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This paper summarizes a number of best practices associated with the use of numerical models of the atmosphere and is motivated by the rapid growth in the number of model users, who have a range of scientific and technical preparations. An underlying important message is that models are complex and imperfect tools, and model users must be aware of their strengths and weaknesses and be thorough in the process of model configuration and verification.
This paper summarizes a number of best practices associated with the use of numerical models of the atmosphere and is motivated by the rapid growth in the number of model users, who have a range of scientific and technical preparations. An underlying important message is that models are complex and imperfect tools, and model users must be aware of their strengths and weaknesses and be thorough in the process of model configuration and verification.
Abstract
A cross-sectional numerical primitive-equation model is used to simulate the summertime airflow pattern in the Los Angeles basin for calm synoptic-scale wind conditions. The contributions of the sea breeze, the urban heat island effect and the mountain-valley wind are quantified. The mountain-valley and sea-breeze circulations are of the same sense (landward at the surface, toward water aloft) and strength (maximum of 5-10 m s−1 at surface), but the urban heat island effect is negligible. Correct specification of the land surface characteristics is found to be important to the quality of the simulation.
Model output is then used to calculate estimates of the space and time variation of boundary-layer ventilation. Ventilation, defined as the product of the height of the planetary boundary layer and the mean wind speed therein, is found to be enhanced in the vicinity of the sea breeze front, and generally increases with distance from the ocean. In the stable marine air layer behind the front, the ventilation is especially low.
Abstract
A cross-sectional numerical primitive-equation model is used to simulate the summertime airflow pattern in the Los Angeles basin for calm synoptic-scale wind conditions. The contributions of the sea breeze, the urban heat island effect and the mountain-valley wind are quantified. The mountain-valley and sea-breeze circulations are of the same sense (landward at the surface, toward water aloft) and strength (maximum of 5-10 m s−1 at surface), but the urban heat island effect is negligible. Correct specification of the land surface characteristics is found to be important to the quality of the simulation.
Model output is then used to calculate estimates of the space and time variation of boundary-layer ventilation. Ventilation, defined as the product of the height of the planetary boundary layer and the mean wind speed therein, is found to be enhanced in the vicinity of the sea breeze front, and generally increases with distance from the ocean. In the stable marine air layer behind the front, the ventilation is especially low.
Abstract
The Penn State/NCAR mesoscale model has been used in a study of special static- and dynamic-initialization techniques that improve a very-short-range forecast of the heavy convective rainfall that occurred in Texas, Oklahoma and Kansas during 9–10 May 1979, the SESAME IV study period. In this study, the model is initialized during the precipitation event. Two types of four-dimensional data assimilation (FDDA) procedures are used in the dynamic-initialization experiments in order to incorporate data during a 12-hour preforecast period. With the first type, FDDA by Newtonian relaxation is used to incorporate sounding data during the preforecast period. With the second FDDA procedure, radar-based precipitation-rate estimates and hourly raingage data are used to define a three-dimensional latent-heating rate field that contributes to the diabatic heating term in the model's thermodynamic equation during the preforecast period. This latent-heating specification procedure is also employed in static-initialization experiments, where the observed rainfall rate and radar echo pattern near the initial time of the forecast are used to infer a latent-heating rate that is specified in the mesoscale model's thermodynamic equation during the early part of the actual forecast. The precipitation forecasts from these static- and dynamic-initialization experiments are compared with the forecast produced when only operational radiosonde data are used in a conventional static initialization.
The conventional (control) initialization procedure that used only operational radiosonde data produced a precipitation prediction for the first three to four hours of the forecast period that would have been inadequate in an operational setting. Whereas at the initial time of the forecast there was substantial convective precipitation observed in a band near the edge of an elevated mixed layer, the model did not initiate the heavy rainfall until about the fourth hour of the forecast.
The use of the experimental static initialization with prescribed latent heating during the first forecast hour produced greatly improved rainfall rates during the first three to four hours. The success of the technique was shown to be not especially sensitive to moderate variations in the duration, intensity and vertical distribution of the imposed heating. Applications of the Newtonian-relaxation procedure during the preforecast period, that relaxed the model solution toward the initial large-scale analysis, produced a better precipitation forecast than did the control, with a maximum in approximately the correct position, but the intensities were too small. Combined use of either the preforecast or in-forecast latent-heat forcing with the Newtonian relaxation produced an improved forecast of rainfall intensity compared to use of the Newtonian relaxation alone. Even though both the experimental static- and dynamic-initialization procedures produced considerably improved very-short-range precipitation forecasts, compared to the control, the experimental static-initialization procedure that used latent-heat forcing during the first forecast hour did slightly better for this case.
Abstract
The Penn State/NCAR mesoscale model has been used in a study of special static- and dynamic-initialization techniques that improve a very-short-range forecast of the heavy convective rainfall that occurred in Texas, Oklahoma and Kansas during 9–10 May 1979, the SESAME IV study period. In this study, the model is initialized during the precipitation event. Two types of four-dimensional data assimilation (FDDA) procedures are used in the dynamic-initialization experiments in order to incorporate data during a 12-hour preforecast period. With the first type, FDDA by Newtonian relaxation is used to incorporate sounding data during the preforecast period. With the second FDDA procedure, radar-based precipitation-rate estimates and hourly raingage data are used to define a three-dimensional latent-heating rate field that contributes to the diabatic heating term in the model's thermodynamic equation during the preforecast period. This latent-heating specification procedure is also employed in static-initialization experiments, where the observed rainfall rate and radar echo pattern near the initial time of the forecast are used to infer a latent-heating rate that is specified in the mesoscale model's thermodynamic equation during the early part of the actual forecast. The precipitation forecasts from these static- and dynamic-initialization experiments are compared with the forecast produced when only operational radiosonde data are used in a conventional static initialization.
The conventional (control) initialization procedure that used only operational radiosonde data produced a precipitation prediction for the first three to four hours of the forecast period that would have been inadequate in an operational setting. Whereas at the initial time of the forecast there was substantial convective precipitation observed in a band near the edge of an elevated mixed layer, the model did not initiate the heavy rainfall until about the fourth hour of the forecast.
The use of the experimental static initialization with prescribed latent heating during the first forecast hour produced greatly improved rainfall rates during the first three to four hours. The success of the technique was shown to be not especially sensitive to moderate variations in the duration, intensity and vertical distribution of the imposed heating. Applications of the Newtonian-relaxation procedure during the preforecast period, that relaxed the model solution toward the initial large-scale analysis, produced a better precipitation forecast than did the control, with a maximum in approximately the correct position, but the intensities were too small. Combined use of either the preforecast or in-forecast latent-heat forcing with the Newtonian relaxation produced an improved forecast of rainfall intensity compared to use of the Newtonian relaxation alone. Even though both the experimental static- and dynamic-initialization procedures produced considerably improved very-short-range precipitation forecasts, compared to the control, the experimental static-initialization procedure that used latent-heat forcing during the first forecast hour did slightly better for this case.
Abstract
The initialization of a three-dimensional model with operational data for Hurricane Eloise (1975) was studied to assess the impact of using bogus storm data, surface winds, rainfall rates, and a high-resolution surface pressure analysis in the initialization of forecasts of hurricane track and intensity.
Because the track and intensity forecasts based on the unaugmented NMC analyses were unsatisfactory, various data improvement procedures were used. Boundary-layer flow was diagnosed from the surface pressure with a primitive equation PBL model, a climatological hurricane circulation was inserted into the NMC wind analysis above the boundary layer, and the three-dimensional moisture field was defined with the aid of visible-image satellite photographs. Model simulations with this improved data set were designed to test the effectiveness of dynamic initialization (DI) and the data enhancement procedures in improving the numerical hurricane forecasts. A 24 h time period, starting at 0000 GMT 21 September 1975, was considered. In procedure A, all data improvements were made and surface pressure was taken directly from a detailed analysis. Procedure B represented what might be done operationally—the only modification to the original NMC data was the insertion of a bogus storm based on composite data and the diagnosis of surface pressure from the 1000 mb heights and temperatures.
For each procedure, three model integrations were made to test the effect of DI by nudging on the forecast. Model results were evaluated in terms of track, the boundary-layer flow, surface pressure and rainfall rates. All forecasts with the improved data were much better than in the preliminary model experiments with the unmodified NMC analysis. Procedure B track predictions, which were based on initial conditions that contained the least amount of mesoscale information, were somewhat better than the others, with vector position errors of <80 km. Dynamic initialization had little effect on the path of the model storm. Intensity forecasts were best using procedure A, in which the greatest amount of hurricane scale information went into the initial conditions, and when DI was employed. However, large-scale mass-momentum adjustment and the proximity of the model storm to the lateral boundaries distorted the predictions of boundary-layer flow and rainfall rates.
A time composite of surface wind reports from land-based stations, buoys, and ships represented the type of data that might be available from future remote sensing satellites like Seasat-A. Because the data were valid at only one synoptic time, a DI could not be performed. The impact of the surface winds on the initialization could only be examined in terms of a 12 h forecast. Several methods of incorporating the surface wind observations into the initial conditions included direct insertion of the data into the NMC wind analysis and a diagnosis of surface pressure from the surface winds through a divergence equation. Although satellite winds improved the mesoscale realism of the initial boundary layer winds and the surface pressure, model forecasts were virtually unimproved. Forecast errors associated with the large-scale mass momentum adjustments, the limitations of the model physics, the data enhancement procedures, and the accuracy of the surface wind analysis, prevented our reaching any definite conclusion about the benefits of supplementary near-surface wind data.
A 12 h DI was performed in which the latent heat release due to convection was externally specified based upon satellite estimates of rainfall rate. A comparison of 12 h forecasts based on this DI and a static initialization showed that this type of DI produced forecasts of surface pressure and precipitation that were greatly improved and which were reflective of observed storm intensity. Track forecasts were not significantly changed.
Abstract
The initialization of a three-dimensional model with operational data for Hurricane Eloise (1975) was studied to assess the impact of using bogus storm data, surface winds, rainfall rates, and a high-resolution surface pressure analysis in the initialization of forecasts of hurricane track and intensity.
Because the track and intensity forecasts based on the unaugmented NMC analyses were unsatisfactory, various data improvement procedures were used. Boundary-layer flow was diagnosed from the surface pressure with a primitive equation PBL model, a climatological hurricane circulation was inserted into the NMC wind analysis above the boundary layer, and the three-dimensional moisture field was defined with the aid of visible-image satellite photographs. Model simulations with this improved data set were designed to test the effectiveness of dynamic initialization (DI) and the data enhancement procedures in improving the numerical hurricane forecasts. A 24 h time period, starting at 0000 GMT 21 September 1975, was considered. In procedure A, all data improvements were made and surface pressure was taken directly from a detailed analysis. Procedure B represented what might be done operationally—the only modification to the original NMC data was the insertion of a bogus storm based on composite data and the diagnosis of surface pressure from the 1000 mb heights and temperatures.
For each procedure, three model integrations were made to test the effect of DI by nudging on the forecast. Model results were evaluated in terms of track, the boundary-layer flow, surface pressure and rainfall rates. All forecasts with the improved data were much better than in the preliminary model experiments with the unmodified NMC analysis. Procedure B track predictions, which were based on initial conditions that contained the least amount of mesoscale information, were somewhat better than the others, with vector position errors of <80 km. Dynamic initialization had little effect on the path of the model storm. Intensity forecasts were best using procedure A, in which the greatest amount of hurricane scale information went into the initial conditions, and when DI was employed. However, large-scale mass-momentum adjustment and the proximity of the model storm to the lateral boundaries distorted the predictions of boundary-layer flow and rainfall rates.
A time composite of surface wind reports from land-based stations, buoys, and ships represented the type of data that might be available from future remote sensing satellites like Seasat-A. Because the data were valid at only one synoptic time, a DI could not be performed. The impact of the surface winds on the initialization could only be examined in terms of a 12 h forecast. Several methods of incorporating the surface wind observations into the initial conditions included direct insertion of the data into the NMC wind analysis and a diagnosis of surface pressure from the surface winds through a divergence equation. Although satellite winds improved the mesoscale realism of the initial boundary layer winds and the surface pressure, model forecasts were virtually unimproved. Forecast errors associated with the large-scale mass momentum adjustments, the limitations of the model physics, the data enhancement procedures, and the accuracy of the surface wind analysis, prevented our reaching any definite conclusion about the benefits of supplementary near-surface wind data.
A 12 h DI was performed in which the latent heat release due to convection was externally specified based upon satellite estimates of rainfall rate. A comparison of 12 h forecasts based on this DI and a static initialization showed that this type of DI produced forecasts of surface pressure and precipitation that were greatly improved and which were reflective of observed storm intensity. Track forecasts were not significantly changed.
Abstract
Experiments are described that provide an example of the baseline skill level for the numerical prediction of cloud ceiling and visibility, where application to aviation-system safety and efficiency is emphasized. Model simulations of a light, mixed-phase, East Coast precipitation event are employed to assess ceiling and visibility predictive skill, and its sensitivity to the use of data assimilation and the use of simple versus complex microphysics schemes. To obtain ceiling and visibility from the model-simulated, state-of-the-atmosphere variables, a translation algorithm was developed based on empirical and theoretical relationships between hydrometeor characteristics and light extinction. The model-simulated ceilings were generally excessively high; however, the visibility simulations were reasonably accurate and comparable to the existing operational terminal forecasts. The benefit of data assimilation for such very short-range forecasts was demonstrated, as was the desirability of employing a reasonably sophisticated microphysics scheme.
Abstract
Experiments are described that provide an example of the baseline skill level for the numerical prediction of cloud ceiling and visibility, where application to aviation-system safety and efficiency is emphasized. Model simulations of a light, mixed-phase, East Coast precipitation event are employed to assess ceiling and visibility predictive skill, and its sensitivity to the use of data assimilation and the use of simple versus complex microphysics schemes. To obtain ceiling and visibility from the model-simulated, state-of-the-atmosphere variables, a translation algorithm was developed based on empirical and theoretical relationships between hydrometeor characteristics and light extinction. The model-simulated ceilings were generally excessively high; however, the visibility simulations were reasonably accurate and comparable to the existing operational terminal forecasts. The benefit of data assimilation for such very short-range forecasts was demonstrated, as was the desirability of employing a reasonably sophisticated microphysics scheme.
Abstract
During Intensive Observation Period 2 of the Genesis of Atlantic Lows Experiment, a number of mesoscale phenomena were observed with special and conventional observing systems over the land and coastal waters. This study involved analysis of these data for the period 24–26 January 1986 in order to define the structure and dynamics of three features: the coastal front; a shallow cyclone that propagated along the coastal front, modifying it as it moved northward; and a low-level jet that formed in the strong coastal pressure-gradient field.
The coastal front formed in an existing pressure trough over the Gulf Stream as a result of both ageostrophic deformation and differential diabatic heating. There existed considerable variability in the frontal strength and position on both the mesoalpha and mesobeta scales. The level of strongest frontogenesis was near the surface, with frontolysis calculated above 950 mb.
The marine atmospheric boundary layer (MABL) over the Gulf Stream was conducive to cyclone formation. Latent and sensible heat fluxes of up to 800 W m−2 and 400 W m−2 respectively, were calculated early in the study period, and a deep, moist conditionally unstable boundary layer was present. Calculation of the vorticity tendency associated with the sensible heating yielded a narrow band of positive values to the east of the coastline. As a weak midtropospheric wave reached this favorable region to the cut of Florida, a shallow cyclone formed along the coastal front. As the cyclone tracked northeastward along the front, geostrophic deformation ahead of it strengthened the front while strong cold-air advection to its rear displaced the coastal front to the east, leaving behind a dry, stable MABL with low-level, cold-air advection and weak descent. As the cyclone moved northward along the front, conditionally unstable, moist, low-level air ahead was forced by the southeasterly flow to rise along the coastal front and its extension over the cold air near the coastline, causing enhanced precipitation.
A low-level northeasterly jet was also observed over the Carolinas, and formed as a result of the strong low- level pressure gradient created by the proximity of the cold continental air over land and the warm air of the Gulf Stream MABL near the coast. This jet, with a maximum near 960 mb, showed a diurnal variation of up to 20 m s−1 which likely resulted from day/night variations in mixing at jet level, an inertial oscillation with the frictional decoupling of the low-level flow at sunset, and isallobaric accelerations.
Abstract
During Intensive Observation Period 2 of the Genesis of Atlantic Lows Experiment, a number of mesoscale phenomena were observed with special and conventional observing systems over the land and coastal waters. This study involved analysis of these data for the period 24–26 January 1986 in order to define the structure and dynamics of three features: the coastal front; a shallow cyclone that propagated along the coastal front, modifying it as it moved northward; and a low-level jet that formed in the strong coastal pressure-gradient field.
The coastal front formed in an existing pressure trough over the Gulf Stream as a result of both ageostrophic deformation and differential diabatic heating. There existed considerable variability in the frontal strength and position on both the mesoalpha and mesobeta scales. The level of strongest frontogenesis was near the surface, with frontolysis calculated above 950 mb.
The marine atmospheric boundary layer (MABL) over the Gulf Stream was conducive to cyclone formation. Latent and sensible heat fluxes of up to 800 W m−2 and 400 W m−2 respectively, were calculated early in the study period, and a deep, moist conditionally unstable boundary layer was present. Calculation of the vorticity tendency associated with the sensible heating yielded a narrow band of positive values to the east of the coastline. As a weak midtropospheric wave reached this favorable region to the cut of Florida, a shallow cyclone formed along the coastal front. As the cyclone tracked northeastward along the front, geostrophic deformation ahead of it strengthened the front while strong cold-air advection to its rear displaced the coastal front to the east, leaving behind a dry, stable MABL with low-level, cold-air advection and weak descent. As the cyclone moved northward along the front, conditionally unstable, moist, low-level air ahead was forced by the southeasterly flow to rise along the coastal front and its extension over the cold air near the coastline, causing enhanced precipitation.
A low-level northeasterly jet was also observed over the Carolinas, and formed as a result of the strong low- level pressure gradient created by the proximity of the cold continental air over land and the warm air of the Gulf Stream MABL near the coast. This jet, with a maximum near 960 mb, showed a diurnal variation of up to 20 m s−1 which likely resulted from day/night variations in mixing at jet level, an inertial oscillation with the frictional decoupling of the low-level flow at sunset, and isallobaric accelerations.
Abstract
An 18-h numerical simulation of the weather associated with the severe-storm outbreak in the region of the Texas-Oklahoma panhandles, during the AVE-SESAME IV study period (9–10 May 1979), was performed using the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model. This simulation and the related sensitivity tests provided the four-dimensional data sets that were used to reach a better understanding of the processes that were involved in this case in the development of severe convection along the edge of the elevated mixed layer (EML).
The sensitivity studies were performed to isolate the contributions of differential surface forcing, latent heating and the low-level moisture gradient to the development of the underrunning, its intensification, and the heavy rainfall. These studies showed that the differential surface heating at the edge of the EML is the most important single factor responsible for initiating the underrunning, and therefore the precipitation, during this case. Compared to the precipitation amounts produced by the complete model during the 9-h period of heavy precipitation (2100–0600 GMT), only 3% was produced after the elimination of the surface differential heating associated with the cloud-cover and soil moisture-availability gradients. The elimination of the latent-heating feedback in the model atmosphere caused a decrease in the 18-h precipitation amounts of ∼50%. Finally, the strong gradient in the low-level mixing ratio along the edge of the EML had a surprisingly important direct dynamic influence on the underrunning, and consequently on the precipitation.
Abstract
An 18-h numerical simulation of the weather associated with the severe-storm outbreak in the region of the Texas-Oklahoma panhandles, during the AVE-SESAME IV study period (9–10 May 1979), was performed using the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model. This simulation and the related sensitivity tests provided the four-dimensional data sets that were used to reach a better understanding of the processes that were involved in this case in the development of severe convection along the edge of the elevated mixed layer (EML).
The sensitivity studies were performed to isolate the contributions of differential surface forcing, latent heating and the low-level moisture gradient to the development of the underrunning, its intensification, and the heavy rainfall. These studies showed that the differential surface heating at the edge of the EML is the most important single factor responsible for initiating the underrunning, and therefore the precipitation, during this case. Compared to the precipitation amounts produced by the complete model during the 9-h period of heavy precipitation (2100–0600 GMT), only 3% was produced after the elimination of the surface differential heating associated with the cloud-cover and soil moisture-availability gradients. The elimination of the latent-heating feedback in the model atmosphere caused a decrease in the 18-h precipitation amounts of ∼50%. Finally, the strong gradient in the low-level mixing ratio along the edge of the EML had a surprisingly important direct dynamic influence on the underrunning, and consequently on the precipitation.
Abstract
A synoptic climatology of the atmospheric conditions associated with the creation of the elevated mixed-layer inversion, or lid, over the southern Great Plains of the U.S. (defined as Kansas, Oklahoma, Texas, and portions of the surrounding states) during four spring (April, May, June) seasons from 1983 through 1986 is presented. The lid sounding, also known as a type 1 tornado sounding, is created through the superposition of a potentially warm, nearly dry-adiabatic elevated mixed layer (EML) over a moist, potentially unstable layer. This study examines the situations which are favorable and unfavorable for creation of the lid stratification, using EML and lid occurrence statistics and analyses of various parameters associated with the EML and lid. In addition, we define a set of synoptic types that prevail in this region during the three-month period. The synoptic types are categorized by simultaneously examining the surface isobaric patterns and the predominant 500-mb flow direction over the study region, and designating the flow as either “favorable” or “unfavorable” for lid formation based on the implied thermal advection in the layer and the number of lid soundings observed over the region.
Our analysis reveals that a typical lid covers only about 20% to 25% of the southern Great Plains, and that a lid coverage greater than 50% occurred on fewer than 2% of the study days. High lid-frequency values expand northward during the season, and the maximum-frequency axis shifts westward. We show that this seasonal change is primarily caused by the northward expansion of the EML-source region from Mexico into the central Rockies and Great Basin, and a westward shift in the mean low-level moist axis. The westward shift in the low-level moist axis is related to the westward expansion of the Bermuda anticyclone. We find that the relative airstream configuration associated with the classic models of lid formation and severe weather over this region occurs most frequently in April and May, and corresponds to a flow type associated with southerly low-level flow and southwest flow aloft. As the season progresses, the expansion of both the EML-source region and low-level moist areas allows the lid to be created with a variety of additional flow configurations. In addition to the classic southwesterly midtropospheric flow type, these configurations include northwest and anticyclonic 500-mb flows by May and June. The dominance in late spring of flow types associated with large scale subsidence leads to an airstream configuration in which the inversion base sinks and the lid strengthens downstream from the source region. This is in stark contrast to the classic lid model where the inversion base rises and the lid weakens in an environment of large-scale vertical ascent.
Abstract
A synoptic climatology of the atmospheric conditions associated with the creation of the elevated mixed-layer inversion, or lid, over the southern Great Plains of the U.S. (defined as Kansas, Oklahoma, Texas, and portions of the surrounding states) during four spring (April, May, June) seasons from 1983 through 1986 is presented. The lid sounding, also known as a type 1 tornado sounding, is created through the superposition of a potentially warm, nearly dry-adiabatic elevated mixed layer (EML) over a moist, potentially unstable layer. This study examines the situations which are favorable and unfavorable for creation of the lid stratification, using EML and lid occurrence statistics and analyses of various parameters associated with the EML and lid. In addition, we define a set of synoptic types that prevail in this region during the three-month period. The synoptic types are categorized by simultaneously examining the surface isobaric patterns and the predominant 500-mb flow direction over the study region, and designating the flow as either “favorable” or “unfavorable” for lid formation based on the implied thermal advection in the layer and the number of lid soundings observed over the region.
Our analysis reveals that a typical lid covers only about 20% to 25% of the southern Great Plains, and that a lid coverage greater than 50% occurred on fewer than 2% of the study days. High lid-frequency values expand northward during the season, and the maximum-frequency axis shifts westward. We show that this seasonal change is primarily caused by the northward expansion of the EML-source region from Mexico into the central Rockies and Great Basin, and a westward shift in the mean low-level moist axis. The westward shift in the low-level moist axis is related to the westward expansion of the Bermuda anticyclone. We find that the relative airstream configuration associated with the classic models of lid formation and severe weather over this region occurs most frequently in April and May, and corresponds to a flow type associated with southerly low-level flow and southwest flow aloft. As the season progresses, the expansion of both the EML-source region and low-level moist areas allows the lid to be created with a variety of additional flow configurations. In addition to the classic southwesterly midtropospheric flow type, these configurations include northwest and anticyclonic 500-mb flows by May and June. The dominance in late spring of flow types associated with large scale subsidence leads to an airstream configuration in which the inversion base sinks and the lid strengthens downstream from the source region. This is in stark contrast to the classic lid model where the inversion base rises and the lid weakens in an environment of large-scale vertical ascent.
Abstract
This study investigates the relationships between the occurrence of the lid (also known as a type 1 tornado sounding) and the occurrence of severe storms over Kansas, Oklahoma, and Texas during the spring season (defined as the months of April, May, and June). The period of this study covers four seasons from 1983 through 1986. The size distribution of severe storm events is examined in relation to the occurrence/size of the antecedent lid over the study region. Days in which no severe weather was observed over the region (defined as “non-event” days) are included in order to examine the occurrence/size of the lid on these days as well. The relationships between the occurrence/size of severe-storm outbreaks and the antecedent lid are also examined using conceptual models of the life cycle of lid development and dissipation, for both early and late spring. Composite mean analyses of key meteorological parameters and geographic frequency composites of the elevated mixed layer, buoyant instability (as defined by an unstable value of the Lifted Index or buoyancy term in the Lid Strength Index), and severe-weather events are constructed for different stages of lid development. These composites are then utilized to determine geographic relationships among these parameters.
The results show that the size distribution of severe-weather events has a peak at the 1600 km2 (a 40- × 40- km grid square) category; this peak strengthens during the season (especially from May to June). Also found is a relationship between the occurrence and size of the lid at 1200 UTC and the occurrence and size of subsequent severe-storm events, where this relationship is most well-defined in April and deteriorates rapidly from May to June. We hypothesize that the deterioration of this relationship is due to factors such as the increasing horizontal extent of the low-level moist layer (and buoyant instability) during the spring, and changes in the synoptic-scale circulation from baroclinic waves in the westerlies to subtropical anticyclones and weak cyclonic disturbances. In some late spring synoptic patterns, severe weather is not only associated with the presence of a widespread lid, but also is found to exist in an environment containing a variety of sounding types; these include lid soundings, uncapped soundings (with a zero or negative lid strength term), and even a subsidence-type sounding that is buoyantly unstable. In such environments, features such as the low-level jet and surface troughs may provide a sufficient lifting mechanism to allow the development of deep convection to occur.
Abstract
This study investigates the relationships between the occurrence of the lid (also known as a type 1 tornado sounding) and the occurrence of severe storms over Kansas, Oklahoma, and Texas during the spring season (defined as the months of April, May, and June). The period of this study covers four seasons from 1983 through 1986. The size distribution of severe storm events is examined in relation to the occurrence/size of the antecedent lid over the study region. Days in which no severe weather was observed over the region (defined as “non-event” days) are included in order to examine the occurrence/size of the lid on these days as well. The relationships between the occurrence/size of severe-storm outbreaks and the antecedent lid are also examined using conceptual models of the life cycle of lid development and dissipation, for both early and late spring. Composite mean analyses of key meteorological parameters and geographic frequency composites of the elevated mixed layer, buoyant instability (as defined by an unstable value of the Lifted Index or buoyancy term in the Lid Strength Index), and severe-weather events are constructed for different stages of lid development. These composites are then utilized to determine geographic relationships among these parameters.
The results show that the size distribution of severe-weather events has a peak at the 1600 km2 (a 40- × 40- km grid square) category; this peak strengthens during the season (especially from May to June). Also found is a relationship between the occurrence and size of the lid at 1200 UTC and the occurrence and size of subsequent severe-storm events, where this relationship is most well-defined in April and deteriorates rapidly from May to June. We hypothesize that the deterioration of this relationship is due to factors such as the increasing horizontal extent of the low-level moist layer (and buoyant instability) during the spring, and changes in the synoptic-scale circulation from baroclinic waves in the westerlies to subtropical anticyclones and weak cyclonic disturbances. In some late spring synoptic patterns, severe weather is not only associated with the presence of a widespread lid, but also is found to exist in an environment containing a variety of sounding types; these include lid soundings, uncapped soundings (with a zero or negative lid strength term), and even a subsidence-type sounding that is buoyantly unstable. In such environments, features such as the low-level jet and surface troughs may provide a sufficient lifting mechanism to allow the development of deep convection to occur.
Abstract
To evaluate the relative importance of different mechanisms responsible for the formation of polar lows, we have developed a three-layer, two-dimensional, quasi-geostrophic model, which includes both the effects of latent heating and baroclinity. Latent heating was parameterized for both stable precipitation associated with moist baroclinic processes as well as for convective precipitation associated with Conditional Instability of the Second Kind (CISK). Seven case studies and other observational studies are used to demonstrate that CISK or dry baroclinity does not individually provide the necessary forcing to allow the instability to grow to the observed wavelengths at the observed rates. It is found that moist baroclinic processes alone may explain the origin of Pacific polar lows, while moist baroclinity and CISK are essential in the genesis of Atlantic polar lows. Thus, there appears to be two types of polar lows. Sensitivity studies reveal the importance of low-level shear for the Atlantic disturbances. A further investigation with the analytic model used Lau's wintertime climatological data to find correlations between growth rates and preferred scales of disturbances and local effects such as latitude and climatology of the base state. The model predicts the preferred geographic regions where polar lows develop—in the Atlantic in the vicinity of Greenland/Iceland, and in the northern Pacific. We suggest that, because the former region is one of the resting places of Sander's “bombs,” polar lows in the Atlantic may result from the residual circulations or baroclinity of occluded systems. As these incipient disturbances move over warmer waters, conditionally unstable lapse rates develop in the lower troposphere and the CISK mechanism then becomes potentially important.
Abstract
To evaluate the relative importance of different mechanisms responsible for the formation of polar lows, we have developed a three-layer, two-dimensional, quasi-geostrophic model, which includes both the effects of latent heating and baroclinity. Latent heating was parameterized for both stable precipitation associated with moist baroclinic processes as well as for convective precipitation associated with Conditional Instability of the Second Kind (CISK). Seven case studies and other observational studies are used to demonstrate that CISK or dry baroclinity does not individually provide the necessary forcing to allow the instability to grow to the observed wavelengths at the observed rates. It is found that moist baroclinic processes alone may explain the origin of Pacific polar lows, while moist baroclinity and CISK are essential in the genesis of Atlantic polar lows. Thus, there appears to be two types of polar lows. Sensitivity studies reveal the importance of low-level shear for the Atlantic disturbances. A further investigation with the analytic model used Lau's wintertime climatological data to find correlations between growth rates and preferred scales of disturbances and local effects such as latitude and climatology of the base state. The model predicts the preferred geographic regions where polar lows develop—in the Atlantic in the vicinity of Greenland/Iceland, and in the northern Pacific. We suggest that, because the former region is one of the resting places of Sander's “bombs,” polar lows in the Atlantic may result from the residual circulations or baroclinity of occluded systems. As these incipient disturbances move over warmer waters, conditionally unstable lapse rates develop in the lower troposphere and the CISK mechanism then becomes potentially important.
