosphere to prescribed diabatic heating in a uniform flow is investigated analytically using perturbation expansion in a small value of the nonlinearity factor for the thermally induced waves. The diabatic heating is assumed to have only a zeroth-order term specified to be vertically homogeneous between the surface and a certain height and bell shaped in the horizontal. The first-order (weakly nonlinear) solutions are obtained using the FFT algorithm after solutions in wavenumber space are obtained analytically. The forcing (F) to the first-order perturbation equation induced by the Jacobian of the zeroth-order (linear) solutions always represents cooling in the lower layer regardless of specified forcing type (cooling or heating). The vertical structure of F is related to the nondimensional heating depth (d) or the inverse Froude number. The first-order solutions are valid for relatively small values (<3) of d. The main nonlinear effect is to produce a strong convergence region near the surface associated with the zeroth-order perturbations regardless of the value of d. This convergence is responsible for producing upward motion in the center of the forcing region that extends upstream. As a result, the zeroth-order downward motion becomes weaker according to a degree of nonlinearity. The relative magnitude of the zeroth-order downward motion and the first-order upward motion upstream of the forcing can be determined by d. The source of the first-order wave energy is found to come mainly from the horizontal advection of the zeroth-order total wave energy by the first-order perturbation horizontal wind.

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Ji-Young Han and Jong-Jin Baik

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One of the important problems in mesoscale atmospheric dynamics is how the atmosphere responds to convective heating or cooling. Here, the authors examine nonlinear effects on convectively forced mesoscale flows in the context of the nonlinear response of a stably stratified flow to elevated steady heating in two dimensions using a nondimensional numerical model. Results of two-dimensional numerical experiments in a uniform flow show that even without vertical wind shear, a separation of an upwind cellular updraft from the steady heating-induced main updraft occurs in a highly nonlinear flow system. This separation occurs as the compensating cellular downdraft associated with a secondary maximum in the main updraft develops. As the nonlinearity of the flow system increases, the upwind cellular updraft is separated earlier and becomes stronger. Smaller viscous terms result in the separation of more cellular updrafts, which become stronger and move farther away from the main updraft region. In particular, in an inviscid flow, cellular updrafts are periodically separated from the main updraft, and the first cellular updraft and downdraft have intensities comparable to the intensity of the main updraft. In a viscid flow with a constant vertical wind shear up to a certain height, the propagating cellular updraft and downdraft are produced when the nonlinearity is large, as in a uniform flow. Stronger vertical wind shear leads to the earlier formation of the cellular updraft and its stronger intensity, faster propagating speed, and longer lifetime. Results of numerical experiments with squall line–type forcing imply that the highly nonlinear state is necessary for the development of cellular updrafts.

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Ji-Young Han and Jong-Jin Baik

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Convectively forced mesoscale flows in a shear flow with a critical level are theoretically investigated by obtaining analytic solutions for a hydrostatic, nonrotating, inviscid, Boussinesq airflow system. The response to surface pulse heating shows that near the center of the moving mode, the magnitude of the vertical velocity becomes constant after some time, whereas the magnitudes of the vertical displacement and perturbation horizontal velocity increase linearly with time. It is confirmed from the solutions obtained in present and previous studies that this result is valid regardless of the basic-state wind profile and dimension. The response to 3D finite-depth steady heating representing latent heating due to cumulus convection shows that, unlike in two dimensions, a low-level updraft that is necessary to sustain deep convection always occurs at the heating center regardless of the intensity of vertical wind shear and the heating depth. For deep heating across a critical level, little change occurs in the perturbation field below the critical level, although the heating top height increases. This is because downward-propagating gravity waves induced by the heating above, but not near, the critical level can hardly affect the flow response field below the critical level. When the basic-state wind backs with height, the vertex of V-shaped perturbations above the heating top points to a direction rotated a little clockwise from the basic-state wind direction. This is because the V-shaped perturbations above the heating top is induced by upward-propagating gravity waves that have passed through the layer below where the basic-state wind direction is clockwise relative to that above.

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