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Abstract
In earlier work, a three-dimensional cloud model was used to simulate the interaction between the sea-breeze front (SBF) and front-parallel horizontal convective rolls (HCRs), resulting in the SBF systematically encountering roll updrafts and downdrafts as it progressed inland. Interestingly, deep convection was spawned above an HCR updraft ahead of the SBF as the front approached, well before the inevitable front–roll merger. Ostensibly, both the sea-breeze and roll circulations were required for deep convection to be present in this case at all because convection was entirely absent when either phenomenon was removed.
Further analysis reveals why both circulations were necessary yet not sufficient for the excitation of deep convection in this case. The sea-breeze circulation (SBC) made its upstream (inland) environment more favorable for convection by bringing about persistent if gentle lifting over an extended region stretching well ahead of the SBF. This persistent ascent established a moist and cool tongue of air, manifested by a visible and/or subvisible cloud feature termed the cloud shelf emanating ahead of the front. Though this lifting moistened and destabilized the environment, the roll’s direct and indirect effects on this moist tongue were also required. The former consisted of a moisture plume lofted by the roll updraft, and the latter consisted of obstacle effect gravity waves generated as the roll drafts penetrated through the top of the boundary layer, into the SBC-associated offshore flow farther aloft. These provided the missing spark, which led to rapid growth of cumulus above the roll updraft, drawing first from air located above the boundary layer.
Once established, deep convection above the roll updraft modulated cloudiness above the approaching SBF, at first suppressing it but subsequently assuring its reestablishment and eventual growth into deep convection, again prior to the front–roll merger. This resulted from the influence of gravity waves excited owing to heating and cooling within the roll cloud.
Abstract
In earlier work, a three-dimensional cloud model was used to simulate the interaction between the sea-breeze front (SBF) and front-parallel horizontal convective rolls (HCRs), resulting in the SBF systematically encountering roll updrafts and downdrafts as it progressed inland. Interestingly, deep convection was spawned above an HCR updraft ahead of the SBF as the front approached, well before the inevitable front–roll merger. Ostensibly, both the sea-breeze and roll circulations were required for deep convection to be present in this case at all because convection was entirely absent when either phenomenon was removed.
Further analysis reveals why both circulations were necessary yet not sufficient for the excitation of deep convection in this case. The sea-breeze circulation (SBC) made its upstream (inland) environment more favorable for convection by bringing about persistent if gentle lifting over an extended region stretching well ahead of the SBF. This persistent ascent established a moist and cool tongue of air, manifested by a visible and/or subvisible cloud feature termed the cloud shelf emanating ahead of the front. Though this lifting moistened and destabilized the environment, the roll’s direct and indirect effects on this moist tongue were also required. The former consisted of a moisture plume lofted by the roll updraft, and the latter consisted of obstacle effect gravity waves generated as the roll drafts penetrated through the top of the boundary layer, into the SBC-associated offshore flow farther aloft. These provided the missing spark, which led to rapid growth of cumulus above the roll updraft, drawing first from air located above the boundary layer.
Once established, deep convection above the roll updraft modulated cloudiness above the approaching SBF, at first suppressing it but subsequently assuring its reestablishment and eventual growth into deep convection, again prior to the front–roll merger. This resulted from the influence of gravity waves excited owing to heating and cooling within the roll cloud.
Abstract
A “consensus clustering” strategy is applied to long-term temperature and precipitation time series data for the purpose of delineating climate zones of the conterminous United States in a “data-driven” (as opposed to “rule-driven”) fashion. Cluster analysis simplifies a dataset by arranging “objects” (here, climate divisions or stations) into a smaller number of relatively homogeneous groups or clusters on the basis of interobject dissimilarities computed using the identified “attributes” (here, temperature and precipitation measurements recorded for the objects). The results demonstrate the spatial scales associated with climatic variability and may suggest climatically justified ways in which the number of objects in a dataset may be reduced. Implicit in this work is the arguable contention that temperature and precipitation data are both necessary and sufficient for the delineation of climatic zones.
In prior work, the temperature and precipitation data were mixed during the computation of the interobject dissimilarities. This allowed the clusters to jointly reflect temperature and precipitation distinctions, but also had inherent problems relating to arbitrary attribute scaling and information redundancy that proved difficult to resolve. In the present approach, the temperature and precipitation data are clustered separately and then categorically intersected to forge consensus clusters. The consensus outcome may be viewed as having identified the temperature subzones of precipitation clusters (or vice versa) or as representing distinct groupings that are relatively homogeneous with respect to both attribute types simultaneously.
The dissimilarity measure employed herein is the Euclidean distance. As it employs only continuous time series data representing a single information type (temperature or precipitation), the consensus approach has the advantage of allowing an attractively simple interpretation of the total Euclidean distance between object pairs. The total squared distance may be subdivided into three components representing object dissimilarity with respect to temporal mean (level), seasonality (variability), and coseasonality (relative temporal phasing). Therefore, concerns about redundancy or arbitrary scaling problems are neutralized. This is seen as the chief advantage of consensus clustering.
The consensus strategy has several disadvantages. It is possible for two (or more) relatively general, undetailed clusterings to produce a very complex and fragmented clustering following categorical intersection. Further, the fact that the analyst chooses the clustering levels of the separate, contributing clusterings means that he or she has considerable freedom in fashioning the consensus outcome, which makes it difficult (if not impossible) to argue that true, “natural” clusters have been identified. The latter often applies to cluster analysis in general, however. It is believed that the consensus approach merits consideration owing to its advantages.
Two consensus outcomes are presented: a lower-order solution with 14 clusters and a higher-order solution with 26 clusters. The sensitivity of these clusterings to perturbations in the input data is assessed. The regionalizations are compared with those presented in prior work.
Abstract
A “consensus clustering” strategy is applied to long-term temperature and precipitation time series data for the purpose of delineating climate zones of the conterminous United States in a “data-driven” (as opposed to “rule-driven”) fashion. Cluster analysis simplifies a dataset by arranging “objects” (here, climate divisions or stations) into a smaller number of relatively homogeneous groups or clusters on the basis of interobject dissimilarities computed using the identified “attributes” (here, temperature and precipitation measurements recorded for the objects). The results demonstrate the spatial scales associated with climatic variability and may suggest climatically justified ways in which the number of objects in a dataset may be reduced. Implicit in this work is the arguable contention that temperature and precipitation data are both necessary and sufficient for the delineation of climatic zones.
In prior work, the temperature and precipitation data were mixed during the computation of the interobject dissimilarities. This allowed the clusters to jointly reflect temperature and precipitation distinctions, but also had inherent problems relating to arbitrary attribute scaling and information redundancy that proved difficult to resolve. In the present approach, the temperature and precipitation data are clustered separately and then categorically intersected to forge consensus clusters. The consensus outcome may be viewed as having identified the temperature subzones of precipitation clusters (or vice versa) or as representing distinct groupings that are relatively homogeneous with respect to both attribute types simultaneously.
The dissimilarity measure employed herein is the Euclidean distance. As it employs only continuous time series data representing a single information type (temperature or precipitation), the consensus approach has the advantage of allowing an attractively simple interpretation of the total Euclidean distance between object pairs. The total squared distance may be subdivided into three components representing object dissimilarity with respect to temporal mean (level), seasonality (variability), and coseasonality (relative temporal phasing). Therefore, concerns about redundancy or arbitrary scaling problems are neutralized. This is seen as the chief advantage of consensus clustering.
The consensus strategy has several disadvantages. It is possible for two (or more) relatively general, undetailed clusterings to produce a very complex and fragmented clustering following categorical intersection. Further, the fact that the analyst chooses the clustering levels of the separate, contributing clusterings means that he or she has considerable freedom in fashioning the consensus outcome, which makes it difficult (if not impossible) to argue that true, “natural” clusters have been identified. The latter often applies to cluster analysis in general, however. It is believed that the consensus approach merits consideration owing to its advantages.
Two consensus outcomes are presented: a lower-order solution with 14 clusters and a higher-order solution with 26 clusters. The sensitivity of these clusterings to perturbations in the input data is assessed. The regionalizations are compared with those presented in prior work.
Abstract
Two-dimensional model simulations were made to gage the effect of the Coriolis force on model squall lines. The case chosen for intensive study had low-to-moderate wind shear confined to low levels. With this wind shear, two Coriolis simulations were made, with and without a geostrophically balanced along-line temperature gradient. Additional simulations were made with other wind shear intensifies to test the sensitivity to low-level shear.
Unlike their nonrotational counterparts, none of the Coriolis model storms were able to attain or maintain a “quasi-equilibrium” state. Quasi-equilibrium storms possess mature phases characterized by essentially statistically steady behavior with respect to storm strength, propagation speed, etc. Instead, the Coriolis storms possessed mature phases marked by gradual but definite decay. These are the first model storms created with the present model sounding and wind profiles that have terminal mature phases due to physically realistic forcings. However, the time scale of the decay, at least in these cases, makes it unlikely that Coriolis forcing is the primary mechanism behind the demise of real long-lived, mature squall line thunderstorms.
In each rotational case, the decay phase was marked by two major temporal trends absent in the mature phase of the nonrotational simulations: the continued contamination of the forward environment with storm-induced subsidence warming and the decline in intensity of the rear inflow current. The subsidence warming was slowly eradicating the convective instability of the air flowing into the storm, and the dissipating inflow current appeared to be at least partially responsible for the progressive collapse of the storm's subcloud cold pool. The accumulation of subsidence warming was clearly injuring the model storm. The role that the declining rear inflow played in the decay phase is less clear and requires additional study.
It was found that the inclusion of the geostrophically balanced along-line temperature gradient had small but measurable consequences in this situation. Warm advection at low levels ahead of the storm worked to negate the effect of warm advection aloft on the convective instability, and cold advection into the cold pool opposed the general decline in pool intensity. The net effect was that the Coriolis-associated mature-phase decaying tendency was stowed somewhat, but not arrested.
Abstract
Two-dimensional model simulations were made to gage the effect of the Coriolis force on model squall lines. The case chosen for intensive study had low-to-moderate wind shear confined to low levels. With this wind shear, two Coriolis simulations were made, with and without a geostrophically balanced along-line temperature gradient. Additional simulations were made with other wind shear intensifies to test the sensitivity to low-level shear.
Unlike their nonrotational counterparts, none of the Coriolis model storms were able to attain or maintain a “quasi-equilibrium” state. Quasi-equilibrium storms possess mature phases characterized by essentially statistically steady behavior with respect to storm strength, propagation speed, etc. Instead, the Coriolis storms possessed mature phases marked by gradual but definite decay. These are the first model storms created with the present model sounding and wind profiles that have terminal mature phases due to physically realistic forcings. However, the time scale of the decay, at least in these cases, makes it unlikely that Coriolis forcing is the primary mechanism behind the demise of real long-lived, mature squall line thunderstorms.
In each rotational case, the decay phase was marked by two major temporal trends absent in the mature phase of the nonrotational simulations: the continued contamination of the forward environment with storm-induced subsidence warming and the decline in intensity of the rear inflow current. The subsidence warming was slowly eradicating the convective instability of the air flowing into the storm, and the dissipating inflow current appeared to be at least partially responsible for the progressive collapse of the storm's subcloud cold pool. The accumulation of subsidence warming was clearly injuring the model storm. The role that the declining rear inflow played in the decay phase is less clear and requires additional study.
It was found that the inclusion of the geostrophically balanced along-line temperature gradient had small but measurable consequences in this situation. Warm advection at low levels ahead of the storm worked to negate the effect of warm advection aloft on the convective instability, and cold advection into the cold pool opposed the general decline in pool intensity. The net effect was that the Coriolis-associated mature-phase decaying tendency was stowed somewhat, but not arrested.
Abstract
A regionalization of the conterminous United States is accomplished using hierarchical cluster analysis on temperature and precipitation data. The “best” combination of clustering method and data preprocessing strategy yields a set of candidate clustering levels, from which the 14-, 25-, and 8-duster solutions are chosen. Collectively, these are termed the “reference clusterings.” At the 14-cluster level, the bulk of the nation is partitioned into four principal climate zones: the Southeast, East Central, Northeastern Tier, and Interior West clusters. Many small clusters are concentrated in the Pacific Northwest. The 25-cluster solution can be used to identify the subzones within the 14 clusters. At that more detailed level, many of the areally more extensive clusters are partitioned into smaller, more internally cohesive subgroups.
The “best” clustering approach is the one that minimizes the influences of three forms of bias-methodological, latent, and information-for the dataset at hand. Sources of, and remedies for, these biases are discussed. Sensitivity tests indicate that some of the clusters in the reference clusterings lack robustness, especially those in the Northeast quadrant of the United States. Some of the tests involve small and large alterations to the data preprocessing strategy.
The major shortcomings of the analysis procedure are that the clusters are unnaturally constrained to he nonoverlapping and also that potentially important data from points outside of the political boundaries of the conterminous United States and over water are not included. Also, other variables that could be important or useful in characterizing climate type could be added to, or used in place of, the temperature and precipitation variables used herein. Further work on data preprocessing techniques is also required. Remedies for these and other shortcomings are proposed.
Abstract
A regionalization of the conterminous United States is accomplished using hierarchical cluster analysis on temperature and precipitation data. The “best” combination of clustering method and data preprocessing strategy yields a set of candidate clustering levels, from which the 14-, 25-, and 8-duster solutions are chosen. Collectively, these are termed the “reference clusterings.” At the 14-cluster level, the bulk of the nation is partitioned into four principal climate zones: the Southeast, East Central, Northeastern Tier, and Interior West clusters. Many small clusters are concentrated in the Pacific Northwest. The 25-cluster solution can be used to identify the subzones within the 14 clusters. At that more detailed level, many of the areally more extensive clusters are partitioned into smaller, more internally cohesive subgroups.
The “best” clustering approach is the one that minimizes the influences of three forms of bias-methodological, latent, and information-for the dataset at hand. Sources of, and remedies for, these biases are discussed. Sensitivity tests indicate that some of the clusters in the reference clusterings lack robustness, especially those in the Northeast quadrant of the United States. Some of the tests involve small and large alterations to the data preprocessing strategy.
The major shortcomings of the analysis procedure are that the clusters are unnaturally constrained to he nonoverlapping and also that potentially important data from points outside of the political boundaries of the conterminous United States and over water are not included. Also, other variables that could be important or useful in characterizing climate type could be added to, or used in place of, the temperature and precipitation variables used herein. Further work on data preprocessing techniques is also required. Remedies for these and other shortcomings are proposed.
Abstract
The authors study herein the convective cell life cycle and the cell generation process in mature, multicellular squall-line storms possessing well-developed subcloud cold pools using two- and three-dimensional models. The multicellular storm establishes new cells on its forward side, in the vicinity of the forced updraft formed at the pool boundary, that first intensify and then decay as they travel rearward within the storm’s upward sloping front-to-rear airflow. The principal effort is expended on the two-dimensional case owing to the strong similarity in basic behavior seen in the two geometries.
The cell life cycle is examined in several complementary fashions. The cells are shown to be convectively active entities that induce local circulations that alternately enhance and suppress the forced updraft, modulating the influx of the potentially warm inflow. This transient circulation also drives the episodic mixing of stable air into the inflow that results in the cell’s ultimate dissipation. The timing of cell regeneration is also examined; an explanation involving two separate and successive phases, each with their own timescales, is proposed. The second of these phases can be shortened if a “convective trigger,” another by-product of the cell’s circulation, is present in the storm’s inflow environment. Sensitivity of the results to strictly numerical model details is also discussed.
Abstract
The authors study herein the convective cell life cycle and the cell generation process in mature, multicellular squall-line storms possessing well-developed subcloud cold pools using two- and three-dimensional models. The multicellular storm establishes new cells on its forward side, in the vicinity of the forced updraft formed at the pool boundary, that first intensify and then decay as they travel rearward within the storm’s upward sloping front-to-rear airflow. The principal effort is expended on the two-dimensional case owing to the strong similarity in basic behavior seen in the two geometries.
The cell life cycle is examined in several complementary fashions. The cells are shown to be convectively active entities that induce local circulations that alternately enhance and suppress the forced updraft, modulating the influx of the potentially warm inflow. This transient circulation also drives the episodic mixing of stable air into the inflow that results in the cell’s ultimate dissipation. The timing of cell regeneration is also examined; an explanation involving two separate and successive phases, each with their own timescales, is proposed. The second of these phases can be shortened if a “convective trigger,” another by-product of the cell’s circulation, is present in the storm’s inflow environment. Sensitivity of the results to strictly numerical model details is also discussed.
Abstract
A three-dimensional, cloud-resolving model is used to investigate the interaction between the sea-breeze circulation and boundary layer roll convection. Horizontal convective rolls (HCRs) develop over land in response to strong daytime surface heating and tend to become aligned parallel to the vertical wind shear vector, whereas the land–sea heating contrast causes the formation of the sea-breeze front (SBF) along the coastline. The ability of HCRs to modulate the along-frontal structure of the SBF is examined, complementing and extending previous observational and numerical studies.
Three simulations are discussed, the first two demonstrating that the model can produce both phenomena independently. The third is initialized with offshore mean flow and vertical shear perpendicular to the coastline, and results in a sharply defined, inland-propagating SBF that encounters HCRs aligned perpendicular to it. Before the interaction takes place, the SBF is nearly two-dimensional and devoid of along-frontal variability. Its subsequent encounter with the HCRs, however, causes enhanced (suppressed) convection at frontal locations where HCR roll updrafts (downdrafts) intersect. The suppressing effect of the roll downdrafts seems particularly striking. The interaction as it relates to vertical and horizontal motion, vorticity, and the cloud field are discussed. In future work, a similar simulation with HCRs oriented parallel to the SBF will be analyzed. These results provide further evidence that HCRs can play an important role in the initiation and modulation of convection along a sea-breeze front.
Abstract
A three-dimensional, cloud-resolving model is used to investigate the interaction between the sea-breeze circulation and boundary layer roll convection. Horizontal convective rolls (HCRs) develop over land in response to strong daytime surface heating and tend to become aligned parallel to the vertical wind shear vector, whereas the land–sea heating contrast causes the formation of the sea-breeze front (SBF) along the coastline. The ability of HCRs to modulate the along-frontal structure of the SBF is examined, complementing and extending previous observational and numerical studies.
Three simulations are discussed, the first two demonstrating that the model can produce both phenomena independently. The third is initialized with offshore mean flow and vertical shear perpendicular to the coastline, and results in a sharply defined, inland-propagating SBF that encounters HCRs aligned perpendicular to it. Before the interaction takes place, the SBF is nearly two-dimensional and devoid of along-frontal variability. Its subsequent encounter with the HCRs, however, causes enhanced (suppressed) convection at frontal locations where HCR roll updrafts (downdrafts) intersect. The suppressing effect of the roll downdrafts seems particularly striking. The interaction as it relates to vertical and horizontal motion, vorticity, and the cloud field are discussed. In future work, a similar simulation with HCRs oriented parallel to the SBF will be analyzed. These results provide further evidence that HCRs can play an important role in the initiation and modulation of convection along a sea-breeze front.
Abstract
A three-dimensional, high-resolution model is employed to examine the interaction between the sea-breeze front (SBF) and horizontal convective rolls (HCRs) aligned parallel to the front. This study extends the perpendicular case that was the focus of Part I. In this situation, the SBF systematically encounters roll downdrafts and updrafts as it propagates inland.
The sea-breeze circulation is found to significantly influence HCR strength and development. In turn, the rolls are found to dramatically modulate the overall convective activity, alternately suppressing and enhancing SBF-associated convection. Suppression occurs as the SBF merges with a roll downdraft. This is in part due to the downdraft's introduction of dry air into the mixed layer that becomes part of the SBF cloud's inflow.
Following suppression, the SBF accelerates as convective heating above the frontal head diminishes. This leads to reinvigorated convection above the front prior to its contact with the next roll updraft, which itself sports a strong, deep cloud of its own by this time. This brings about two strong updrafts obscured by a single, merged cloud shield. During this time, a strong yet brief midlevel downdraft occurs in between the two updrafts; forcing mechanisms for this feature are discussed. The SBF propagation speed also declines significantly during this period; the near-surface portion of the front actually becoming retrograde for a period of a few minutes. Two other, less dramatic roll encounters are also examined.
Abstract
A three-dimensional, high-resolution model is employed to examine the interaction between the sea-breeze front (SBF) and horizontal convective rolls (HCRs) aligned parallel to the front. This study extends the perpendicular case that was the focus of Part I. In this situation, the SBF systematically encounters roll downdrafts and updrafts as it propagates inland.
The sea-breeze circulation is found to significantly influence HCR strength and development. In turn, the rolls are found to dramatically modulate the overall convective activity, alternately suppressing and enhancing SBF-associated convection. Suppression occurs as the SBF merges with a roll downdraft. This is in part due to the downdraft's introduction of dry air into the mixed layer that becomes part of the SBF cloud's inflow.
Following suppression, the SBF accelerates as convective heating above the frontal head diminishes. This leads to reinvigorated convection above the front prior to its contact with the next roll updraft, which itself sports a strong, deep cloud of its own by this time. This brings about two strong updrafts obscured by a single, merged cloud shield. During this time, a strong yet brief midlevel downdraft occurs in between the two updrafts; forcing mechanisms for this feature are discussed. The SBF propagation speed also declines significantly during this period; the near-surface portion of the front actually becoming retrograde for a period of a few minutes. Two other, less dramatic roll encounters are also examined.
Abstract
The “Santa Ana” wind is an offshore flow that affects Southern California periodically during the winter half of the year, typically between September and May. The winds can be locally gusty, particularly in the complex terrain of San Diego County, where the winds have characteristics of downslope windstorms. These winds can cause and/or rapidly spread wildfires, the threat of which is particularly acute during the autumn season before the onset of winter rains. San Diego’s largest fires, including the Cedar fire of 2003 and Witch Creek fire of 2007, occurred during Santa Ana wind events.
A case study of downslope flow during a moderately intense Santa Ana event during mid-February 2013 is presented. Motivated by the need to forecast winds impinging on electrical lines, the authors make use of an exceptionally dense network of near-surface observations in San Diego County to calibrate and verify simulations made utilizing the Advanced Research version of the Weather Research and Forecasting (WRF) Model, which in turn is employed to augment the observations. Results demonstrate that this particular Santa Ana episode consists of two pulses separated by a protracted lull. During the first pulse, the downslope flow is characterized by a prominent hydraulic jumplike feature, while during the second one the flow possesses a clear temporal progression of winds downslope. WRF has skill in capturing the evolution and magnitude of the event at most locations, although most model configurations overpredict the observed sustained wind and the forecast bias is itself biased.
Abstract
The “Santa Ana” wind is an offshore flow that affects Southern California periodically during the winter half of the year, typically between September and May. The winds can be locally gusty, particularly in the complex terrain of San Diego County, where the winds have characteristics of downslope windstorms. These winds can cause and/or rapidly spread wildfires, the threat of which is particularly acute during the autumn season before the onset of winter rains. San Diego’s largest fires, including the Cedar fire of 2003 and Witch Creek fire of 2007, occurred during Santa Ana wind events.
A case study of downslope flow during a moderately intense Santa Ana event during mid-February 2013 is presented. Motivated by the need to forecast winds impinging on electrical lines, the authors make use of an exceptionally dense network of near-surface observations in San Diego County to calibrate and verify simulations made utilizing the Advanced Research version of the Weather Research and Forecasting (WRF) Model, which in turn is employed to augment the observations. Results demonstrate that this particular Santa Ana episode consists of two pulses separated by a protracted lull. During the first pulse, the downslope flow is characterized by a prominent hydraulic jumplike feature, while during the second one the flow possesses a clear temporal progression of winds downslope. WRF has skill in capturing the evolution and magnitude of the event at most locations, although most model configurations overpredict the observed sustained wind and the forecast bias is itself biased.
Abstract
The temporal behavior of mature multicellular model storms, created in an experiment that varied the vertical wind shear layer depth, is examined herein. These storms form new cells at low levels on the storm's forward side, in or near the forced lifting zone at the edge of the evaporationally chilled subcloud cold-air pool. Each moves upward and rearward within the storm as it intensifies, matures, and decays and becomes replaced by a new cell development. As a result, the storms oscillate in time with respect to updraft intensity and the generation of condensation and surface rainfall.
A few model storms oscillate in a simply periodic fashion during maturity, generating a series of nearly identical cells separated by a nearly constant period. Other storms are still periodic but in a more complex fashion, manifesting repeat cycles consisting of two or more cells. Several simulations appear quite aperiodic. Spectral analyses of temporal statistics reveal the existence of a fundamental period of oscillation in every (simple or complex) periodic case. Further, this period varies little among the simulations in the present experiment and averages about 15 minutes, a realistic cell production period according to observations. In this paper, the authors examine the various modes of mature storm behavior, laying the foundation for a discussion of the forcings and factors responsible for determining the period and temporal behavior of multicell-type storms to come in future work.
Abstract
The temporal behavior of mature multicellular model storms, created in an experiment that varied the vertical wind shear layer depth, is examined herein. These storms form new cells at low levels on the storm's forward side, in or near the forced lifting zone at the edge of the evaporationally chilled subcloud cold-air pool. Each moves upward and rearward within the storm as it intensifies, matures, and decays and becomes replaced by a new cell development. As a result, the storms oscillate in time with respect to updraft intensity and the generation of condensation and surface rainfall.
A few model storms oscillate in a simply periodic fashion during maturity, generating a series of nearly identical cells separated by a nearly constant period. Other storms are still periodic but in a more complex fashion, manifesting repeat cycles consisting of two or more cells. Several simulations appear quite aperiodic. Spectral analyses of temporal statistics reveal the existence of a fundamental period of oscillation in every (simple or complex) periodic case. Further, this period varies little among the simulations in the present experiment and averages about 15 minutes, a realistic cell production period according to observations. In this paper, the authors examine the various modes of mature storm behavior, laying the foundation for a discussion of the forcings and factors responsible for determining the period and temporal behavior of multicell-type storms to come in future work.
Abstract
A strictly two-dimensional cloud model was used to gauge the effect of vertical wind shear on the mature phase behavior of model-simulated multicellular storms, extending the previous work of the authors. We specifically examined the propagation speed, quasi-equilibrium behavior, storm scale and updraft orientation of the model storms as a function of shear intensity. We also considered the precipitation efficiencies of our. model storms and applied density current and Rotunno–Klemp–Weisman theories to our results.
Our previous work revealed that model storms could achieve a mature phase consisting of repetitive multicellular development when certain numerical obstacles were overcome. This was referred to as a “quasi-equilibrium state.” We found herein that this state was also reached by model storms even when subjected to a very wide range of low-level wind shear intensities, although the temporal behavior during this stage was clearly dependent on the shear. We also found a very systematic relationship between the storm speed and the shear strength. Therefore, small shear values produced slowly moving storms which generally exhibited simple oscillations with time, fitting the classic multicell model. Larger shears resulted in complex oscillations similar to what has been termed “weak evolution,” culminating in a nearly unicellular storm in the most extreme case.
The transition between the strongly and weakly evolving modes was abrupt in the wind shear spectrum, and the temporal behavior of the precipitation production was quite different between the two regimes. Yet, we also found that the precipitation efficiencies of these model storms were roughly constant among the simulations, irrespective of the low-level shear. The larger shear storms typically produced more precipitation, because they were processing water vapor at faster rates due to their more rapid propagation speeds, but were neither identifiably more nor less efficient in doing so. The rear inflow current feature, present in each case, appeared to play a major role in creating the colder subcloud cold pools which helped the storms formed in larger shear to move faster.
An important result is that none of the model storms suffered a terminal decaying phase, certainly not within a reasonable period of time. This suggests that the storm itself does not sow the seeds of its own demise, at least for the favorable, homogeneous environmental conditions considered and the simple, strictly two-dimensional framework adopted for this study.
Abstract
A strictly two-dimensional cloud model was used to gauge the effect of vertical wind shear on the mature phase behavior of model-simulated multicellular storms, extending the previous work of the authors. We specifically examined the propagation speed, quasi-equilibrium behavior, storm scale and updraft orientation of the model storms as a function of shear intensity. We also considered the precipitation efficiencies of our. model storms and applied density current and Rotunno–Klemp–Weisman theories to our results.
Our previous work revealed that model storms could achieve a mature phase consisting of repetitive multicellular development when certain numerical obstacles were overcome. This was referred to as a “quasi-equilibrium state.” We found herein that this state was also reached by model storms even when subjected to a very wide range of low-level wind shear intensities, although the temporal behavior during this stage was clearly dependent on the shear. We also found a very systematic relationship between the storm speed and the shear strength. Therefore, small shear values produced slowly moving storms which generally exhibited simple oscillations with time, fitting the classic multicell model. Larger shears resulted in complex oscillations similar to what has been termed “weak evolution,” culminating in a nearly unicellular storm in the most extreme case.
The transition between the strongly and weakly evolving modes was abrupt in the wind shear spectrum, and the temporal behavior of the precipitation production was quite different between the two regimes. Yet, we also found that the precipitation efficiencies of these model storms were roughly constant among the simulations, irrespective of the low-level shear. The larger shear storms typically produced more precipitation, because they were processing water vapor at faster rates due to their more rapid propagation speeds, but were neither identifiably more nor less efficient in doing so. The rear inflow current feature, present in each case, appeared to play a major role in creating the colder subcloud cold pools which helped the storms formed in larger shear to move faster.
An important result is that none of the model storms suffered a terminal decaying phase, certainly not within a reasonable period of time. This suggests that the storm itself does not sow the seeds of its own demise, at least for the favorable, homogeneous environmental conditions considered and the simple, strictly two-dimensional framework adopted for this study.
