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- Author or Editor: Hirotaka Kamahori x
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Abstract
Using the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) four-dimensional data assimilation (4DDA) upper-air data, the large-scale heat source (Q 1) and moisture sink (Q 2) over the western and eastern Tibetan Plateau are examined for a 4-month period from 1 May to 31 August 1998. The computations were performed on the sigma–pressure hybrid coordinate, named η-coordinate, since the analysis area includes high-elevation mountains.
Over the western Tibetan Plateau in May, there is a deep layer of heating occupying the whole troposphere with the maximum value exceeding 3 K day−1 around 400–600 hPa. The smaller magnitude of the apparent moisture sink is confined in the lower troposphere 1 km above the ground surface. Vertically integrated heat sources of 103 W m−2 over the western Tibetan Plateau are accompanied by a moisture sink of about half that (60 W m−2). These results indicate that the latent heat release associated with condensation plays an important role in the total heating besides the sensible heat supply from the land surface. Later in July, the moisture sink over the eastern Tibetan Plateau nearly equals the heat source indicating the dominance of moist processes associated with summertime monsoon rains.
The contrasting features of the heat source and moisture sink are closely related to the circulation fields. Throughout May and June, we observe strong upward motion along the western and southwestern slopes of the western Tibetan Plateau, while there is salient subsidence motion over the eastern Tibetan Plateau. The analyses of static stability and lifting condensation level indicate that the release of latent heat relevant to moist convection is a dominant factor for tropospheric heating after the monsoon onset, while the premonsoon period (May) is composed of both convective rainfall and dry thermal convection.
Thus, the heating mechanism prior to the onset of the monsoon, especially over the western Tibetan Plateau, can be characterized by the hybrid nature of “wet” processes due to condensation heating and “dry” processes associated with the sensible heat flux from the elevated mountain surface.
Abstract
Using the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) four-dimensional data assimilation (4DDA) upper-air data, the large-scale heat source (Q 1) and moisture sink (Q 2) over the western and eastern Tibetan Plateau are examined for a 4-month period from 1 May to 31 August 1998. The computations were performed on the sigma–pressure hybrid coordinate, named η-coordinate, since the analysis area includes high-elevation mountains.
Over the western Tibetan Plateau in May, there is a deep layer of heating occupying the whole troposphere with the maximum value exceeding 3 K day−1 around 400–600 hPa. The smaller magnitude of the apparent moisture sink is confined in the lower troposphere 1 km above the ground surface. Vertically integrated heat sources of 103 W m−2 over the western Tibetan Plateau are accompanied by a moisture sink of about half that (60 W m−2). These results indicate that the latent heat release associated with condensation plays an important role in the total heating besides the sensible heat supply from the land surface. Later in July, the moisture sink over the eastern Tibetan Plateau nearly equals the heat source indicating the dominance of moist processes associated with summertime monsoon rains.
The contrasting features of the heat source and moisture sink are closely related to the circulation fields. Throughout May and June, we observe strong upward motion along the western and southwestern slopes of the western Tibetan Plateau, while there is salient subsidence motion over the eastern Tibetan Plateau. The analyses of static stability and lifting condensation level indicate that the release of latent heat relevant to moist convection is a dominant factor for tropospheric heating after the monsoon onset, while the premonsoon period (May) is composed of both convective rainfall and dry thermal convection.
Thus, the heating mechanism prior to the onset of the monsoon, especially over the western Tibetan Plateau, can be characterized by the hybrid nature of “wet” processes due to condensation heating and “dry” processes associated with the sensible heat flux from the elevated mountain surface.
Abstract
This study examines disastrous historical precipitation cases that generate extreme precipitation simultaneously over a wide area in Japan (as in July 2018), defined as widespread extreme precipitation events. A statistically significant large-scale environment conducive for widespread extreme precipitation events over western Japan is investigated based on composite analysis. During a widespread precipitation event, a zonally elongated positive anomaly of the column-integrated water vapor extends from East China to western Japan. In the lower troposphere, a dipole of a geopotential height anomaly exists with positive and negative values at the east and west of the precipitation area, respectively. It is found that the negative geopotential anomaly is enhanced over East China at 2 days before the event and moves toward the precipitating area mainly due to the potential vorticity (PV) production term by diabatic heating, analogous to a diabatic Rossby wave. The temporal evolution of the dynamical forced vertical velocity is well in phase with that the PV production term, suggesting the importance of the coupling between the dynamical forced motion and diabatic heating. This result provides a physical understanding of the reason why both the background moisture and the baroclinicity are essential in the composited atmospheric fields and another view to the importance of the feedback parameter between the dynamical motion and diabatic heating.
Abstract
This study examines disastrous historical precipitation cases that generate extreme precipitation simultaneously over a wide area in Japan (as in July 2018), defined as widespread extreme precipitation events. A statistically significant large-scale environment conducive for widespread extreme precipitation events over western Japan is investigated based on composite analysis. During a widespread precipitation event, a zonally elongated positive anomaly of the column-integrated water vapor extends from East China to western Japan. In the lower troposphere, a dipole of a geopotential height anomaly exists with positive and negative values at the east and west of the precipitation area, respectively. It is found that the negative geopotential anomaly is enhanced over East China at 2 days before the event and moves toward the precipitating area mainly due to the potential vorticity (PV) production term by diabatic heating, analogous to a diabatic Rossby wave. The temporal evolution of the dynamical forced vertical velocity is well in phase with that the PV production term, suggesting the importance of the coupling between the dynamical forced motion and diabatic heating. This result provides a physical understanding of the reason why both the background moisture and the baroclinicity are essential in the composited atmospheric fields and another view to the importance of the feedback parameter between the dynamical motion and diabatic heating.
