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- Author or Editor: V. Brahmananda Rao x
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Abstract
The Mak parameterization of latent heat release is discussed using a finite difference version of a quasigeostrophic β-plane model. It is found that his analytical results can be well reproduced with 20-layer vertical resolution. This finite difference version can be used for the study of baroclinic instability of observed (arbitrary) zonal wind and static stability vertical profiles with more realistic heating. This model with Chang profiles of heating is applied to the North Pacific oceanic polar air cyclones studied by Mullen and to an intermediate-scale (inverted) comma cloud system observed over central South America. The principal features of these disturbances are reproduced by the model. Thus, the generation mechanism of polar cyclones and comma cloud disturbances seems to be the baroclinic instability modified by the latent heat release.
Abstract
The Mak parameterization of latent heat release is discussed using a finite difference version of a quasigeostrophic β-plane model. It is found that his analytical results can be well reproduced with 20-layer vertical resolution. This finite difference version can be used for the study of baroclinic instability of observed (arbitrary) zonal wind and static stability vertical profiles with more realistic heating. This model with Chang profiles of heating is applied to the North Pacific oceanic polar air cyclones studied by Mullen and to an intermediate-scale (inverted) comma cloud system observed over central South America. The principal features of these disturbances are reproduced by the model. Thus, the generation mechanism of polar cyclones and comma cloud disturbances seems to be the baroclinic instability modified by the latent heat release.
Abstract
A long-lasting blocking event occurred over the southeast Pacific Ocean near the west coast of South America from 29 July through 14 August 1986. This blocking happened in a recently found new region of blocking in the southeast Pacific. During the blocking event the transient eddies were forced to move to the north and south of the blocking. This caused precipitation over South America to increase to the north of the blocking high and diminish over southern South America.
The blocking event is analyzed in detail and its impact on the winter general circulation is discussed. Change in amplitude of several wavenumbers and the zonal wind (zero wavenumber) suggest that the blocking event is essentially a local phenomenon. Since transient eddies are prevented from moving through the blocking high the variance of meridional wind is small. However, the variance of geopotential height is maximized because of the persistence of the block. This signature of the blocking on the general circulation is similar to what is seen over the New Zealand region. Calculation of local Eliassen–Palm flux showed that in the region of the split jet both the barotropic and baroclinic components complement each other to decelerate the westerlies and maintain the block. This seems to fit into the “eddy straining” concept. The principal difference between blocking over the southeast Pacific and the New Zealand regions is in barotropic energy exchange between the eddies and zonal flow. In the present case eddies maintain two branches of the jet (subtropical and subpolar) by converting eddy kinetic energy into zonal kinetic energy. At the location of the split jet, zonal kinetic energy is converted into eddy kinetic energy, thus maintaining the split jet. A composite of four blocking events over the southeast Pacific confirmed these results. This is opposite to what was found for a blocking episode near New Zealand.
Abstract
A long-lasting blocking event occurred over the southeast Pacific Ocean near the west coast of South America from 29 July through 14 August 1986. This blocking happened in a recently found new region of blocking in the southeast Pacific. During the blocking event the transient eddies were forced to move to the north and south of the blocking. This caused precipitation over South America to increase to the north of the blocking high and diminish over southern South America.
The blocking event is analyzed in detail and its impact on the winter general circulation is discussed. Change in amplitude of several wavenumbers and the zonal wind (zero wavenumber) suggest that the blocking event is essentially a local phenomenon. Since transient eddies are prevented from moving through the blocking high the variance of meridional wind is small. However, the variance of geopotential height is maximized because of the persistence of the block. This signature of the blocking on the general circulation is similar to what is seen over the New Zealand region. Calculation of local Eliassen–Palm flux showed that in the region of the split jet both the barotropic and baroclinic components complement each other to decelerate the westerlies and maintain the block. This seems to fit into the “eddy straining” concept. The principal difference between blocking over the southeast Pacific and the New Zealand regions is in barotropic energy exchange between the eddies and zonal flow. In the present case eddies maintain two branches of the jet (subtropical and subpolar) by converting eddy kinetic energy into zonal kinetic energy. At the location of the split jet, zonal kinetic energy is converted into eddy kinetic energy, thus maintaining the split jet. A composite of four blocking events over the southeast Pacific confirmed these results. This is opposite to what was found for a blocking episode near New Zealand.
Abstract
The location and movement of 500-hPa troughs using an automated method are studied with data from a 24-yr period with the objective of determining the trough formation and dissipation regions in the Southern Hemisphere. To identify the 500-hPa mobile troughs, an objective method that uses the Eulerian centripetal acceleration (ECA) is developed. On average, 868 troughs per year were identified by the method, with an increase in trend during the period studied. The troughs have an average lifetime of 4.3 days, being longer (shorter) in subtropical (high) latitudes. The average calculated phase velocity was 13.6 m s−1, being higher (lower) in middle (high) latitudes. The troughs are normally found in the 60°–40°S latitudinal band, with maximum occurrence at 50°S. The longitudinal distribution of trough genesis has three maximum regions: over the Drake Strait and the South Atlantic Ocean, over the Indian Ocean around 50°S, and over the southwestern Pacific Ocean between 150°E and 150°W. The trough dissipation regions are less concentrated than the genesis regions and also show three maxima: over the west of the Andes, south of the African continent, and south of Australia. The seasonal variation in the trough dissipation shows that the 30°–40°S band is more active during winter than in summer. The difference between the genesis and dissipation location is that formation occurs more in middle and high latitudes, while dissipation is more common in the 40°–30°S latitude belt.
Abstract
The location and movement of 500-hPa troughs using an automated method are studied with data from a 24-yr period with the objective of determining the trough formation and dissipation regions in the Southern Hemisphere. To identify the 500-hPa mobile troughs, an objective method that uses the Eulerian centripetal acceleration (ECA) is developed. On average, 868 troughs per year were identified by the method, with an increase in trend during the period studied. The troughs have an average lifetime of 4.3 days, being longer (shorter) in subtropical (high) latitudes. The average calculated phase velocity was 13.6 m s−1, being higher (lower) in middle (high) latitudes. The troughs are normally found in the 60°–40°S latitudinal band, with maximum occurrence at 50°S. The longitudinal distribution of trough genesis has three maximum regions: over the Drake Strait and the South Atlantic Ocean, over the Indian Ocean around 50°S, and over the southwestern Pacific Ocean between 150°E and 150°W. The trough dissipation regions are less concentrated than the genesis regions and also show three maxima: over the west of the Andes, south of the African continent, and south of Australia. The seasonal variation in the trough dissipation shows that the 30°–40°S band is more active during winter than in summer. The difference between the genesis and dissipation location is that formation occurs more in middle and high latitudes, while dissipation is more common in the 40°–30°S latitude belt.
Abstract
The energetics and behavior of midtropospheric troughs over the Southern Hemisphere and their relationship with South America surface cyclogenesis were studied during the winters of 1999–2003. All surface cyclogenesis situations over Uruguay and adjacent areas associated with 500-hPa troughs were analyzed. The atmospheric circulation associated with type-B and type-C cyclones form the basis for two composites: composite B (with 25 cases) and composite C (with 13 cases). The results showed that the midtropospheric troughs were more intense in composite C than in composite B before the surface cyclogenesis and that the opposite occurred during the surface cyclogenesis. The baroclinic conversion was dominant in both composites. In composite B, the ageostrophic flux convergence (AFC) contributed positively to the intensification of the surface cyclone since it imported energy into the area before the cyclogenesis started. But in composite C, the AFC served as a sink because it exported energy. Based on these results, it can be concluded that (i) the trough was crucial for the cyclogenesis; (ii) the variables in the mid- and upper levels did not differ significantly from one composite to another; (iii) the northerly heat and moisture flow acted as a preconditioning for the cyclogenesis, mainly for composite C; (iv) the baroclinic conversion dominated the energetics; and (v) the AFC had only a secondary role, contributing negatively to the development of the cyclone in composite C and positively to the initial development in composite B.
Abstract
The energetics and behavior of midtropospheric troughs over the Southern Hemisphere and their relationship with South America surface cyclogenesis were studied during the winters of 1999–2003. All surface cyclogenesis situations over Uruguay and adjacent areas associated with 500-hPa troughs were analyzed. The atmospheric circulation associated with type-B and type-C cyclones form the basis for two composites: composite B (with 25 cases) and composite C (with 13 cases). The results showed that the midtropospheric troughs were more intense in composite C than in composite B before the surface cyclogenesis and that the opposite occurred during the surface cyclogenesis. The baroclinic conversion was dominant in both composites. In composite B, the ageostrophic flux convergence (AFC) contributed positively to the intensification of the surface cyclone since it imported energy into the area before the cyclogenesis started. But in composite C, the AFC served as a sink because it exported energy. Based on these results, it can be concluded that (i) the trough was crucial for the cyclogenesis; (ii) the variables in the mid- and upper levels did not differ significantly from one composite to another; (iii) the northerly heat and moisture flow acted as a preconditioning for the cyclogenesis, mainly for composite C; (iv) the baroclinic conversion dominated the energetics; and (v) the AFC had only a secondary role, contributing negatively to the development of the cyclone in composite C and positively to the initial development in composite B.
Abstract
A primitive equation global zonally averaged general circulation model is used to study the effects of the topography on the atmospheric annual cycle. A smoothed zonally averaged topography that has a form similar to that observed was used. The control experiment showed that the model was capable of capturing the zonally averaged behavior of the annual cycle. The model is able to capture some characteristics of the monsoonlike circulation such as the seasonal wind reversal and the easterly jet in the boreal summer. Even in the absence of topography the model was able to reproduce the monsoonlike features. However, the circulation was weak and the position of its components was altered. This suggests that the topography has an important role in modifying the intensity and position of the monsoon circulation. Sensitivity tests were made in order to investigate the effects of high elevation and its steep southern slope. Two experiments were performed: 1) increasing the elevation of orography without changing the steepness of the slope, and 2) increasing both the elevation and the steepness of the slope. The results indicated that the steepness of the southern slope seems to control the monsoonlike flow in the model. The model was also capable of reproducing a monsoonlike response to changed external conditions. When the values of the earth’s orbital parameters (precession, obliquity, and eccentricity) were changed to those of 9000 yr BP, the precipitation and circulation intensified, which seems to agree with paleoclimatic evidence.
Abstract
A primitive equation global zonally averaged general circulation model is used to study the effects of the topography on the atmospheric annual cycle. A smoothed zonally averaged topography that has a form similar to that observed was used. The control experiment showed that the model was capable of capturing the zonally averaged behavior of the annual cycle. The model is able to capture some characteristics of the monsoonlike circulation such as the seasonal wind reversal and the easterly jet in the boreal summer. Even in the absence of topography the model was able to reproduce the monsoonlike features. However, the circulation was weak and the position of its components was altered. This suggests that the topography has an important role in modifying the intensity and position of the monsoon circulation. Sensitivity tests were made in order to investigate the effects of high elevation and its steep southern slope. Two experiments were performed: 1) increasing the elevation of orography without changing the steepness of the slope, and 2) increasing both the elevation and the steepness of the slope. The results indicated that the steepness of the southern slope seems to control the monsoonlike flow in the model. The model was also capable of reproducing a monsoonlike response to changed external conditions. When the values of the earth’s orbital parameters (precession, obliquity, and eccentricity) were changed to those of 9000 yr BP, the precipitation and circulation intensified, which seems to agree with paleoclimatic evidence.
Abstract
A coupled biosphere–atmosphere statistical–dynamical model is used to study the relative roles of the impact of the land change caused by tropical deforestation and global warming on energy balance and climate. Three experiments were made: 1) deforestation, 2) deforestation + 2 × CO2, and 3) deforestation + CO2, CH4, N2O, and O3 for 2100. In experiment 1, the climatic impact of the Amazonian deforestation is studied. In experiment 2, the effect of doubling CO2 is included. In experiment 3, the concentrations of the greenhouse gases (GHGs) correspond to the A1FI scenario from the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios. The results showed that the percentage of the warming caused by deforestation relative to the warming when the increase in GHG concentrations is included is higher than 60% in the tropical region. On the other hand, with the increase in GHG concentrations, a reduction in the decrease of evapotranspiration and precipitation in the tropical region occurs when compared with the deforestation case. Because of an increase in the net longwave flux at the surface, there is a reduction in the decrease of the surface net radiation flux when compared with the case of only deforestation. This leads to an increase in the surface temperature. Although the changes are higher at 5°S, the percentage of them when the increase in GHG concentrations is included together with deforestation relative to the case of only deforestation is higher at 5°N (higher than 50% for the surface temperature and higher than 90% for the foliage and air foliage temperatures) in both experiments 2 and 3.
Abstract
A coupled biosphere–atmosphere statistical–dynamical model is used to study the relative roles of the impact of the land change caused by tropical deforestation and global warming on energy balance and climate. Three experiments were made: 1) deforestation, 2) deforestation + 2 × CO2, and 3) deforestation + CO2, CH4, N2O, and O3 for 2100. In experiment 1, the climatic impact of the Amazonian deforestation is studied. In experiment 2, the effect of doubling CO2 is included. In experiment 3, the concentrations of the greenhouse gases (GHGs) correspond to the A1FI scenario from the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios. The results showed that the percentage of the warming caused by deforestation relative to the warming when the increase in GHG concentrations is included is higher than 60% in the tropical region. On the other hand, with the increase in GHG concentrations, a reduction in the decrease of evapotranspiration and precipitation in the tropical region occurs when compared with the deforestation case. Because of an increase in the net longwave flux at the surface, there is a reduction in the decrease of the surface net radiation flux when compared with the case of only deforestation. This leads to an increase in the surface temperature. Although the changes are higher at 5°S, the percentage of them when the increase in GHG concentrations is included together with deforestation relative to the case of only deforestation is higher at 5°N (higher than 50% for the surface temperature and higher than 90% for the foliage and air foliage temperatures) in both experiments 2 and 3.
Abstract
The primitive equation barotropic unstable linear normal modes are computed using an eigenvalue approach for daily latitudinal profiles of zonal flow in the upper-tropospheric layer of 100–350 hPa before and after formation of cyclonic vortices during January 1993 and November 2001 off the coast of northeast Brazil. The wave kinetic energy equation for u- and υ-motion is presented. Equations are derived to isolate the contribution of divergence and other dynamical processes in the movement and growth of unstable modes. Numerical accuracy and physical nature of unstable modes are tested.
In a short span of 2–3 days, prior to formation of vortices, a progressive and a sharp intensification of the basic flow shear zone and its barotropic instability are seen with time. The horizontal structure, momentum transport, and zonal and meridional scales of the most unstable normalized wave are obtained and compared with the vortex extracted from the 200-hPa observed winds using a bandpass smoother. A close agreement is found between them. It is shown that the zonal and meridional scales of the preferred wave are related to the length scale of the shear zone. The wave is confined to the shear zone and its maximum amplitude is located at the latitude of maximum β −
It is emphasized that a deeper insight regarding the genesis of the cyclonic vortex can be gained on the basis of stability analysis of daily observed zonal flow profiles, which may not be possible using idealized or mean profiles. An explanation for nonmanifestation of the instability in the monthly mean flow is provided.
Abstract
The primitive equation barotropic unstable linear normal modes are computed using an eigenvalue approach for daily latitudinal profiles of zonal flow in the upper-tropospheric layer of 100–350 hPa before and after formation of cyclonic vortices during January 1993 and November 2001 off the coast of northeast Brazil. The wave kinetic energy equation for u- and υ-motion is presented. Equations are derived to isolate the contribution of divergence and other dynamical processes in the movement and growth of unstable modes. Numerical accuracy and physical nature of unstable modes are tested.
In a short span of 2–3 days, prior to formation of vortices, a progressive and a sharp intensification of the basic flow shear zone and its barotropic instability are seen with time. The horizontal structure, momentum transport, and zonal and meridional scales of the most unstable normalized wave are obtained and compared with the vortex extracted from the 200-hPa observed winds using a bandpass smoother. A close agreement is found between them. It is shown that the zonal and meridional scales of the preferred wave are related to the length scale of the shear zone. The wave is confined to the shear zone and its maximum amplitude is located at the latitude of maximum β −
It is emphasized that a deeper insight regarding the genesis of the cyclonic vortex can be gained on the basis of stability analysis of daily observed zonal flow profiles, which may not be possible using idealized or mean profiles. An explanation for nonmanifestation of the instability in the monthly mean flow is provided.
Abstract
A comparison between the National Centers for Environmental Predictions–National Center for Atmospheric Research (NCEP–NCAR) reanalysis rainfall data and the Agência Nacional de Energia Elétrica (ANEEL) rain gauge data over Brazil is made. It is found that over northeast Brazil, NCEP–NCAR rainfall is overestimated. But over south and southeast Brazil, the correlation between the two datasets is highly significant showing the utility of NCEP–NCAR rainfall data. Over other parts of Brazil the validity of NCEP–NCAR rainfall data is questionable. A detailed comparison between NCEP–NCAR rainfall data over northwest South America and rain gauge data showed that NCEP–NCAR rainfall data are useful despite important differences between the characteristics in the two data sources. NCEP–NCAR reanalysis data seem to have difficulty in correctly reproducing the strength and orientation of the South Atlantic convergence zone.
Abstract
A comparison between the National Centers for Environmental Predictions–National Center for Atmospheric Research (NCEP–NCAR) reanalysis rainfall data and the Agência Nacional de Energia Elétrica (ANEEL) rain gauge data over Brazil is made. It is found that over northeast Brazil, NCEP–NCAR rainfall is overestimated. But over south and southeast Brazil, the correlation between the two datasets is highly significant showing the utility of NCEP–NCAR rainfall data. Over other parts of Brazil the validity of NCEP–NCAR rainfall data is questionable. A detailed comparison between NCEP–NCAR rainfall data over northwest South America and rain gauge data showed that NCEP–NCAR rainfall data are useful despite important differences between the characteristics in the two data sources. NCEP–NCAR reanalysis data seem to have difficulty in correctly reproducing the strength and orientation of the South Atlantic convergence zone.
Abstract
The effect of latent heat of condensation on baroclinic instability process is studied using the model developed by Bonatti and Rao. This model includes the effect of latent heat of condensation as parameterized by Mak, a zonal wind profile that varies linearly with pressure, and a constant lapse rate of −6°C km−1. The effect of dissipative terms and vertical variation of heating on baroclinic instability with positive and negative zonal wind shears is studied.
Dissipative terms and the type and intensity of heating change the selection of the most unstable wave. Viscosity and Newtonian cooling stabilize short waves. The characteristics of the most unstable wave obtained using a double maximum in the vertical distribution of heating and observed vertical profiles of zonal wind and temperature representative of the summer monsoon of India resemble those of observed monsoon depressions. The inclusion of dissipative effects makes the Green mode the most unstable wave.
Abstract
The effect of latent heat of condensation on baroclinic instability process is studied using the model developed by Bonatti and Rao. This model includes the effect of latent heat of condensation as parameterized by Mak, a zonal wind profile that varies linearly with pressure, and a constant lapse rate of −6°C km−1. The effect of dissipative terms and vertical variation of heating on baroclinic instability with positive and negative zonal wind shears is studied.
Dissipative terms and the type and intensity of heating change the selection of the most unstable wave. Viscosity and Newtonian cooling stabilize short waves. The characteristics of the most unstable wave obtained using a double maximum in the vertical distribution of heating and observed vertical profiles of zonal wind and temperature representative of the summer monsoon of India resemble those of observed monsoon depressions. The inclusion of dissipative effects makes the Green mode the most unstable wave.
