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Hirohiko Ishikawa

Abstract

Atmospheric transport models usually require the mass conservation of the advective meteorological field. Even if the advection field is provided by sophisticated initialization or prediction models of meteorological centers, some mass imbalance may be introduced when the data are converted to the grid system specific to each tracer transport model. The mass-consistent wind model described here is used to impose mass continuity on the meteorological field, which is converted to a new grid system. The model is formulated in map coordinates. In the vertical, a terrain-following height coordinate is used. The model is unique in that a compressible continuity equation is used as a strong constraint, and in that the map scale factor, which is associated with conformal mapping, is incorporated. The weight on the adjustment of each wind component can be a spatial function. Some testing results of the model and an application are presented.

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Hirohiko Ishikawa

Abstract

The effect of horizontal diffusion on the long-range transport simulation is examined with a Lagrangian particle transport model. The transport of radioactivity released from Chernobyl is simulated by the model with different values of horizontal diffusivity. The computed concentrations are statistically compared with measured concentration. The best simulation is found when the magnitude of the horizontal diffusivity is between 3.3 × 104 and 1.0 × 105 m2 s−1. The performance of empirical formulas of horizontal diffusion, in which mean-square displacement σy is specified as a function of time, is also examined.

A part of measured concentrations, which are relatively low concentrations, cannot be explained by transport and diffusion only. It is shown that these measured concentrations can be explained by resuspension of deposited radioactivity.

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Toshihisa Itano and Hirohiko Ishikawa

Abstract

If a cyclonic vortex is bounded by a zero isotach surrounding its center at some radius, it is inevitably accompanied by negative vorticity regions somewhere within it. Since the radial gradient of vorticity changes its sign at the negative vorticity regions, it is thought to cause instability of the vortex and the resultant secondary circulation. It is attempted to attribute the origin of the oscillatory motion and/or the multiple structure observed in natural vortices to this instability. For this purpose, a simple axisymmetric vortex having a negative vorticity region between the solidly rotating core and a potentially rotating outer part is assumed, and its linear stability to asymmetric disturbances is examined with a quasigeostrophic model. The results indicate that the disturbances with azimuthal wavenumber m ≥ 1 are destabilized under certain conditions. The most unstable mode changes from an internal disturbance for m = 1 to an internal or an external disturbance for m ≥ 2 dependent on the magnitude and width of the negative vorticity region. Such effects of the negative vorticity may be relevant to the formation of the oscillatory motion and the multiple structures of natural vortices.

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Ryuji Yoshida and Hirohiko Ishikawa

Abstract

The flow environment associated with tropical cyclone genesis (TCG) over the western North Pacific was assessed via categorization into five flow patterns: monsoon shear line (SL), monsoon confluence region (CR), monsoon gyre (GY), easterly wave (EW), and preexisting tropical cyclone (PTC). Using reanalysis data and an objective algorithm, the authors defined “contribution scores” for the five flow patterns. Each score represents the contribution to TCG from each flow pattern, and scores were calculated for 908 TCG cases from 1979 to 2008 (30 yr). Of the major contribution flow patterns, SL accounted for 42% of TCGs, EW for 18%, CR for 16%, PTC for 11%, and GY for 6%. Seasonal variations in the occurrence frequency of these five patterns were clear, but interannual variations were not as apparent. Tropical cyclones often appear to be generated in conditions with multiple flow patterns. Thus, relationships between multiple flow patterns were investigated by comparing contribution scores. The SL and CR patterns were strongly correlated to each other, which can be explained by the monsoon southwesterly that organizes both patterns. The EW pattern tends to be independent of the other flow patterns. The PTC pattern has a relatively high correlation with CR, but does not have a correlation with SL or EW. Thus, the characteristics of flow patterns for the occurrence frequency of TCG are derived for a longer period than in previous studies, and correlations among flow patterns are also investigated.

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Yuichiro Oku and Hirohiko Ishikawa

Abstract

Geostationary Meteorological Satellite Visible/Infrared Spin-Scan Radiometer (GMS VISSR) images have been used to estimate diurnal variations of land surface temperature distributions over the Tibetan Plateau. The infrared split-window algorithm developed for NOAA Advanced Very High Resolution Radiometer (AVHRR) has been adapted for this purpose. Radiative transfer simulations are carried out to obtain the atmospheric transmittances and the difference temperatures that are involved in the internal coefficients of the split-window algorithm. Precipitable water distribution that is required by this algorithm is estimated from 6.7-μm brightness temperature utilizing spectral characteristics of the GMS water vapor channel. Cloud removal plays an important role in the surface temperature retrieval process. To identify convective cloud activity, many researchers use satellite infrared measurements with a fixed threshold technique. In this study, it is necessary to remove not only convective clouds but also warm clouds. For this purpose, a variable threshold technique is proposed. The threshold varies both seasonally and diurnally, and its value is determined on the basis of surface observations. With a variable threshold, it becomes possible to remove relatively warmer clouds in summer and detect colder ground surfaces at nighttime in the winter. The results of comparing estimated surface temperature from GMS data using this algorithm with in situ surface measurements show correlations around 0.8.

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Yuichiro Oku, Hirohiko Ishikawa, and Zhongbo Su

Abstract

A Surface Energy Balance System (SEBS) originally developed for the NOAA Advanced Very High Resolution Radiometer was applied to Geostationary Meteorological Satellite (GMS)-5 Visible/Infrared Spin-Scan Radiometer data that were supplemented with other meteorological data. GMS-5, which is a geostationary satellite, recorded continuous hourly information. Surface temperatures obtained from the GMS-5 data were entered into SEBS to estimate the hourly regional distribution of the surface heat fluxes over the Tibetan Plateau. The estimated fluxes are verified by using corresponding field observations. The diurnal cycle of estimated fluxes agreed well with the field measurements. For example, the diurnal range of the estimated sensible heat flux decreases from June to August. This reflects the change of dry to wet surface characteristics resulting from frequent precipitation during the summer monsoon. Over the Tibetan Plateau, the diurnal range of the surface temperature is as large as the annual range, so that the resultant sensible heat flux has a large diurnal variation. Thus, the hourly estimation based on the GMS data may contribute to a better understanding of the land surface–atmosphere interaction in this critical area.

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Yuichiro Oku, Hirohiko Ishikawa, Shigenori Haginoya, and Yaoming Ma

Abstract

The diurnal, seasonal, and interannual variations in land surface temperature (LST) on the Tibetan Plateau from 1996 to 2002 are analyzed using the hourly LST dataset obtained by Japanese Geostationary Meteorological Satellite 5 (GMS-5) observations. Comparing LST retrieved from GMS-5 with independent precipitation amount data demonstrates the consistent and complementary relationship between them. The results indicate an increase in the LST over this period. The daily minimum has risen faster than the daily maximum, resulting in a narrowing of the diurnal range of LST. This is in agreement with the observed trends in both global and plateau near-surface air temperature. Since the near-surface air temperature is mainly controlled by LST, this result ensures a warming trend in near-surface air temperature.

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Tian-You Yu, David Parsons, Eiichi Nakakita, Toshitaka Tsuda, and Hirohiko Ishikawa
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Pierre-Emmanuel Kirstetter, Tian-You Yu, Robert Palmer, David Parsons, Hirohiko Ishikawa, and Jessica M. Erlingis
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Kun Yang, Toshio Koike, Hirohiko Ishikawa, Joon Kim, Xin Li, Huizhi Liu, Shaomin Liu, Yaoming Ma, and Jieming Wang

Abstract

Parameterization of turbulent flux from bare-soil and undercanopy surfaces is imperative for modeling land–atmosphere interactions in arid and semiarid regions, where flux from the ground is dominant or comparable to canopy-sourced flux. This paper presents the major characteristics of turbulent flux transfers over seven bare-soil surfaces. These sites are located in arid, semiarid, and semihumid regions in Asia and represent a variety of conditions for aerodynamic roughness length (z 0 m; from <1 to 10 mm) and sensible heat flux (from −50 to 400 W m−2). For each site, parameter kB −1 [=ln(z 0 m/z 0 h), where z 0 h is the thermal roughness length] exhibits clear diurnal variations with higher values during the day and lower values at night. Mean values of z 0 h for the individual sites do not change significantly with z 0 m, resulting in kB −1 increasing with z 0 m, and thus the momentum transfer coefficient increases faster than the heat transfer coefficient with z 0 m. The term kB −1 often becomes negative at night for relatively smooth surfaces (z 0 m ∼ 1 mm), indicating that the widely accepted excess resistance for heat transfer can be negative, which cannot be explained by current theories for aerodynamically rough surfaces. Last, several kB −1 schemes are evaluated using the same datasets. The results indicate that a scheme that can reproduce the diurnal variation of kB −1 generally performs better than schemes that cannot.

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