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- Author or Editor: Jin Huang x
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Abstract
An analysis of the large-scale atmospheric circulation patterns over the South Pacific during part of FGGE SOP-1, 10–27 January 1979, is presented. Results, which are derived from Level III-b analyses produced at ECMWF, are composited for three time periods, based on changing characteristics of the South Pacific Convergence Zone (SPCZ): 0000 GMT 10 January–1200 GMT 18 January, when the SPCZ was a quasistationary persistent feature of the circulation; 0000 GMT 19 January–0000 GMT 24 January, when the SPCZ propagated westward and began to weaken; and 1200 GMT 24 January–1200 GMT 27 January, when it disappeared.
The major findings include 1) the buildup of high pressure in the eastern Pacific coincident with the westward movement of the SPCZ, followed by a rapid buildup of high pressure over the central Pacific and demise of the SPCZ; 2) a trend from middle and upper tropospheric wavelike patterns in wind, temperature and height to more zonally-oriented patterns when the SPCZ disappears; and 3) strong cross-equatorial flow from the SPCZ into the Northern Hemisphere during the first period and strong poleward flow from the SPCZ into middle latitudes during the first two periods.
Abstract
An analysis of the large-scale atmospheric circulation patterns over the South Pacific during part of FGGE SOP-1, 10–27 January 1979, is presented. Results, which are derived from Level III-b analyses produced at ECMWF, are composited for three time periods, based on changing characteristics of the South Pacific Convergence Zone (SPCZ): 0000 GMT 10 January–1200 GMT 18 January, when the SPCZ was a quasistationary persistent feature of the circulation; 0000 GMT 19 January–0000 GMT 24 January, when the SPCZ propagated westward and began to weaken; and 1200 GMT 24 January–1200 GMT 27 January, when it disappeared.
The major findings include 1) the buildup of high pressure in the eastern Pacific coincident with the westward movement of the SPCZ, followed by a rapid buildup of high pressure over the central Pacific and demise of the SPCZ; 2) a trend from middle and upper tropospheric wavelike patterns in wind, temperature and height to more zonally-oriented patterns when the SPCZ disappears; and 3) strong cross-equatorial flow from the SPCZ into the Northern Hemisphere during the first period and strong poleward flow from the SPCZ into middle latitudes during the first two periods.
Abstract
A modified set of Level III-b grid point analyses, originally produced by ECMWF, is used to diagnose the circulation features and energy conversions in the Southern Hemisphere during the FGGE SOP-1 period of 10–27 January 1979. One of the dominant features during the period was the South Pacific Convergence Zone (SPCZ), a large-scale, quasi-stationary, convectively-active cloud band over the South Pacific Ocean. The study focuses on the significance of the SPCZ on Southern Hemisphere energy conversions by partitioning the conversions into zonal and eddy (transient and standing) components. The mean state is examined for a 15-day period, 10–24 January, when the SPCZ was most active. After 24 January it dissipated. In addition, daily variations are examined for the entire period and a zonal wavenumber analysis fox. wavenumbers 1–15 is performed.
The major findings are that 1) the baroclinic conversion of eddy potential to eddy kinetic energy (CE) is the dominant conversion term in the tropics (0–30°S), and it is particularly important in the vicinity of the SPCZ; 2) all conversion terms in middle latitudes (30–60°S) are comparable and equally important; 3) standing (transient) eddies make the most significant contribution to CE (all eddy conversion terms) in the tropics and SPCZ area (midlatitudes); 4) wavenumber 4 dominates the CE conversion in the tropics, whereas wavenumbers 5–8 dominate all the eddy conversions in middle latitudes; 5) one of the four waves in the n=4CE conversion in the tropics coincides with the SPCZ, while the remaining three correspond to the continental areas of Africa, Australia and South America; and 6) during the last three days, when the SPCZ is decaying, the importance of the n=4 contribution to CE is negligible.
Abstract
A modified set of Level III-b grid point analyses, originally produced by ECMWF, is used to diagnose the circulation features and energy conversions in the Southern Hemisphere during the FGGE SOP-1 period of 10–27 January 1979. One of the dominant features during the period was the South Pacific Convergence Zone (SPCZ), a large-scale, quasi-stationary, convectively-active cloud band over the South Pacific Ocean. The study focuses on the significance of the SPCZ on Southern Hemisphere energy conversions by partitioning the conversions into zonal and eddy (transient and standing) components. The mean state is examined for a 15-day period, 10–24 January, when the SPCZ was most active. After 24 January it dissipated. In addition, daily variations are examined for the entire period and a zonal wavenumber analysis fox. wavenumbers 1–15 is performed.
The major findings are that 1) the baroclinic conversion of eddy potential to eddy kinetic energy (CE) is the dominant conversion term in the tropics (0–30°S), and it is particularly important in the vicinity of the SPCZ; 2) all conversion terms in middle latitudes (30–60°S) are comparable and equally important; 3) standing (transient) eddies make the most significant contribution to CE (all eddy conversion terms) in the tropics and SPCZ area (midlatitudes); 4) wavenumber 4 dominates the CE conversion in the tropics, whereas wavenumbers 5–8 dominate all the eddy conversions in middle latitudes; 5) one of the four waves in the n=4CE conversion in the tropics coincides with the SPCZ, while the remaining three correspond to the continental areas of Africa, Australia and South America; and 6) during the last three days, when the SPCZ is decaying, the importance of the n=4 contribution to CE is negligible.
Abstract
A multigrid finite-difference solver is developed for the Helmholtz equation on the sphere. The finite-difference grid resolution is constant in the latitudinal direction and variable in the longitudinal direction so as to keep the physical gridpoint spacing approximately uniform over the sphere. The cpu time per grid point required to reduce the residual by a given amount is independent of grid resolution. The discretization error is slightly worse than second order as a result of the variable grid spacing. The method should be applicable to general elliptic equations on the sphere and should be useful for problems where uniform grid spacing is disadvantageous.
Abstract
A multigrid finite-difference solver is developed for the Helmholtz equation on the sphere. The finite-difference grid resolution is constant in the latitudinal direction and variable in the longitudinal direction so as to keep the physical gridpoint spacing approximately uniform over the sphere. The cpu time per grid point required to reduce the residual by a given amount is independent of grid resolution. The discretization error is slightly worse than second order as a result of the variable grid spacing. The method should be applicable to general elliptic equations on the sphere and should be useful for problems where uniform grid spacing is disadvantageous.
Abstract
The deep circulation in the South Atlantic is studied through numerical experiments, using an oceanic general circulation model based on z coordinates. The new feature of these numerical experiments is that diapycnal mixing is idealized as strong bottom-intensified mixing on both sides of the midocean ridge; elsewhere diapycnal mixing is set at a low background value.
The bottom-intensified diapycnal mixing induces strong equatorward (poleward) flow along the western (eastern) slope of the midocean ridge. In addition, the strong vertical gradient of diapycnal mixing rate induces downwelling in the basin interior and intensifies the upwelling near the axis of the midocean ridge. The strong ridge-following currents induce anticlockwise circulation in both the eastern and western deep basins, and such circulation is opposite to the classical Stommel–Arons circulation. This study indicates that bottom topography, strong mixing over the midocean ridge, and strong localized mixing associated with overflows can have major impact on the strength of the deep meridional overturning cell and deep water properties in the whole deep basin.
Abstract
The deep circulation in the South Atlantic is studied through numerical experiments, using an oceanic general circulation model based on z coordinates. The new feature of these numerical experiments is that diapycnal mixing is idealized as strong bottom-intensified mixing on both sides of the midocean ridge; elsewhere diapycnal mixing is set at a low background value.
The bottom-intensified diapycnal mixing induces strong equatorward (poleward) flow along the western (eastern) slope of the midocean ridge. In addition, the strong vertical gradient of diapycnal mixing rate induces downwelling in the basin interior and intensifies the upwelling near the axis of the midocean ridge. The strong ridge-following currents induce anticlockwise circulation in both the eastern and western deep basins, and such circulation is opposite to the classical Stommel–Arons circulation. This study indicates that bottom topography, strong mixing over the midocean ridge, and strong localized mixing associated with overflows can have major impact on the strength of the deep meridional overturning cell and deep water properties in the whole deep basin.
Abstract
Sea surface elevation and bottom pressure anomalies due to thermohaline forcing are examined through analytical and numerical models, including Boussinesq and non-Boussinesq models. It is shown that Boussinesq approximations can introduce noticeable errors, depending on the spatial and temporal scales of the perturbations. According to the theory of geostrophic adjustment, when the initial perturbations have horizontal scales comparable to the barotropic radius of deformation, the initial pressure perturbations will be basically retained through the adjustment. On the other hand, if the initial perturbations have horizontal scales much smaller than the barotropic radius of deformation, the initial pressure perturbations will be largely lost. Precipitation has horizontal scales on the order of 10–100 km, much smaller than the barotropic radius of deformation. Thus, for timescales longer than days, the contribution from individual precipitation events to the local free surface elevation and bottom pressure is small and is difficult to identify from satellite data. On the other hand, thermal forcing has horizontal scales comparable to the barotropic radius of deformation, so its long-term contribution to sea surface height anomaly is noticeable and is easily identified from satellite data. Because Boussinesq models induce faulty sea surface height and bottom pressure signals, the errors introduced by these models are noticeable for anomalies in large-scale [O(1000 km)] thermohaline forcing.
Abstract
Sea surface elevation and bottom pressure anomalies due to thermohaline forcing are examined through analytical and numerical models, including Boussinesq and non-Boussinesq models. It is shown that Boussinesq approximations can introduce noticeable errors, depending on the spatial and temporal scales of the perturbations. According to the theory of geostrophic adjustment, when the initial perturbations have horizontal scales comparable to the barotropic radius of deformation, the initial pressure perturbations will be basically retained through the adjustment. On the other hand, if the initial perturbations have horizontal scales much smaller than the barotropic radius of deformation, the initial pressure perturbations will be largely lost. Precipitation has horizontal scales on the order of 10–100 km, much smaller than the barotropic radius of deformation. Thus, for timescales longer than days, the contribution from individual precipitation events to the local free surface elevation and bottom pressure is small and is difficult to identify from satellite data. On the other hand, thermal forcing has horizontal scales comparable to the barotropic radius of deformation, so its long-term contribution to sea surface height anomaly is noticeable and is easily identified from satellite data. Because Boussinesq models induce faulty sea surface height and bottom pressure signals, the errors introduced by these models are noticeable for anomalies in large-scale [O(1000 km)] thermohaline forcing.
Abstract
The gravitational potential energy balance of the thermal circulation in a simple rectangular model basin is diagnosed from numerical experiments based on a mass-conserving oceanic general circulation model. The vertical mixing coefficient is assumed to be a given constant. The model ocean is heated/cooled from the upper surface or bottom, and the equation of state is linear or nonlinear. Although the circulation patterns obtained from these cases look rather similar, the energetics of the circulation may be very different. For cases of differential heating from the bottom with a nonlinear equation of state, the circulation is driven by mechanical energy generated by heating from the bottom. On the other hand, circulation for three other cases is driven by external mechanical energy, which is implicitly provided by tidal dissipation and wind stress. The major balance of gravitational energy in this model ocean is between the source of energy due to vertical mixing and the conversion from kinetic energy at low latitudes and the sink of energy due to convection adjustment and conversion to kinetic energy at high latitudes.
Abstract
The gravitational potential energy balance of the thermal circulation in a simple rectangular model basin is diagnosed from numerical experiments based on a mass-conserving oceanic general circulation model. The vertical mixing coefficient is assumed to be a given constant. The model ocean is heated/cooled from the upper surface or bottom, and the equation of state is linear or nonlinear. Although the circulation patterns obtained from these cases look rather similar, the energetics of the circulation may be very different. For cases of differential heating from the bottom with a nonlinear equation of state, the circulation is driven by mechanical energy generated by heating from the bottom. On the other hand, circulation for three other cases is driven by external mechanical energy, which is implicitly provided by tidal dissipation and wind stress. The major balance of gravitational energy in this model ocean is between the source of energy due to vertical mixing and the conversion from kinetic energy at low latitudes and the sink of energy due to convection adjustment and conversion to kinetic energy at high latitudes.
Abstract
The structure of the Equatorial Undercurrent in a two-hemisphere ocean is studied, using a simple ideal-fluid model in which both potential vorticity and the Bernoulli function are conserved along streamlines. For the case of a symmetric forcing, the solution is reduced to the case discussed in previous studies. For the case of asymmetric forcing, the western boundary current from the hemisphere with stronger forcing overshoots the equator where the two western boundary currents merge and form an undercurrent that is asymmetric with respect to the equator. Layer thicknesses are continuous across the matching streamline, but zonal velocity can be discontinuous. Both the wind stress pattern and the Indonesian Throughflow are the most important factors dictating the asymmetric nature of the undercurrent.
Abstract
The structure of the Equatorial Undercurrent in a two-hemisphere ocean is studied, using a simple ideal-fluid model in which both potential vorticity and the Bernoulli function are conserved along streamlines. For the case of a symmetric forcing, the solution is reduced to the case discussed in previous studies. For the case of asymmetric forcing, the western boundary current from the hemisphere with stronger forcing overshoots the equator where the two western boundary currents merge and form an undercurrent that is asymmetric with respect to the equator. Layer thicknesses are continuous across the matching streamline, but zonal velocity can be discontinuous. Both the wind stress pattern and the Indonesian Throughflow are the most important factors dictating the asymmetric nature of the undercurrent.
Abstract
The monthly mean precipitation-air temperature (MMP-MMAT) relation over the United States has been examined by analyzing the observed MMP and MMAT during the period of 1931–87. The authors’ main purpose is to examine the possibility of using MMP as a second predictor in addition to the MMAT itself in predicting the next month's MMAT and to shed light on the physical relationship between MMP and MMAT. Both station and climate division data are used.
It was found that the lagged MMP-MMAT correlation with MMP leading by a month is generally negative, with the strongest negative correlation in summer and in the interior United States continent. Over large areas of the interior United States in summer, predictions of MMAT based on either antecedent MMP alone or on a combination of antecedent MMP and MMAT are better than a Prediction scheme based on MMAT alone. On the whole, even in the interior United States though, including MMP as a second predictor does not improve the skill of MMAT forecasts on either dependent or independent data dramatically because the first predictor (temperature persistence) has accounted for most of the MMP's predictive variance. For a verification performed separately for antecedent wet and dry months, much larger skill was found following wet than dry Julys for both one- and two-predictor schemes. Upon further analysis, we attribute this to the differences in the climate between the dependent (1931–60) and independent (1961–87) periods (the second being considerably colder in August) rather than to a true wetness dependence in the predictability.
We found some evidence for the role of soil moisture in explaining negative MMP-MMAT and positive MMAT-MMAT lagged correlations both from observed data and from output of multiyear runs with the National Meteorological Center model. This suggests that we should use some direct measure of soil moisture to improve MMAT forecasts instead of using the MMP as a proxy.
Abstract
The monthly mean precipitation-air temperature (MMP-MMAT) relation over the United States has been examined by analyzing the observed MMP and MMAT during the period of 1931–87. The authors’ main purpose is to examine the possibility of using MMP as a second predictor in addition to the MMAT itself in predicting the next month's MMAT and to shed light on the physical relationship between MMP and MMAT. Both station and climate division data are used.
It was found that the lagged MMP-MMAT correlation with MMP leading by a month is generally negative, with the strongest negative correlation in summer and in the interior United States continent. Over large areas of the interior United States in summer, predictions of MMAT based on either antecedent MMP alone or on a combination of antecedent MMP and MMAT are better than a Prediction scheme based on MMAT alone. On the whole, even in the interior United States though, including MMP as a second predictor does not improve the skill of MMAT forecasts on either dependent or independent data dramatically because the first predictor (temperature persistence) has accounted for most of the MMP's predictive variance. For a verification performed separately for antecedent wet and dry months, much larger skill was found following wet than dry Julys for both one- and two-predictor schemes. Upon further analysis, we attribute this to the differences in the climate between the dependent (1931–60) and independent (1961–87) periods (the second being considerably colder in August) rather than to a true wetness dependence in the predictability.
We found some evidence for the role of soil moisture in explaining negative MMP-MMAT and positive MMAT-MMAT lagged correlations both from observed data and from output of multiyear runs with the National Meteorological Center model. This suggests that we should use some direct measure of soil moisture to improve MMAT forecasts instead of using the MMP as a proxy.
Abstract
A new nonlinear artificial intelligence ensemble prediction (NAIEP) model has been developed for predicting typhoon intensity based on multiple neural networks with the same expected output and using an evolutionary genetic algorithm (GA). The model is validated with short-range forecasts of typhoon intensity in the South China Sea (SCS); results show that the NAIEP model is clearly better than the climatology and persistence (CLIPER) model for 24-h forecasts of typhoon intensity. Using identical predictors and sample cases, predictions of the genetic neural network (GNN) ensemble prediction (GNNEP) model are compared with the single-GNN prediction model, and it has been proven theoretically that the former is more accurate. Computation and analysis of the generalization capacity of GNNEP also demonstrate that the prediction of the ensemble model integrates predictions of its optimized ensemble members, so the generalization capacity of the ensemble prediction model is also enhanced. This model better addresses the “overfitting” problem that generally exists in the traditional neural network approach to practical weather prediction.
Abstract
A new nonlinear artificial intelligence ensemble prediction (NAIEP) model has been developed for predicting typhoon intensity based on multiple neural networks with the same expected output and using an evolutionary genetic algorithm (GA). The model is validated with short-range forecasts of typhoon intensity in the South China Sea (SCS); results show that the NAIEP model is clearly better than the climatology and persistence (CLIPER) model for 24-h forecasts of typhoon intensity. Using identical predictors and sample cases, predictions of the genetic neural network (GNN) ensemble prediction (GNNEP) model are compared with the single-GNN prediction model, and it has been proven theoretically that the former is more accurate. Computation and analysis of the generalization capacity of GNNEP also demonstrate that the prediction of the ensemble model integrates predictions of its optimized ensemble members, so the generalization capacity of the ensemble prediction model is also enhanced. This model better addresses the “overfitting” problem that generally exists in the traditional neural network approach to practical weather prediction.
Abstract
This study is intended to determine the spatially varying optimal time periods for calculating seasonal climate normals over the entire United States based on temperature data at 344 United States climate divisions during the period of 1931–1993. This is done by verifying the seasonal climate normals as a forecast for the same season next year, The forecast skill is measured by the correlation between the predicted and observed anomalies relative to the 30-yr normal. The optimal time periods are chosen to produce the highest correlation between the forecasts and the observation.
The results indicate that generally (all seasons and all locations) annually updated climate normals averaged over shorter than 30-yr periods are better than the WMO specified 30-yr normal (updated only every 10 years), in terms of the skill in predicting the upcoming year. The spatial pattern of the optimal averaging time periods changes with season. The skill of optimal normals comes from both the annual updating and the shorter averaging time periods of these normals. Using optimal climate normals turns out to be a reasonably successful forecast method. Utility is further enhanced by realizing that the lead time of this forecast is almost one year. Forecasts at leads beyond one year (skipping a year) are also reasonably skillful.
The skill obtained from the dependent verification is lowered to take account of the degradation expected on independent data.
In practice the optimal climate normals with a variable averaging period were found to be somewhat problematic. The problems had to do primarily with the temporal continuity and spatial consistency of the forecasts. For the time being, a constant time period of 10 years is used in the operational seasonal temperature forecasts for all seasons and locations.
Abstract
This study is intended to determine the spatially varying optimal time periods for calculating seasonal climate normals over the entire United States based on temperature data at 344 United States climate divisions during the period of 1931–1993. This is done by verifying the seasonal climate normals as a forecast for the same season next year, The forecast skill is measured by the correlation between the predicted and observed anomalies relative to the 30-yr normal. The optimal time periods are chosen to produce the highest correlation between the forecasts and the observation.
The results indicate that generally (all seasons and all locations) annually updated climate normals averaged over shorter than 30-yr periods are better than the WMO specified 30-yr normal (updated only every 10 years), in terms of the skill in predicting the upcoming year. The spatial pattern of the optimal averaging time periods changes with season. The skill of optimal normals comes from both the annual updating and the shorter averaging time periods of these normals. Using optimal climate normals turns out to be a reasonably successful forecast method. Utility is further enhanced by realizing that the lead time of this forecast is almost one year. Forecasts at leads beyond one year (skipping a year) are also reasonably skillful.
The skill obtained from the dependent verification is lowered to take account of the degradation expected on independent data.
In practice the optimal climate normals with a variable averaging period were found to be somewhat problematic. The problems had to do primarily with the temporal continuity and spatial consistency of the forecasts. For the time being, a constant time period of 10 years is used in the operational seasonal temperature forecasts for all seasons and locations.
