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- Author or Editor: Thomas R. Parish x
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Abstract
Katabatic winds are a common feature of the lower Antarctic atmosphere. Although these drainage flows are quite shallow, there is increasing evidence that the low-level circulations are an important component in establishing large-scale tropospheric motions in the high southern latitudes. Three-dimensional numerical simulations of the Antarctic katabatic wind regime and attendant tropospheric circulations have been conducted over the entire continent to depict the topographically forced drainage patterns in the near-surface layer of the atmosphere. Results of the simulation enable a mapping of katabatic wind potential and identification of coastal regions which may experience anomalously intense katabatic winds. A large upper-level cyclonic circulation forms rapidly in response to the evolving katabatic wind structure in the lower atmosphere, suggesting that the drainage circulations are an important component in prescribing the resulting resulting circumpolar vortex. These results imply that some representation of the Antarctic katabatic wind regime is necessary in general circulation models in order to properly simulate the large-scale circulations about the continent.
Abstract
Katabatic winds are a common feature of the lower Antarctic atmosphere. Although these drainage flows are quite shallow, there is increasing evidence that the low-level circulations are an important component in establishing large-scale tropospheric motions in the high southern latitudes. Three-dimensional numerical simulations of the Antarctic katabatic wind regime and attendant tropospheric circulations have been conducted over the entire continent to depict the topographically forced drainage patterns in the near-surface layer of the atmosphere. Results of the simulation enable a mapping of katabatic wind potential and identification of coastal regions which may experience anomalously intense katabatic winds. A large upper-level cyclonic circulation forms rapidly in response to the evolving katabatic wind structure in the lower atmosphere, suggesting that the drainage circulations are an important component in prescribing the resulting resulting circumpolar vortex. These results imply that some representation of the Antarctic katabatic wind regime is necessary in general circulation models in order to properly simulate the large-scale circulations about the continent.
Abstract
Five-year seasonal cycle output produced by the NCAR Community Climate Model Version 1 (CCM 1) with R15 resolution is used to evaluate the ability of the model to simulate the present-day climate of Antarctica. The model results are compared with observed horizontal syntheses and point data.
Katabatic winds, surface temperatures over the continent, the circumpolar trough, the vertical motion field, the split jet stream over the Pacific Ocean, and the snowfall accumulation are analyzed. The results show that the CCM1 with R15 resolution can well simulate to some extent the dynamics of Antarctic climate not only for the synoptic scale, but also for some mesoscale features (mesoscale cyclogenesis). This is reflected in the zonal-mean pattern of vertical motion by the presence of two convergence centers. The finding suggests that the CCM1 might also capture the split jet stream over New Zealand in winter, but the evidence is mixed. This is inferred to be due to inadequate simulation of the thermal forcing over high southern latitudes. The CCM1 can also capture the phase and amplitude of the annual and semiannual variation of temperature, sea level pressure, and zonally averaged zonal (E-W) wind. That the CCM1 can simulate some characteristics of the semiannual variation may be due to the improved radiation treatment compared to the earlier CCM0.
The most dramatic shortcomings were associated with the model's anomalously large precipitation amounts at high latitudes, which result from the scheme to suppress negative moisture values. The simulations of cloudiness and the atmospheric heat balance are adversely affected. A greatly refined moisture budget scheme is needed to eliminate these problems and may allow the split jet-stream feature over the New Zealand area in winter to be accurately reproduced. A coupled mesoscale-CCM1 model may be needed to adequately simulate the feedback from mesoscale cyclones to synoptic-scale weather systems, and the katabatic wind circulation.
Abstract
Five-year seasonal cycle output produced by the NCAR Community Climate Model Version 1 (CCM 1) with R15 resolution is used to evaluate the ability of the model to simulate the present-day climate of Antarctica. The model results are compared with observed horizontal syntheses and point data.
Katabatic winds, surface temperatures over the continent, the circumpolar trough, the vertical motion field, the split jet stream over the Pacific Ocean, and the snowfall accumulation are analyzed. The results show that the CCM1 with R15 resolution can well simulate to some extent the dynamics of Antarctic climate not only for the synoptic scale, but also for some mesoscale features (mesoscale cyclogenesis). This is reflected in the zonal-mean pattern of vertical motion by the presence of two convergence centers. The finding suggests that the CCM1 might also capture the split jet stream over New Zealand in winter, but the evidence is mixed. This is inferred to be due to inadequate simulation of the thermal forcing over high southern latitudes. The CCM1 can also capture the phase and amplitude of the annual and semiannual variation of temperature, sea level pressure, and zonally averaged zonal (E-W) wind. That the CCM1 can simulate some characteristics of the semiannual variation may be due to the improved radiation treatment compared to the earlier CCM0.
The most dramatic shortcomings were associated with the model's anomalously large precipitation amounts at high latitudes, which result from the scheme to suppress negative moisture values. The simulations of cloudiness and the atmospheric heat balance are adversely affected. A greatly refined moisture budget scheme is needed to eliminate these problems and may allow the split jet-stream feature over the New Zealand area in winter to be accurately reproduced. A coupled mesoscale-CCM1 model may be needed to adequately simulate the feedback from mesoscale cyclones to synoptic-scale weather systems, and the katabatic wind circulation.
Abstract
The NCAR CCM1's simulation of the modern arctic climate is evaluated by comparing a five-year seasonal cycle simulation with the ECMWF global analyses. The sea level pressure (SLP), storm tracks, vertical cross section of height, 500-hPa height, total energy budget, and moisture budget are analyzed to investigate the biases in the simulated arctic climate.
The results show that the model simulates anomalously low SLP, too much storm activity, and anomalously strong baroclinicity to the west of Greenland and vice versa to the east of Greenland. This bias is mainly attributed to the model's topographic representation of Greenland. First, the broadened Greenland topography in the model distorts the path of cyclone waves over the North Atlantic Ocean. Second, the model oversimulates the ridge over Greenland, which intensifies its blocking effect and steers the cyclone waves clockwise around it and hence produces an artificial “circum-Greenland” trough. These biases are significantly alleviated when the horizontal resolution increases to T42.
Over the Arctic basin, the model simulates large amounts of low-level (stratus) clouds in winter and almost no stratus in summer, which is opposite to the observations. This bias is mainly due to the location of the simulated SLP features and the negative anomaly of storm activity, which prevent the transport of moisture into this region during summer but favor this transport in winter.
The moisture budget analysis shows that the model's net annual precipitation ([P - E]) between 70°N and the North Pole is 6.6 times larger than the observations and the model transports six times more moisture into this region. The bias in the advection term is attributed to the positive moisture fixer scheme and the distorted flow pattern. However, the excessive moisture transport into the Arctic basin does not solely result from the advection term. The contribution by the moisture fixer is as large as from advection. By contrast, the semi-Lagrangian transport scheme used in the CCM2 significantly improves the moisture simulation for this region; however, globally the error is as serious as for the positive moisture fixer scheme.
Finally, because the model has such serious problems in simulating the present arctic climate, its simulations of past and future climate change for this region are questionable.
Abstract
The NCAR CCM1's simulation of the modern arctic climate is evaluated by comparing a five-year seasonal cycle simulation with the ECMWF global analyses. The sea level pressure (SLP), storm tracks, vertical cross section of height, 500-hPa height, total energy budget, and moisture budget are analyzed to investigate the biases in the simulated arctic climate.
The results show that the model simulates anomalously low SLP, too much storm activity, and anomalously strong baroclinicity to the west of Greenland and vice versa to the east of Greenland. This bias is mainly attributed to the model's topographic representation of Greenland. First, the broadened Greenland topography in the model distorts the path of cyclone waves over the North Atlantic Ocean. Second, the model oversimulates the ridge over Greenland, which intensifies its blocking effect and steers the cyclone waves clockwise around it and hence produces an artificial “circum-Greenland” trough. These biases are significantly alleviated when the horizontal resolution increases to T42.
Over the Arctic basin, the model simulates large amounts of low-level (stratus) clouds in winter and almost no stratus in summer, which is opposite to the observations. This bias is mainly due to the location of the simulated SLP features and the negative anomaly of storm activity, which prevent the transport of moisture into this region during summer but favor this transport in winter.
The moisture budget analysis shows that the model's net annual precipitation ([P - E]) between 70°N and the North Pole is 6.6 times larger than the observations and the model transports six times more moisture into this region. The bias in the advection term is attributed to the positive moisture fixer scheme and the distorted flow pattern. However, the excessive moisture transport into the Arctic basin does not solely result from the advection term. The contribution by the moisture fixer is as large as from advection. By contrast, the semi-Lagrangian transport scheme used in the CCM2 significantly improves the moisture simulation for this region; however, globally the error is as serious as for the positive moisture fixer scheme.
Finally, because the model has such serious problems in simulating the present arctic climate, its simulations of past and future climate change for this region are questionable.