Search Results
You are looking at 1 - 10 of 10 items for :
- Author or Editor: Stephen J. Lord x
- Monthly Weather Review x
- Refine by Access: All Content x
Abstract
Synoptic-wale thermodynamic fields in the environment of Hurricane Debby (1982) determined from two sets of VAS soundings (VAS1, VAS2) are compared with those obtained from in-situ data (INS). VAS1 sounding were derived from an iterative solution of the radiative transfer equation with manual quality control. VAS2 soundings, which represent the present state-of-the-art, were derived from a simultaneous solution of the transfer equation with objective quality control. In situ data were obtained primarily from Omega dropwindsondes. Comparisons are made for 0000 UTC 16 September 1982 at the mandatory pressure levels up to 400 mb. The integrated effect of VAS-INS differences is estimated by comparing 400 mb geopotential height fields and their associated gradient winds.
The comparisons show that the VAS1-INS temperature differences are not spatially uniform at most levels, due largely to the influence of moisture. The quality of the VAS2 data is much improved over VAS1; the effect of moisture is not noticeable. However, the VAS2 analyses still show spatially nonuniform differences from INS at some levels. Thus, VAS gradient data may be of irregular quality on the synoptic scale. Geopotential height fields at 400 mb imply gradient wind differences from INS of up to 12 m s−1 for VAS 1 and 6 m s−1 for VAS2. The VAS2 sounding set could be improved further by the use of manual data editing, and a more accurate first-guess of the surface temperature analysis.
Abstract
Synoptic-wale thermodynamic fields in the environment of Hurricane Debby (1982) determined from two sets of VAS soundings (VAS1, VAS2) are compared with those obtained from in-situ data (INS). VAS1 sounding were derived from an iterative solution of the radiative transfer equation with manual quality control. VAS2 soundings, which represent the present state-of-the-art, were derived from a simultaneous solution of the transfer equation with objective quality control. In situ data were obtained primarily from Omega dropwindsondes. Comparisons are made for 0000 UTC 16 September 1982 at the mandatory pressure levels up to 400 mb. The integrated effect of VAS-INS differences is estimated by comparing 400 mb geopotential height fields and their associated gradient winds.
The comparisons show that the VAS1-INS temperature differences are not spatially uniform at most levels, due largely to the influence of moisture. The quality of the VAS2 data is much improved over VAS1; the effect of moisture is not noticeable. However, the VAS2 analyses still show spatially nonuniform differences from INS at some levels. Thus, VAS gradient data may be of irregular quality on the synoptic scale. Geopotential height fields at 400 mb imply gradient wind differences from INS of up to 12 m s−1 for VAS 1 and 6 m s−1 for VAS2. The VAS2 sounding set could be improved further by the use of manual data editing, and a more accurate first-guess of the surface temperature analysis.
Abstract
A three-dimensional analysis of temperature and relative humidity in the environment of Hurricane Debby (1982) has been completed. Observations from Omega dropwindsondes (ODWs) within 1000 km of the storm have been combined with rawinsondes over the continental United States and the Caribbean and with observations from surface ships and aircraft data where possible.
The temperature and relative humidity analyses, together with wind analyses from a previous study, form a dataset that can be used an an initial condition in a multilevel prognostic model when combined with analyses over area larger than our analysis domain. In this paper a series of diagnostic tests has been applied to the dataset to evaluate its performance without using a prognostic model. These tests include horizontal maps of the moist convective instability, calculation of the heat and moisture budgets in the vicinity of Bermuda, which was 350 km to the northeast of the storm center, and diagnosis of precipitation from these budgets and from the Arakawa-Schubert cumulus parameterization.
Results show that the horizontal distribution of moist convective instability is strongly affected by the low-level moisture field upstream of the main inflow region to the storm. The total surface heat flux, estimated with a bulk aerodynamic method, matches the vertically integrated eddy flux of moist static energy to within observational errors. Precipitation estimates from the budgets give rates of approximately 20 mm day−1, which are consistent with an estimated rate from radar. Partition of the rainfall rate into convective scale and resolvable scale (stratiform) shows about equal contributions.
Our results lead us to believe that, within the limitations determined by the horizontal distribution of the observations, the final dataset for Hurricane Debby provides a realistic depiction of the various physical processes that were occurring in Debby's environment. Future work will include data sensitivity experiments with a three-dimensional forecast model.
Abstract
A three-dimensional analysis of temperature and relative humidity in the environment of Hurricane Debby (1982) has been completed. Observations from Omega dropwindsondes (ODWs) within 1000 km of the storm have been combined with rawinsondes over the continental United States and the Caribbean and with observations from surface ships and aircraft data where possible.
The temperature and relative humidity analyses, together with wind analyses from a previous study, form a dataset that can be used an an initial condition in a multilevel prognostic model when combined with analyses over area larger than our analysis domain. In this paper a series of diagnostic tests has been applied to the dataset to evaluate its performance without using a prognostic model. These tests include horizontal maps of the moist convective instability, calculation of the heat and moisture budgets in the vicinity of Bermuda, which was 350 km to the northeast of the storm center, and diagnosis of precipitation from these budgets and from the Arakawa-Schubert cumulus parameterization.
Results show that the horizontal distribution of moist convective instability is strongly affected by the low-level moisture field upstream of the main inflow region to the storm. The total surface heat flux, estimated with a bulk aerodynamic method, matches the vertically integrated eddy flux of moist static energy to within observational errors. Precipitation estimates from the budgets give rates of approximately 20 mm day−1, which are consistent with an estimated rate from radar. Partition of the rainfall rate into convective scale and resolvable scale (stratiform) shows about equal contributions.
Our results lead us to believe that, within the limitations determined by the horizontal distribution of the observations, the final dataset for Hurricane Debby provides a realistic depiction of the various physical processes that were occurring in Debby's environment. Future work will include data sensitivity experiments with a three-dimensional forecast model.
Abstract
A three-dimensional, nested analysis of wind fields in the environment of Hurricane Debby (1982) has been completed. The basic analysis tool uses a two-dimensional least-squares fitting algorithm combined with a derivative constraint that acts as a spatial low-pass filter on the analyzed field. Gridded results of horizontally analyzed fields are combined into vertical cross sections and then analyzed to produce vertical continuity. Consequently, a three-dimensional analysis is obtained.
The database for the analysis comes primarily from Omega dropwindsondes (ODWs), rawinsondes, and satellite-derived winds above 400 mb in the environment of Hurricane Debby near 0000 UTC 16 September 1982. Since these data come from many different sources, and thus are not evenly distributed in the horizontal or vertical, techniques have been developed to alleviate difficulties associated with inhomogeneous data. The analyzed wind fields provide an independent evaluation of satellite-derived winds at and below 400 mb.
General features of the environmental wind fields surrounding Debby are described. The wind analyses are then used to diagnose terms in the vorticity equation. The spatial orientation of a calculated dipole in the horizontal vorticity flux convergence term indicates that it is an approximate indicator of Debby's observed short-term motion.
Finally, to provide an initial assessment of the wind analysis quality, experimental track forecasts with a barotropic model are performed with the layer-mean wind fields and operationally available data outside the analysis domain. Initial errors in the forecast tracks are directly related to the orientation of the diagnosed vorticity flux convergence dipole. The research wind analysis results in a substantial reduction in track error for short-term (12 h) forecasts compared to analyses from operationally available data. This reduction is due to an improved representation of the wind fields in the near-storm environment.
Abstract
A three-dimensional, nested analysis of wind fields in the environment of Hurricane Debby (1982) has been completed. The basic analysis tool uses a two-dimensional least-squares fitting algorithm combined with a derivative constraint that acts as a spatial low-pass filter on the analyzed field. Gridded results of horizontally analyzed fields are combined into vertical cross sections and then analyzed to produce vertical continuity. Consequently, a three-dimensional analysis is obtained.
The database for the analysis comes primarily from Omega dropwindsondes (ODWs), rawinsondes, and satellite-derived winds above 400 mb in the environment of Hurricane Debby near 0000 UTC 16 September 1982. Since these data come from many different sources, and thus are not evenly distributed in the horizontal or vertical, techniques have been developed to alleviate difficulties associated with inhomogeneous data. The analyzed wind fields provide an independent evaluation of satellite-derived winds at and below 400 mb.
General features of the environmental wind fields surrounding Debby are described. The wind analyses are then used to diagnose terms in the vorticity equation. The spatial orientation of a calculated dipole in the horizontal vorticity flux convergence term indicates that it is an approximate indicator of Debby's observed short-term motion.
Finally, to provide an initial assessment of the wind analysis quality, experimental track forecasts with a barotropic model are performed with the layer-mean wind fields and operationally available data outside the analysis domain. Initial errors in the forecast tracks are directly related to the orientation of the diagnosed vorticity flux convergence dipole. The research wind analysis results in a substantial reduction in track error for short-term (12 h) forecasts compared to analyses from operationally available data. This reduction is due to an improved representation of the wind fields in the near-storm environment.
Abstract
Data from a numerical simulation of a moving barotropic vortex on a sphere with 10-km resolution are used to assess the ability of a state-of-the-art objective analysis scheme to detect certain large-scale tropical cyclone asymmetries, the so-called “beta gyres” to which the cyclone motion appears to be attributed.
A series of analyses is conducted, first using the entire dataset and then taking subsets of it. Four subsets were considered in which data at a regular array of points were extracted, progressively increasing the separation between points. A fifth calculation was considered in which data were selected at points corresponding to the proposed upgraded upper-air network for a tropical cyclone motion experiment in the northwest Pacific region.
It is shown that for a moderate-sized tropical cyclone-scale vortex, a regular grid spacing on the order of 100–150 km is required to adequately define the gyres, at least when the ambient flow is weak. The upgraded upper-air network was found to be inadequate by itself for this purpose, suggesting that aircraft dropwindsonde data are a prerequisite for this task.
Abstract
Data from a numerical simulation of a moving barotropic vortex on a sphere with 10-km resolution are used to assess the ability of a state-of-the-art objective analysis scheme to detect certain large-scale tropical cyclone asymmetries, the so-called “beta gyres” to which the cyclone motion appears to be attributed.
A series of analyses is conducted, first using the entire dataset and then taking subsets of it. Four subsets were considered in which data at a regular array of points were extracted, progressively increasing the separation between points. A fifth calculation was considered in which data were selected at points corresponding to the proposed upgraded upper-air network for a tropical cyclone motion experiment in the northwest Pacific region.
It is shown that for a moderate-sized tropical cyclone-scale vortex, a regular grid spacing on the order of 100–150 km is required to adequately define the gyres, at least when the ambient flow is weak. The upgraded upper-air network was found to be inadequate by itself for this purpose, suggesting that aircraft dropwindsonde data are a prerequisite for this task.
Abstract
An evaluation of the performance of the National Centers for Environmental Prediction Medium-Range Forecast Model was made for the large-scale tropical forecasts and hurricane track forecasts during the 1995 hurricane season. The assessment of the model was based on changes to the deep convection and planetary boundary layer parameterizations to determine their impact on some of the model deficiencies identified during previous hurricane seasons. Some of the deficiencies in the hurricane forecasts included a weakening of the storm circulation with time that seriously degraded the track forecasts. In the larger-scale forecasts, an upper-level easterly wind bias was identified in association with the failure of the model to maintain the midoceanic upper-tropical upper-tropospheric trough.
An overall modest improvement is shown in the large-scale upper-level tropical winds from root-mean-square-error calculations. Within a diagnostic framework, an improved simulation of the midoceanic tropical trough has contributed to a better forecast of the upper-level westerly flow. In the hurricane forecasts, enhanced diabatic heating in the model vortex has significantly improved the vertical structure of the forecast storm. This is shown to contribute to a substantial improvement in the track forecasts.
Abstract
An evaluation of the performance of the National Centers for Environmental Prediction Medium-Range Forecast Model was made for the large-scale tropical forecasts and hurricane track forecasts during the 1995 hurricane season. The assessment of the model was based on changes to the deep convection and planetary boundary layer parameterizations to determine their impact on some of the model deficiencies identified during previous hurricane seasons. Some of the deficiencies in the hurricane forecasts included a weakening of the storm circulation with time that seriously degraded the track forecasts. In the larger-scale forecasts, an upper-level easterly wind bias was identified in association with the failure of the model to maintain the midoceanic upper-tropical upper-tropospheric trough.
An overall modest improvement is shown in the large-scale upper-level tropical winds from root-mean-square-error calculations. Within a diagnostic framework, an improved simulation of the midoceanic tropical trough has contributed to a better forecast of the upper-level westerly flow. In the hurricane forecasts, enhanced diabatic heating in the model vortex has significantly improved the vertical structure of the forecast storm. This is shown to contribute to a substantial improvement in the track forecasts.
Abstract
Two soundings from the eye of Hurricane Gloria (1985) during a period of rapid deepening are described. The soundings were made by Omega dropwindsondes (ODWs) during research flights of the NOAA Hurricane Research Division on 24–25 September 1985. During the 4.7 hours between the two ODW drops. Gloria's minimum sea-level pressure fell from 932 to 922 mb.
The ODWs indicate substantial warming due to dry-adiabatic descent from 580 to 660 mb. Descent rates are estimated to be about 11 cm s−1. Near 500 mb, ascent is indicated. Approximately 60% of the 10 mb pressure fall is associated with thermodynamic changes below 500 mb.
Abstract
Two soundings from the eye of Hurricane Gloria (1985) during a period of rapid deepening are described. The soundings were made by Omega dropwindsondes (ODWs) during research flights of the NOAA Hurricane Research Division on 24–25 September 1985. During the 4.7 hours between the two ODW drops. Gloria's minimum sea-level pressure fell from 932 to 922 mb.
The ODWs indicate substantial warming due to dry-adiabatic descent from 580 to 660 mb. Descent rates are estimated to be about 11 cm s−1. Near 500 mb, ascent is indicated. Approximately 60% of the 10 mb pressure fall is associated with thermodynamic changes below 500 mb.
Abstract
This study evaluates the performance of the National Centers for Environmental Prediction Global Forecast System (GFS) against observations made by the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program at the southern Great Plains site for the years 2001–04. The spatial and temporal scales of the observations are examined to search for an optimum approach for comparing grid-mean model forecasts with single-point observations. A single-column model (SCM) based upon the GFS was also used to aid in understanding certain forecast errors. The investigation is focused on the surface energy fluxes and clouds. Results show that the overall performance of the GFS model has been improving, although certain forecast errors remain. The model overestimated the daily maximum latent heat flux by 76 W m−2 and the daily maximum surface downward solar flux by 44 W m−2, and underestimated the daily maximum sensible heat flux by 44 W m−2. The model’s surface energy balance was reached by a cancellation of errors. For clouds, the GFS was able to capture the observed evolutions of cloud systems during major synoptic events. However, on average, the model largely underestimated cloud fraction in the lower and midtroposphere, especially for daytime nonprecipitating low clouds because shallow convection in the GFS does not produce clouds. Analyses of surface radiative fluxes revealed that the diurnal cycle of the model’s surface downward longwave flux (SDLW) was not in phase with that of the ARM-observed SDLW. SCM experiments showed that this error was caused by an inaccurate scaling factor, which was a function of ground skin temperature and was used to adjust the SDLW at each model time step to that computed by the model’s longwave radiative transfer routine once every 3 h. A method has been proposed to correct this error in the operational forecast model. It was also noticed that the SDLW biases changed from mostly negative in 2003 to slightly positive in 2004. This change was traced back to errors in the near-surface air temperature. In addition, the SDLW simulated with the newly implemented Rapid Radiative Transfer Model longwave routine in the GFS is usually 5–10 W m−2 larger than that simulated with the previous routine. The forecasts of surface downward shortwave flux (SDSW) were relatively accurate under clear-sky conditions. The errors in SDSW were primarily caused by inaccurate forecasts of cloud properties. Results from this study can be used as guidance for the further development of the GFS.
Abstract
This study evaluates the performance of the National Centers for Environmental Prediction Global Forecast System (GFS) against observations made by the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program at the southern Great Plains site for the years 2001–04. The spatial and temporal scales of the observations are examined to search for an optimum approach for comparing grid-mean model forecasts with single-point observations. A single-column model (SCM) based upon the GFS was also used to aid in understanding certain forecast errors. The investigation is focused on the surface energy fluxes and clouds. Results show that the overall performance of the GFS model has been improving, although certain forecast errors remain. The model overestimated the daily maximum latent heat flux by 76 W m−2 and the daily maximum surface downward solar flux by 44 W m−2, and underestimated the daily maximum sensible heat flux by 44 W m−2. The model’s surface energy balance was reached by a cancellation of errors. For clouds, the GFS was able to capture the observed evolutions of cloud systems during major synoptic events. However, on average, the model largely underestimated cloud fraction in the lower and midtroposphere, especially for daytime nonprecipitating low clouds because shallow convection in the GFS does not produce clouds. Analyses of surface radiative fluxes revealed that the diurnal cycle of the model’s surface downward longwave flux (SDLW) was not in phase with that of the ARM-observed SDLW. SCM experiments showed that this error was caused by an inaccurate scaling factor, which was a function of ground skin temperature and was used to adjust the SDLW at each model time step to that computed by the model’s longwave radiative transfer routine once every 3 h. A method has been proposed to correct this error in the operational forecast model. It was also noticed that the SDLW biases changed from mostly negative in 2003 to slightly positive in 2004. This change was traced back to errors in the near-surface air temperature. In addition, the SDLW simulated with the newly implemented Rapid Radiative Transfer Model longwave routine in the GFS is usually 5–10 W m−2 larger than that simulated with the previous routine. The forecasts of surface downward shortwave flux (SDSW) were relatively accurate under clear-sky conditions. The errors in SDSW were primarily caused by inaccurate forecasts of cloud properties. Results from this study can be used as guidance for the further development of the GFS.
Abstract
A set of three-dimensional, filtered, multiply nested objective analyses has been completed for the wind field of Hurricane Gloria for 0000 UTC 25 September 1985. At this time Gloria was one of the most intense hurricanes ever observed in the Atlantic basin, with a minimum sea level pressure of 919 mb. The nested analyses, based on observations from airborne Doppler radar and Omega dropwindsondes, simultaneously describe eyewall and synoptic-scale features, and are the most comprehensive analyses of a single hurricane constructed to date. The analyses have been used to document the multiscale kinematic structure of Gloria and to investigate the relationship between the kinematic fields and the motion of the vortex.
The analyses indicate that the vortex was unusually barotropic. The radius of maximum wind (RMW) was nearly vertical below 500 mb, with a slight inward slope with height between 750 and 550 mb. The strongest azimuthal mean tangential winds were found well above the boundary layer, near 550 mb, where the RMW was smallest. We speculate that this unusual structure was associated with a concentric eye cycle. A persistent asymmetry in the distribution of eyewall convection was associated with the vertical shear of the environmental flow.
The vortex moved approximately 2.5 m s−1 faster than the deep layer mean flow averaged at 667-km radius from the center. Barotropic models have predicted a relationship between the relative motion of the vortex and the gradients of absolute vorticity in the cyclone's environment; however, the predicted relationship was not found for Gloria. The vortex also did not move with the mean flow in the immediate vicinity of the center; the motion of the hurricane was most consistent with the 300–850-mb layer mean flow well outside the eyewall, at a radius of 65 km. The analyses suggest that the environmental flow near the center had been distorted by eyewall convection, with the scale of the distortion determined by the local Rossby radius of deformation.
Abstract
A set of three-dimensional, filtered, multiply nested objective analyses has been completed for the wind field of Hurricane Gloria for 0000 UTC 25 September 1985. At this time Gloria was one of the most intense hurricanes ever observed in the Atlantic basin, with a minimum sea level pressure of 919 mb. The nested analyses, based on observations from airborne Doppler radar and Omega dropwindsondes, simultaneously describe eyewall and synoptic-scale features, and are the most comprehensive analyses of a single hurricane constructed to date. The analyses have been used to document the multiscale kinematic structure of Gloria and to investigate the relationship between the kinematic fields and the motion of the vortex.
The analyses indicate that the vortex was unusually barotropic. The radius of maximum wind (RMW) was nearly vertical below 500 mb, with a slight inward slope with height between 750 and 550 mb. The strongest azimuthal mean tangential winds were found well above the boundary layer, near 550 mb, where the RMW was smallest. We speculate that this unusual structure was associated with a concentric eye cycle. A persistent asymmetry in the distribution of eyewall convection was associated with the vertical shear of the environmental flow.
The vortex moved approximately 2.5 m s−1 faster than the deep layer mean flow averaged at 667-km radius from the center. Barotropic models have predicted a relationship between the relative motion of the vortex and the gradients of absolute vorticity in the cyclone's environment; however, the predicted relationship was not found for Gloria. The vortex also did not move with the mean flow in the immediate vicinity of the center; the motion of the hurricane was most consistent with the 300–850-mb layer mean flow well outside the eyewall, at a radius of 65 km. The analyses suggest that the environmental flow near the center had been distorted by eyewall convection, with the scale of the distortion determined by the local Rossby radius of deformation.
Abstract
A numerical method for analysing and forecasting a wide range of horizontal scales of motion is tested in a barotropic hurricane track forecast model. The numerical method uses cubic B-spline representations of variables on nested domains. The spline representation is used for the objective analysis of observations and the solution of the prediction equations (shallow-water equations on a Mercator projection). This analysis and forecasting system is referred to as VICBAR (Vic Ooyama barotropic model).
The VICBAR model was tested in near real time during the 1989 and 1990 Atlantic hurricane seasons. For the 1989 season, VICBAR had skill comparable to, or greater than, that of the operational track forecast models. For the, 1990 season, VICBAR had skill comparable to that of the operational track-forecast models. During both 1989 and 1990, VICBAR had considerably more skill for forecasts of hurricanes than for forecasts of tropical storms.
For the 1990 season, VICBAR was generalized to include time-dependent boundary conditions from a global forecast model. These boundary conditions improve the longer-range forecasts (60–72 h). The skill of VICBAR is sensitive to the choice of the background field used in the objective analysis and the fields used to apply the boundary conditions. The use of background fields and boundary-condition fields from a 12-h-old global model forecast significantly reduces the VICBAR skill (versus the use of fields from the current global forecast).
Abstract
A numerical method for analysing and forecasting a wide range of horizontal scales of motion is tested in a barotropic hurricane track forecast model. The numerical method uses cubic B-spline representations of variables on nested domains. The spline representation is used for the objective analysis of observations and the solution of the prediction equations (shallow-water equations on a Mercator projection). This analysis and forecasting system is referred to as VICBAR (Vic Ooyama barotropic model).
The VICBAR model was tested in near real time during the 1989 and 1990 Atlantic hurricane seasons. For the 1989 season, VICBAR had skill comparable to, or greater than, that of the operational track forecast models. For the, 1990 season, VICBAR had skill comparable to that of the operational track-forecast models. During both 1989 and 1990, VICBAR had considerably more skill for forecasts of hurricanes than for forecasts of tropical storms.
For the 1990 season, VICBAR was generalized to include time-dependent boundary conditions from a global forecast model. These boundary conditions improve the longer-range forecasts (60–72 h). The skill of VICBAR is sensitive to the choice of the background field used in the objective analysis and the fields used to apply the boundary conditions. The use of background fields and boundary-condition fields from a 12-h-old global model forecast significantly reduces the VICBAR skill (versus the use of fields from the current global forecast).
