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Abstract
A two-dimensional, hydrostatic, nearly adiabatic primitive equation model is used to study the evolution of a front passing across topography. Frontogenesis is forced by shearing deformation associated with the nonlinear evolution of an Eady wave. This study extends previous work by including an upper-level potential vorticity (PV) anomaly and a growing baroclinic wave in a baroclinically unstable basic state.
Results for the Eady wave simulations show that the mountain retards and blocks the approaching front at the surface while the upper-level PV anomaly associated with the front moves across the domain unaffected. Warm advection ahead of the lee trough forces convergence and cyclonic vorticity growth near the base of the lee slope. This vorticity growth is further encouraged by the approach of the upper-level PV anomaly. The upper-level PV anomaly then couples with this new surface vorticity center and propagates downstream. The original surface front remains trapped on the windward slope. Thus when the upstream blocking is strong, frontal propagation is discontinuous across the ridge. This evolution occurs for tall mountains and narrow mountains, as well as weak fronts. For low mountains, wide mountains, and strong fronts, only weak retardation is observed on the windward slope. The surface front remains coupled with the upper-level PV anomaly. The front moves continuously across the mountain.
The net result, regardless of mountain size and shape, is that the front reaches the base of the lee slope stronger, sooner, and with a decreased cross-front scale compared to the “no-mountain” case. Well downstream of the mountain, no position change of the surface front is observed.
Abstract
A two-dimensional, hydrostatic, nearly adiabatic primitive equation model is used to study the evolution of a front passing across topography. Frontogenesis is forced by shearing deformation associated with the nonlinear evolution of an Eady wave. This study extends previous work by including an upper-level potential vorticity (PV) anomaly and a growing baroclinic wave in a baroclinically unstable basic state.
Results for the Eady wave simulations show that the mountain retards and blocks the approaching front at the surface while the upper-level PV anomaly associated with the front moves across the domain unaffected. Warm advection ahead of the lee trough forces convergence and cyclonic vorticity growth near the base of the lee slope. This vorticity growth is further encouraged by the approach of the upper-level PV anomaly. The upper-level PV anomaly then couples with this new surface vorticity center and propagates downstream. The original surface front remains trapped on the windward slope. Thus when the upstream blocking is strong, frontal propagation is discontinuous across the ridge. This evolution occurs for tall mountains and narrow mountains, as well as weak fronts. For low mountains, wide mountains, and strong fronts, only weak retardation is observed on the windward slope. The surface front remains coupled with the upper-level PV anomaly. The front moves continuously across the mountain.
The net result, regardless of mountain size and shape, is that the front reaches the base of the lee slope stronger, sooner, and with a decreased cross-front scale compared to the “no-mountain” case. Well downstream of the mountain, no position change of the surface front is observed.
Abstract
A two-dimensional, hydrostatic, primitive-equation model is used to investigate the dynamics of frontogenesis in a moist atmosphere. The development of a cold front is simulated through shear-deformation associated with the non-linear evolution of an Eady wave. Simulations are performed with 5, 10, 40 and 80 km horizontal resolutions and 14 levels in the vertical (four in the boundary layer).
Compared to the dry case, the inclusion of moisture in the model produces a stronger low-level jet ahead of the front and a stronger upper-level jet. Moisture also produces a stronger ageostrophic circulation across the front and a more concentrated updraft just ahead of the surface front. The updraft develops a banded structure above and behind the surface front, with a wavelength of about 70 km. Bands form near the back edge of the cloud shield and move toward the surface front with a relative velocity of ∼1 m s−1. These characteristics agree with observations of wide cold-frontal rainbands.
The banded structures form in a convectively stable region. The first band that appears in the numerical simulation forms and intensifies in a region of negative equivalent potential vorticity. Subsequent bands form behind the first and intensify as they move into the region of negative equivalent potential vorticity, indicating that conditional symmetric instability (CSI) may play an important role in their formation and intensification. Many of the characteristics of the bands agree with the theory of CSI. The bands disappear when equivalent potential vorticity is everywhere positive. The bands are poorly resolved when the horizontal resolution (Δx) of the model is 40 km, and they are absent with Δx = 80 km. However, the strength and horizontal scale of the bands is about the same with Δx = 5 km and Δx = 10 km. This indicates that the banded structure is not an artifact of the model.
Frictional convergence in the boundary layer forces a narrow cold-frontal rainband (NCFR) just above the surface front. The horizontal dimension of this band is greater than that for observed NCFR, presumably because of limited resolution in the model.
Abstract
A two-dimensional, hydrostatic, primitive-equation model is used to investigate the dynamics of frontogenesis in a moist atmosphere. The development of a cold front is simulated through shear-deformation associated with the non-linear evolution of an Eady wave. Simulations are performed with 5, 10, 40 and 80 km horizontal resolutions and 14 levels in the vertical (four in the boundary layer).
Compared to the dry case, the inclusion of moisture in the model produces a stronger low-level jet ahead of the front and a stronger upper-level jet. Moisture also produces a stronger ageostrophic circulation across the front and a more concentrated updraft just ahead of the surface front. The updraft develops a banded structure above and behind the surface front, with a wavelength of about 70 km. Bands form near the back edge of the cloud shield and move toward the surface front with a relative velocity of ∼1 m s−1. These characteristics agree with observations of wide cold-frontal rainbands.
The banded structures form in a convectively stable region. The first band that appears in the numerical simulation forms and intensifies in a region of negative equivalent potential vorticity. Subsequent bands form behind the first and intensify as they move into the region of negative equivalent potential vorticity, indicating that conditional symmetric instability (CSI) may play an important role in their formation and intensification. Many of the characteristics of the bands agree with the theory of CSI. The bands disappear when equivalent potential vorticity is everywhere positive. The bands are poorly resolved when the horizontal resolution (Δx) of the model is 40 km, and they are absent with Δx = 80 km. However, the strength and horizontal scale of the bands is about the same with Δx = 5 km and Δx = 10 km. This indicates that the banded structure is not an artifact of the model.
Frictional convergence in the boundary layer forces a narrow cold-frontal rainband (NCFR) just above the surface front. The horizontal dimension of this band is greater than that for observed NCFR, presumably because of limited resolution in the model.
Abstract
A comparison between the climatological structure of retarded and unretarded fronts aligned parallel to the Appalachian Mountains is investigated. With the average height of the Appalachians being 1 km, retarded and unretarded fronts are common occurrences during the cold season. Because of the narrow half-width of 100 km and the 1000-km length of the mountain chain, a comparison to two- and three-dimensional numerical studies can be performed. Of the 142 cases of frontal passages over the Appalachians during the winters between October 1984 and April 1990, over 55% of all cold fronts were retarded by the mountains. Statistical analysis showed that retarded fronts have a stronger cross-front temperature gradient and a weaker cross-front pressure gradient. Composite fields of sea level pressure, 850-, 500-, and 200-mb heights; quasigeostrophic potential vorticity and its advection, and potential height (U/N) were computed for all retarded and unretarded fronts. Unretarded fronts were associated with stronger cyclones, larger potential vorticity anomalies, larger positive potential vorticity advection, and more amplified flow at all levels. There was no significant difference between the potential height fields of the two types of fronts. In addition the average potential height, for both groups of fronts, easily met the criteria for retardation. Rather than depending upon the Froude number of the flow, it is hypothesized that the strength of the synoptic-scale circulations in the middle and upper troposphere primarily determines whether or not a front will be retarded by the Appalachian Mountains.
Abstract
A comparison between the climatological structure of retarded and unretarded fronts aligned parallel to the Appalachian Mountains is investigated. With the average height of the Appalachians being 1 km, retarded and unretarded fronts are common occurrences during the cold season. Because of the narrow half-width of 100 km and the 1000-km length of the mountain chain, a comparison to two- and three-dimensional numerical studies can be performed. Of the 142 cases of frontal passages over the Appalachians during the winters between October 1984 and April 1990, over 55% of all cold fronts were retarded by the mountains. Statistical analysis showed that retarded fronts have a stronger cross-front temperature gradient and a weaker cross-front pressure gradient. Composite fields of sea level pressure, 850-, 500-, and 200-mb heights; quasigeostrophic potential vorticity and its advection, and potential height (U/N) were computed for all retarded and unretarded fronts. Unretarded fronts were associated with stronger cyclones, larger potential vorticity anomalies, larger positive potential vorticity advection, and more amplified flow at all levels. There was no significant difference between the potential height fields of the two types of fronts. In addition the average potential height, for both groups of fronts, easily met the criteria for retardation. Rather than depending upon the Froude number of the flow, it is hypothesized that the strength of the synoptic-scale circulations in the middle and upper troposphere primarily determines whether or not a front will be retarded by the Appalachian Mountains.
Abstract
This paper examines the climatological, large-scale, and synoptic-scale aspects of South American cold surges using NCEP–NCAR gridded reanalyses for the 1992–96 period. Three common cold surge types are identified on the basis of a thickness (1000–850 hPa) criteria: type 1—a transient surge associated with weak anticyclone development east of the Andes in the absence of ridging aloft, type 2—a strong and persistent surge associated with dynamic anticyclogenesis aloft and strong surface anticyclone development east of the Andes, and type 3—a surge east of the Brazilian coastal mountains. Cold surges are most common during the winter and spring (Jun–Nov), accounting for 189 of the 256 events (74%).
Case studies of two events (19–22 Jul 1992 and 12–14 Apr 1993) are conducted from both a conventional isobaric and a potential vorticity (PV) perspective. The upper-air flow pattern in the July 1992 type 2 case is characterized by the presence of a strong ridge–trough couplet, which amplifies and becomes quasi-stationary, allowing for a deep layer of equatorward flow over South America. Dynamically, this flow pattern favors the development of a very strong surface anticyclone to the east of the Andes in response to a combination of differential anticyclonic vorticity advection, low-level cold advection, and, equivalently, positive PV advection. Because of the associated cold air damming east of the Andes, modified cool air is transported into the western part of Amazonia. Cold air damming east of the Brazilian coastal mountains is associated with the transition of the July 1992 type 2 surge into a type 3 surge.
The cold surge of April 1993 is examined as a rare event that does not fit the above classification. It is characterized by explosive cyclogenesis close to the coast of Argentina. Unlike the representative type 2 cold surge of July 1992, which tends to occur in association with southwesterly flow aloft, the April 1993 cold surge occurs beneath westerly and northwesterly flow aloft. Cold air penetration into lower latitudes is restricted because the geostrophic wind has a component directed away from the Andes equatorward of the cyclone. The dynamical forcing mechanisms associated with the April 1993 event are of smaller scale than those of the much more common surges typified by the July 1992 event.
Abstract
This paper examines the climatological, large-scale, and synoptic-scale aspects of South American cold surges using NCEP–NCAR gridded reanalyses for the 1992–96 period. Three common cold surge types are identified on the basis of a thickness (1000–850 hPa) criteria: type 1—a transient surge associated with weak anticyclone development east of the Andes in the absence of ridging aloft, type 2—a strong and persistent surge associated with dynamic anticyclogenesis aloft and strong surface anticyclone development east of the Andes, and type 3—a surge east of the Brazilian coastal mountains. Cold surges are most common during the winter and spring (Jun–Nov), accounting for 189 of the 256 events (74%).
Case studies of two events (19–22 Jul 1992 and 12–14 Apr 1993) are conducted from both a conventional isobaric and a potential vorticity (PV) perspective. The upper-air flow pattern in the July 1992 type 2 case is characterized by the presence of a strong ridge–trough couplet, which amplifies and becomes quasi-stationary, allowing for a deep layer of equatorward flow over South America. Dynamically, this flow pattern favors the development of a very strong surface anticyclone to the east of the Andes in response to a combination of differential anticyclonic vorticity advection, low-level cold advection, and, equivalently, positive PV advection. Because of the associated cold air damming east of the Andes, modified cool air is transported into the western part of Amazonia. Cold air damming east of the Brazilian coastal mountains is associated with the transition of the July 1992 type 2 surge into a type 3 surge.
The cold surge of April 1993 is examined as a rare event that does not fit the above classification. It is characterized by explosive cyclogenesis close to the coast of Argentina. Unlike the representative type 2 cold surge of July 1992, which tends to occur in association with southwesterly flow aloft, the April 1993 cold surge occurs beneath westerly and northwesterly flow aloft. Cold air penetration into lower latitudes is restricted because the geostrophic wind has a component directed away from the Andes equatorward of the cyclone. The dynamical forcing mechanisms associated with the April 1993 event are of smaller scale than those of the much more common surges typified by the July 1992 event.
Abstract
A full understanding of the causes of the severe drought seen in the Sahel in the latter part of the twentieth-century remains elusive some 25 yr after the height of the event. Previous studies have suggested that this drying trend may be explained by either decadal modes of natural variability or by human-driven emissions (primarily aerosols), but these studies lacked a sufficiently large number of models to attribute one cause over the other. In this paper, signatures of both aerosol and greenhouse gas changes on Sahel rainfall are illustrated. These idealized responses are used to interpret the results of historical Sahel rainfall changes from two very large ensembles of fully coupled climate models, which both sample uncertainties arising from internal variability and model formulation. The sizes of these ensembles enable the relative role of human-driven changes and natural variability on historic Sahel rainfall to be assessed. The paper demonstrates that historic aerosol changes are likely to explain most of the underlying 1940–80 drying signal and a notable proportion of the more pronounced 1950–80 drying.
Abstract
A full understanding of the causes of the severe drought seen in the Sahel in the latter part of the twentieth-century remains elusive some 25 yr after the height of the event. Previous studies have suggested that this drying trend may be explained by either decadal modes of natural variability or by human-driven emissions (primarily aerosols), but these studies lacked a sufficiently large number of models to attribute one cause over the other. In this paper, signatures of both aerosol and greenhouse gas changes on Sahel rainfall are illustrated. These idealized responses are used to interpret the results of historical Sahel rainfall changes from two very large ensembles of fully coupled climate models, which both sample uncertainties arising from internal variability and model formulation. The sizes of these ensembles enable the relative role of human-driven changes and natural variability on historic Sahel rainfall to be assessed. The paper demonstrates that historic aerosol changes are likely to explain most of the underlying 1940–80 drying signal and a notable proportion of the more pronounced 1950–80 drying.
Abstract
This paper describes the construction and results of a comprehensive, three-dimensional general circulation model (GCM) of the earth's climate. The model, developed at the National Center for Atmospheric Research (NCAR), links separate existing models of the atmosphere, ocean and sea ice. The atmospheric model is a version of the third-generation NCAR GCM which has a relatively complete treatment of physical processes. It uses a generalized vertical coordinate with eight layers (∼3 km thick) and 5° horizontal grid spacing over the entire globe. The ocean model, using the primitive equations and the hydrostatic and Boussinesq approximations, was changed to the world domain from an earlier model developed by Bryan (1969) and reprogrammed by Semtner (1974). The model has four unequally spaced vertical layers and 5° horizontal grid structure. The sea ice model is a simple thermodynamic model using a simplified calculation of heat flux through sea ice (Semtner, 1976).
The method of coupling the atmosphere and ocean models is an attempt to deal with the two different time scales of the atmosphere and ocean in a computationally efficient fashion. By means of four relatively short integrations, the atmospheric model provides samples (10–30 days in length) of four seasonal months—January, April, July and October. The data from the four atmospheric model months are fitted to annual and semiannual harmonics and are used to drive the ocean model for five years. The process is iterated for a number of cycles to achieve an approximate equilibrium.
The atmospheric circulation in the coupled model is similar to that obtained previously by Washington et al. (1979) with climatological ocean forcing. The simulated ocean surface temperature pattern is reasonably similar to the observed pattern, but the calculated ocean temperatures tend to be as much as 3°C too cold locally in the tropics and up to 4°C too warm in the midlatitudes. Possible reasons for these discrepancies are discussed. The major mean ocean current gyre systems are reproduced in the ocean model second layer where effects of non-geostrophic Ekman drift and short-term wind-stress averaging bias are not felt. These effects, however, tend to complicate somewhat the computed surface current pattern. The computed horizontal oceanic heat flux compares favorably with the observed of Oort and Vonder Haar (1976) in phase and amplitude. Vertical velocities at the bottom of the 50 m surface layer, which can be considered a simple mixed layer, have the same general pattern as those calculated using observed wind stress. The simulation of sea ice thickness and seasonal geographical extent is closer to the observed in the Arctic than in the Antarctic region.
The experiment described here must be regarded as preliminary; even though many first-order aspects of the climate system are simulated, improvements are still needed.
Abstract
This paper describes the construction and results of a comprehensive, three-dimensional general circulation model (GCM) of the earth's climate. The model, developed at the National Center for Atmospheric Research (NCAR), links separate existing models of the atmosphere, ocean and sea ice. The atmospheric model is a version of the third-generation NCAR GCM which has a relatively complete treatment of physical processes. It uses a generalized vertical coordinate with eight layers (∼3 km thick) and 5° horizontal grid spacing over the entire globe. The ocean model, using the primitive equations and the hydrostatic and Boussinesq approximations, was changed to the world domain from an earlier model developed by Bryan (1969) and reprogrammed by Semtner (1974). The model has four unequally spaced vertical layers and 5° horizontal grid structure. The sea ice model is a simple thermodynamic model using a simplified calculation of heat flux through sea ice (Semtner, 1976).
The method of coupling the atmosphere and ocean models is an attempt to deal with the two different time scales of the atmosphere and ocean in a computationally efficient fashion. By means of four relatively short integrations, the atmospheric model provides samples (10–30 days in length) of four seasonal months—January, April, July and October. The data from the four atmospheric model months are fitted to annual and semiannual harmonics and are used to drive the ocean model for five years. The process is iterated for a number of cycles to achieve an approximate equilibrium.
The atmospheric circulation in the coupled model is similar to that obtained previously by Washington et al. (1979) with climatological ocean forcing. The simulated ocean surface temperature pattern is reasonably similar to the observed pattern, but the calculated ocean temperatures tend to be as much as 3°C too cold locally in the tropics and up to 4°C too warm in the midlatitudes. Possible reasons for these discrepancies are discussed. The major mean ocean current gyre systems are reproduced in the ocean model second layer where effects of non-geostrophic Ekman drift and short-term wind-stress averaging bias are not felt. These effects, however, tend to complicate somewhat the computed surface current pattern. The computed horizontal oceanic heat flux compares favorably with the observed of Oort and Vonder Haar (1976) in phase and amplitude. Vertical velocities at the bottom of the 50 m surface layer, which can be considered a simple mixed layer, have the same general pattern as those calculated using observed wind stress. The simulation of sea ice thickness and seasonal geographical extent is closer to the observed in the Arctic than in the Antarctic region.
The experiment described here must be regarded as preliminary; even though many first-order aspects of the climate system are simulated, improvements are still needed.
An education-oriented workshop for college faculty in the atmospheric and related sciences was held in Boulder, Colorado, during June 1997 by three programs of the University Corporation for Atmospheric Research. The objective of this workshop was to provide faculty with hands-on training in the use of Web-based instructional methods for specific application to the teaching of satellite remote sensing in their subject areas. More than 150 faculty and associated scientists participated, and postworkshop evaluation showed it to have been a very successful integration of information and activities related to computer-based instruction, educational principles, and scientific lectures.
An education-oriented workshop for college faculty in the atmospheric and related sciences was held in Boulder, Colorado, during June 1997 by three programs of the University Corporation for Atmospheric Research. The objective of this workshop was to provide faculty with hands-on training in the use of Web-based instructional methods for specific application to the teaching of satellite remote sensing in their subject areas. More than 150 faculty and associated scientists participated, and postworkshop evaluation showed it to have been a very successful integration of information and activities related to computer-based instruction, educational principles, and scientific lectures.
During May–July 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) occurred in the High Plains, near the Colorado–Kansas border. STEPS aimed to achieve a better understanding of the interactions between kinematics, precipitation, and electrification in severe thunderstorms. Specific scientific objectives included 1) understanding the apparent major differences in precipitation output from supercells that have led to them being classified as low precipitation (LP), classic or medium precipitation, and high precipitation; 2) understanding lightning formation and behavior in storms, and how lightning differs among storm types, particularly to better understand the mechanisms by which storms produce predominantly positive cloud-to-ground (CG) lightning; and 3) verifying and improving microphysical interpretations from polarimetric radar. The project involved the use of a multiple-Doppler polarimetric radar network, as well as a time-of-arrival very high frequency (VHF) lightning mapping system, an armored research aircraft, electric field meters carried on balloons, mobile mesonet vehicles, instruments to detect and classify transient luminous events (TLEs; e.g., sprites and blue jets) over thunderstorms, and mobile atmospheric sounding equipment. The project featured significant collaboration with the local National Weather Service office in Goodland, Kansas, as well as outreach to the general public. The project gathered data on a number of different cases, including LP storms, supercells, and mesoscale convective systems, among others. Many of the storms produced mostly positive CG lightning during significant portions of their lifetimes and also exhibited unusual electrical structures with opposite polarity to ordinary thunderstorms. The field data from STEPS is expected to bring new advances to understanding of supercells, positive CG lightning, TLEs, and precipitation formation in convective storms.
During May–July 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) occurred in the High Plains, near the Colorado–Kansas border. STEPS aimed to achieve a better understanding of the interactions between kinematics, precipitation, and electrification in severe thunderstorms. Specific scientific objectives included 1) understanding the apparent major differences in precipitation output from supercells that have led to them being classified as low precipitation (LP), classic or medium precipitation, and high precipitation; 2) understanding lightning formation and behavior in storms, and how lightning differs among storm types, particularly to better understand the mechanisms by which storms produce predominantly positive cloud-to-ground (CG) lightning; and 3) verifying and improving microphysical interpretations from polarimetric radar. The project involved the use of a multiple-Doppler polarimetric radar network, as well as a time-of-arrival very high frequency (VHF) lightning mapping system, an armored research aircraft, electric field meters carried on balloons, mobile mesonet vehicles, instruments to detect and classify transient luminous events (TLEs; e.g., sprites and blue jets) over thunderstorms, and mobile atmospheric sounding equipment. The project featured significant collaboration with the local National Weather Service office in Goodland, Kansas, as well as outreach to the general public. The project gathered data on a number of different cases, including LP storms, supercells, and mesoscale convective systems, among others. Many of the storms produced mostly positive CG lightning during significant portions of their lifetimes and also exhibited unusual electrical structures with opposite polarity to ordinary thunderstorms. The field data from STEPS is expected to bring new advances to understanding of supercells, positive CG lightning, TLEs, and precipitation formation in convective storms.
