Toshihiro Kitada

Numerical Analysis of Air Pollution in a Combined Field of Land/Sea Breeze and Mountain/Valley Wind

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 d...

Effects of Topography and Urbanization on Local Winds and Thermal Environment in the Nohbi Plain, Coastal Region of Central Japan: A Numerical Analysis by Mesoscale Meteorological Model with a k−ε Turbulence Model

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−e 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 urbani...

Estimation of vertical air motion from limited horizontal wind data—a numerical experiment

Abstract The variational calculus and direct-differencing objective analysis procedures, which are formulated to produce mass conservative wind fields from horizontal wind data, were evaluated in numerical experiments. In the initial tests, the variational calculus procedure was evaluated using a test flow field generated by a land/sea breeze numerical model. The results indicated that this procedure reproduced the complex vertical flow field quite well if a minimum measurement density level was retained. A second set of numerical experiments employed the variational calculus procedure and direct-differencing method to generate a three-dimensional wind field for the Mikawa Bay area in Japan, using horizontal surface winds as input. The weighting parameter in the variational procedure, ( α 1 α 2 )2, affected both the resulting vertical motion and the residual divergence. The vertical motion was lower for smaller values of ( α 1 α 2 )2 and there was an optimum value of ( α 1 α 2 )2 to minimize the residual divergence. The direct-differencing method yielded quite similar flow patterns to those resulting from the variational method. However, this method requires very accurate horizontal wind field data as input to obtain vertical winds of reasonable magnitude.

Dynamics of air pollution transport in late wintertime over Kathmandu valley, Nepal: As revealed with numerical simulation

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 doub...

Numerical analysis of the role of sea breeze fronts on air quality in coastal and inland polluted areas

Abstract The role of moving sea breeze fronts on air quality in coastal and inland polluted areas was investigated by the numerical simulation of the transport/chemistry of air pollutants, using detailed structures of eddy diffusivity, temperature and flow fields associated with the moving sea breeze fronts. The eddy diffusivity field used was the output of a two-equation turbulence model (i.e. the k-ϵ model) and reasonably well expressed dynamical nature of turbulence in sea breeze, at its front and in an inland mixed layer (Kitada et al., 1987, Proc. Envir. Sani. Eng. Res.23, 103–113). The transport/chemistry of pollutants was calculated using a comprehensive Eulerian model, which adopts more than twenty chemical species advected (Kitada et al., 1983, Proc. 3rd. Int. Symp. on Numer. Methods in Engng, pp. 223–233; Carmichael et al., 1986, Atmospheric Environment20, 173–188). Several cases of numerical simulations were performed, being characterized by the locations of emission sources (i.e. coastal or inland), and the temporal activities of those. Results showed the following. In the coastal-source cases, (1) a circulation behind the sea breeze front was responsible for maintaining high concentration zone of photochemical product, just behind the front; (2) the local maximum of the products concentration appeared at the upper part of the circulation (i.e. around a height of 500 m), where the temperature profile was stably stratified; (3) the concentration profile was vertically-uniform in the lower part of the circulation, where the thermal internal boundary layer extended over; in the inland-source cases, (4) updraft at the front swept away pollutants into the upper layer. Thus the vertical profile of the pollutant, having a peak at a higher level of 900–1000 m above ground, was formed; (5) this polluted zone at the higher layer remained for several hours after the front passed over. Finally, these indicate that the passage of the sea breeze front can result in complex layering of pollutants.

Numerical Simulation of the Transport of Chemically Reactive Species under Land- and Sea-Breeze Circulations

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 red...

Numerical modeling of long-range transport of acidic species in association with meso-β- convective-clouds across the Japan sea resulting in acid snow over coastal Japan-I. model description and qualitative verifications

Abstract This is Part I of two papers in series. Part I is dedicated to descriptions and qualitative verifications of the model, and Part II [Kitada and Lee, Atmospheric Environment 27 A, 1077–1090, 1993] to discussion of model results. The model is 3-D Eulerian and includes transport/gas- and aqueous-phase chemistry/dry and wet deposition processes for chemical species in gas-phase and hydrometeor-phases of cloud water, cloud ice, rain, snow and graupel. Six aqueous species of NO 3 − , NH 4 + , S(IV), SO 4 2− , H 2 O 2 and O 3 are accounted for, and the rates of interphase mass transfer processes among hydrometeors are calculated using the cloud-microphysics model presented in Rutledge and Hobbs [ J. atmos. Sci. 41, 2949–2972, 1984]. The gas-phase part of the model is based on that in Kitada et al. [ J. Clim. appl. Met. 23 , 1153–1172, 1984] and Carmichael et al . [ Atmospheric Environment 20 , 173–188, 1986]. The model is applied to a mid-winter snowfall event in association with meso-β-convective-clouds induced in a cold continental air flow over the warmer Japan Sea. The predicted snowfall intensities along the course of the flow agreed with those by another independent modeling study, proving the soundness of model output of the cloud microphysics fields. The predicted chemical characteristics of the event are compared with recent measurements of acid rain/snow episodes, with emphasis made to interpret those taken along the Japanese coast. Model outputs of in-cloud-air, cloud-phase, precipitated-snow-phase concentrations of the N- and S-compounds are in qualitative agreement with the data. The model results suggest that between 30 and 40% of the wintertime sulfate deposition in Kanazawa, a Japanese coastal city, can be explained by long-range transport of air pollutants from the continent.

Numerical modeling of long-range transport of acidic species in association with mesoβ-convective-clouds across the Japan sea resulting in acid snow over coastal Japan—II. Results and discussion

Abstract The acid snow/rain model [describedin Part I, Kitada et al., Atmospheric Environment 27A , 1061–1076, 1993] was applied to investigate transport/transformation/deposition of acidic species in association with snow-precipitating cloud over the Japan Sea in winter. The model results showed: (1) The snow-precipitating clouds generated by relatively weak convective motions tend to trap aerosols of sulfate and nitrate and soluble gases such as SO 2 and HNO 3 below cloud levels, thus keeping their concentrations at higher levels than those for no-cloud situations. The mechanisms involved are: transfer of gas- and aerosol-phase species to cloud-phase through absorption and nucleation scavenging, then their transfer from cloud to snow through riming, and subsequent release from sublimating snow back to gas- and aerosol-phases below cloud base. (2) In-cloud oxidation enhanced the overall conversion of SO 2 to SO 4 2− by some 25% with respect to no-cloud situation after 12 h. Furthermore, contributions to the oxidation were 77.4%, 21.1% and 1.5% for S(IV)H 2 O 2 , S(IV)O 2 with catalysts of Fe 3+ + Mn 2+ and S(IV)O 3 reactions, respectively. (3) The sulfate wet deposited by precipitating snow for 12 h was due mostly to in-cloud scavenging and in-cloud oxidation, i.e. 66% by nucleation scavenging and the remaining by in-cloud oxidation of S(IV), while the contribution of below-cloud scavenging was negligible. (4) The adsorption process of HNO 3 onto the surface of falling snow was found to account for major below-cloud scavenging of snow, and thus in contrast to SO 4 2− , the below-cloud scavenging contributed very significantly to the nitrate wet deposition. Throughout the stimulation, below-cloud scavenging was responsible for 75% of the snow-NO 3 − formation. Therefore, taking account of this process in acid snow models is important.

Effects of dry deposition on the concentration-distributions of atmospheric pollutants within land- and sea-breeze circulations

Abstract In a land- and sea-breeze situation, effects of dry deposition on the dynamics of the concentrations of chemically reacting air pollutants are investigated using a transport/transformation/removal model with diurnally varying deposition velocities modeled in terms of the aerodynamic, surface, and residual resistances. The results show that the diurnally varying flows and eddy diffusivities, which are characteristic of the landand sea-breeze system, transfer the effects of dry deposition on the concentrations quickly to the upper layer over the land and sea surfaces. The dry deposition effect on one species can be transmitted to others through the network of chemical reactions, e.g. inclusion of dry deposition into the simulation resulted in the increase of hydrocarbon concentrations. It is also predicted that the dry deposition processes could remove a considerable part of emitted NO x , and SO 2 from the local circulations, e.g. for 2 days about 40% of the emitted NO x was removed by the dry deposition of NO, NO 2 , HNO 3 and PAN and in the case of SO 2 , 25 % by that of SO 2 and SO 4 2− .

Effect of non-zero divergence wind fields on atmospheric transport calculations

Abstract The use of wind fields that do not satisfy the equation of continuity exactly can introduce significant errors in the calculation of the atmospheric transport of chemical species. Results are presented showing that non-zero divergence winds, used as inputs to a transport equation of conservation form, can introduce local fictitious production or destruction rates into the numerical calculations. A divergence-corrected form of the equation, which is mathematically equivalent but not numerically equivalent to an advection form of the equation, can eliminate these undesirable effects as well as the advection form equation can. The divergence-corrected form of the equation leads exactly to the conservation form when massconsistent winds are used. Also, it has the desirable property that the numerical form can preserve, to a large extent, integral conservation relations of the original mass balance equation even when Δ · V ≠ 0. The effects of using non-zero divergence winds appear as first-order chemical reactions. These terms are compared quantitatively with the rates of several tropospheric chemical reactions.