The Effects of Mountains on the General Circulation of the Atmosphere as Identified by Numerical Experiments

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  • 1 Geophysical Fluid Dynamics Laboratory/NOAA, Princeton University, Princeton, N. J. 08540
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Abstract

In order to identify the effects of mountains upon the general circulation of the atmosphere, a set of numerical experiments is performed by use of a general circulation model developed at the Geophysical Fluid Dynamics Laboratory of NOAA. The numerical time integrations of the model are performed with and without the effects of mountains. By comparing the structure of the model atmospheres that emerged from these two numerical experiments, it is possible to discuss the role of mountains in maintaining the stationary and transient disturbances in the atmosphere.

The model adopted for this study has a global computational domain and covers both the troposphere and stratosphere. For the computation of radiative transfer, the distribution of incoming solar radiation in January is assumed. Over the ocean, the observed distribution of the sea surface temperature of February is assumed as a lower boundary condition of the model. Over the continental surface, temperature is determined such that the condition of heat balance at the ground surface is satisfied. The mountain topography is taken into consideration using the so-called σ-coordinate system in which pressure normalized by surface pressure is used as a vertical coordinate. The grid size for the computation of horizontal finite differences is chosen to be about 250 km. Nine finite-difference levels are chosen in unequal pressure intervals so that these levels can represent not only the structure of the mid-troposphere but also that of the stratosphere and the planetary boundary layer.

The results of the numerical experiments indicate that it is necessary to consider the effects of mountains for the successful simulation of the stationary flow field in the atmosphere, particularly in the upper troposphere and stratosphere. As predicted by Bolin, the flow field in the upper troposphere of the mountain model has a stationary trough in the lees of major mountain ranges such as the Rocky Mountains and the Tibetan Plateau. To the east of the trough, an intense westerly flow predominates. In the stratosphere, an anticyclone develops over the Aleutian Archipelago. These features of the mountain model, which are missing in the model without mountains, are in good qualitative agreement with the features of the actual atmosphere in winter.

In the model troposphere, mountains increase markedly the kinetic energy of stationary disturbances by increasing the stationary component of the eddy conversion of potential energy, whereas mountains decrease the kinetic energy of transient disturbances. The sum of the stationary and transient eddy kinetic energy is affected little by mountains. In the model stratosphere, mountains increase the amplitude of stationary disturbances partly because they enhance the energy supply from the model troposphere to the stratosphere.

According to wavenumber analysis, the longitudinal scale of eddy conversion in the model atmosphere increases significantly due to the effects of mountains. This increase results mainly from the large increase of stationary eddy conversion which takes place at very low wavenumbers.

The results of the analysis reveal other important effects of mountains. For example, the probability of cyclogenesis in the model atmosphere increases significantly on the lee side of major mountain ranges where the core of the westerly jet is located. Also, mountains affect the hydrologic processes in the model atmosphere by modifying the field of three-dimensional advection of moisture, and alter the global distribution of precipitation very significantly. In general, the distribution of the model with mountains is less zonal and more realistic than that of the model without mountains.

Abstract

In order to identify the effects of mountains upon the general circulation of the atmosphere, a set of numerical experiments is performed by use of a general circulation model developed at the Geophysical Fluid Dynamics Laboratory of NOAA. The numerical time integrations of the model are performed with and without the effects of mountains. By comparing the structure of the model atmospheres that emerged from these two numerical experiments, it is possible to discuss the role of mountains in maintaining the stationary and transient disturbances in the atmosphere.

The model adopted for this study has a global computational domain and covers both the troposphere and stratosphere. For the computation of radiative transfer, the distribution of incoming solar radiation in January is assumed. Over the ocean, the observed distribution of the sea surface temperature of February is assumed as a lower boundary condition of the model. Over the continental surface, temperature is determined such that the condition of heat balance at the ground surface is satisfied. The mountain topography is taken into consideration using the so-called σ-coordinate system in which pressure normalized by surface pressure is used as a vertical coordinate. The grid size for the computation of horizontal finite differences is chosen to be about 250 km. Nine finite-difference levels are chosen in unequal pressure intervals so that these levels can represent not only the structure of the mid-troposphere but also that of the stratosphere and the planetary boundary layer.

The results of the numerical experiments indicate that it is necessary to consider the effects of mountains for the successful simulation of the stationary flow field in the atmosphere, particularly in the upper troposphere and stratosphere. As predicted by Bolin, the flow field in the upper troposphere of the mountain model has a stationary trough in the lees of major mountain ranges such as the Rocky Mountains and the Tibetan Plateau. To the east of the trough, an intense westerly flow predominates. In the stratosphere, an anticyclone develops over the Aleutian Archipelago. These features of the mountain model, which are missing in the model without mountains, are in good qualitative agreement with the features of the actual atmosphere in winter.

In the model troposphere, mountains increase markedly the kinetic energy of stationary disturbances by increasing the stationary component of the eddy conversion of potential energy, whereas mountains decrease the kinetic energy of transient disturbances. The sum of the stationary and transient eddy kinetic energy is affected little by mountains. In the model stratosphere, mountains increase the amplitude of stationary disturbances partly because they enhance the energy supply from the model troposphere to the stratosphere.

According to wavenumber analysis, the longitudinal scale of eddy conversion in the model atmosphere increases significantly due to the effects of mountains. This increase results mainly from the large increase of stationary eddy conversion which takes place at very low wavenumbers.

The results of the analysis reveal other important effects of mountains. For example, the probability of cyclogenesis in the model atmosphere increases significantly on the lee side of major mountain ranges where the core of the westerly jet is located. Also, mountains affect the hydrologic processes in the model atmosphere by modifying the field of three-dimensional advection of moisture, and alter the global distribution of precipitation very significantly. In general, the distribution of the model with mountains is less zonal and more realistic than that of the model without mountains.

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