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Abstract
Previous cloud modeling studies have noted difficulty in producing strong, sustained deep convection in environments with convective inhibition and/or midlevel dryness when the thermal bubble technique is used to initiate convection. This difficulty is also demonstrated herein, using 113 supercell proximity soundings—most of which contain capping inversions and some amount of convective inhibition. Instead, by using an updraft nudging initiation technique, substantially more supercells result and for a longer period. Additionally, the number of supercell-producing cases is maximized when updraft nudging is applied for only the first 15 min of cloud time near the top of the boundary layer instead of longer/shorter periods or when nudging is applied near the surface.
Abstract
Previous cloud modeling studies have noted difficulty in producing strong, sustained deep convection in environments with convective inhibition and/or midlevel dryness when the thermal bubble technique is used to initiate convection. This difficulty is also demonstrated herein, using 113 supercell proximity soundings—most of which contain capping inversions and some amount of convective inhibition. Instead, by using an updraft nudging initiation technique, substantially more supercells result and for a longer period. Additionally, the number of supercell-producing cases is maximized when updraft nudging is applied for only the first 15 min of cloud time near the top of the boundary layer instead of longer/shorter periods or when nudging is applied near the surface.
Abstract
This work studies the relationship between midtropospheric dryness and supercell thunderstorm morphology and evolution using a three-dimensional, nonhydrostatic cloud model. Environments that differ only in midtropospheric dryness are found to produce supercells having different low-level outflow and rotational characteristics. Thunderstorms forming in environments with moderate vertical wind shear, large instability, and very dry midtropospheric air produce strong low-level outflow. When this low-level outflow propagates faster than the midlevel mesocyclone, the storm updraft and low-level mesocyclone weaken. However, in environments with larger vertical wind shear or with higher-altitude dry midtropospheric air, the low-level outflow is not as detrimental to the supercell. This provides a possible explanation for why some environments that appear favorable for the development of strong low-level mesocyclones in supercells fail to do so.
Downdraft convective available potential energy (DCAPE) is also investigated as one possible index for estimating potential downdraft strength. Trajectory analysis shows that the strongest downdrafts are subsaturated and diluted due to mixing between the downdraft and the surrounding environment. These significant violations of parcel theory make DCAPE a worse estimate for supercell downdraft intensity than convective available potential energy is for the updraft. A more sophisticated parameter is needed in order to determine downdraft intensity and low-level outflow strength within supercells.
Abstract
This work studies the relationship between midtropospheric dryness and supercell thunderstorm morphology and evolution using a three-dimensional, nonhydrostatic cloud model. Environments that differ only in midtropospheric dryness are found to produce supercells having different low-level outflow and rotational characteristics. Thunderstorms forming in environments with moderate vertical wind shear, large instability, and very dry midtropospheric air produce strong low-level outflow. When this low-level outflow propagates faster than the midlevel mesocyclone, the storm updraft and low-level mesocyclone weaken. However, in environments with larger vertical wind shear or with higher-altitude dry midtropospheric air, the low-level outflow is not as detrimental to the supercell. This provides a possible explanation for why some environments that appear favorable for the development of strong low-level mesocyclones in supercells fail to do so.
Downdraft convective available potential energy (DCAPE) is also investigated as one possible index for estimating potential downdraft strength. Trajectory analysis shows that the strongest downdrafts are subsaturated and diluted due to mixing between the downdraft and the surrounding environment. These significant violations of parcel theory make DCAPE a worse estimate for supercell downdraft intensity than convective available potential energy is for the updraft. A more sophisticated parameter is needed in order to determine downdraft intensity and low-level outflow strength within supercells.
Abstract
Radar, cloud-to-ground (CG) lightning characteristics, and storm reports were documented for 20 long-lived supercell thunderstorms that occurred during a 6-h period in the west Texas Panhandle on 2–3 June 1995. These thunderstorms occurred in proximity to a preexisting mesoscale outflow boundary.
Storms that remained on the warm side of the mesoscale outflow boundary and storms that formed directly on the boundary tended to produce weaker low-level rotation, lower maximum heights for the 40-dBZ echo top, and had the largest negative CG flash rates. The largest negative flash rate was produced as each storm was gradually weakening. In contrast, out of 11 boundary-crossing storms, several important radar-based measurands increased unambiguously after storms crossed the boundary: 40-dBZ echo-top height in 5 cases, radar reflectivity above the environmental freezing level in 6 cases, and low-level mesocyclone strength in 9 cases. Trends of the first two measurands were ambiguous for 4 of 11 cases affected by a ±15 min estimated boundary-position uncertainty. Five out of 11 storms dramatically increased their positive flash rate within 60 min after crossing the outflow boundary. These large positive flash rates were associated with descending reflectivity cores that were larger in magnitude and areal extent compared to other storms in this study.
The local mesoscale environment and its horizontal variations of 0–3-km vertical wind profile, CAPE below the in-cloud freezing level, and boundary layer mixing ratio appeared to greatly influence storm structure and evolution. The observed environmental variations are hypothesized to support changes in charge structure that might lead to the observed changes in flash rate and polarity.
Abstract
Radar, cloud-to-ground (CG) lightning characteristics, and storm reports were documented for 20 long-lived supercell thunderstorms that occurred during a 6-h period in the west Texas Panhandle on 2–3 June 1995. These thunderstorms occurred in proximity to a preexisting mesoscale outflow boundary.
Storms that remained on the warm side of the mesoscale outflow boundary and storms that formed directly on the boundary tended to produce weaker low-level rotation, lower maximum heights for the 40-dBZ echo top, and had the largest negative CG flash rates. The largest negative flash rate was produced as each storm was gradually weakening. In contrast, out of 11 boundary-crossing storms, several important radar-based measurands increased unambiguously after storms crossed the boundary: 40-dBZ echo-top height in 5 cases, radar reflectivity above the environmental freezing level in 6 cases, and low-level mesocyclone strength in 9 cases. Trends of the first two measurands were ambiguous for 4 of 11 cases affected by a ±15 min estimated boundary-position uncertainty. Five out of 11 storms dramatically increased their positive flash rate within 60 min after crossing the outflow boundary. These large positive flash rates were associated with descending reflectivity cores that were larger in magnitude and areal extent compared to other storms in this study.
The local mesoscale environment and its horizontal variations of 0–3-km vertical wind profile, CAPE below the in-cloud freezing level, and boundary layer mixing ratio appeared to greatly influence storm structure and evolution. The observed environmental variations are hypothesized to support changes in charge structure that might lead to the observed changes in flash rate and polarity.
Abstract
The simplified version of the Berry and Reinhardt parameterization used for initiating rain from cloud droplets is presented and is compared with 12 other versions of itself from the literature. Many of the versions that appear to be different from each other can be brought into agreement with the original parameterization by making the same assumptions: a mean diameter based upon mass or volume and distribution shape parameters chosen to give the same cloud mass relative variance as the original Berry and Reinhardt parameterization. However, there are differences in how authors have chosen to parameterize the cloud number concentration sink and rain number concentration source, and those choices, along with model limitations, have important impacts on rain development within the scheme. These differences among versions are shown to have important time-integrated feedbacks upon the developing initial rain distribution. Three of 12 implementations of the bulk scheme are shown to be able to reproduce the original Berry and Reinhardt bin-model solutions very well, and about 6 of 12 do poorly.
Abstract
The simplified version of the Berry and Reinhardt parameterization used for initiating rain from cloud droplets is presented and is compared with 12 other versions of itself from the literature. Many of the versions that appear to be different from each other can be brought into agreement with the original parameterization by making the same assumptions: a mean diameter based upon mass or volume and distribution shape parameters chosen to give the same cloud mass relative variance as the original Berry and Reinhardt parameterization. However, there are differences in how authors have chosen to parameterize the cloud number concentration sink and rain number concentration source, and those choices, along with model limitations, have important impacts on rain development within the scheme. These differences among versions are shown to have important time-integrated feedbacks upon the developing initial rain distribution. Three of 12 implementations of the bulk scheme are shown to be able to reproduce the original Berry and Reinhardt bin-model solutions very well, and about 6 of 12 do poorly.
Abstract
A three-dimensional idealized cloud model was used to study the storm-scale differences between simulated supercells that produce tornado-like vortices and those that do not. Each simulation was initialized with a different Rapid Update Cycle, version 2 (RUC-2), sounding that was associated with tornadic and nontornadic supercells in nature. The focus is an analysis of vorticity along backward-integrated trajectories leading up to tornadogenesis (19 simulations) and tornadogenesis failure (14 simulations). In so doing, the differences between the nontornadic and tornadic cases can be explored in relation to their associated environmental sounding.
Backward-integrated trajectories seeded in the near-surface circulation indicate that the largest differences in vertical vorticity production between the tornadic and nontornadic simulations occur in parcels that descend to the surface from aloft (i.e., descending). Thus, the results from this study support the hypothesis that descending air in the rear of the storm is crucial to tornadogenesis. In the tornadic simulations, the descending parcels experience more negative vertical vorticity production during descent and larger tilting of horizontal vorticity into positive vertical vorticity after reaching the surface, owing to stronger horizontal gradients of vertical velocity. The larger vertical velocities experienced by the trajectories just prior to tornadogenesis in the tornadic simulations are associated with environmental soundings of larger CAPE, smaller convective inhibition (CIN), and larger 0–1-km storm-relative environmental helicity. Furthermore, in contrast with what might be expected from previous works, trajectories entering the incipient tornadic circulations are more negatively buoyant than those entering the nontornadic circulations.
Abstract
A three-dimensional idealized cloud model was used to study the storm-scale differences between simulated supercells that produce tornado-like vortices and those that do not. Each simulation was initialized with a different Rapid Update Cycle, version 2 (RUC-2), sounding that was associated with tornadic and nontornadic supercells in nature. The focus is an analysis of vorticity along backward-integrated trajectories leading up to tornadogenesis (19 simulations) and tornadogenesis failure (14 simulations). In so doing, the differences between the nontornadic and tornadic cases can be explored in relation to their associated environmental sounding.
Backward-integrated trajectories seeded in the near-surface circulation indicate that the largest differences in vertical vorticity production between the tornadic and nontornadic simulations occur in parcels that descend to the surface from aloft (i.e., descending). Thus, the results from this study support the hypothesis that descending air in the rear of the storm is crucial to tornadogenesis. In the tornadic simulations, the descending parcels experience more negative vertical vorticity production during descent and larger tilting of horizontal vorticity into positive vertical vorticity after reaching the surface, owing to stronger horizontal gradients of vertical velocity. The larger vertical velocities experienced by the trajectories just prior to tornadogenesis in the tornadic simulations are associated with environmental soundings of larger CAPE, smaller convective inhibition (CIN), and larger 0–1-km storm-relative environmental helicity. Furthermore, in contrast with what might be expected from previous works, trajectories entering the incipient tornadic circulations are more negatively buoyant than those entering the nontornadic circulations.
Abstract
This note documents the results of more exact parameterizations for continuous-collection growth and evaporation against simpler traditional ones. Although the main focus is on improving research models, the research results also apply to high-resolution forecast models because the use of lookup tables can make the proposed evaporation, terminal velocity, and collection parameterizations as fast as or faster than proposed ones. It is shown that the older method of ignoring oblate-like distortions of shapes in raindrops, truncated at a maximum diameter of 8 mm, gives a solution like that including oblate-like distortions but only because of two large errors that nearly cancel. The biggest differences from the solutions using oblate-like distortions in shape arise from parameterizations that incorporate more exact approximations (e.g., sweep-out diameter) that are not combined with appropriately more exact approximations for other variables dependent on diameter (e.g., terminal velocity).
Abstract
This note documents the results of more exact parameterizations for continuous-collection growth and evaporation against simpler traditional ones. Although the main focus is on improving research models, the research results also apply to high-resolution forecast models because the use of lookup tables can make the proposed evaporation, terminal velocity, and collection parameterizations as fast as or faster than proposed ones. It is shown that the older method of ignoring oblate-like distortions of shapes in raindrops, truncated at a maximum diameter of 8 mm, gives a solution like that including oblate-like distortions but only because of two large errors that nearly cancel. The biggest differences from the solutions using oblate-like distortions in shape arise from parameterizations that incorporate more exact approximations (e.g., sweep-out diameter) that are not combined with appropriately more exact approximations for other variables dependent on diameter (e.g., terminal velocity).
The authors surveyed 55 university departments in the United States and Canada that grant doctor of philosophy and/or master of science degrees in meteorology or the atmospheric sciences. Two-thirds of university departments responded. Survey topics included graduate student income (stipends and health insurance benefits) and mandatory costs (tuition, fees, and health insurance costs) incurred for fall 1996.
Results show that most graduate students do have funding but only one-quarter of departments indicate that health insurance benefits are provided to graduate assistants. The largest mandatory cost is typically housing, which was estimated (except for Canadian schools) with 1996 Fair Market Rent data from the U.S. Department of Housing and Urban Development. For schools not providing it, the second largest cost is typically health insurance. The smallest costs are typically tuition (waived for graduate assistants in most cases) and fees.
The difference between income and mandatory costs over a nine-month period gives an “effective income.” Evidence was found associating greater effective income with larger departments and with locations where housing costs are larger. No significant evidence was found to associate differences in effective income with city size or geographic region. The broad range in effective income between the departments suggests that some graduate programs may be much more affordable than others.
This information can aid university departments in planning budgets that keep them competitive with one another. This paper will also help prospective graduate students by raising awareness about important issues of graduate program affordability.
The authors surveyed 55 university departments in the United States and Canada that grant doctor of philosophy and/or master of science degrees in meteorology or the atmospheric sciences. Two-thirds of university departments responded. Survey topics included graduate student income (stipends and health insurance benefits) and mandatory costs (tuition, fees, and health insurance costs) incurred for fall 1996.
Results show that most graduate students do have funding but only one-quarter of departments indicate that health insurance benefits are provided to graduate assistants. The largest mandatory cost is typically housing, which was estimated (except for Canadian schools) with 1996 Fair Market Rent data from the U.S. Department of Housing and Urban Development. For schools not providing it, the second largest cost is typically health insurance. The smallest costs are typically tuition (waived for graduate assistants in most cases) and fees.
The difference between income and mandatory costs over a nine-month period gives an “effective income.” Evidence was found associating greater effective income with larger departments and with locations where housing costs are larger. No significant evidence was found to associate differences in effective income with city size or geographic region. The broad range in effective income between the departments suggests that some graduate programs may be much more affordable than others.
This information can aid university departments in planning budgets that keep them competitive with one another. This paper will also help prospective graduate students by raising awareness about important issues of graduate program affordability.
Abstract
Idealized simulations using the Weather Research and Forecasting Model (WRF) were performed to examine the role of capping inversions on the near-surface thermodynamic structure of outflow from simulated supercells. Two simulations were performed: one with the traditional noncapped Weisman and Klemp (WK) analytic sounding and the second with a WK sounding modified to contain a capping inversion. Both sounding environments favor splitting storms and a right-moving supercell by 90 min into the simulation. These two supercell simulations evolve in a qualitatively similar fashion, with both storms exhibiting large, quasi-steady updrafts, hook-shaped appendages in the precipitation mixing ratio field, and prominent localized downdrafts.
Results show that the supercell simulated in the capped environment has a surface cold pool with larger values of pseudoequivalent potential temperature (θ ep) than the cold pool of the supercell produced in the noncapped simulation. Parcels in the surface cold pool of the supercell produced in the capped sounding simulation have a lower origin height than those in the surface cold pool of the supercell produced in the noncapped simulation for all times. Although θ ep values in the surface cold pool are primarily associated with the origin height of downdraft parcels and the environmental θ ep at that level, it is shown that nonconservation of θ ep primarily associated with hydrometeor melting can decrease θ ep values of downdraft parcels as they descend by several degrees.
Abstract
Idealized simulations using the Weather Research and Forecasting Model (WRF) were performed to examine the role of capping inversions on the near-surface thermodynamic structure of outflow from simulated supercells. Two simulations were performed: one with the traditional noncapped Weisman and Klemp (WK) analytic sounding and the second with a WK sounding modified to contain a capping inversion. Both sounding environments favor splitting storms and a right-moving supercell by 90 min into the simulation. These two supercell simulations evolve in a qualitatively similar fashion, with both storms exhibiting large, quasi-steady updrafts, hook-shaped appendages in the precipitation mixing ratio field, and prominent localized downdrafts.
Results show that the supercell simulated in the capped environment has a surface cold pool with larger values of pseudoequivalent potential temperature (θ ep) than the cold pool of the supercell produced in the noncapped simulation. Parcels in the surface cold pool of the supercell produced in the capped sounding simulation have a lower origin height than those in the surface cold pool of the supercell produced in the noncapped simulation for all times. Although θ ep values in the surface cold pool are primarily associated with the origin height of downdraft parcels and the environmental θ ep at that level, it is shown that nonconservation of θ ep primarily associated with hydrometeor melting can decrease θ ep values of downdraft parcels as they descend by several degrees.
Abstract
Weisman and Klemp suggested that their liquid-only, deep convective storm experiments should be repeated with a liquid-ice microphysics scheme to determine if the solutions are qualitatively the same. Using a three-dimensional, nonhydrostatic cloud model, such results are compared between three microphysics schemes: the “Kessler” liquid-only scheme (used by Weisman and Klemp), a Lin–Farley–Orville-like scheme with liquid and ice parameterization (Li), and the same Lin–Farley–Orville-like microphysics scheme but with only liquid processes turned on (Lr). Convection is simulated using a single thermodynamic profile and a variety of shear profiles. The shear profiles are represented by five idealized half-circle wind hodographs with arc lengths (U s ) of 20, 25, 30, 40, and 50 m s−1. The precipitation, cold pool characteristics, and storm evolution produced by the different schemes are compared.
The Kessler scheme produces similar accumulated precipitation over 2 h compared to Lr for all shear regimes. Although Kessler's rain evaporation rate is 1.5–1.8 times faster in the lower troposphere, rain production is also faster via accretion and autoconversion of cloud water. In addition, nearly ∼40% more accumulated precipitation occurs in Li compared to Lr. This can be attributed primarily to increased precipitation production rates and enhanced low-level precipitation fluxes in Li for all shear regimes. Differences in the amount of precipitation reaching ground and the low-level cooling rates also cause differences in storm cold pools.
For the U s = 25 shear regime, microphysics cases with colder low-level outflow are shown to be associated with temporarily weaker (Li) or shorter-lived (Kessler) supercells as compared to cases with warmer outflow (Lr). This is consistent with a previous study showing that the cold pool has a greater relative impact on the storm updraft compared to dynamic forcing for weaker shear.
Abstract
Weisman and Klemp suggested that their liquid-only, deep convective storm experiments should be repeated with a liquid-ice microphysics scheme to determine if the solutions are qualitatively the same. Using a three-dimensional, nonhydrostatic cloud model, such results are compared between three microphysics schemes: the “Kessler” liquid-only scheme (used by Weisman and Klemp), a Lin–Farley–Orville-like scheme with liquid and ice parameterization (Li), and the same Lin–Farley–Orville-like microphysics scheme but with only liquid processes turned on (Lr). Convection is simulated using a single thermodynamic profile and a variety of shear profiles. The shear profiles are represented by five idealized half-circle wind hodographs with arc lengths (U s ) of 20, 25, 30, 40, and 50 m s−1. The precipitation, cold pool characteristics, and storm evolution produced by the different schemes are compared.
The Kessler scheme produces similar accumulated precipitation over 2 h compared to Lr for all shear regimes. Although Kessler's rain evaporation rate is 1.5–1.8 times faster in the lower troposphere, rain production is also faster via accretion and autoconversion of cloud water. In addition, nearly ∼40% more accumulated precipitation occurs in Li compared to Lr. This can be attributed primarily to increased precipitation production rates and enhanced low-level precipitation fluxes in Li for all shear regimes. Differences in the amount of precipitation reaching ground and the low-level cooling rates also cause differences in storm cold pools.
For the U s = 25 shear regime, microphysics cases with colder low-level outflow are shown to be associated with temporarily weaker (Li) or shorter-lived (Kessler) supercells as compared to cases with warmer outflow (Lr). This is consistent with a previous study showing that the cold pool has a greater relative impact on the storm updraft compared to dynamic forcing for weaker shear.
