Hideji Kida

Local circulations developed in the vicinity of both coastal and inland urban areas: A numerical study with a mesoscale atmospheric model

Abstract A numerical study was conducted of the effects of two nearby urban areas, one coastal and one inland, on the local circulations and transport of urban pollutants, using a 3D mesoscale incompressible and hydrostatic atmospheric model. The Lagrangian Particle Dispersion Model was applied in the atmospheric model to study the transport of pollutants. The main experimental results are as follows: 1) A “chain flow,” which flows downward from the upper layer over the coastal urban area to the lower layer over the inland urban area, is formed as the sea-breeze front moves inland. 2) If an inland urban area exists, the induced sea-breeze–heat-island circulation system is stronger and is maintained longer in time. This feature becomes clearer as the distance between the two urban areas widens. 3) The chain flow efficiently transports coastal urban pollutants into the lower layer over the suburban area, located between the two urban areas, and also to the farther-inland urban area, prior to the arrival of ...

Climatic Impact of Vegetation Change in the Asian Tropical Region. Part I: Case of the Northern Hemisphere Summer

Abstract Several numerical simulations were performed, using a global climate model that includes a realistic land surface model, to investigate the impact of Asian tropical vegetation changes on the climate. The control simulation, under conditions of the actual vegetation, and three vegetation-change impact experiments were performed. The results of the impact experiments were compared with those of the control simulation. The horizontal resolution of the model used in these simulations was 1.875°, being finer than that of the models used in previous vegetation-change impact studies. As a result, it was determined that the effects of vegetation changes in the Asian tropical region had spatially different features. The morphological, physiological, and physical changes of the land surface vegetation in the Asian tropical region certainly induce statistically significant climate changes in these and the surrounding areas. That is, from the results of the bare soil and C4 grass experiments, the decrease in...

Climatic Impact of Vegetation Change in the Asian Tropical Region. Part II: Case of the Northern Hemisphere Winter and Impact on the Extratropical Circulation

Abstract Several numerical simulations were performed, using a global climate model that includes a realistic land surface model, to investigate the impact of Asian tropical vegetation changes on the climate. The control simulation, under conditions of the actual vegetation, and three vegetation-change impact experiments were performed. The horizontal resolution of the model used in these simulations was finer than those of the models used in previous vegetation-change impact studies. In Part I, which is a companion of this paper, the results of the Northern Hemisphere summer June–July–August (JJA) case were described. In the present paper, the results of the analysis concern the Northern Hemisphere winter; that is, the December–January–February (DJF) case are discussed as Part II. It was clarified, from the results of the bare soil and C4 grass experiments, that the decrease in the roughness length, and from the results of the green-less experiment, that the decrease in the latent heat flux exert strong ...

Local Circulations Developed in the Vicinity of Both Coastal and Inland Urban Areas. Part II: Effects of Urban and Mountain Areas on Moisture Transport

The moisture distribution near the ground surface in and around the Japanese cities of Osaka and Kyoto was investigated. From the analysis of observed data, the atmosphere over the suburban areas between coastal Osaka and inland Kyoto was drier than that over Osaka and Kyoto during the daytime hours. This feature differs from results in previous studies and from expectations based on urban and suburban surface heat budgets. To understand the drying mechanism, numerical experiments were performed, using a simplified geometrical model consisting of a straight coastline, a square urban area on the coast, a square inland urban area, and a plateau mountain surrounding the urban areas. The following main results were obtained. First, suburban drying during the daytime was mainly caused by a valley circulation that developed over the surrounding mountain area. In addition, the two heat island circulations that developed over the two urban areas also caused suburban drying. As a consequence, the coexistence of mountain and urban areas caused more notable suburban drying. Second, the amount of suburban drying was greatest when the urban distance was 40‐50 km, which is roughly equal to the actual distance between the Osaka and Kyoto urban areas. Last, temporal changes in moisture and those of suspended particulate matter, SO 2, and NOx concentrations decreased before the arrival of the sea-breeze front. Thus, it is argued that moisture and pollutants were transported by the two heat island circulations that developed over Osaka and Kyoto and by the valley circulations and then were modified by the sea-breeze circulation.

Seasonality of Decadal Sea Surface Temperature Anomalies in the Northwestern Pacific

Abstract The seasonality of the decadal sea surface temperature (SST) anomalies and the related physical processes in the northwestern Pacific were investigated using a three-dimensional bulk mixed layer model. In the Kuroshio–Oyashio Extension (KOE) region, the strongest decadal SST anomaly was observed during December–February, while that of the central North Pacific occurred during February–April. From an examination of the seasonal heat budget of the ocean mixed layer, it was revealed that the seasonal-scale enhancement of the decadal SST anomaly in the KOE region was controlled by horizontal Ekman temperature transport in early winter and by vertical entrainment in autumn. The temperature transport by the geostrophic current made only a slight contribution to the seasonal variation of the decadal SST anomaly, despite controlling the upper-ocean thermal conditions on decadal time scales through the slow Rossby wave adjustment to the wind stress curl. When averaging over the entire KOE region, the cont...

Roles of Low- and High-Frequency Eddies in the Transitional Process of the Southern Hemisphere Annular Mode

Abstract The effects of low- and high-frequency eddies (time scales longer and shorter than 10 days, respectively) on the transitional processes of the Southern Hemisphere “Annular Mode” are investigated, based on NCEP–NCAR daily reanalysis data for the period 1979–2001. Special attention is focused on the zonal symmetry/asymmetry and the temporal evolution of the eddy forcing. For the poleward transitional process, the effects of low-frequency eddies precede those of high-frequency eddies in driving the jet transition. Quasi-stationary Rossby waves propagating along the polar jet with wavelengths of 7000 km play an important role. The waves, originally come from the Indian Ocean through the waveguide associated with the polar jet, dissipate equatorward over the eastern Pacific Ocean. This anomalous equatorward dissipation of wave activity induces an anomalous poleward momentum flux, which is responsible for changes in the polar jet over the Pacific Ocean during the beginning stage. Following the low-freq...