Search Results
You are looking at 1 - 10 of 11 items for
- Author or Editor: Sergio H. Franchito x
- Refine by Access: All Content x
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
A biosphere model based on BATS (Biosphere–Atmosphere Transfer Scheme) is coupled to a primitive equation global statistical–dynamical model in order to study the climatic impact due to land surface alterations. The fraction of the earth’s surface covered by each vegetation type according to BATS is obtained for each latitude belt. In the control experiment, the mean annual zonally averaged climate is well simulated when compared with observations. Deforestation and desertification experiments are performed. In the deforestation experiment, the evergreen broadleaf tree in the Amazonian region is substituted by short grass; in the desertification experiment the semidesert, and the tall grass and deciduous shrubs are substituted by desert and semidesert in the African continent, respectively.
The results show that in both the experiments there is a reduction in evapotranspiration and precipitation in the perturbed region and an increase in the soil surface temperature, the temperature of the foliage air layer, and the foliage temperature. Also, the latent heat flux decreased in the perturbed cases relative to the control case. To partially compensate for the decrease in latent heating, sensible heat flux increased in the perturbed cases compared with the control case. The changes in the deforestation case are greater in the latitude belt centered at 5°S, where in most part the Amazonian forest is situated. Otherwise, the changes in the desertification are greater in the latitude belt centered at 15°N. When there is also degradation of the African tropical forest (substitution of evergreen broadleaf trees by short grass), the greatest changes occur southward from that region (in the latitude belt centered at 5°N), and the magnitude of the changes are also increased. This shows the important role of the modification of tropical forest when there is degradation of the vegetation in the African region from 20°N to 0°°.
The results regarding the changes in the temperature and in the energy fluxes are in agreement with those of earlier experiments carried out with sophisticated general circulation models, which shows the usefulness of this kind of simple model.
Abstract
A biosphere model based on BATS (Biosphere–Atmosphere Transfer Scheme) is coupled to a primitive equation global statistical–dynamical model in order to study the climatic impact due to land surface alterations. The fraction of the earth’s surface covered by each vegetation type according to BATS is obtained for each latitude belt. In the control experiment, the mean annual zonally averaged climate is well simulated when compared with observations. Deforestation and desertification experiments are performed. In the deforestation experiment, the evergreen broadleaf tree in the Amazonian region is substituted by short grass; in the desertification experiment the semidesert, and the tall grass and deciduous shrubs are substituted by desert and semidesert in the African continent, respectively.
The results show that in both the experiments there is a reduction in evapotranspiration and precipitation in the perturbed region and an increase in the soil surface temperature, the temperature of the foliage air layer, and the foliage temperature. Also, the latent heat flux decreased in the perturbed cases relative to the control case. To partially compensate for the decrease in latent heating, sensible heat flux increased in the perturbed cases compared with the control case. The changes in the deforestation case are greater in the latitude belt centered at 5°S, where in most part the Amazonian forest is situated. Otherwise, the changes in the desertification are greater in the latitude belt centered at 15°N. When there is also degradation of the African tropical forest (substitution of evergreen broadleaf trees by short grass), the greatest changes occur southward from that region (in the latitude belt centered at 5°N), and the magnitude of the changes are also increased. This shows the important role of the modification of tropical forest when there is degradation of the vegetation in the African region from 20°N to 0°°.
The results regarding the changes in the temperature and in the energy fluxes are in agreement with those of earlier experiments carried out with sophisticated general circulation models, which shows the usefulness of this kind of simple model.
Abstract
The relationships between coastal upwelling and local winds at Cabo Frio (Brazil) are studied using SST and time series of surface wind for a 10-yr period (1971–80). The results show that the seasonal variations of SST and local winds are closely related. Sea-breeze circulation is intensified by the enhancement of the land–sea temperature gradient due to cold water upwelling near the coast; coastal upwelling, in turn, is associated with strong northeasterlies. This result confirms the conclusions of earlier modeling studies. Interannual variability is also apparent in the results. During the period from 1971 to 1980, the highest SST values occur during the years 1972–73 (strong El Niño event) and the lowest occur in 1977 (moderate El Niño event). This suggests some possible effects of atmospheric teleconnections on South Atlantic SSTs. However, a record longer than 10 yr is needed to confirm the connection with El Niño and La Niña events. Time–frequency analyses of the SST and zonal wind series for 1975–77 are done using Morlet wavelet analysis. The global wavelet spectra for these variables show strong peaks at 24 and 157 h (approximately 6.6 days). These analyses also indicate that the sea breeze occurs at Cabo Frio almost year-round and confirm the relationships with the coastal upwelling in the region.
Abstract
The relationships between coastal upwelling and local winds at Cabo Frio (Brazil) are studied using SST and time series of surface wind for a 10-yr period (1971–80). The results show that the seasonal variations of SST and local winds are closely related. Sea-breeze circulation is intensified by the enhancement of the land–sea temperature gradient due to cold water upwelling near the coast; coastal upwelling, in turn, is associated with strong northeasterlies. This result confirms the conclusions of earlier modeling studies. Interannual variability is also apparent in the results. During the period from 1971 to 1980, the highest SST values occur during the years 1972–73 (strong El Niño event) and the lowest occur in 1977 (moderate El Niño event). This suggests some possible effects of atmospheric teleconnections on South Atlantic SSTs. However, a record longer than 10 yr is needed to confirm the connection with El Niño and La Niña events. Time–frequency analyses of the SST and zonal wind series for 1975–77 are done using Morlet wavelet analysis. The global wavelet spectra for these variables show strong peaks at 24 and 157 h (approximately 6.6 days). These analyses also indicate that the sea breeze occurs at Cabo Frio almost year-round and confirm the relationships with the coastal upwelling in the region.
Abstract
Large precipitation deficits observed during the 2001 austral summer over the southeast region of Brazil contributed to the worsening of the energy crisis that was occurring in the country, with unprecedented social and economic consequences. Reliable information on the beginning of the rainy season was essential for the Brazilian government to manage the energy crisis. The purpose of this study is to determine the rainy season in this region and to point out the risk of using outgoing longwave radiation (OLR) data to estimate the beginning of it. The results show that when OLR data are used the beginning and the end dates of the rainy season are wrongly anticipated and delayed, respectively. The present study aims to provide useful information for the management of the impact of adverse climate conditions such as the one in 2001 by basing the analysis on rainfall data instead of on OLR.
Abstract
Large precipitation deficits observed during the 2001 austral summer over the southeast region of Brazil contributed to the worsening of the energy crisis that was occurring in the country, with unprecedented social and economic consequences. Reliable information on the beginning of the rainy season was essential for the Brazilian government to manage the energy crisis. The purpose of this study is to determine the rainy season in this region and to point out the risk of using outgoing longwave radiation (OLR) data to estimate the beginning of it. The results show that when OLR data are used the beginning and the end dates of the rainy season are wrongly anticipated and delayed, respectively. The present study aims to provide useful information for the management of the impact of adverse climate conditions such as the one in 2001 by basing the analysis on rainfall data instead of on OLR.
Abstract
Seasonal variations in the Southern Hemisphere (SH) storm track characteristics and associated wave propagation are studied using 19 years of NCEP–NCAR gridded data. It is found that the SH storm track is strongest in the austral autumn season and weakest in spring. The characteristics of wave packets are studied by computing 1-point lag correlation maps with unfiltered meridional wind at 300 hPa. It is found that the eastward group velocities of waves are much higher than the phase velocities in the transition seasons. This shows that the downstream development occurs throughout the year and is a basic feature of upper-tropospheric waves in the midlatitudes of the SH. This extends previous studies made for winter and summer seasons. Based on the indices that show wave coherence and correlation maps, it is found that the pathways in the transition seasons split into two branches east of Australia, in a way similar to what happens in the zonal wind distribution.
Abstract
Seasonal variations in the Southern Hemisphere (SH) storm track characteristics and associated wave propagation are studied using 19 years of NCEP–NCAR gridded data. It is found that the SH storm track is strongest in the austral autumn season and weakest in spring. The characteristics of wave packets are studied by computing 1-point lag correlation maps with unfiltered meridional wind at 300 hPa. It is found that the eastward group velocities of waves are much higher than the phase velocities in the transition seasons. This shows that the downstream development occurs throughout the year and is a basic feature of upper-tropospheric waves in the midlatitudes of the SH. This extends previous studies made for winter and summer seasons. Based on the indices that show wave coherence and correlation maps, it is found that the pathways in the transition seasons split into two branches east of Australia, in a way similar to what happens in the zonal wind distribution.
Abstract
A diagnosis of rainfall over South America (SA) during the 1997/98 El Niño year is made examining the roles of water vapor transport and stationary waves. It is found that the low-level jet (LLJ) on the eastern side of the central Andes is stronger during the El Niño event and transports more moisture. This seems to be the source for higher rainfall over southeast SA noted during the El Niño years.
A calculation of three-dimensional stationary wave activity (F s ) for 1997 and 1998 showed that in the summer of 1996/97 stationary waves propagate poleward and eastward from midlatitudes into the higher latitudes to the west of south SA and then propagate equatorward to the east of SA. During the autumn of 1997, the vertical component of F s is consistent with a blocking high over the southeast Pacific. To the east of this high cold air advection from Antarctica occurs, and to the west warm air advection occurs. This is consistent with negative and positive centers seen in the vertical component of F s to the east and west of south SA. The rainfall anomalies during a particular season seem to be due to multiple causes and this complicates a direct connection between them. Although over the southeast of SA, the higher rainfall during 1997 and 1998 can be attributed to some specific causes such as higher water vapor transport by LLJ in summer of 1997/98, in other parts such an association was not possible.
Abstract
A diagnosis of rainfall over South America (SA) during the 1997/98 El Niño year is made examining the roles of water vapor transport and stationary waves. It is found that the low-level jet (LLJ) on the eastern side of the central Andes is stronger during the El Niño event and transports more moisture. This seems to be the source for higher rainfall over southeast SA noted during the El Niño years.
A calculation of three-dimensional stationary wave activity (F s ) for 1997 and 1998 showed that in the summer of 1996/97 stationary waves propagate poleward and eastward from midlatitudes into the higher latitudes to the west of south SA and then propagate equatorward to the east of SA. During the autumn of 1997, the vertical component of F s is consistent with a blocking high over the southeast Pacific. To the east of this high cold air advection from Antarctica occurs, and to the west warm air advection occurs. This is consistent with negative and positive centers seen in the vertical component of F s to the east and west of south SA. The rainfall anomalies during a particular season seem to be due to multiple causes and this complicates a direct connection between them. Although over the southeast of SA, the higher rainfall during 1997 and 1998 can be attributed to some specific causes such as higher water vapor transport by LLJ in summer of 1997/98, in other parts such an association was not possible.
