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Soon-Ung Park

Abstract

The method for estimating dry deposition velocity using local routine surface measurements with some empirical constants used in the Regional Acid Deposition Model is developed and implemented to the Lagrangian particle dispersion model to more accurately estimate near-surface concentrations of the SO2 pollutant. A test is performed for the synoptic case of a weak westerly geostrophic wind at 850 hPa with the cloud amount of less than 5/10. Hourly surface data from 64 sites located in South Korea are hourly averaged for five spring seasons from 1989 to 1993 for the chosen synoptic situation and used to construct 3D meteorological fields and turbulent fields in the boundary layer. The dry deposition velocity of SO2 estimated by the present model ranges from 0.01 to 1.4 cm s−1 with relatively large values in the daytime. The estimated near-surface concentrations at the height of 1.5 m above the ground through the Lagrangian particle dispersion model including the effect of dry deposition indicate that the impact of the dry deposition on the near-surface concentration is significant during the day when the convective turbulent intensity is strong and the deposition velocity is large. The maximum reduction of the near-surface concentration due to the dry deposition process is more than 10% of the estimated concentration with the perfect reflection of the Lagrangian particle at the surface within 70 km in the downwind direction from the source during the daytime, while less than 5% reduction occurs within 50 km from the source during the night.

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L. Mahrt and Soon-Ung Park

Abstract

The influence of boundary layer pumping on an externally forced, synoptic-scale flow is examined. The results follow earlier theories of stratified incompressible Boussinesq flow theories in that the spin-down time scale and the penetration depth of the influence of boundary layer pumping are inversely proportional to the stratification and proportional to the horizontal length scale of the flow. The present development is performed in isentropic coordinates to construct estimates applicable to the atmosphere. This analysis indicates that boundary layer pumping could be synoptically important in the lower troposphere under conditions of significant surface stress and tropospheric stratification.

The estimate of the stratified penetration depth scale is used to construct a simple homogeneous model to examine order-of-Rossby-number corrections to the quasi-geostrophic vorticity dynamics. Such corrections result from the influence of accelerations in both the free flow and boundary layer. It is found, for example, that vorticity adjustments due to various interactions between boundary layer pumping and accelerations are less important than predicted by scale analyses. Results are interpreted for the case of topographically forced flow.

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Soon-Ung Park and Dhirendra N. Sikdar

Abstract

A well-organized severe storm complex on 30 May 1976 developed in the southeastern part of the mesonetwork of the National Severe Storms Laboratory (NSSL) in central Oklahoma and merged, in its mature stage, with a band of thunderstorms along a stationary front. Rawinsonde observations at nine stations within the mesonetwork, supplemented by satellite time lapse images, have been analyzed to depict the kinematic and thermodynamic structure of the storm and its near environment. An objective analysis technique is used to describe the evolving storm structure.

Among the significant features the analysis revealed, the low-level horizontal convergence was confined to the southeast prior to the appearance of the first cloud in the satellite images, and a well-defined mixed layer, capped by a strong stable layer, developed with northwest to southeast horizontal temperature and moisture, gradients. The height of the mixed layer increased with time in the dry northwest side, while it decreased in the southeast part accompanied by a developing low-level southerly jet. Then, the differential mixed-layer growth became accelerated. The low-level southerly jet pushed the moist air into the strong stable layer above a shallow mixed layer, and brought about the destruction of the stable layer. The mesoscale convergence and the heat flux from the superadiabatic layer in the low levels in the southeast region caused a rapid growth of the mixed layer to the lifting condensation level, releasing potential instability, and storm development ensued. The southerly air transported further northwest along the sloping mixed-layer top lost its momentum in the strong stable layer and descended with the air coming from the northwest at middle levels. In the cloud-free region, the strength of the inversion layer was maintained by the growth of the mixed layer and the differential mesoscale sinking enhanced by nearby active convection.

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Pao-Shin Chu and Soon-Ung Park

Abstract

A case study is presented of the cold surge over East Asia during 9–13 December 1978; using nine vertical levels of Winter MONEX data. The surge event is manifested by a rapid meridional mass flow in the lower troposphere from the midlatitudes (∼30°N) to the equator within three days. During the period studied, the lower tropospheric circulation dramatically changed in extent and intensity. With the onset of the surge event, the main divergent maximum began shifting from the South China Sea to southeast China. At the same time, the upper tropospheric circulation correspondingly changed in a reversed order from that of the lower troposphere, and a direct vertical coupling between flows in the low and high troposphere was observed.

The time-averaged meridional mass circulation between 100 and 126°E reveals a two-cell structure; the southern cell is located between the northern South China Sea and the equator, and the northern cell between midlatitudes and the northern South China Sea. Analysis of sensible heat transport indicates that the southern cell is associated with warm air in the south and cold air in the north; thus it resembles a thermally-direct local meridional circulation. Moisture transport analysis shows that the moisture source is found in the southern branch of this cell, and the sink in the northern branch. Conversely, the northern cell is thermally indirect.

The time-mean zonal mass circulation between 32°N and 4°S is marked by two cells, linked by subsidence near the longitudes of the South China Sea. The eastern cell is accompanied by heat and moisture sources while the western cell is associated with heat and moisture sinks.

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Soon-Ung Park, D. N. Sikdar, and V. E. Suomi

Abstract

Nimbus 4 data (11.5-μm) channel and ATS 3 digitized data were analyzed for three days during April and May, 1970. Cloud-top temperature, as measured by the 11.5-μm channel of the satellite radiometer, shows a close negative correlation with cloud brightness measured by the ATS 3 satellite. This is especially true in well-developed cumulonimbus clouds.

An estimate of cloud heights corresponding to cloud-top temperature was obtained by using radiosonde data and radar images, and the relationships between the cloud thickness and brightness counts were derived for the cumulonimbus clouds.

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Hyun-Ju Ahn, Soon-Ung Park, and Lim-Seok Chang

Abstract

Coupled and noncoupled models in a grid of 60 × 60 km2 in the eastern Asian domain have been employed to examine the effect of the direct radiative forcing of the Asian dust aerosol on meteorological fields for an intense Asian dust event observed in eastern Asia on 18–23 March 2002. The coupled model consists of the modified fifth-generation Pennsylvania State University–National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5) with direct radiative forcing of the Asian dust aerosol through the NCAR Column Radiation Model and the Asian Dust Aerosol Model. The noncoupled model is MM5 without the direct radiative forcing of the Asian dust aerosol. The results indicate that the radiative cooling of the dust aerosol over the high-dust-concentration (HDC) region induces sinking motion and positive pressure perturbation near the surface, whereas the radiative warming of the aerosol near the top of the HDC layer induces rising motion and negative pressure perturbation, thereby enhancing the stable stratification in the dust layer. The induced positive pressure perturbation near the surface causes a dipole shape of the pressure perturbation field by forming a negative pressure perturbation toward the low pressure center downstream near the surface. This negative pressure perturbation is so deep and wide that the perturbation of the meteorological fields extends in the wide and deep regions away from the HDC region. The associated secondary circulation of this pressure perturbation reduces the low-level wind speed in the upstream HDC region but strengthens the downstream wind speed, resulting in reduction of dust emission in the upstream source region and downstream enhancement. Some of the dust aerosols lifted to the higher level by the synoptic rising motion are transported to the downstream direction more quickly with the upper-level strong wind, resulting in a bent-over shape of dust cloud in the downstream region with a maximum concentration near the surface and a secondary maximum in the upper level. The radiative flux convergence between these maximum dust layers produces a warm layer, thereby further enhancing the stable stratification.

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