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Abstract
A particularly intense case of western Atlantic baroclinic cyclogenesis was investigated in this study. Specifically, the roles of latent heat of condensation and surface friction were examined from the potential vorticity or “PV thinking” perspective. The methodology used for this study involves three key components: 1) a full-physics mesoscale model, which provides a continuous and dynamically consistent dataset and provides full user control over physical processes; 2) a partitioned PV integration, which temporally integrates the accumulation of PV due to various physical processes in the model's Eulerian framework: and 3) the piecewise inversion method of Davis and Emanuel, which calculates the balanced wind and mass field associated with particular PV anomalies. Potential vorticity features obtained through the partitioned integration technique were inverted to yield their direct contributions to the total circulation. In addition, sensitivity studies were carried out to determine the overall impact of various nonconservative processes on the cyclone development.
Results of the PV integration showed that latent heating created a significant positive anomaly above the surface warm and bent-back fronts at the level of maximum heating. Inversion of this feature showed that it explained approximately 70% of the total balanced nondivergent circulation at low levels during the mature stage of the storm. The circulation associated with latent-heating-generated PV also enhanced the coupling between the surface and upper-level waves, both by hastening the eastward propagation of the surface wave and by slowing the eastward propagation of the upper-level wave. Comparison of the control experiment with a sensitivity test, in which latent heating was withheld, showed that latent heating also enhanced upper-level divergence, which expanded the downstream ridge and kept an upper-level small-scale PV anomaly coupled to the low-level disturbance. However, cyclogenesis still occurred in the absence of latent heating, due to a second, larger-scale upper PV anomaly that approached from the northwest. Surface friction caused the formation of mainly positive PV at low levels, primarily in the easterly flow of the warm frontal zone, where the dominant mechanism was frictional formation of southward-oriented horizontal vorticity in the presence of a strong southward temperature gradient. Inversion of this PV yielded a small cyclonic circulation centered on the surface low. However, a frictionless simulation produced a slightly stronger cyclone, due to indirect enhancement of the upper-level PV anomaly and the generation of low-level PV by thermal diffusion in the narrow warm sector of the storm.
Abstract
A particularly intense case of western Atlantic baroclinic cyclogenesis was investigated in this study. Specifically, the roles of latent heat of condensation and surface friction were examined from the potential vorticity or “PV thinking” perspective. The methodology used for this study involves three key components: 1) a full-physics mesoscale model, which provides a continuous and dynamically consistent dataset and provides full user control over physical processes; 2) a partitioned PV integration, which temporally integrates the accumulation of PV due to various physical processes in the model's Eulerian framework: and 3) the piecewise inversion method of Davis and Emanuel, which calculates the balanced wind and mass field associated with particular PV anomalies. Potential vorticity features obtained through the partitioned integration technique were inverted to yield their direct contributions to the total circulation. In addition, sensitivity studies were carried out to determine the overall impact of various nonconservative processes on the cyclone development.
Results of the PV integration showed that latent heating created a significant positive anomaly above the surface warm and bent-back fronts at the level of maximum heating. Inversion of this feature showed that it explained approximately 70% of the total balanced nondivergent circulation at low levels during the mature stage of the storm. The circulation associated with latent-heating-generated PV also enhanced the coupling between the surface and upper-level waves, both by hastening the eastward propagation of the surface wave and by slowing the eastward propagation of the upper-level wave. Comparison of the control experiment with a sensitivity test, in which latent heating was withheld, showed that latent heating also enhanced upper-level divergence, which expanded the downstream ridge and kept an upper-level small-scale PV anomaly coupled to the low-level disturbance. However, cyclogenesis still occurred in the absence of latent heating, due to a second, larger-scale upper PV anomaly that approached from the northwest. Surface friction caused the formation of mainly positive PV at low levels, primarily in the easterly flow of the warm frontal zone, where the dominant mechanism was frictional formation of southward-oriented horizontal vorticity in the presence of a strong southward temperature gradient. Inversion of this PV yielded a small cyclonic circulation centered on the surface low. However, a frictionless simulation produced a slightly stronger cyclone, due to indirect enhancement of the upper-level PV anomaly and the generation of low-level PV by thermal diffusion in the narrow warm sector of the storm.
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
Linear and nonlinear simulations of idealized baroclinic waves interacting with topography are examined in the context of quasigeostrophy. The purpose is to provide a simple conceptual interpretation of the transients resulting from this interaction. A perturbation expansion is employed, with the small parameter being proportional to topographic slope, to isolate fundamentally different topographic effects, and show how they enter systematically at each order. First- and second-order corrections appear to capture the essence of the topographic effect for all cases considered, even for values of the “small” parameter as large as 0.5, and are qualitatively useful for a parameter value of unity.
Results indicate the importance of surface Rossby wave dynamics at first order near the mountain and downshear from the mountain a distance inversely proportional to the growth rate of the most unstable mode of the system. The second-order correction projects onto the initial baroclinic wave. Being primarily out of phase with the initial wave, it contributes systematically to weakening the initial disturbance. This behavior changes notably for meridionally localized topography offset from the symmetry axis of the initial zonally invariant jet flow. The first-order correction affects the translational speed of the initial wave and, downshear from the mountain, grows as an unstable mode projecting strongly onto the scale of the initial wave. For a mountain to the south of the jet, the incident baroclinic wave is accelerated; for a mountain to the north, it is slowed. The dominant effect at second order is still a weakening of the initial wave. In the nonlinear regime, with a meridionally invariant mountain, the total topographic perturbation can be decomposed into a part excited by the wave-induced zonal-mean flow, and a part excited by the remaining transients whose interaction with topography qualitatively resembles that of the linear solution.
Abstract
Linear and nonlinear simulations of idealized baroclinic waves interacting with topography are examined in the context of quasigeostrophy. The purpose is to provide a simple conceptual interpretation of the transients resulting from this interaction. A perturbation expansion is employed, with the small parameter being proportional to topographic slope, to isolate fundamentally different topographic effects, and show how they enter systematically at each order. First- and second-order corrections appear to capture the essence of the topographic effect for all cases considered, even for values of the “small” parameter as large as 0.5, and are qualitatively useful for a parameter value of unity.
Results indicate the importance of surface Rossby wave dynamics at first order near the mountain and downshear from the mountain a distance inversely proportional to the growth rate of the most unstable mode of the system. The second-order correction projects onto the initial baroclinic wave. Being primarily out of phase with the initial wave, it contributes systematically to weakening the initial disturbance. This behavior changes notably for meridionally localized topography offset from the symmetry axis of the initial zonally invariant jet flow. The first-order correction affects the translational speed of the initial wave and, downshear from the mountain, grows as an unstable mode projecting strongly onto the scale of the initial wave. For a mountain to the south of the jet, the incident baroclinic wave is accelerated; for a mountain to the north, it is slowed. The dominant effect at second order is still a weakening of the initial wave. In the nonlinear regime, with a meridionally invariant mountain, the total topographic perturbation can be decomposed into a part excited by the wave-induced zonal-mean flow, and a part excited by the remaining transients whose interaction with topography qualitatively resembles that of the linear solution.
Abstract
The present note proposes an explanation of topographic normal modes that grow in a flow with a zonally symmetric mountain. An issue of practical importance is whether one expects the maximum amplitude to occur poleward or equatorward of a mountain localized in the cross-stream direction. Using a quasigeostrophic model, it is shown that the occurrence of maximum amplitude to the south of the mountain occurs only for longer waves. For a given wave of medium scale, the amplitude is more likely to maximize to the south of the mountain for steeper slopes.
The transient behavior resulting from the sudden appearance of a mountain in the presence of an Eady normal mode is discussed first. The topographic correction to the Eady mode quickly acquires a negative meridional tilt at the surface. This tilted structure penetrates upward to the lid and advects basic-state temperature. Because the phase tilt of the Eady mode varies with zonal wavelength, the phase of the topographic correction at the lid relative to the initial wave will vary similarly. For shorter zonal scales the primary cancellation of the initial wave is equatorward of the channel center, thus biasing the total disturbance amplitude poleward. The opposite occurs for longer waves.
At long times, topographic modes emerge. These modes involve the interaction of gravest (symmetric) and first asymmetric modes meridionally through the topographic term in the lower boundary condition. The phase displacement of the symmetric and asymmetric modes is such as to damp the symmetric mode and amplify the asymmetric mode so that the two maintain the same growth rate. The phase relation of the symmetric and asymmetric modes varies with zonal wavelength and determines the meridional bias in amplitude of the total solution.
Abstract
The present note proposes an explanation of topographic normal modes that grow in a flow with a zonally symmetric mountain. An issue of practical importance is whether one expects the maximum amplitude to occur poleward or equatorward of a mountain localized in the cross-stream direction. Using a quasigeostrophic model, it is shown that the occurrence of maximum amplitude to the south of the mountain occurs only for longer waves. For a given wave of medium scale, the amplitude is more likely to maximize to the south of the mountain for steeper slopes.
The transient behavior resulting from the sudden appearance of a mountain in the presence of an Eady normal mode is discussed first. The topographic correction to the Eady mode quickly acquires a negative meridional tilt at the surface. This tilted structure penetrates upward to the lid and advects basic-state temperature. Because the phase tilt of the Eady mode varies with zonal wavelength, the phase of the topographic correction at the lid relative to the initial wave will vary similarly. For shorter zonal scales the primary cancellation of the initial wave is equatorward of the channel center, thus biasing the total disturbance amplitude poleward. The opposite occurs for longer waves.
At long times, topographic modes emerge. These modes involve the interaction of gravest (symmetric) and first asymmetric modes meridionally through the topographic term in the lower boundary condition. The phase displacement of the symmetric and asymmetric modes is such as to damp the symmetric mode and amplify the asymmetric mode so that the two maintain the same growth rate. The phase relation of the symmetric and asymmetric modes varies with zonal wavelength and determines the meridional bias in amplitude of the total solution.
Abstract
A coupled air–sea numerical model comprising a mesoscale atmospheric model, a marine circulation model, and a surface wave model is presented. The coupled model is tested through simulations of an event of frontal passage through the Lake Erie region. Experiments investigate the effects of different sea surface roughness parameterizations on the atmospheric simulations.
The coupled system’s components are the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5), the Princeton Numerical Ocean Model (POM), and the GLERL–Donelan Wave Model (GDM). The finest of the MM5’s three nested grids covers Lake Erie, on which the POM and GDM operate. The MM5 provides surface heat and momentum fluxes to the POM, and the POM returns lake surface temperatures to the MM5. The MM5 provides 10-m winds to the GDM, and the GDM returns sea state information to the MM5. The MM5 uses this sea state information in calculating overwater roughness lengths (z 0’s).
Experiments varying the MM5’s roughness parameterization over Lake Erie are performed, resulting in a broad range of z 0’s. It is found that wave model coupling can significantly increase overwater roughnesses in the MM5, leading to increased surface heat and moisture fluxes and to changes in PBL characteristics. The impacts on the atmosphere from marine model coupling can appear far downstream of the coupled zones.
The accuracy of the mesoscale atmospheric simulation appears sensitive to the assumptions behind the marine roughness parameterizations used. The results suggest that, for consistent forecast improvement, marine roughness parameterizations should account for wave age. In addition, it is found that accounting for wave movement in an air–sea coupling scheme can be a significant factor in the calculation of surface stresses and, with them, surface heat fluxes over marine areas. Thus, the approach with which a coupling scheme implements sea-state-dependent roughness parameterizations can be as influential as the parameterizations themselves.
Abstract
A coupled air–sea numerical model comprising a mesoscale atmospheric model, a marine circulation model, and a surface wave model is presented. The coupled model is tested through simulations of an event of frontal passage through the Lake Erie region. Experiments investigate the effects of different sea surface roughness parameterizations on the atmospheric simulations.
The coupled system’s components are the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5), the Princeton Numerical Ocean Model (POM), and the GLERL–Donelan Wave Model (GDM). The finest of the MM5’s three nested grids covers Lake Erie, on which the POM and GDM operate. The MM5 provides surface heat and momentum fluxes to the POM, and the POM returns lake surface temperatures to the MM5. The MM5 provides 10-m winds to the GDM, and the GDM returns sea state information to the MM5. The MM5 uses this sea state information in calculating overwater roughness lengths (z 0’s).
Experiments varying the MM5’s roughness parameterization over Lake Erie are performed, resulting in a broad range of z 0’s. It is found that wave model coupling can significantly increase overwater roughnesses in the MM5, leading to increased surface heat and moisture fluxes and to changes in PBL characteristics. The impacts on the atmosphere from marine model coupling can appear far downstream of the coupled zones.
The accuracy of the mesoscale atmospheric simulation appears sensitive to the assumptions behind the marine roughness parameterizations used. The results suggest that, for consistent forecast improvement, marine roughness parameterizations should account for wave age. In addition, it is found that accounting for wave movement in an air–sea coupling scheme can be a significant factor in the calculation of surface stresses and, with them, surface heat fluxes over marine areas. Thus, the approach with which a coupling scheme implements sea-state-dependent roughness parameterizations can be as influential as the parameterizations themselves.
Abstract
This study examines the changes in Cascade Mountain spring snowpack since 1930. Three new time series facilitate this analysis: a water-balance estimate of Cascade snowpack from 1930 to 2007 that extends the observational record 20 years earlier than standard snowpack measurements; a radiosonde-based time series of lower-tropospheric temperature during onshore flow, to which Cascade snowpack is well correlated; and a new index of the North Pacific sea level pressure pattern that encapsulates modes of variability to which Cascade spring snowpack is particularly sensitive.
Cascade spring snowpack declined 23% during 1930–2007. This loss is nearly statistically significant at the 5% level. The snowpack increased 19% during the recent period of most rapid global warming (1976–2007), though this change is not statistically significant because of large annual variability. From 1950 to 1997, a large and statistically significant decline of 48% occurred. However, 80% of this decline is connected to changes in the circulation patterns over the North Pacific Ocean that vary naturally on annual to interdecadal time scales. The residual time series of Cascade snowpack after Pacific variability is removed displays a relatively steady loss rate of 2.0% decade−1, yielding a loss of 16% from 1930 to 2007. This loss is very nearly statistically significant and includes the possible impacts of anthropogenic global warming.
The dates of maximum snowpack and 90% melt out have shifted 5 days earlier since 1930. Both shifts are statistically insignificant. A new estimate of the sensitivity of Cascade spring snowpack to temperature of −11% per °C, when combined with climate model projections of 850-hPa temperatures offshore of the Pacific Northwest, yields a projected 9% loss of Cascade spring snowpack due to anthropogenic global warming between 1985 and 2025.
Abstract
This study examines the changes in Cascade Mountain spring snowpack since 1930. Three new time series facilitate this analysis: a water-balance estimate of Cascade snowpack from 1930 to 2007 that extends the observational record 20 years earlier than standard snowpack measurements; a radiosonde-based time series of lower-tropospheric temperature during onshore flow, to which Cascade snowpack is well correlated; and a new index of the North Pacific sea level pressure pattern that encapsulates modes of variability to which Cascade spring snowpack is particularly sensitive.
Cascade spring snowpack declined 23% during 1930–2007. This loss is nearly statistically significant at the 5% level. The snowpack increased 19% during the recent period of most rapid global warming (1976–2007), though this change is not statistically significant because of large annual variability. From 1950 to 1997, a large and statistically significant decline of 48% occurred. However, 80% of this decline is connected to changes in the circulation patterns over the North Pacific Ocean that vary naturally on annual to interdecadal time scales. The residual time series of Cascade snowpack after Pacific variability is removed displays a relatively steady loss rate of 2.0% decade−1, yielding a loss of 16% from 1930 to 2007. This loss is very nearly statistically significant and includes the possible impacts of anthropogenic global warming.
The dates of maximum snowpack and 90% melt out have shifted 5 days earlier since 1930. Both shifts are statistically insignificant. A new estimate of the sensitivity of Cascade spring snowpack to temperature of −11% per °C, when combined with climate model projections of 850-hPa temperatures offshore of the Pacific Northwest, yields a projected 9% loss of Cascade spring snowpack due to anthropogenic global warming between 1985 and 2025.
Abstract
A numerical simulation using the Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5) was run on a rainband associated with a cold front aloft (CFA) in a warm occluded structure on the U.S. east coast. The storm originally developed in the lee of the Rocky Mountains as a Pacific cold front overtook a Rocky Mountain lee trough. This formed a warm-type, occluded structure that was essentially maintained as the storm proceeded to the East Coast.
The CFA was a thermal front and therefore dynamically active. The prominence of the CFA in the equivalent potential temperature field was due primarily to the strong upward transport of water vapor from lower levels in the updraft associated with the CFA. The baroclinic zone was characterized by a tipped-forward lower region, where the CFA coincided with a maximum in potential temperature, and a tipped-backward upper region, where the CFA coincided with the leading (warm-side) edge of a zone of enhanced thermal gradient. The tipped-backward upper region displayed many of the characteristics of a vertically propagating gravity wave. In both of these regions, the potential temperature pattern produced a corresponding change in pressure gradient within the baroclinic zone; the imbalance of forces acting on air parcels as they moved through this pressure gradient produced the convergence in the lower baroclinic zone that was responsible for the CFA rainband.
Neither the dry quasigeostrophic nor dry Sawyer–Eliassen diagnosis resolved the details of the simulated mesoscale lifting associated with the CFA rainband. This is because the baroclinic zone of the CFA was mesoscale and structurally complex.
Abstract
A numerical simulation using the Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5) was run on a rainband associated with a cold front aloft (CFA) in a warm occluded structure on the U.S. east coast. The storm originally developed in the lee of the Rocky Mountains as a Pacific cold front overtook a Rocky Mountain lee trough. This formed a warm-type, occluded structure that was essentially maintained as the storm proceeded to the East Coast.
The CFA was a thermal front and therefore dynamically active. The prominence of the CFA in the equivalent potential temperature field was due primarily to the strong upward transport of water vapor from lower levels in the updraft associated with the CFA. The baroclinic zone was characterized by a tipped-forward lower region, where the CFA coincided with a maximum in potential temperature, and a tipped-backward upper region, where the CFA coincided with the leading (warm-side) edge of a zone of enhanced thermal gradient. The tipped-backward upper region displayed many of the characteristics of a vertically propagating gravity wave. In both of these regions, the potential temperature pattern produced a corresponding change in pressure gradient within the baroclinic zone; the imbalance of forces acting on air parcels as they moved through this pressure gradient produced the convergence in the lower baroclinic zone that was responsible for the CFA rainband.
Neither the dry quasigeostrophic nor dry Sawyer–Eliassen diagnosis resolved the details of the simulated mesoscale lifting associated with the CFA rainband. This is because the baroclinic zone of the CFA was mesoscale and structurally complex.
Abstract
On 8–9 March 1992, a long-lived squall line traversed the state of Kansas, producing hail and damaging winds. It was shown previously that this squall line was part of a synoptic-scale rainband 2000 km in length that was associated with a cold front aloft (CFA). The present study is concerned with the detailed mesoscale structure of this squall line and its relationship to the CFA.
Examination of synoptic-scale cross sections based on rawinsonde ascents, and a mesoscale cross section of winds derived from dual-Doppler radar measurements, shows that the squall line was exactly coincident with the “nose” of the CFA. The dual-Doppler analysis also shows that the inflow of air to the squall line was elevated, drawing in air from the potentially unstable layer within the weak warm frontal–like feature that was being occluded by the CFA. The stability analysis of the air in the pre-squall-line environment shows that when the CFA overtook the surface position of the drytrough, the thermal and moisture structure of the atmosphere was such that a moderate amount of lifting provided by the CFA could have released convective instability within an elevated layer approximately 1–2 km above ground.
The mesoscale structure of the squall line, derived from the radar reflectivity and dual-Doppler wind fields, differs substantially from the “leading line/trailing stratiform” conceptual model for midlatitude squall lines. The lack of a strong cold pool, and the presence of strong low-level shear, indicates that the squall line described here was able to persist in its mature stage in an environment that was “greater than optimal” in terms of the balance of the vorticity of the cold pool to that of the low-level shear. However, in view of 1) the weakness of the surface cold pool, 2) the elevated inflow and convergence associated with the convection, and 3) the collocation of the large rainband in which the squall line was embedded and the CFA, it seems likely that the CFA (rather than the cold pool) provided the driving force for the squall line.
Abstract
On 8–9 March 1992, a long-lived squall line traversed the state of Kansas, producing hail and damaging winds. It was shown previously that this squall line was part of a synoptic-scale rainband 2000 km in length that was associated with a cold front aloft (CFA). The present study is concerned with the detailed mesoscale structure of this squall line and its relationship to the CFA.
Examination of synoptic-scale cross sections based on rawinsonde ascents, and a mesoscale cross section of winds derived from dual-Doppler radar measurements, shows that the squall line was exactly coincident with the “nose” of the CFA. The dual-Doppler analysis also shows that the inflow of air to the squall line was elevated, drawing in air from the potentially unstable layer within the weak warm frontal–like feature that was being occluded by the CFA. The stability analysis of the air in the pre-squall-line environment shows that when the CFA overtook the surface position of the drytrough, the thermal and moisture structure of the atmosphere was such that a moderate amount of lifting provided by the CFA could have released convective instability within an elevated layer approximately 1–2 km above ground.
The mesoscale structure of the squall line, derived from the radar reflectivity and dual-Doppler wind fields, differs substantially from the “leading line/trailing stratiform” conceptual model for midlatitude squall lines. The lack of a strong cold pool, and the presence of strong low-level shear, indicates that the squall line described here was able to persist in its mature stage in an environment that was “greater than optimal” in terms of the balance of the vorticity of the cold pool to that of the low-level shear. However, in view of 1) the weakness of the surface cold pool, 2) the elevated inflow and convergence associated with the convection, and 3) the collocation of the large rainband in which the squall line was embedded and the CFA, it seems likely that the CFA (rather than the cold pool) provided the driving force for the squall line.
Abstract
The outbreak of tornadoes from the Mississippi River to just east of the Appalachian Mountains on 2–5 April 1974 is analyzed using conventional techniques and the Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5). The MM5 was run for 48 h using the NCEP–NCAR reanalysis dataset for initial conditions. It is suggested that the first damaging squall line within the storm of 2–5 April 1974 (herein referred to as the Super Outbreak storm) was initiated by updrafts associated with an undular bore. The bore resulted from the forward advance of a Pacific cold front into a stable air mass. The second major squall line within the Super Outbreak storm, which produced the strongest and most numerous tornadoes, was directly connected with the lifting associated with a cold front aloft. This second squall line was located along the farthest forward protrusion of a Pacific cold front as it occluded with a lee trough/dryline. An important factor in the formation of this occluded structure was the diabatic effects of evaporative cooling ahead of the Pacific cold front and daytime surface heating behind the Pacific cold front. These effects combined to lessen the horizontal temperature gradient across the cold front within the boundary layer. Although daytime surface heating and evaporative cooling are considered to be essential ingredients in the formation and maintenance of organized convection, the MM5 produced a strong squall line along the leading edge of the Pacific cold front even with the effects of surface heating and evaporational cooling removed from the model simulations.
Abstract
The outbreak of tornadoes from the Mississippi River to just east of the Appalachian Mountains on 2–5 April 1974 is analyzed using conventional techniques and the Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5). The MM5 was run for 48 h using the NCEP–NCAR reanalysis dataset for initial conditions. It is suggested that the first damaging squall line within the storm of 2–5 April 1974 (herein referred to as the Super Outbreak storm) was initiated by updrafts associated with an undular bore. The bore resulted from the forward advance of a Pacific cold front into a stable air mass. The second major squall line within the Super Outbreak storm, which produced the strongest and most numerous tornadoes, was directly connected with the lifting associated with a cold front aloft. This second squall line was located along the farthest forward protrusion of a Pacific cold front as it occluded with a lee trough/dryline. An important factor in the formation of this occluded structure was the diabatic effects of evaporative cooling ahead of the Pacific cold front and daytime surface heating behind the Pacific cold front. These effects combined to lessen the horizontal temperature gradient across the cold front within the boundary layer. Although daytime surface heating and evaporative cooling are considered to be essential ingredients in the formation and maintenance of organized convection, the MM5 produced a strong squall line along the leading edge of the Pacific cold front even with the effects of surface heating and evaporational cooling removed from the model simulations.
