Search Results
You are looking at 1 - 10 of 22 items for
- Author or Editor: Morris A. Bender x
- Refine by Access: All Content x
Abstract
Asymmetric structure of tropical cyclones simulated by the Geophysical Fluid Dynamics Laboratory high-resolution triply nested movable-mesh hurricane model was analyzed. Emphasis was placed on the quasi-steady component of the asymmetric structure in the region of the eyewall. It was found that the asymmetry was primarily caused by the relative wind, that is, the flow entering and leaving the storm region relative to the moving storm. A set of idealized numerical experiments was first performed both with a constant and a variable Coriolis parameter ( f ) and the addition of basic flows that were either constant or sheared with height. Analysis was then made for one case of Hurricane Gilbert (1988) to demonstrate that the quasi-steady asymmetric structure analyzed in the idealized studies could be identified in this real data case.
Vorticity analysis in the variable f experiment indicated that quasi-steady asymmetries resulted in the eyewall region through the effect of vorticity advection due to differences between the beta gyre flow in the lower free atmosphere and the storm motion. This was roughly matched with a persistent area of divergence and vorticity compression in the lower free atmosphere ahead of the storm and enhanced convergence and vorticity stretching to the rear. An asymmetric structure in the upward motion and accumulated precipitation, when averaged over a sufficiently long period of time, exhibited a corresponding maximum in the eyewall’s rear quadrant.
With the addition of an easterly basic flow, a pronounced change in the asymmetry of the time-averaged boundary layer convergence resulted, with maximum convergence located ahead of the storm. However, the asymmetries in the average vertical motion in the middle troposphere and accumulated precipitation were more affected by the convergence field in the lower free atmosphere produced by the relative flow there. The relative flow depended on both the basic and beta gyre flow. With the addition of an easterly vertical shear to the easterly basic flow, the storm moved faster than the lower-level winds, and strong relative wind was from the front to the rear in the lower free atmosphere and from the opposite direction in the outflow layer aloft. As a result, the upward motion was significantly increased in the front of the storm and reduced in the rear, and the precipitation maximum shifted to the left front quadrant.
Overall, analysis results suggest that the flow relative to the storm motion is an important factor contributing to the formation of quasi-steady asymmetries in the convergence and vertical motion fields, as well as in the mean precipitation pattern of tropical cyclones.
Abstract
Asymmetric structure of tropical cyclones simulated by the Geophysical Fluid Dynamics Laboratory high-resolution triply nested movable-mesh hurricane model was analyzed. Emphasis was placed on the quasi-steady component of the asymmetric structure in the region of the eyewall. It was found that the asymmetry was primarily caused by the relative wind, that is, the flow entering and leaving the storm region relative to the moving storm. A set of idealized numerical experiments was first performed both with a constant and a variable Coriolis parameter ( f ) and the addition of basic flows that were either constant or sheared with height. Analysis was then made for one case of Hurricane Gilbert (1988) to demonstrate that the quasi-steady asymmetric structure analyzed in the idealized studies could be identified in this real data case.
Vorticity analysis in the variable f experiment indicated that quasi-steady asymmetries resulted in the eyewall region through the effect of vorticity advection due to differences between the beta gyre flow in the lower free atmosphere and the storm motion. This was roughly matched with a persistent area of divergence and vorticity compression in the lower free atmosphere ahead of the storm and enhanced convergence and vorticity stretching to the rear. An asymmetric structure in the upward motion and accumulated precipitation, when averaged over a sufficiently long period of time, exhibited a corresponding maximum in the eyewall’s rear quadrant.
With the addition of an easterly basic flow, a pronounced change in the asymmetry of the time-averaged boundary layer convergence resulted, with maximum convergence located ahead of the storm. However, the asymmetries in the average vertical motion in the middle troposphere and accumulated precipitation were more affected by the convergence field in the lower free atmosphere produced by the relative flow there. The relative flow depended on both the basic and beta gyre flow. With the addition of an easterly vertical shear to the easterly basic flow, the storm moved faster than the lower-level winds, and strong relative wind was from the front to the rear in the lower free atmosphere and from the opposite direction in the outflow layer aloft. As a result, the upward motion was significantly increased in the front of the storm and reduced in the rear, and the precipitation maximum shifted to the left front quadrant.
Overall, analysis results suggest that the flow relative to the storm motion is an important factor contributing to the formation of quasi-steady asymmetries in the convergence and vertical motion fields, as well as in the mean precipitation pattern of tropical cyclones.
Abstract
In order to investigate the effect of tropical cyclone–ocean interaction on the intensity of observed hurricanes, the GFDL movable triply nested mesh hurricane model was coupled with a high-resolution version of the Princeton Ocean Model. The ocean model had 1/6° uniform resolution, which matched the horizontal resolution of the hurricane model in its innermost grid. Experiments were run with and without inclusion of the coupling for two cases of Hurricane Opal (1995) and one case of Hurricane Gilbert (1988) in the Gulf of Mexico and two cases each of Hurricanes Felix (1995) and Fran (1996) in the western Atlantic. The results confirmed the conclusions suggested by the earlier idealized studies that the cooling of the sea surface induced by the tropical cyclone will have a significant impact on the intensity of observed storms, particularly for slow moving storms where the SST decrease is greater. In each of the seven forecasts, the ocean coupling led to substantial improvements in the prediction of storm intensity measured by the storm’s minimum sea level pressure.
Without the effect of coupling the GFDL model incorrectly forecasted 25-hPa deepening of Gilbert as it moved across the Gulf of Mexico. With the coupling included, the model storm deepened only 10 hPa, which was much closer to the observed amount of 4 hPa. Similarly, during the period that Opal moved very slowly in the southern Gulf of Mexico, the coupled model produced a large SST decrease northwest of the Yucatan and slow deepening consistent with the observations. The uncoupled model using the initial NCEP SSTs predicted rapid deepening of 58 hPa during the same period.
Improved intensity prediction was achieved both for Hurricanes Felix and Fran in the western Atlantic. For the case of Hurricane Fran, the coarse resolution of the NCEP SST analysis could not resolve Hurricane Edouard’s wake, which was produced when Edouard moved in nearly an identical path to Fran four days earlier. As a result, the operational GFDL forecast using the operational SSTs and without coupling incorrectly forecasted 40-hPa deepening while Fran remained at nearly constant intensity as it crossed the wake. When the coupled model was run with Edouard’s cold wake generated by imposing hurricane wind forcing during the ocean initialization, the intensity prediction was significantly improved. The model also correctly predicted the rapid deepening that occurred as Fran began to move away from the cold wake. These results suggest the importance of an accurate initial SST analysis as well as the inclusion of the ocean coupling, for accurate hurricane intensity prediction with a dynamical model.
Recently, the GFDL hurricane–ocean coupled model used in these case studies was run on 163 forecasts during the 1995–98 seasons. Improved intensity forecasts were again achieved with the mean absolute error in the forecast of central pressure reduced by about 26% compared to the operational GFDL model. During the 1998 season, when the system was run in near–real time, the coupled model improved the intensity forecasts for all storms with central pressure higher than 940 hPa although the most significant improvement (∼60%) occurred in the intensity range of 960–970 hPa. These much larger sample sets confirmed the conclusion from the case studies, that the hurricane–ocean interaction is an important physical mechanism in the intensity of observed tropical cyclones.
Abstract
In order to investigate the effect of tropical cyclone–ocean interaction on the intensity of observed hurricanes, the GFDL movable triply nested mesh hurricane model was coupled with a high-resolution version of the Princeton Ocean Model. The ocean model had 1/6° uniform resolution, which matched the horizontal resolution of the hurricane model in its innermost grid. Experiments were run with and without inclusion of the coupling for two cases of Hurricane Opal (1995) and one case of Hurricane Gilbert (1988) in the Gulf of Mexico and two cases each of Hurricanes Felix (1995) and Fran (1996) in the western Atlantic. The results confirmed the conclusions suggested by the earlier idealized studies that the cooling of the sea surface induced by the tropical cyclone will have a significant impact on the intensity of observed storms, particularly for slow moving storms where the SST decrease is greater. In each of the seven forecasts, the ocean coupling led to substantial improvements in the prediction of storm intensity measured by the storm’s minimum sea level pressure.
Without the effect of coupling the GFDL model incorrectly forecasted 25-hPa deepening of Gilbert as it moved across the Gulf of Mexico. With the coupling included, the model storm deepened only 10 hPa, which was much closer to the observed amount of 4 hPa. Similarly, during the period that Opal moved very slowly in the southern Gulf of Mexico, the coupled model produced a large SST decrease northwest of the Yucatan and slow deepening consistent with the observations. The uncoupled model using the initial NCEP SSTs predicted rapid deepening of 58 hPa during the same period.
Improved intensity prediction was achieved both for Hurricanes Felix and Fran in the western Atlantic. For the case of Hurricane Fran, the coarse resolution of the NCEP SST analysis could not resolve Hurricane Edouard’s wake, which was produced when Edouard moved in nearly an identical path to Fran four days earlier. As a result, the operational GFDL forecast using the operational SSTs and without coupling incorrectly forecasted 40-hPa deepening while Fran remained at nearly constant intensity as it crossed the wake. When the coupled model was run with Edouard’s cold wake generated by imposing hurricane wind forcing during the ocean initialization, the intensity prediction was significantly improved. The model also correctly predicted the rapid deepening that occurred as Fran began to move away from the cold wake. These results suggest the importance of an accurate initial SST analysis as well as the inclusion of the ocean coupling, for accurate hurricane intensity prediction with a dynamical model.
Recently, the GFDL hurricane–ocean coupled model used in these case studies was run on 163 forecasts during the 1995–98 seasons. Improved intensity forecasts were again achieved with the mean absolute error in the forecast of central pressure reduced by about 26% compared to the operational GFDL model. During the 1998 season, when the system was run in near–real time, the coupled model improved the intensity forecasts for all storms with central pressure higher than 940 hPa although the most significant improvement (∼60%) occurred in the intensity range of 960–970 hPa. These much larger sample sets confirmed the conclusion from the case studies, that the hurricane–ocean interaction is an important physical mechanism in the intensity of observed tropical cyclones.
Abstract
A scheme is presented for improving the previously proposed method of dynamic initialization of the boundary layer in a primitive equation model (Kurihara and Tuleya, 1978). Performance of the revised scheme is shown for the case of a strong vortex superposed on a zonal flow.
Abstract
A scheme is presented for improving the previously proposed method of dynamic initialization of the boundary layer in a primitive equation model (Kurihara and Tuleya, 1978). Performance of the revised scheme is shown for the case of a strong vortex superposed on a zonal flow.
Abstract
The mesh nesting strategy proposed by Kurihara et al.(1979) was used to construct a movable, nested-mesh, 11-level primitive equation model. The framework of the model is described in detail.
With the use of a triply nested mesh system with 1°,⅓° and ⅙° longitude-latitude resolution, a small intense dry vortex in a zonal flow of 10 m s-1 was successfully advected for 48 h. The shape of the vortex was well preserved during the time integration which involved over 50 movements of the innermost mesh. The noise, which was excited when a mesh moved, was suppressed in ∼4 min after the movement. For comparison. the results from similar experiments performed with reduced inner mesh resolutions are also presented.
Abstract
The mesh nesting strategy proposed by Kurihara et al.(1979) was used to construct a movable, nested-mesh, 11-level primitive equation model. The framework of the model is described in detail.
With the use of a triply nested mesh system with 1°,⅓° and ⅙° longitude-latitude resolution, a small intense dry vortex in a zonal flow of 10 m s-1 was successfully advected for 48 h. The shape of the vortex was well preserved during the time integration which involved over 50 movements of the innermost mesh. The noise, which was excited when a mesh moved, was suppressed in ∼4 min after the movement. For comparison. the results from similar experiments performed with reduced inner mesh resolutions are also presented.
Abstract
A numerical scheme to treat the open lateral boundary of a limited-area primitive equation model was formulated. Although overspecification of the boundary condition is inevitable in the pointwise boundary setting, the scheme was designed to keep the overspecification to a minimum degree. To impose the boundary conditions, a damping technique was used. Special care was taken to deal with the boundary layer winds at the lateral boundary. The above scheme is most suitable when gravity waves do not prevail in the vicinity of the open boundary.
The scheme was tested in the numerical integrations of prognostic equations for a Haurwitz-type wave. Experimental results are presented which indicate the utility of the proposed method.
Abstract
A numerical scheme to treat the open lateral boundary of a limited-area primitive equation model was formulated. Although overspecification of the boundary condition is inevitable in the pointwise boundary setting, the scheme was designed to keep the overspecification to a minimum degree. To impose the boundary conditions, a damping technique was used. Special care was taken to deal with the boundary layer winds at the lateral boundary. The above scheme is most suitable when gravity waves do not prevail in the vicinity of the open boundary.
The scheme was tested in the numerical integrations of prognostic equations for a Haurwitz-type wave. Experimental results are presented which indicate the utility of the proposed method.
Abstract
From the end of October 1973 to the beginning of January 1974, Continental Airlines operated one of its Boeing 747 aircraft with special instrumentation for the study of clear-air turbulence (CAT). The observations were compared with satellite-derived radiance gradients, conventional temperature gradients from analyzed maps, and temperature gradients obtained from a Rosemount total air temperature sensor on the plane. The results led to the following conclusions:
1) In regions of weak gradients of temperature or of CO2 band radiance, the probability of CAT is extremely small.
2) CAT probabilities are significantly higher over mountains than flat terrain.
3) Even over mountains the probability of CAT is greatly increased by large gradients of temperature or radiance.
4) Satellite radiance gradients appear to discriminate between CAT and no CAT better than conventional temperature gradients over flat lands, whereas the reverse is true over mountains—although the differences between the two techniques are not large over mountains. Since most of the flights over flat terrain were flown over the Pacific Ocean, the result, if significant, may suggest that conventional temperature gradients over regions of sparse data are not as accurate as temperature gradients which can be inferred from satellites.
5) Temperature gradients obtainable from aircraft temperature sensors are not correlated with CAT statistics.
Abstract
From the end of October 1973 to the beginning of January 1974, Continental Airlines operated one of its Boeing 747 aircraft with special instrumentation for the study of clear-air turbulence (CAT). The observations were compared with satellite-derived radiance gradients, conventional temperature gradients from analyzed maps, and temperature gradients obtained from a Rosemount total air temperature sensor on the plane. The results led to the following conclusions:
1) In regions of weak gradients of temperature or of CO2 band radiance, the probability of CAT is extremely small.
2) CAT probabilities are significantly higher over mountains than flat terrain.
3) Even over mountains the probability of CAT is greatly increased by large gradients of temperature or radiance.
4) Satellite radiance gradients appear to discriminate between CAT and no CAT better than conventional temperature gradients over flat lands, whereas the reverse is true over mountains—although the differences between the two techniques are not large over mountains. Since most of the flights over flat terrain were flown over the Pacific Ocean, the result, if significant, may suggest that conventional temperature gradients over regions of sparse data are not as accurate as temperature gradients which can be inferred from satellites.
5) Temperature gradients obtainable from aircraft temperature sensors are not correlated with CAT statistics.
Abstract
A triply-nested, movable mesh model was used to study the effects of a mountain range on a landfalling tropical cyclone embedded in an easterly flow of ∼10 m s−1. The integration domain consisted of a 37° wide and 45° long channel, with an innermost mesh resolution of 1/6°. An idealized mountain range with maximum height of ∼958 meters was placed parallel to the shoreline. The mountain range, which spanned 19° in the north–south direction and 5° in the east–west direction, was centered in the middle of the channel. Results obtained were compared with a previous landfall simulation, performed without the effect of the mountain range included. In particular, comparison was made of the total storm rainfall, maximum wind distribution and storm decay rate. It was found that the storm filled much more rapidly in the simulation run with the mountain included. The mountain range affected the decay rate through reduction in the supply of latent and kinetic energy into the storm circulation during, as well as after, passage of the storm over the mountain. It was found that a low-level, warm and dry region was produced where the storm winds descended the mountain slope.
In order to better isolate the effect of the mountain on the basic easterly flow, a supplemental integration was performed for the flow without the storm. It revealed that the mountain range caused a significant change in the basic flow over the mountain as well as up to several hundred kilometers downstream and extending considerably above the mountain top. A low-level southerly jet was observed to the west of the mountain base.
Abstract
A triply-nested, movable mesh model was used to study the effects of a mountain range on a landfalling tropical cyclone embedded in an easterly flow of ∼10 m s−1. The integration domain consisted of a 37° wide and 45° long channel, with an innermost mesh resolution of 1/6°. An idealized mountain range with maximum height of ∼958 meters was placed parallel to the shoreline. The mountain range, which spanned 19° in the north–south direction and 5° in the east–west direction, was centered in the middle of the channel. Results obtained were compared with a previous landfall simulation, performed without the effect of the mountain range included. In particular, comparison was made of the total storm rainfall, maximum wind distribution and storm decay rate. It was found that the storm filled much more rapidly in the simulation run with the mountain included. The mountain range affected the decay rate through reduction in the supply of latent and kinetic energy into the storm circulation during, as well as after, passage of the storm over the mountain. It was found that a low-level, warm and dry region was produced where the storm winds descended the mountain slope.
In order to better isolate the effect of the mountain on the basic easterly flow, a supplemental integration was performed for the flow without the storm. It revealed that the mountain range caused a significant change in the basic flow over the mountain as well as up to several hundred kilometers downstream and extending considerably above the mountain top. A low-level southerly jet was observed to the west of the mountain base.
Abstract
A triply nested, movable mesh model was used to study the behavior of tropical cyclones encountering island mountain ranges. The integration domain consisted of a 37° wide and 45° long channel, with an innermost mesh resolution of 1/6°. The storms used for this study were embedded in easterly flows of ∼5 and ∼10 m s−1 initially. Realistic distributions of island topography at 1/6° resolution were inserted into the model domain for the region of the Caribbean, including the islands of Cuba, Hispaniola, and Puerto Rico; the island of Taiwan; and the region of Luzon in the northern Philippines.
It was found that the islands affected the basic flow as well as the wind field directly associated with the storm system. The combination of these effects caused changes in the track and translational speed of the storm. In particular, in the case of the 5 m s−1 easterly flow, the storm accelerated and veered to the north well before reaching Taiwan. For the other island distributions, the northward deflection of the track and the increase of translational speed occurred near and over the islands. After landfall, the surface pressure underwent rapid filling. As the tropical cyclone passed over Hispaniola, the surface low continued to move along with the upper level vortex as it transversed the mountain range, while over Luzon it became obscure before reforming on the lee side slope of the mountain. In case of Taiwan and the 10 m s−1 easterly zonal flow, secondary surface lows developed behind the mountain range. The upper level vortex in this case became detached from the original surface low and eventually coupled with a secondary one.
The intensity changes of the storm near and over the islands were strongly related to the latent energy supply and the vertical coherence of the storm system. Advection of dry air from near or above the mountain tops into the storm area caused significant weakening of all the storms moving with the weaker easterly flow. Storms leaving Hispaniola and moving over open sea quickly reintensified as their vertical structure remained coherent. On the other hand, storms leaving Luzon were disorganized and did not reintensify until several hours later when the vertical coherence of the systems was reestablished.
Although these experiments were performed for an idealized experimental design and basic flow, many observed storms have exhibited similar behavior in track deviation and decay. This implies that the effect of detailed topography should be considered if an accurate forecast of the storm direction and behavior is to be made.
Abstract
A triply nested, movable mesh model was used to study the behavior of tropical cyclones encountering island mountain ranges. The integration domain consisted of a 37° wide and 45° long channel, with an innermost mesh resolution of 1/6°. The storms used for this study were embedded in easterly flows of ∼5 and ∼10 m s−1 initially. Realistic distributions of island topography at 1/6° resolution were inserted into the model domain for the region of the Caribbean, including the islands of Cuba, Hispaniola, and Puerto Rico; the island of Taiwan; and the region of Luzon in the northern Philippines.
It was found that the islands affected the basic flow as well as the wind field directly associated with the storm system. The combination of these effects caused changes in the track and translational speed of the storm. In particular, in the case of the 5 m s−1 easterly flow, the storm accelerated and veered to the north well before reaching Taiwan. For the other island distributions, the northward deflection of the track and the increase of translational speed occurred near and over the islands. After landfall, the surface pressure underwent rapid filling. As the tropical cyclone passed over Hispaniola, the surface low continued to move along with the upper level vortex as it transversed the mountain range, while over Luzon it became obscure before reforming on the lee side slope of the mountain. In case of Taiwan and the 10 m s−1 easterly zonal flow, secondary surface lows developed behind the mountain range. The upper level vortex in this case became detached from the original surface low and eventually coupled with a secondary one.
The intensity changes of the storm near and over the islands were strongly related to the latent energy supply and the vertical coherence of the storm system. Advection of dry air from near or above the mountain tops into the storm area caused significant weakening of all the storms moving with the weaker easterly flow. Storms leaving Hispaniola and moving over open sea quickly reintensified as their vertical structure remained coherent. On the other hand, storms leaving Luzon were disorganized and did not reintensify until several hours later when the vertical coherence of the systems was reestablished.
Although these experiments were performed for an idealized experimental design and basic flow, many observed storms have exhibited similar behavior in track deviation and decay. This implies that the effect of detailed topography should be considered if an accurate forecast of the storm direction and behavior is to be made.
Abstract
A numerical scheme to construct a two-way, movable, nested-mesh primitive equation model is proposed. Dynamical coupling in a two-way nesting system is performed at a dynamical interface which is separated from a mesh interface by two coarse-grid intervals. Dynamical interaction is achieved by a method which conserves mass, momentum and internal energy of the system. During the course 'Of integration, the nested mesh moves so that the central position of the disturbance contained in The fine-mesh area never deviates from the center of the nest by more than one coarse-mesh interval. New grid data near the leading and trailing edges of the moving nest are obtained by an interpolation method which has a conservation property. The proposed methods of dynamical coupling and mesh movement were extensively tested by a one-dimensional shallow water equation model. Numerical results of these experiments are presented.
Abstract
A numerical scheme to construct a two-way, movable, nested-mesh primitive equation model is proposed. Dynamical coupling in a two-way nesting system is performed at a dynamical interface which is separated from a mesh interface by two coarse-grid intervals. Dynamical interaction is achieved by a method which conserves mass, momentum and internal energy of the system. During the course 'Of integration, the nested mesh moves so that the central position of the disturbance contained in The fine-mesh area never deviates from the center of the nest by more than one coarse-mesh interval. New grid data near the leading and trailing edges of the moving nest are obtained by an interpolation method which has a conservation property. The proposed methods of dynamical coupling and mesh movement were extensively tested by a one-dimensional shallow water equation model. Numerical results of these experiments are presented.
Abstract
A numerical scheme proposed by Kurihara and Bender is modified so as to improve the behavior of open lateral boundaries of a regional model. In the new scheme, both the local values and the gradients of fields from a larger model are used to define the time-dependent reference values toward which the boundary gridpoint values of the regional model prediction are relaxed at each step of the model integration. Use of the gradients in the boundary forcing imposes constraints on the vorticity, divergence and baroclinicity fields for the regional model. The relaxation time of forcing is set to be short for the normal component of wind. For other variables, the relaxation time at a given boundary gridpoint depends on the wind direction at that gridpoint, with a minimum at a point of normal inflow and a maximum at a point of normal outflow. The forcing strength is reduced in the planetary boundary layer so that the boundary layer structure is determined mainly by the surface condition of the regional model. Also, a simple method to control the total mass in the regional model is described. Numerical results from 96-hour integrations with the improved scheme are compared with those from the previous scheme for the cases of the propagations of a wave and a vortex. The behavior of the model at the lateral boundary was noticeably improved with the use of the new scheme, while the solution in the interior domain was little affected by the scheme modification.
Abstract
A numerical scheme proposed by Kurihara and Bender is modified so as to improve the behavior of open lateral boundaries of a regional model. In the new scheme, both the local values and the gradients of fields from a larger model are used to define the time-dependent reference values toward which the boundary gridpoint values of the regional model prediction are relaxed at each step of the model integration. Use of the gradients in the boundary forcing imposes constraints on the vorticity, divergence and baroclinicity fields for the regional model. The relaxation time of forcing is set to be short for the normal component of wind. For other variables, the relaxation time at a given boundary gridpoint depends on the wind direction at that gridpoint, with a minimum at a point of normal inflow and a maximum at a point of normal outflow. The forcing strength is reduced in the planetary boundary layer so that the boundary layer structure is determined mainly by the surface condition of the regional model. Also, a simple method to control the total mass in the regional model is described. Numerical results from 96-hour integrations with the improved scheme are compared with those from the previous scheme for the cases of the propagations of a wave and a vortex. The behavior of the model at the lateral boundary was noticeably improved with the use of the new scheme, while the solution in the interior domain was little affected by the scheme modification.
