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- Author or Editor: Peter S. Dailey x
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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 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
In the recent literature, considerable attention has been paid to the relationship between climate signals and tropical cyclone activity. Much of the research has focused on Atlantic Ocean basin activity while less attention has been given to landfall frequency and the geographic distribution of risk to life and property. However, recent active seasons like 2004 and 2005 and the resulting damage and economic loss have generated significant interest in the relationship between climate and landfall risk. This study focuses on sea surface temperatures (SST) and examines modulation of landfall activity occurring in anomalously warm-SST seasons. The objective of the study is to evaluate the effect of warmer ocean conditions on U.S. landfall risk. The study is broken into two parts–—statistical and physical. The statistical analysis categorizes historical hurricane seasons as either warm or cool and then estimates shifts in landfall frequency under these two climate modes. The analysis is carried out for overall U.S. landfall risk and then for logical subregions along the U.S. coastline. The climatological behavior for warm-SST conditions is developed across the intensity spectrum, from weak tropical storms to major hurricanes, using wind speed as an intensity measure. The analysis suggests that landfall risk is sensitive to SST conditions but that sensitivity varies by region and intensity. The uncertainty associated with these estimates is discussed. The physical analysis is carried out to understand better why landfall risk is not affected uniformly along the U.S. coastline and to reinforce the reasonability of the statistical results. The study involves a detailed examination of the complete life cycle of historical storms. Results indicate that storms making landfall along the East Coast have different genesis and intensification characteristics relative to storms making landfall along the Gulf Coast. As SSTs warm, the genesis pattern shifts, greatly influencing regional landfall risk. Further, hurricane landfalls may react not only to warm-SST conditions, but also to the effect of ocean temperature anomalies on the atmosphere’s general circulation. There are implications that complex feedback mechanisms play a role in modulating the probability of landfall, especially from certain parts of the Atlantic basin. Such physical theories provide added confidence in statistical estimates of elevated risk for certain breeds of tropical cyclones.
Abstract
In the recent literature, considerable attention has been paid to the relationship between climate signals and tropical cyclone activity. Much of the research has focused on Atlantic Ocean basin activity while less attention has been given to landfall frequency and the geographic distribution of risk to life and property. However, recent active seasons like 2004 and 2005 and the resulting damage and economic loss have generated significant interest in the relationship between climate and landfall risk. This study focuses on sea surface temperatures (SST) and examines modulation of landfall activity occurring in anomalously warm-SST seasons. The objective of the study is to evaluate the effect of warmer ocean conditions on U.S. landfall risk. The study is broken into two parts–—statistical and physical. The statistical analysis categorizes historical hurricane seasons as either warm or cool and then estimates shifts in landfall frequency under these two climate modes. The analysis is carried out for overall U.S. landfall risk and then for logical subregions along the U.S. coastline. The climatological behavior for warm-SST conditions is developed across the intensity spectrum, from weak tropical storms to major hurricanes, using wind speed as an intensity measure. The analysis suggests that landfall risk is sensitive to SST conditions but that sensitivity varies by region and intensity. The uncertainty associated with these estimates is discussed. The physical analysis is carried out to understand better why landfall risk is not affected uniformly along the U.S. coastline and to reinforce the reasonability of the statistical results. The study involves a detailed examination of the complete life cycle of historical storms. Results indicate that storms making landfall along the East Coast have different genesis and intensification characteristics relative to storms making landfall along the Gulf Coast. As SSTs warm, the genesis pattern shifts, greatly influencing regional landfall risk. Further, hurricane landfalls may react not only to warm-SST conditions, but also to the effect of ocean temperature anomalies on the atmosphere’s general circulation. There are implications that complex feedback mechanisms play a role in modulating the probability of landfall, especially from certain parts of the Atlantic basin. Such physical theories provide added confidence in statistical estimates of elevated risk for certain breeds of tropical cyclones.
