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Abstract
This study investigates the evolution of a mesoscale convective system that formed during the Taiwan Area Mesoscale Experiment (TAMEX) on the 19 June 1987. With respect to the upstream flow, the convective system formed in the lee with rainfall totals from this system exceeding 100 mm. Any event that produces over 100 mm of rain in 24 h is thought to be capable of producing, flooding over the steep orography of Taiwan. Analysis of Doppler radar data showed that the convective cells during this heavy rainfall event repeatedly formed near a fixed location over the foothills and moved slowly (˜4 m s−1) northward. Although the radar reflectivities within these cells were relatively modest (35–40 dBZ), the repeated passage of slowly moving cells partly supplemented by relatively steady, stratiform rainfall during the later stages of the event resulted in the high precipitation totals. The heavy rain in this event resulted from a number of factors including 1) a moist, convectively unstable southerly flow of tropical origin, 2) a shallow convergence zone on the western side of the island dividing flow with a northerly component from that with a southerly component, 3) a quasi-stationary area of storm formation, and 4) a mesoscale environment that produced convective systems with a favorable storm structure and movement. Due to the fact that the large-scale forcing—as evidenced by vertical ascent calculated using rawinsonde data—was small, the authors believe that the conceptual model of low-Froude number flow around the island of Taiwan in the presence of heating can be used to explain the local convergent region that initiated this convective event. A numerical simulation of flow around the island of Taiwan in the presence of surface heating predicted a persistent quasi-stationary area of convergence in the foothills near the location of the observed convection. In this study the hypothesis is discussed that this persistent, quasi-stationary area of convergence may have also played a role in maintaining this convective system. These results and the application of this conceptual model will he discussed within the more general context of forecasting flash floods in Taiwan and the United States.
Abstract
This study investigates the evolution of a mesoscale convective system that formed during the Taiwan Area Mesoscale Experiment (TAMEX) on the 19 June 1987. With respect to the upstream flow, the convective system formed in the lee with rainfall totals from this system exceeding 100 mm. Any event that produces over 100 mm of rain in 24 h is thought to be capable of producing, flooding over the steep orography of Taiwan. Analysis of Doppler radar data showed that the convective cells during this heavy rainfall event repeatedly formed near a fixed location over the foothills and moved slowly (˜4 m s−1) northward. Although the radar reflectivities within these cells were relatively modest (35–40 dBZ), the repeated passage of slowly moving cells partly supplemented by relatively steady, stratiform rainfall during the later stages of the event resulted in the high precipitation totals. The heavy rain in this event resulted from a number of factors including 1) a moist, convectively unstable southerly flow of tropical origin, 2) a shallow convergence zone on the western side of the island dividing flow with a northerly component from that with a southerly component, 3) a quasi-stationary area of storm formation, and 4) a mesoscale environment that produced convective systems with a favorable storm structure and movement. Due to the fact that the large-scale forcing—as evidenced by vertical ascent calculated using rawinsonde data—was small, the authors believe that the conceptual model of low-Froude number flow around the island of Taiwan in the presence of heating can be used to explain the local convergent region that initiated this convective event. A numerical simulation of flow around the island of Taiwan in the presence of surface heating predicted a persistent quasi-stationary area of convergence in the foothills near the location of the observed convection. In this study the hypothesis is discussed that this persistent, quasi-stationary area of convergence may have also played a role in maintaining this convective system. These results and the application of this conceptual model will he discussed within the more general context of forecasting flash floods in Taiwan and the United States.
Abstract
Time continuous data assimilation or four-dimensional data assimilation (FDDA) is a collection of techniques where observations are ingested into a numerical model during the simulation in order to produce a physically balanced estimate of the true state of the atmosphere. Application of FDDA to the mesoalpha and subalpha scales is relatively new. One of many strategies for undertaking FDDA on the mesoscale is to employ Newtonian relaxation on increasingly finer horizontal grids. Encouraging results were found using this technique by Kuo et al. on a 40-km grid and by Stauffer and Seaman in a nested model with a 10-km inner grid. In these studies, the model is nudged toward the observations through adding an extra term(s) based on the difference between observations and the model predictions to the model’s prognostic equation(s). Since the model must retain a balance, this adjustment is spread over relatively large spatial and long temporal scales, and the nudging term is also multiplied by a coefficient that keeps the adjustment relatively small. Despite the positive findings of past studies, a number of questions arise in the application of this technique to fine grids. One area yet to be tested is how nudging will behave on fine grids under conditions with sharp horizontal and temporal gradients. Little improvement or even degradation of the model by the nudging might be expected as the timescale of nudging is relatively slow compared to the rapid evolution of the atmosphere, and spreading the observations out in time and space may not be representative of the actual atmospheric conditions. Other questions include 1) how the behavior of nudging at these scales and in active convection depends on boundary conditions, network density, and areal extent; 2) how the results depend on variations in the nudging coefficients; and 3) how nudging compares to simple objective analysis of the observations. In this study, Newtonian relaxation is used in a moist, full physics, nonhydrostatic mesoscale model to conduct simulations with horizontal resolutions as fine as 5 km in environments with deep convection and in mountainous terrain. Observing system simulation experiments were designed to address the previously mentioned questions. The authors show that nudging on these scales and in these conditions tends not to produce any large degradations but instead leads to improvements in the simulations even with a small number of observing sites. In applying nudging to a limited mesoscale area, the authors found that the results were more favorable if the nudging was undertaken over larger regions, which supports the nested approach used by Stauffer and Seaman. Some negative aspects of nudging were also uncovered with locally high rms errors due to data representativity problems and predictability issues. The accuracy of objective analysis was also explored and discussed in the context of the Atmospheric Radiation Measurement (ARM) Program. In agreement with Mace and Ackerman, the errors associated with objective analysis can be too large for the goals of ARM. However, the authors also found that a method proposed by Mace and Ackerman to detect time periods where significant errors exist in the objective analysis was not valid for this case. Based on this work, the authors propose that for a modest network of observing sites FDDA has a number of advantages over objective analysis.
Abstract
Time continuous data assimilation or four-dimensional data assimilation (FDDA) is a collection of techniques where observations are ingested into a numerical model during the simulation in order to produce a physically balanced estimate of the true state of the atmosphere. Application of FDDA to the mesoalpha and subalpha scales is relatively new. One of many strategies for undertaking FDDA on the mesoscale is to employ Newtonian relaxation on increasingly finer horizontal grids. Encouraging results were found using this technique by Kuo et al. on a 40-km grid and by Stauffer and Seaman in a nested model with a 10-km inner grid. In these studies, the model is nudged toward the observations through adding an extra term(s) based on the difference between observations and the model predictions to the model’s prognostic equation(s). Since the model must retain a balance, this adjustment is spread over relatively large spatial and long temporal scales, and the nudging term is also multiplied by a coefficient that keeps the adjustment relatively small. Despite the positive findings of past studies, a number of questions arise in the application of this technique to fine grids. One area yet to be tested is how nudging will behave on fine grids under conditions with sharp horizontal and temporal gradients. Little improvement or even degradation of the model by the nudging might be expected as the timescale of nudging is relatively slow compared to the rapid evolution of the atmosphere, and spreading the observations out in time and space may not be representative of the actual atmospheric conditions. Other questions include 1) how the behavior of nudging at these scales and in active convection depends on boundary conditions, network density, and areal extent; 2) how the results depend on variations in the nudging coefficients; and 3) how nudging compares to simple objective analysis of the observations. In this study, Newtonian relaxation is used in a moist, full physics, nonhydrostatic mesoscale model to conduct simulations with horizontal resolutions as fine as 5 km in environments with deep convection and in mountainous terrain. Observing system simulation experiments were designed to address the previously mentioned questions. The authors show that nudging on these scales and in these conditions tends not to produce any large degradations but instead leads to improvements in the simulations even with a small number of observing sites. In applying nudging to a limited mesoscale area, the authors found that the results were more favorable if the nudging was undertaken over larger regions, which supports the nested approach used by Stauffer and Seaman. Some negative aspects of nudging were also uncovered with locally high rms errors due to data representativity problems and predictability issues. The accuracy of objective analysis was also explored and discussed in the context of the Atmospheric Radiation Measurement (ARM) Program. In agreement with Mace and Ackerman, the errors associated with objective analysis can be too large for the goals of ARM. However, the authors also found that a method proposed by Mace and Ackerman to detect time periods where significant errors exist in the objective analysis was not valid for this case. Based on this work, the authors propose that for a modest network of observing sites FDDA has a number of advantages over objective analysis.
Abstract
A large nocturnal mesoscale convective complex (MCC) developed on 4 June 1985 during the Oklaboma-Kansas Preliminary Regional Experiment for STORM-Central (PRE-STORM) field phase. It occurred near the climatological center of the nocturnal maximum in warm-season precipitation situated over the central United States. In this study special rawinsonde and surface mesonet data have been used to examine how the environmental conditions, which supported MCC development, evolved at night over this region. The MCC of interest was the fourth in a series of MCCS, three of which propagated east-northeastward, 100–300 km north of a quasi-stationary surface front. The region where the MCC experienced its most intensive growth was initially characterized by dry and hydrostatically stable conditions (associated with the passage of the previous MCC) above the shallow, wedge-shaped cold air mass. In less than 3 h, interaction between the diurnally varying low-level jet and the frontal boundary led to a local increase in convective available potential energy (CAPE) of over 2000 J kg−1 for air parcels averaged through a 50-mb-deep layer immediately above the Frontal surface.
Our analysis shows that the region north of the quasi-stationary surface front became a favored zone for nocturnal MCC development when 1) particularly high CAPE arose due to the transport of moist, high-θ E air northward above the frontal surface by the diurnally modulated low-level jet into a region of significantly colder midtropospheric conditions, and 2) adiabatic mesoscale ascent, which was particularly strong near the northern terminus of the low-level jet, resulted in significant cooling above the jet axis. The cooling acted together with the strong moisture advection to eliminate convective inhibition [negative CAPE below the level of free convection (LFC)], thus enabling air parcels over a mesoscale region to more easily attain their LFC. Strong and deep mesoscale ascent was absent south of the front. In this region the surface-based deep convection that was supported during the evening hours weakened overnight as the low-level jet veered to a southwesterly direction, resulting in less favorable vertical shear for the sustenance of convective updrafts, while diurnal cooling increased the convective inhibition and raised the height of the LFC.
Abstract
A large nocturnal mesoscale convective complex (MCC) developed on 4 June 1985 during the Oklaboma-Kansas Preliminary Regional Experiment for STORM-Central (PRE-STORM) field phase. It occurred near the climatological center of the nocturnal maximum in warm-season precipitation situated over the central United States. In this study special rawinsonde and surface mesonet data have been used to examine how the environmental conditions, which supported MCC development, evolved at night over this region. The MCC of interest was the fourth in a series of MCCS, three of which propagated east-northeastward, 100–300 km north of a quasi-stationary surface front. The region where the MCC experienced its most intensive growth was initially characterized by dry and hydrostatically stable conditions (associated with the passage of the previous MCC) above the shallow, wedge-shaped cold air mass. In less than 3 h, interaction between the diurnally varying low-level jet and the frontal boundary led to a local increase in convective available potential energy (CAPE) of over 2000 J kg−1 for air parcels averaged through a 50-mb-deep layer immediately above the Frontal surface.
Our analysis shows that the region north of the quasi-stationary surface front became a favored zone for nocturnal MCC development when 1) particularly high CAPE arose due to the transport of moist, high-θ E air northward above the frontal surface by the diurnally modulated low-level jet into a region of significantly colder midtropospheric conditions, and 2) adiabatic mesoscale ascent, which was particularly strong near the northern terminus of the low-level jet, resulted in significant cooling above the jet axis. The cooling acted together with the strong moisture advection to eliminate convective inhibition [negative CAPE below the level of free convection (LFC)], thus enabling air parcels over a mesoscale region to more easily attain their LFC. Strong and deep mesoscale ascent was absent south of the front. In this region the surface-based deep convection that was supported during the evening hours weakened overnight as the low-level jet veered to a southwesterly direction, resulting in less favorable vertical shear for the sustenance of convective updrafts, while diurnal cooling increased the convective inhibition and raised the height of the LFC.
Abstract
In this study a three-dimensional numerical cloud model is used to examine the early evolution of deep convective rainbands that occur in an environment of weak to moderate buoyancy and directionally varying lower-tropospheric vertical wind shear. A simulation based on a case observed on 8 June 1987 during the Taiwan Area Mesoscale Experiment produced a narrow bow-shaped rainband that comprised 1) short-lived updrafts along the downshear portion of the weak rain-induced cold pool and 2) more persistent updrafts along its southern flank, which were highly correlated with vertical vorticity. Trajectory calculations and an analysis of the dynamic portion of the perturbation pressure field are presented to illustrate the hybrid dynamical character of the simulated rainband. The shorter-lived updrafts were associated with weak upward-directed pressure gradient forces at the leading edge of the surface-based cold pool. The more persistent updrafts exhibited much stronger upward-directed pressure gradient forces, which have previously been noted to play an important role in the longevity and propagation of updrafts in midlatitude supercell storms.
While this work was motivated by the desire to better understand mechanisms important to the. formation of heavy rainfall that occurs in association with prefrontal low-level jets over Taiwan, direct verification of the control simulation was hindered by the lack of available Doppler radar observations and difficulties in unambiguously determining initial conditions. Therefore, the simulation results were viewed as idealized and interpreted within the context of a series of sensitivity experiments. These experiments revealed that updraft dynamics and convective organization were strongly dependent on the magnitude of the ambient vertical shear. At weaker vertical shears, low-level updrafts were generally weaker and not associated with strong vertical vorticity. Maximum rainwater mixing ratios were also significantly weaker for less ambient vertical shears despite the specification of identical initial profiles of temperature and moisture for all simulations. This suggests that the strong vertical shear associated with the low-level jet provides a mechanism for producing greater local rainfall rates by allowing enhanced forcing for low-level updrafts in the nearly saturated ambient environment.
Abstract
In this study a three-dimensional numerical cloud model is used to examine the early evolution of deep convective rainbands that occur in an environment of weak to moderate buoyancy and directionally varying lower-tropospheric vertical wind shear. A simulation based on a case observed on 8 June 1987 during the Taiwan Area Mesoscale Experiment produced a narrow bow-shaped rainband that comprised 1) short-lived updrafts along the downshear portion of the weak rain-induced cold pool and 2) more persistent updrafts along its southern flank, which were highly correlated with vertical vorticity. Trajectory calculations and an analysis of the dynamic portion of the perturbation pressure field are presented to illustrate the hybrid dynamical character of the simulated rainband. The shorter-lived updrafts were associated with weak upward-directed pressure gradient forces at the leading edge of the surface-based cold pool. The more persistent updrafts exhibited much stronger upward-directed pressure gradient forces, which have previously been noted to play an important role in the longevity and propagation of updrafts in midlatitude supercell storms.
While this work was motivated by the desire to better understand mechanisms important to the. formation of heavy rainfall that occurs in association with prefrontal low-level jets over Taiwan, direct verification of the control simulation was hindered by the lack of available Doppler radar observations and difficulties in unambiguously determining initial conditions. Therefore, the simulation results were viewed as idealized and interpreted within the context of a series of sensitivity experiments. These experiments revealed that updraft dynamics and convective organization were strongly dependent on the magnitude of the ambient vertical shear. At weaker vertical shears, low-level updrafts were generally weaker and not associated with strong vertical vorticity. Maximum rainwater mixing ratios were also significantly weaker for less ambient vertical shears despite the specification of identical initial profiles of temperature and moisture for all simulations. This suggests that the strong vertical shear associated with the low-level jet provides a mechanism for producing greater local rainfall rates by allowing enhanced forcing for low-level updrafts in the nearly saturated ambient environment.
Abstract
The International H2O Project (IHOP_2002) included four complementary research components: quantitative precipitation forecasting, convection initiation, atmospheric boundary layer processes, and instrumentation. This special issue introductory paper will review the current state of knowledge on surface-forced convection initiation and then describe some of the outstanding issues in convection initiation that partially motivated IHOP_2002. Subsequent papers in this special issue will illustrate the value of combining varied and complementary datasets to study convection initiation in order to address the outstanding issues discussed in this paper and new questions that arose from IHOP_2002 observations.
The review will focus on convection initiation by boundaries that are prevalent in the U.S. southern Great Plains. Boundary layer circulations, which are sometimes precursors to deep convective development, are clearly observed by radar as reflectivity fine lines and/or convergence in Doppler velocity. The corresponding thermodynamic distribution, particularly the moisture field, is not as readily measured. During IHOP_2002, a variety of sensors capable of measuring atmospheric water vapor were brought together in an effort to sample the three-dimensional time-varying moisture field and determine its impact on forecasting convection initiation. The strategy included examining convection initiation with targeted observations aimed at sampling regions forecast to be ripe for initiation, primarily along frontal zones, drylines, and their mergers.
A key aspect of these investigations was the combination of varied moisture measurements with the detailed observations of the wind field, as presented in many of the subsequent papers in this issue. For example, the high-resolution measurements are being used to better understand the role of misocyclones on convection initiation. The analyses are starting to elucidate the value of new datasets, including satellite products and radar refractivity retrievals. Data assimilation studies using some of the state-of-the-art datasets from IHOP_2002 are already proving to be quite promising.
Abstract
The International H2O Project (IHOP_2002) included four complementary research components: quantitative precipitation forecasting, convection initiation, atmospheric boundary layer processes, and instrumentation. This special issue introductory paper will review the current state of knowledge on surface-forced convection initiation and then describe some of the outstanding issues in convection initiation that partially motivated IHOP_2002. Subsequent papers in this special issue will illustrate the value of combining varied and complementary datasets to study convection initiation in order to address the outstanding issues discussed in this paper and new questions that arose from IHOP_2002 observations.
The review will focus on convection initiation by boundaries that are prevalent in the U.S. southern Great Plains. Boundary layer circulations, which are sometimes precursors to deep convective development, are clearly observed by radar as reflectivity fine lines and/or convergence in Doppler velocity. The corresponding thermodynamic distribution, particularly the moisture field, is not as readily measured. During IHOP_2002, a variety of sensors capable of measuring atmospheric water vapor were brought together in an effort to sample the three-dimensional time-varying moisture field and determine its impact on forecasting convection initiation. The strategy included examining convection initiation with targeted observations aimed at sampling regions forecast to be ripe for initiation, primarily along frontal zones, drylines, and their mergers.
A key aspect of these investigations was the combination of varied moisture measurements with the detailed observations of the wind field, as presented in many of the subsequent papers in this issue. For example, the high-resolution measurements are being used to better understand the role of misocyclones on convection initiation. The analyses are starting to elucidate the value of new datasets, including satellite products and radar refractivity retrievals. Data assimilation studies using some of the state-of-the-art datasets from IHOP_2002 are already proving to be quite promising.
Abstract
A 24-h nested-grid simulation of an intense squall line during the 1985 PRE-STORM experiment is presented using an improved version of the Pennsylvania State University/National Center for Atmospheric Research three-dimensional mesoscale model. Although the model is initialized at 1200 UTC 10 June 1985 with conventional meteorological observations, it reproduces remarkably well many observed meso-β scale features that are analyzed from the high-resolution network data. These include 1) the generation of two areas of deep convection at the model initial time; 2) the timing of the initiation of the squall line along a surface front 9 h into the model integration; 3) the development of several convective bands at 2100 UTC; 4) the rapid intensification and rapid dissipation processes of the squall line as it entered and moved out of the network, respectively; 5) the generation of a presquall mesolow, a squall-induced mesohigh and a wake low as well as corresponding multiple surface convergence-divergence flow structure; 6) the evolution of a traveling 700 mb shortwave; 7) the development of a rear-inflow jet; 8) the leading convective rainfall followed by a transition zone and trailing stratiform precipitation; 9) the observed configuration of front-to-rear relative flow at both upper and lower levels separated by the rear-to-front flow at midlevels; 10) the simulation of “onion-shaped” soundings; 11) the splitting of the wake low; 12) the maintenance and intensification of a mesovortex; 13) the distribution and magnitude of convective and stratiform rainfall; and 14) the diurnal cycle of the planetary boundary layer.
One of the encouraging results is that the model accurately simulates the rear-inflow jet as verified against Doppler windprofiler data after the 18-h integration from essentially synoptic-scale initial conditions. The results confirm the previously proposed hypothesis that the wake low develops hydrostatically as a consequence of adiabatic warming by descending flow entering the squall line within the rear-inflow jet The observed “onion-shaped” soundings are a manifestation of the warming and drying of air within the descending rear inflow jet. It is found that the present wake low is not a transient meso-β scale phenomenon, but has a time scale of more than 50% of the squall line lifetime. Another finding is that the present mesovortex is not produced by latent heat release associated with the squall line but was in existence prior to the model initialization time. The vortex appears to have a significant effect on the distribution of the rainfall associated with the squall line and on the intensity of the rear-inflow jet. Other mesoscale circulation features are also documented in this paper.
This study, along with previous investigations using the model, indicates that the meso-β scale structure and evolution of MCSs under certain synoptic-scale environmental conditions can be well simulated using the standard network observations if compatible grid resolution, reasonable model physics and initial conditions are utilized.
Abstract
A 24-h nested-grid simulation of an intense squall line during the 1985 PRE-STORM experiment is presented using an improved version of the Pennsylvania State University/National Center for Atmospheric Research three-dimensional mesoscale model. Although the model is initialized at 1200 UTC 10 June 1985 with conventional meteorological observations, it reproduces remarkably well many observed meso-β scale features that are analyzed from the high-resolution network data. These include 1) the generation of two areas of deep convection at the model initial time; 2) the timing of the initiation of the squall line along a surface front 9 h into the model integration; 3) the development of several convective bands at 2100 UTC; 4) the rapid intensification and rapid dissipation processes of the squall line as it entered and moved out of the network, respectively; 5) the generation of a presquall mesolow, a squall-induced mesohigh and a wake low as well as corresponding multiple surface convergence-divergence flow structure; 6) the evolution of a traveling 700 mb shortwave; 7) the development of a rear-inflow jet; 8) the leading convective rainfall followed by a transition zone and trailing stratiform precipitation; 9) the observed configuration of front-to-rear relative flow at both upper and lower levels separated by the rear-to-front flow at midlevels; 10) the simulation of “onion-shaped” soundings; 11) the splitting of the wake low; 12) the maintenance and intensification of a mesovortex; 13) the distribution and magnitude of convective and stratiform rainfall; and 14) the diurnal cycle of the planetary boundary layer.
One of the encouraging results is that the model accurately simulates the rear-inflow jet as verified against Doppler windprofiler data after the 18-h integration from essentially synoptic-scale initial conditions. The results confirm the previously proposed hypothesis that the wake low develops hydrostatically as a consequence of adiabatic warming by descending flow entering the squall line within the rear-inflow jet The observed “onion-shaped” soundings are a manifestation of the warming and drying of air within the descending rear inflow jet. It is found that the present wake low is not a transient meso-β scale phenomenon, but has a time scale of more than 50% of the squall line lifetime. Another finding is that the present mesovortex is not produced by latent heat release associated with the squall line but was in existence prior to the model initialization time. The vortex appears to have a significant effect on the distribution of the rainfall associated with the squall line and on the intensity of the rear-inflow jet. Other mesoscale circulation features are also documented in this paper.
This study, along with previous investigations using the model, indicates that the meso-β scale structure and evolution of MCSs under certain synoptic-scale environmental conditions can be well simulated using the standard network observations if compatible grid resolution, reasonable model physics and initial conditions are utilized.
Abstract
On 30 June 1982, a multicellular storm in Colorado produced four downbursts, three misocyclones, a miso-anticyclone, and horizontal vortex circulations within a relatively small area of the storm. Weather events associated with this storm included hail, heavy rain, and strong winds. A sounding taken two hours before storm formation showed the mixed layer was characterized by a nearly dry adiabatic lapse rate to ∼2 km and was relatively moist for eastern Colorado. A hodograph showed the environment had weak to moderate vertical shear of the horizontal wind, a condition conducive to the formation of downdraft misocyclones. The four-dimensional structure of this storm is documented below cloud base using winds, reflectivity, and thermodynamic data derived front multiple Doppler analysis.
One misocyclone (<4 km scale) is particularly intense with a peak vorticity of ≈100 × 10−3 s−1 near cloud base. Despite the intense rotation, no tornadoes or funnels were observed and no damage was reported. Radar characteristics of this misocyclone are similar to those of mesocyclones that produce tornadoes or funnels except that vorticity is a maximum near cloud base and the low-level divergence created by the downbursts weakens the low-level, positive vorticity. While the misocyclone is initially separated from the downdraft, the two features evolve to become collocated. Each misocyclone becomes associated with a local downdraft maximum, suggesting that the misocyclones are important to downdraft development.
Pressure perturbation analysis does not show any evidence for strong, downward-directed pressure gradient forces below cloud base that would act to accelerate a downdraft. Since the downdraft is observed to accelerate below cloud base, other forces must be important. Observations and buoyancy estimates calculated from radar reflectivity show negative buoyancy is playing a role in downdraft intensification. Despite the lack of dynamical forcing of the downdraft by the misocyclone below cloud base, dynamical forces may be playing a role in accelerating the downdraft above cloud base.
Horizontal vortex circulations, or rotors, form along the edge of the misocyclone and downdraft and propagate away from their source region. Strongest surface winds are associated with the rotors. Pressure perturbation analysis shows that a low forms at the center of the circulation that may cause an acceleration of the low-level outflow into the rotor and may explain the strong winds. Rotors may be an integral part of downburst outflows and perhaps multiple rotors are created by pulsating downdrafts. An explanation of these circulations is important since they seem to have been involved in the Dallas-Fort Worth Regional Airport crash of an L-1011 jet.
Abstract
On 30 June 1982, a multicellular storm in Colorado produced four downbursts, three misocyclones, a miso-anticyclone, and horizontal vortex circulations within a relatively small area of the storm. Weather events associated with this storm included hail, heavy rain, and strong winds. A sounding taken two hours before storm formation showed the mixed layer was characterized by a nearly dry adiabatic lapse rate to ∼2 km and was relatively moist for eastern Colorado. A hodograph showed the environment had weak to moderate vertical shear of the horizontal wind, a condition conducive to the formation of downdraft misocyclones. The four-dimensional structure of this storm is documented below cloud base using winds, reflectivity, and thermodynamic data derived front multiple Doppler analysis.
One misocyclone (<4 km scale) is particularly intense with a peak vorticity of ≈100 × 10−3 s−1 near cloud base. Despite the intense rotation, no tornadoes or funnels were observed and no damage was reported. Radar characteristics of this misocyclone are similar to those of mesocyclones that produce tornadoes or funnels except that vorticity is a maximum near cloud base and the low-level divergence created by the downbursts weakens the low-level, positive vorticity. While the misocyclone is initially separated from the downdraft, the two features evolve to become collocated. Each misocyclone becomes associated with a local downdraft maximum, suggesting that the misocyclones are important to downdraft development.
Pressure perturbation analysis does not show any evidence for strong, downward-directed pressure gradient forces below cloud base that would act to accelerate a downdraft. Since the downdraft is observed to accelerate below cloud base, other forces must be important. Observations and buoyancy estimates calculated from radar reflectivity show negative buoyancy is playing a role in downdraft intensification. Despite the lack of dynamical forcing of the downdraft by the misocyclone below cloud base, dynamical forces may be playing a role in accelerating the downdraft above cloud base.
Horizontal vortex circulations, or rotors, form along the edge of the misocyclone and downdraft and propagate away from their source region. Strongest surface winds are associated with the rotors. Pressure perturbation analysis shows that a low forms at the center of the circulation that may cause an acceleration of the low-level outflow into the rotor and may explain the strong winds. Rotors may be an integral part of downburst outflows and perhaps multiple rotors are created by pulsating downdrafts. An explanation of these circulations is important since they seem to have been involved in the Dallas-Fort Worth Regional Airport crash of an L-1011 jet.
Abstract
The passage of shallow cold fronts during the late spring and early summer months over the island of Taiwan is often accompanied by heavy rainfall and occasional flash flood episodes. Previous studies have emphasized the weak baroclinicity of these fronts and their possible modification by fluxes from the air-sea interface. In this study a cold frontal passage in the vicinity of Taiwan is analyzed using data gathered during the Taiwan Area Mesoscale Experiment (TAMEX) on 8 June 1987. At the northern extent of the TAMEX network the cold front was shallow (1–2 km deep) and moderately baroclinic with 5°-7°C temperature contrasts at the surface. A Doppler radar cross section of radial velocity reveals a structure similar to that of a density current at the leading edge of the shallow front. The postfrontal air man was substantially modified by oceanic heat fluxes as it moved southward over the warm ocean waters. This led to a 60%–70% decrease in the temperature contrast across the front between ocean stations at the northern and southern ends of the island, a distance of ∼400 km.
Frontal passages across Taiwan are also influenced by the presence of the Central Mountain Range (CMR), which has an average ridge elevation of ∼2500 m, and is oriented NNE-SSW along the major axis of the island. In the case described in this paper the CMR, 1) acts as a barrier to both the pre- and postfrontal flows, and 2) is influential by inducing thermally-driven diurnal circulations associated with differential heating of the sloped terrain and the nearby ocean. Terrain influences on the kinematics of the flow in the vicinity of the front are also shown to locally modify the frontal intensity.
The inhomogeneous distribution of precipitation attending the frontal passage is related to strong regional variations in thermodynamic stability across the island. These variations in stability are linked to the mesoscale effects of terrain, and to the larger-scale influence of advection of an unstable tropical air mass into the region by a low-level wind maximum.
Abstract
The passage of shallow cold fronts during the late spring and early summer months over the island of Taiwan is often accompanied by heavy rainfall and occasional flash flood episodes. Previous studies have emphasized the weak baroclinicity of these fronts and their possible modification by fluxes from the air-sea interface. In this study a cold frontal passage in the vicinity of Taiwan is analyzed using data gathered during the Taiwan Area Mesoscale Experiment (TAMEX) on 8 June 1987. At the northern extent of the TAMEX network the cold front was shallow (1–2 km deep) and moderately baroclinic with 5°-7°C temperature contrasts at the surface. A Doppler radar cross section of radial velocity reveals a structure similar to that of a density current at the leading edge of the shallow front. The postfrontal air man was substantially modified by oceanic heat fluxes as it moved southward over the warm ocean waters. This led to a 60%–70% decrease in the temperature contrast across the front between ocean stations at the northern and southern ends of the island, a distance of ∼400 km.
Frontal passages across Taiwan are also influenced by the presence of the Central Mountain Range (CMR), which has an average ridge elevation of ∼2500 m, and is oriented NNE-SSW along the major axis of the island. In the case described in this paper the CMR, 1) acts as a barrier to both the pre- and postfrontal flows, and 2) is influential by inducing thermally-driven diurnal circulations associated with differential heating of the sloped terrain and the nearby ocean. Terrain influences on the kinematics of the flow in the vicinity of the front are also shown to locally modify the frontal intensity.
The inhomogeneous distribution of precipitation attending the frontal passage is related to strong regional variations in thermodynamic stability across the island. These variations in stability are linked to the mesoscale effects of terrain, and to the larger-scale influence of advection of an unstable tropical air mass into the region by a low-level wind maximum.
Abstract
Observations from the Oklahoma-Kansas Preliminary Regional Experiment for STORM-CENTRAL (OK PRE-STORM) have been used to study the evolution and propagation characteristics of a long-lived (≥16 h) mesoscale convective system (MCS) that produced locally heavy (50–100 mm) rainfall during 26–27 June 1985. The MCS formed in association with a synoptic-scale cold front and upper-level trough system. Mesoscale ascent contributed to an increase in convective available potential energy (CAPE) and a decrease in convective inhibition, facilitating the development of deep convection.
During the late morning and early afternoon hours convection was present along and within an ∼200-km zone in advance of the cold front. In advance of the main precipitation area, a series of nearly parallel rainbands formed from in situ boundary-layer cloud streets. The development and organization of these rainbands was aided by the moderate-to-large CAPE, small convective inhibition, and moderate unidirectional shear at low levels that characterized the preconvective environment over the ∼200-km region ahead of the cold front. The discrete eastward progression of convection afforded by the formation of the rainbands in advance of the main precipitation area represents a distinct departure from the propagation characteristics of many previously observed cases and idealized simulations of linearly oriented MCSs, where system propagation depends crucially on periodic regeneration of multicell convection along a storm-induced cold pool.
The MCS weakened over southern Kansas after the merger of the main precipitation area with the quasi-stationary presquall rainbands. During its dissipating stages, it exhibited circulation and surface pressure features commonly reported during the mature-to-decaying stages of previously observed systems. These features included a surface mesohigh to the rear of the leading edge of the precipitation, and regions of mesoscale ascent and subsidence associated with a trailing anvil and a sloping rear inflow jet. The presence of these features, despite a system evolution and precursor environment different from those of a more classical linearly oriented MCS supports the consensus from earlier studies that internal processes such as spatial variations in diabatic heating are likely responsible for the observed mesoscale flows in the mature-to-decaying stages of large MCSs.
Abstract
Observations from the Oklahoma-Kansas Preliminary Regional Experiment for STORM-CENTRAL (OK PRE-STORM) have been used to study the evolution and propagation characteristics of a long-lived (≥16 h) mesoscale convective system (MCS) that produced locally heavy (50–100 mm) rainfall during 26–27 June 1985. The MCS formed in association with a synoptic-scale cold front and upper-level trough system. Mesoscale ascent contributed to an increase in convective available potential energy (CAPE) and a decrease in convective inhibition, facilitating the development of deep convection.
During the late morning and early afternoon hours convection was present along and within an ∼200-km zone in advance of the cold front. In advance of the main precipitation area, a series of nearly parallel rainbands formed from in situ boundary-layer cloud streets. The development and organization of these rainbands was aided by the moderate-to-large CAPE, small convective inhibition, and moderate unidirectional shear at low levels that characterized the preconvective environment over the ∼200-km region ahead of the cold front. The discrete eastward progression of convection afforded by the formation of the rainbands in advance of the main precipitation area represents a distinct departure from the propagation characteristics of many previously observed cases and idealized simulations of linearly oriented MCSs, where system propagation depends crucially on periodic regeneration of multicell convection along a storm-induced cold pool.
The MCS weakened over southern Kansas after the merger of the main precipitation area with the quasi-stationary presquall rainbands. During its dissipating stages, it exhibited circulation and surface pressure features commonly reported during the mature-to-decaying stages of previously observed systems. These features included a surface mesohigh to the rear of the leading edge of the precipitation, and regions of mesoscale ascent and subsidence associated with a trailing anvil and a sloping rear inflow jet. The presence of these features, despite a system evolution and precursor environment different from those of a more classical linearly oriented MCS supports the consensus from earlier studies that internal processes such as spatial variations in diabatic heating are likely responsible for the observed mesoscale flows in the mature-to-decaying stages of large MCSs.
Abstract
A horizontal gradient in moisture, termed the dryline, is often detected at the surface over the southern Great Plains of the United States during the spring and early summer. The dryline exhibits distinct diurnal variations in both its movement and structure. Recent research has focused on dryline structure during the afternoon and evening, particularly showing how strong (∼1–5 m s−1) ascent frequently creates an environment favorable to the initiation of convection, quite close (within ∼10 km) to the dryline interface. To date, however, there have been very few detailed analyses of the dryline interface at night, so that the nocturnal behavior of the interface predicted by theory and numerical studies is relatively poorly evaluated. In this study, special observations taken by a Doppler lidar, serial rawinsonde ascents, and a dual-channel microwave radiometer are utilized to describe the behavior of a nocturnal dryline observed on 12–13 May 1985. The analysis presented here reveals that the mesoscale structure of the nocturnal dryline prior to the formation of deep convection is a gently sloping, slow-moving interface. The movement of the dryline at night was related to the evolution of the low-level jet within the moist air. Wavelike structures and evidence for vertical mixing were observed in the moist air as low Richardson numbers occurred below the height of the jet. The previously discussed strong ascent is largely lacking in the present nocturnal case so that the circulations inherent to an undisturbed dryline at night are far less favorable for the initiation of deep convection than in the afternoon and early evening.
In the present case, severe convection developed as a weak cold front approached and merged with the nocturnal dryline and the environment rapidly destabilized. Between soundings taken 2.5 h apart, the convective available potential energy increased from 524 to 3417 J kg−1 and the absolute value of the convective inhibition decreased from 412 to 9 J kg−1. The vertical shear of the horizontal wind also dramatically increased with time, so that the bulk Richardson number was within values normally associated with supercell convection. The timescale of the changes in stability and in the moisture field (∼1–2.5 h) has implications for the type of observing network needed to nowcast severe convection and for assessing the performance of research and operational numerical models.
Abstract
A horizontal gradient in moisture, termed the dryline, is often detected at the surface over the southern Great Plains of the United States during the spring and early summer. The dryline exhibits distinct diurnal variations in both its movement and structure. Recent research has focused on dryline structure during the afternoon and evening, particularly showing how strong (∼1–5 m s−1) ascent frequently creates an environment favorable to the initiation of convection, quite close (within ∼10 km) to the dryline interface. To date, however, there have been very few detailed analyses of the dryline interface at night, so that the nocturnal behavior of the interface predicted by theory and numerical studies is relatively poorly evaluated. In this study, special observations taken by a Doppler lidar, serial rawinsonde ascents, and a dual-channel microwave radiometer are utilized to describe the behavior of a nocturnal dryline observed on 12–13 May 1985. The analysis presented here reveals that the mesoscale structure of the nocturnal dryline prior to the formation of deep convection is a gently sloping, slow-moving interface. The movement of the dryline at night was related to the evolution of the low-level jet within the moist air. Wavelike structures and evidence for vertical mixing were observed in the moist air as low Richardson numbers occurred below the height of the jet. The previously discussed strong ascent is largely lacking in the present nocturnal case so that the circulations inherent to an undisturbed dryline at night are far less favorable for the initiation of deep convection than in the afternoon and early evening.
In the present case, severe convection developed as a weak cold front approached and merged with the nocturnal dryline and the environment rapidly destabilized. Between soundings taken 2.5 h apart, the convective available potential energy increased from 524 to 3417 J kg−1 and the absolute value of the convective inhibition decreased from 412 to 9 J kg−1. The vertical shear of the horizontal wind also dramatically increased with time, so that the bulk Richardson number was within values normally associated with supercell convection. The timescale of the changes in stability and in the moisture field (∼1–2.5 h) has implications for the type of observing network needed to nowcast severe convection and for assessing the performance of research and operational numerical models.
