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Toshihiro Kitada
,
Kiyomi Igarashi
, and
Michio Owada

Abstract

Air pollution in the presence of two types of local flows (i.e., land/sea breeze and mountain/valley wind) was studied by advection simulation of the cluster of hypothetical fluid particles, and transport/chemistry calculation employing a three-dimensional Eulerian model for 20 advected species and about 90 chemical reactions. Three-dimensional flow fields over the River Yahagi basin in Japan were estimated for 48 h using an observe method with routine wind observations. Those obtained showed characteristics of the combined local flows such that in the daytime sea breeze and valley wind tend to form one united flow with substantial wind velocity in the whole region and, in contrast land breeze and mountain wind during the nighttime form two separated circulating flows with a clear weak-wind area between the two local flow regimes. The results of the advection simulation of fluid particles and the transport/elements calculation using those flows as inputs elucidated how the features found in the diurnally varying, complex local flows contribute to produce characteristic time-variations of the concentrations of both primary and secondary pollutants. Among others, dynamics of NO2, HNO3, PAN, O3, SO2, and SO4 = concentrations are discussed.

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Toshihiro Kitada
,
Kiyoshi Okamura
, and
Setsu Tanaka

Abstract

Influence of both urbanization in modified land use in a plain area, the Nohbi Plain of central Japan, and surrounding large-scale topography, such as the Japanese Alps, on the temperature and local wind over the plain has been investigated utilizing numerical simulations with a mesoscale meteorological model that uses the k−ε model for turbulence. Obtained results are as follows. 1) Relative importance of natural topography and human-modified land use in various spatial scales has been clarified in the formation of characteristic diurnal patterns of sea breeze and temperature in the plain area. The Japanese Alps, which are the largest topographic feature in central Japan and are located far from the Nohbi Plain, around 100–200 km away, gave the most important influence on the wind over the plain area. The effect of the high mountains on the wind was caused by heating of the air mass over the plain due to weak subsidence associated with the return flow of the plain–plateau circulation. The urbanization in the Nohbi Plain showed little significant effect on the diurnal flow pattern. 2) The mechanism of the formation of an inland high-temperature zone associated with coastal urbanization under sea-breeze situations has been explained: an urban area, as a local heat source, placed in the topographically induced sea-breeze/valley wind, forms a weak wind zone at the downwind side of the urban area due to the pressure gradient adverse to the sea breeze. In the weak wind convergence zone, the mixed layer rapidly develops and the air mass is strongly heated there from the surface before the arrival of the sea breeze. This high-temperature zone moves inland with an advancing sea-breeze front.

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Toshihiro Kitada
and
Ram P. Regmi

Abstract

Air pollution characteristics over the Kathmandu Valley in wintertime were numerically investigated by using a comprehensive transport–chemistry–deposition model of air pollutants together with the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). In Kathmandu, Nepal, it is known that double-layered local flows, that is, the southwesterly and northwesterly winds, formed as combined valley wind and plain-to-plateau wind, develop every day in the afternoon. In this study, the effect of local flows on air pollution in Kathmandu has been clarified. Detailed analysis of diurnal variation of air pollution transport elucidated the basic nature of the air pollution: 1) the vertical spreading of the pollutants, accumulated during the nighttime, by mixing-layer activity in the late morning, before the intrusion of the two local flows; 2) the late afternoon redevelopment of a shallow polluted layer and the pollutant transport toward the eastern neighboring valley, caused by the double-layered local flows; and 3) the nighttime partial comeback of the pollutants from the eastern mountain pass to the Kathmandu Valley and the gradual spreading of the pollutants over the valley caused by intermittent but organized horizontal flow caused by mountain winds, which also suppresses excess accumulation of fresh pollutants in the source area. Mass balance of sulfur compounds in the Kathmandu Valley is also discussed.

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Ram P. Regmi
,
Toshihiro Kitada
, and
Gakuji Kurata

Abstract

Air pollution transport in the Kathmandu valley/basin has been investigated by numerical simulation of local flows and the observation of NO2 and SO2. The observation was performed at 22 sites with passive samplers from February to April 2001, and the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) was utilized for the flow simulation. The calculation reproduced reasonably well the surface wind and temperature at the Tribhuvan International Airport (TIA) as well as the vertical wind profile taken at the center of the valley by sodar observation. The calculation showed that two characteristic local flows tend to intrude into the valley/basin in the afternoon through the mountain gaps surrounding Kathmandu, that is, the southwesterly from the Indian Plain and the northwesterly from the valley west to Kathmandu. These cool wind layers meet at the center of the Kathmandu basin and form a double-layering structure there. The lower layer is shallow with a depth of about 250 m, being composed of the cooler southwesterly air mass from the Indian Plain. It was concluded that this local flow structure suppresses vertical mixing and leads to high air pollution by decreasing the daytime ventilation of air mass over the valley. The observations performed during the period confirmed it.

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Toshihiro Kitada
,
Gregory R. Carmichael
, and
Leonard K. Peters

Abstract

The characteristics of the transport of chemically reactive species under land- and sea-breeze (LSB) circulations are investigated using a detailed transport/chemistry model, which includes 84 gas-phase and 10 heterogeneous chemical reactions. Model applications are presented which use flow fields derived from a modified version of the Asai and Mitsumoto model and eddy diffusivity profiles predicted by the boundary-layer model of Yamada and Mellor as inputs. The effects of nonprecipitating clouds associated with the LSB circulation on the calculated concentration fields are also studied.

Mass transports by updrafts and counterflows associated with the LSB circulation and diurnally varying eddy diffusion processes show transitions between double and single maxima within a 24-hour cycle. The vertical profiles of some secondary pollutants such as O3 generally agree with field observations. Clouds are also shown to affect the predicted distributions of both the soluble and less soluble species by reducing the below-cloud photon flux, by removing soluble species from the air at cloud level, and/or by in-cloud production processes. Deposition processes reduce the species concentrations near the surface, and these effects propagate upward through mass transport processes. However, the qualitative characteristic vertical concentration profiles are similar to the cases where deposition is not included. Finally, the results demonstrate the effectiveness of the divergence correction method used in the numerical calculations in eliminating the fictitious production and consumption reactions introduced by nonzero divergence wind fields.

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Ram P. Regmi
,
Toshihiro Kitada
,
Jimy Dudhia
, and
Sangeeta Maharjan

Abstract

Nepal has been the location of a series of fatal aircraft accidents, raising serious concerns about civil aviation security and the safety of passengers. However, significant studies on weather patterns associated with the airports and air routes of the Himalayan complex terrain and their implications for aviation activities are yet to be carried out. The present study numerically reconstructs the prevailing weather conditions and puts forward some possible causes behind the most recent fatal aircraft accident in the foothills of the western Nepal Himalaya at 0730 UTC (1315 LST) 16 February 2014. The weather patterns have been numerically simulated at 1-km2 horizontal grid resolution using the Weather Research and Forecasting (WRF) modeling system. The reconstructed weather situation shows the existence of a low-level cloud ceiling, supercooled cloud water and hail, trapped mountain waves, supercritical descent of a strong tail wind, and the development of turbulence at the altitude of the flight path followed by the aircraft. The aircraft might have gone through a series of weather hazards including visibility obstruction, moderate turbulence, abnormal loss in altitude, and icing. It is concluded that the weather situation over the region was adverse enough to affect small aircraft and therefore that it might have played an important role leading to the fatal accident. The development of hazardous weather over the region may be attributed to a previously unanticipated large-scale easterly gravity current over the middle hills of the Nepal Himalaya. The gravity current originated from the central high Himalayan mountainous region located northeast of the Kathmandu valley and traveled more than 200 km, reaching the foothills of the western Nepal Himalaya.

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