Jaemyeong Mango Seo

Relationship between rooftop and on-road concentrations of traffic-related pollutants in a busy street canyon: Ambient wind effects

Rooftop and on-road measurements of O3, NO2, NOx, and CO concentrations were conducted to investigate the relationship between rooftop and on-road concentrations in a busy and shallow street canyon with an aspect ratio of ∼0.3 in Seoul, Republic of Korea, from 15 April to 1 May 2014. The median road-to-roof concentration ratios, correlation coefficients between rooftop and on-road concentrations, and temporal variations of rooftop and on-road concentrations are analyzed according to the rooftop wind directions which are two cross-canyon and two along-canyon directions. The analysis results indicate that the relationship is strong when the rooftop is situated on the downwind side rather than on the upwind side. Relative to the cross-canyon wind directions, one of the along-canyon wind directions can more enhance the relationship. A conceptual framework is proposed to explain the effect of ambient wind direction on the relationship between rooftop and on-road concentrations in a street canyon.

Theoretical calculations of interactions between urban breezes and mountain slope winds in the presence of basic-state wind

Many big cities around the world are located near mountains. In city-mountain regions, thermally and topographically forced local winds are produced and they affect the transport of pollutants emitted into the urban atmosphere. A better understanding of the dynamics of thermally and topographically forced local winds is necessary to improve the prediction of local winds and to cope with environmental problems. In this study, we theoretically examine the interactions of urban breezes with mountain slope winds in the presence of basic-state wind within the context of the response of a stably stratified atmosphere to prescribed thermal and mechanical forcing. The interactions between urban breezes and mountain slope winds are viewed through the linear superposition of individual analytical solutions for urban thermal forcing, mountain thermal forcing, and mountain mechanical forcing. A setting is considered in which a city is located downwind of a mountain. In the nighttime, in the mountain-side urban area, surface/near-surface horizontal flows induced by mountain cooling and mountain mechanical forcing cooperatively interact with urban breezes, resulting in strengthened winds. In the daytime, in the urban area, surface/near-surface horizontal flows induced by mountain heating are opposed to urban breezes, giving rise to weakened winds. It is shown that the degree of interactions between urban breezes and mountain slope winds is sensitive to mountain height and basic-state wind speed. Particularly, a change in basic-state wind speed affects not only the strength of thermally and mechanically induced flows (internal gravity waves) but also their vertical wavelength and decaying rate. The examination of a case in a setting in which a city is located upwind of a mountain reveals that basic-state wind direction is an important factor that significantly affects the interactions of urban breezes with mountain slope winds.

Periodicity and Chaos of High-Order Lorenz Systems

High-order Lorenz systems with five, six, eight, nine, and eleven equations are derived by choosing different numbers of Fourier modes upon truncation. For the original Lorenz system and for the five high-order Lorenz systems, solutions are numerically computed, and periodicity diagrams are plotted on (σ,r) parameter planes with σ,r ∈ [0, 1000], and b = 8/3. Dramatic shifts of patterns are observed among periodicity diagrams of different systems, including the appearance of expansive areas of period 2 in the fifth-, eighth-, ninth-, and 11th-order systems and the disappearance of the onion-like structure beyond order 5. Bifurcation diagrams along with phase portraits offer a closer look at the two phenomena.

Orographic–convective flows, wave reflection, and gravity-wave momentum fluxes in a two-layer hydrostatic atmosphere

ABSTRACT Dynamical aspects of flows forced by either convective heating or a mountain have been extensively studied, but those forced by both convective heating and a mountain have been less studied. Here, we theoretically examine orographic–convective flows, gravity-wave reflection, and gravity-wave momentum fluxes in a stably stratified two-layer hydrostatic atmosphere. The upper layer (stratosphere) has a larger static stability than the lower layer (troposphere), and the basic-state wind has a constant shear in the troposphere and is uniform in the stratosphere. The equations governing small-amplitude perturbations in a two-dimensional, steady-state, inviscid, non-rotating system in the presence of orographic forcing and convective forcing are analytically solved. Then, the analytic solutions are analysed to understand how orographically and convectively forced flows vary with changes in basic-state wind speed, stratospheric static stability, and the location of the convection relative to the mountain. Over the upslope of the mountain, the convectively forced deep upward motion is positively combined with the orographic uplift, thus giving rise to enhanced upward motion there. The ratio of the convectively forced vertical velocity to the orographically forced vertical velocity at the cloud base height over an upslope location of the mountain is analysed to further understand the linear interaction between orographically and convectively forced flows. The gravity-wave reflection at the tropopause plays an important role in orographic–convective flows. The gravity-wave reflection at the tropopause amplifies the symmetric (anti-symmetric) structure of orographically (convectively) forced waves. The vertical fluxes of the horizontal momentum are analytically obtained. The total momentum flux contains the component resulting from the non-linear interaction between orographically and convectively forced waves. It is found that the non-linear interaction component can be as important as each of the orographic and convective components in the total momentum flux depending on the location of the convection relative to the mountain.

Dynamics of Reversed Urban Breeze Circulation

AbstractThe urban breeze circulation (UBC) is a thermally forced mesoscale circulation that is characterized by low-level inward flow toward the urban center, updrafts near the urban center, upper-level outward flows, and weak downdrafts outside the urban area. Previous numerical modeling studies indicate that in the early morning the direction of the UBC can be reversed. Here, the dynamics of a reversed UBC is studied in the context of the response of the atmosphere to specified thermal forcing, which represents diurnally varying urban heating. For this, a linearized, two-dimensional, hydrostatic, Boussinesq airflow system in a rotating frame with specified thermal forcing is solved using the Fourier transform method. The occurrence of a reversed UBC in the early morning is confirmed. The Coriolis parameter affects the strength and vertical structure of the UBC, whose role is similar to that of the coefficient of Rayleigh friction and Newtonian cooling. The occurrence condition, strength, and vertical st...