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Abstract
This study investigates the role of the parameterized boundary layer structure in hurricane intensity change using two retrospective HWRF forecasts of Hurricane Earl (2010) in which the vertical eddy diffusivity K m was modified during physics upgrades. Earl undergoes rapid intensification (RI) in the low-Km forecast as observed in nature, while it weakens briefly before resuming a slow intensification at the RI onset in the high-Km forecast. Angular momentum budget analysis suggests that K m modulates the convergence of angular momentum in the boundary layer, which is a key component of the hurricane spinup dynamics. Reducing K m in the boundary layer causes enhancement of both the inflow and convergence, which in turn leads to stronger and more symmetric deep convection in the low-Km forecast than in the high-Km forecast. The deeper and stronger hurricane vortex with lower static stability in the low-Km forecast is more resilient to shear than that in the high-Km forecast. With a smaller vortex tilt in the low-Km forecast, downdrafts associated with the vortex tilt are reduced, bringing less low-entropy air from the midlevels to the boundary layer, resulting in a less stable boundary layer. Future physics upgrades in operational hurricane models should consider this chain of multiscale interactions to assess their impact on model RI forecasts.
Abstract
This study investigates the role of the parameterized boundary layer structure in hurricane intensity change using two retrospective HWRF forecasts of Hurricane Earl (2010) in which the vertical eddy diffusivity K m was modified during physics upgrades. Earl undergoes rapid intensification (RI) in the low-Km forecast as observed in nature, while it weakens briefly before resuming a slow intensification at the RI onset in the high-Km forecast. Angular momentum budget analysis suggests that K m modulates the convergence of angular momentum in the boundary layer, which is a key component of the hurricane spinup dynamics. Reducing K m in the boundary layer causes enhancement of both the inflow and convergence, which in turn leads to stronger and more symmetric deep convection in the low-Km forecast than in the high-Km forecast. The deeper and stronger hurricane vortex with lower static stability in the low-Km forecast is more resilient to shear than that in the high-Km forecast. With a smaller vortex tilt in the low-Km forecast, downdrafts associated with the vortex tilt are reduced, bringing less low-entropy air from the midlevels to the boundary layer, resulting in a less stable boundary layer. Future physics upgrades in operational hurricane models should consider this chain of multiscale interactions to assess their impact on model RI forecasts.
Abstract
A general framework for the trigger function used in convective parameterization routines in mesoscale models is proposed. The framework is based on the diagnosis of the accessibility of potential buoyant energy. Specifically, the trigger function 1) estimates the magnitude of the largest vertical velocity perturbation from a source layer and 2) calculates the total amount of inhibition between the source layer and the level of free convection. The calculation of perturbation magnitude accounts for such factors as subgrid-scale inhomogeneities, a convective boundary layer, and convergence within the source layer. Specific formulations to quantify these factors are proposed.
The trigger is tested in a simulation using the PSU–NCAR mesoscale model MM5. The event chosen for simulation is a summertime case exhibiting a variety of environments. The results of this simulation are compared with a simulation using the Fritsch–Chappell (FC) trigger function. It is found that decisions made by the new trigger function are more physically consistent with the local environment than decisions made by the FC trigger.
Abstract
A general framework for the trigger function used in convective parameterization routines in mesoscale models is proposed. The framework is based on the diagnosis of the accessibility of potential buoyant energy. Specifically, the trigger function 1) estimates the magnitude of the largest vertical velocity perturbation from a source layer and 2) calculates the total amount of inhibition between the source layer and the level of free convection. The calculation of perturbation magnitude accounts for such factors as subgrid-scale inhomogeneities, a convective boundary layer, and convergence within the source layer. Specific formulations to quantify these factors are proposed.
The trigger is tested in a simulation using the PSU–NCAR mesoscale model MM5. The event chosen for simulation is a summertime case exhibiting a variety of environments. The results of this simulation are compared with a simulation using the Fritsch–Chappell (FC) trigger function. It is found that decisions made by the new trigger function are more physically consistent with the local environment than decisions made by the FC trigger.
Abstract
Mesoscale convective vortices (MCVs) are midtropospheric warm-core cyclonic circulations that often develop in the stratiform region of mesoscale convective systems. Typically, divergent, anticyclonically circulating, mesoscale cold anomalies appear both above and below the MCV. The upper-level cold anomaly is usually found near the tropopause while the low-level anomaly is surface based and exhibits locally higher pressure. One aspect of MCVs that has received much attention recently is the role that they may play in tropical cyclogenesis. Of special interest is how an MCV amplifies when deep convection redevelops within the borders of its midlevel cyclonic circulation and how the amplified MCV transforms the divergent surface-based cold pool with anomalously high surface pressure into a convergent cyclonic circulation with anomalously low pressure.
The Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model is used to simulate an MCV that was instrumental in initiating, within the borders of the midlevel vortex’s circulation, several successive cycles of convective development and decay over a 2-day period. After each cycle of convection, both the horizontal size of the cyclonic circulation and the magnitude of the potential vorticity associated with the vortex were observed to increase. The simulation reproduces the development and evolution of the MCV and associated convective cycles. Mesoscale features responsible for the initiation of convection within the circulation of the vortex and the impact of this convection on the structure and evolution of the vortex are investigated. A conceptual model is presented to explain how convective redevelopment within the MCV causes low-level heights to fall and cyclonic vorticity to grow downward to the surface. Applying this conceptual model to a tropical marine environment is also considered.
Abstract
Mesoscale convective vortices (MCVs) are midtropospheric warm-core cyclonic circulations that often develop in the stratiform region of mesoscale convective systems. Typically, divergent, anticyclonically circulating, mesoscale cold anomalies appear both above and below the MCV. The upper-level cold anomaly is usually found near the tropopause while the low-level anomaly is surface based and exhibits locally higher pressure. One aspect of MCVs that has received much attention recently is the role that they may play in tropical cyclogenesis. Of special interest is how an MCV amplifies when deep convection redevelops within the borders of its midlevel cyclonic circulation and how the amplified MCV transforms the divergent surface-based cold pool with anomalously high surface pressure into a convergent cyclonic circulation with anomalously low pressure.
The Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model is used to simulate an MCV that was instrumental in initiating, within the borders of the midlevel vortex’s circulation, several successive cycles of convective development and decay over a 2-day period. After each cycle of convection, both the horizontal size of the cyclonic circulation and the magnitude of the potential vorticity associated with the vortex were observed to increase. The simulation reproduces the development and evolution of the MCV and associated convective cycles. Mesoscale features responsible for the initiation of convection within the circulation of the vortex and the impact of this convection on the structure and evolution of the vortex are investigated. A conceptual model is presented to explain how convective redevelopment within the MCV causes low-level heights to fall and cyclonic vorticity to grow downward to the surface. Applying this conceptual model to a tropical marine environment is also considered.
Abstract
Understanding the structure and evolution of the tropical cyclone (TC) inner core remains an elusive challenge in tropical meteorology, especially the role of transient asymmetric features such as localized strong updrafts known as convective bursts (CBs). This study investigates the formation of CBs and their role in TC structure and evolution using high-resolution simulations of two Atlantic hurricanes (Dean in 2007 and Bill in 2009) with the Weather Research and Forecasting (WRF) Model.
Several different aspects of the dynamics and thermodynamics of the TC inner-core region are investigated with respect to their influence on TC convective burst development. Composites with CBs show stronger radial inflow in the lowest 2 km, and stronger radial outflow from the eye to the eyewall around z = 2–4 km, than composites without CBs. Asymmetric vorticity associated with eyewall mesovortices appears to be a major factor in some of the radial flow anomalies that lead to CB development. The anomalous outflow from these mesovortices, along with outflow from supergradient parcels above the boundary layer, favors low-level convergence and also appears to mix high-θ e air from the eye into the eyewall. Analyses of individual CBs and parcel trajectories show that parcels are pulled into the eye and briefly mix with the eye air. The parcels then rapidly move outward into the eyewall, and quickly ascend in CBs, in some cases with vertical velocities of over 20 m s−1. These results support the importance of horizontal asymmetries in forcing extreme asymmetric vertical velocity in tropical cyclones.
Abstract
Understanding the structure and evolution of the tropical cyclone (TC) inner core remains an elusive challenge in tropical meteorology, especially the role of transient asymmetric features such as localized strong updrafts known as convective bursts (CBs). This study investigates the formation of CBs and their role in TC structure and evolution using high-resolution simulations of two Atlantic hurricanes (Dean in 2007 and Bill in 2009) with the Weather Research and Forecasting (WRF) Model.
Several different aspects of the dynamics and thermodynamics of the TC inner-core region are investigated with respect to their influence on TC convective burst development. Composites with CBs show stronger radial inflow in the lowest 2 km, and stronger radial outflow from the eye to the eyewall around z = 2–4 km, than composites without CBs. Asymmetric vorticity associated with eyewall mesovortices appears to be a major factor in some of the radial flow anomalies that lead to CB development. The anomalous outflow from these mesovortices, along with outflow from supergradient parcels above the boundary layer, favors low-level convergence and also appears to mix high-θ e air from the eye into the eyewall. Analyses of individual CBs and parcel trajectories show that parcels are pulled into the eye and briefly mix with the eye air. The parcels then rapidly move outward into the eyewall, and quickly ascend in CBs, in some cases with vertical velocities of over 20 m s−1. These results support the importance of horizontal asymmetries in forcing extreme asymmetric vertical velocity in tropical cyclones.
Abstract
This paper investigates convective burst (CB) evolution in Weather Research and Forecasting (WRF) Model simulations of two tropical cyclones (TCs), focusing on the relationship between CBs and TC intensity change. Analysis of intensity change in the simulations shows that there are more CBs inside the radius of maximum winds (RMW) during times when the TCs are about to intensify, while weakening/steady times are associated with more CBs outside the RMW, consistent with past observational and theoretical studies. The vertical mass flux distributions show greater vertical mass flux at upper levels both from weaker updrafts and CBs for intensifying cases. The TC simulations are further dissected by past intensity change, and times of sustained intensification have more CBs than times when the TC has been weakening but then intensifies. This result suggests that CB development may not always be predictive of intensification, but rather may occur as a result of ongoing intensification and contribute to ongoing intensification. Abrupt short-term intensification is found to be associated with an even higher density of CBs inside the RMW than is slower intensification. Lag correlations between CBs and intensity reveal a broad peak, with the CBs leading pressure falls by 0–3 h. These relationships are further confirmed by analysis of individual simulation periods, although the relationship can vary depending on environmental conditions and the previous evolution of the TC. These results show that increased convection due to both weak updrafts and CBs inside the RMW is favorable for sustained TC intensification and show many details of the typical short-term response of the TC core to CBs.
Abstract
This paper investigates convective burst (CB) evolution in Weather Research and Forecasting (WRF) Model simulations of two tropical cyclones (TCs), focusing on the relationship between CBs and TC intensity change. Analysis of intensity change in the simulations shows that there are more CBs inside the radius of maximum winds (RMW) during times when the TCs are about to intensify, while weakening/steady times are associated with more CBs outside the RMW, consistent with past observational and theoretical studies. The vertical mass flux distributions show greater vertical mass flux at upper levels both from weaker updrafts and CBs for intensifying cases. The TC simulations are further dissected by past intensity change, and times of sustained intensification have more CBs than times when the TC has been weakening but then intensifies. This result suggests that CB development may not always be predictive of intensification, but rather may occur as a result of ongoing intensification and contribute to ongoing intensification. Abrupt short-term intensification is found to be associated with an even higher density of CBs inside the RMW than is slower intensification. Lag correlations between CBs and intensity reveal a broad peak, with the CBs leading pressure falls by 0–3 h. These relationships are further confirmed by analysis of individual simulation periods, although the relationship can vary depending on environmental conditions and the previous evolution of the TC. These results show that increased convection due to both weak updrafts and CBs inside the RMW is favorable for sustained TC intensification and show many details of the typical short-term response of the TC core to CBs.
Abstract
Airborne Doppler radar data collected in tropical cyclones by National Oceanic and Atmospheric Administration WP-3D aircraft over an 8-yr period (2003–10) are used to statistically analyze the vertical structure of tropical cyclone eyewalls with reference to the deep-layer shear. Convective evolution within the inner core conforms to patterns shown by previous studies: convection initiates downshear right, intensifies downshear left, and weakens upshear. Analysis of the vertical distribution of radar reflectivity and vertical air motion indicates the development of upper-level downdrafts in conjunction with strong convection downshear left and a maximum in frequency upshear left. Intense updrafts and downdrafts both conform to the shear asymmetry pattern. While strong updrafts occur in the eyewall, intense downdrafts show far more radial variability, particularly in the upshear-left quadrant, though they concentrate along the eyewall edges. Strong updrafts are collocated with low-level inflow and upper-level outflow superimposed on the background flow. In contrast, strong downdrafts occur in association with low-level outflow and upper-level inflow.
Abstract
Airborne Doppler radar data collected in tropical cyclones by National Oceanic and Atmospheric Administration WP-3D aircraft over an 8-yr period (2003–10) are used to statistically analyze the vertical structure of tropical cyclone eyewalls with reference to the deep-layer shear. Convective evolution within the inner core conforms to patterns shown by previous studies: convection initiates downshear right, intensifies downshear left, and weakens upshear. Analysis of the vertical distribution of radar reflectivity and vertical air motion indicates the development of upper-level downdrafts in conjunction with strong convection downshear left and a maximum in frequency upshear left. Intense updrafts and downdrafts both conform to the shear asymmetry pattern. While strong updrafts occur in the eyewall, intense downdrafts show far more radial variability, particularly in the upshear-left quadrant, though they concentrate along the eyewall edges. Strong updrafts are collocated with low-level inflow and upper-level outflow superimposed on the background flow. In contrast, strong downdrafts occur in association with low-level outflow and upper-level inflow.
Abstract
The initiation of a rapid intensification (RI) event for a tropical cyclone (TC) at major hurricane intensity is a rare event in the North Atlantic basin. This study examined the environmental and vortex-scale processes related to such an RI event observed in Hurricane Irma (2017) using a combination of flight-level and airborne radar aircraft reconnaissance observations, microwave satellite observations, and model environmental analyses. The onset of RI was linked to an increase in sea surface temperatures and ocean heat content toward levels more commonly associated with North Atlantic RI episodes. Remarkably, Irma’s RI event comprised two rapidly evolving eyewall replacement cycle (ERC) episodes that each completed in less than 12 h. The two ERC events displayed different secondary eyewall formation (SEF) mechanisms and vortex evolutions. During the first SEF event, a secondary maximum in ascent and tangential wind was observed at the leading edge of a mesoscale descending inflow jet. During the ensuing ERC event, the primary eyewall weakened and ultimately collapsed, resulting in a brief period of weakening. The second SEF event displayed characteristics consistent with unbalanced boundary layer dynamics. Additionally, it is plausible that both SEF events were affected by the stagnation and axisymmeterization of outward-propagating vortex Rossby waves. During the second ERC event, the TC continued to rapidly intensify, which is a stark contrast to the ERC paradigm described in the literature. The differing ERC evolutions appear linked to the vortex response to changing environmental conditions. The results presented here underscore the utility of frequent aircraft reconnaissance observations for an improved understanding of TC dynamics.
Abstract
The initiation of a rapid intensification (RI) event for a tropical cyclone (TC) at major hurricane intensity is a rare event in the North Atlantic basin. This study examined the environmental and vortex-scale processes related to such an RI event observed in Hurricane Irma (2017) using a combination of flight-level and airborne radar aircraft reconnaissance observations, microwave satellite observations, and model environmental analyses. The onset of RI was linked to an increase in sea surface temperatures and ocean heat content toward levels more commonly associated with North Atlantic RI episodes. Remarkably, Irma’s RI event comprised two rapidly evolving eyewall replacement cycle (ERC) episodes that each completed in less than 12 h. The two ERC events displayed different secondary eyewall formation (SEF) mechanisms and vortex evolutions. During the first SEF event, a secondary maximum in ascent and tangential wind was observed at the leading edge of a mesoscale descending inflow jet. During the ensuing ERC event, the primary eyewall weakened and ultimately collapsed, resulting in a brief period of weakening. The second SEF event displayed characteristics consistent with unbalanced boundary layer dynamics. Additionally, it is plausible that both SEF events were affected by the stagnation and axisymmeterization of outward-propagating vortex Rossby waves. During the second ERC event, the TC continued to rapidly intensify, which is a stark contrast to the ERC paradigm described in the literature. The differing ERC evolutions appear linked to the vortex response to changing environmental conditions. The results presented here underscore the utility of frequent aircraft reconnaissance observations for an improved understanding of TC dynamics.
Abstract
Prior studies have shown an association between symmetrically distributed precipitation and tropical cyclone (TC) intensification. Although environmental vertical wind shear typically forces an asymmetric precipitation distribution in TCs, the magnitude of this asymmetry can exhibit considerable variability, even among TCs that experience similar shear magnitudes. This observational study examines the thermodynamic and kinematic influences on precipitation symmetry in two such cases: Bertha and Cristobal (2014). Consistent with the impact of the shear, both TCs exhibited a tilted vortex as well as a pronounced azimuthal asymmetry, with the maximum precipitation occurring in the downshear-left quadrant. However, Bertha was characterized by more symmetrically distributed precipitation and relatively modest vertical motions, while Cristobal was characterized by more azimuthally confined precipitation and much more vigorous vertical motions. Observations showed three potential hindrances to precipitation symmetry that were more prevalent in Cristobal than in Bertha: (i) convective downdrafts that transported low entropy air downward into the boundary layer, cooling and stabilizing the lower troposphere downstream in the left-of-shear and upshear quadrants; (ii) subsidence in the upshear quadrants, which acted to increase the temperature and decrease the relative humidity of the midtroposphere, resulting in capping of the boundary layer; and (iii) lateral advection of midtropospheric dry air from the environment, which dried the TC’s upshear quadrants.
Abstract
Prior studies have shown an association between symmetrically distributed precipitation and tropical cyclone (TC) intensification. Although environmental vertical wind shear typically forces an asymmetric precipitation distribution in TCs, the magnitude of this asymmetry can exhibit considerable variability, even among TCs that experience similar shear magnitudes. This observational study examines the thermodynamic and kinematic influences on precipitation symmetry in two such cases: Bertha and Cristobal (2014). Consistent with the impact of the shear, both TCs exhibited a tilted vortex as well as a pronounced azimuthal asymmetry, with the maximum precipitation occurring in the downshear-left quadrant. However, Bertha was characterized by more symmetrically distributed precipitation and relatively modest vertical motions, while Cristobal was characterized by more azimuthally confined precipitation and much more vigorous vertical motions. Observations showed three potential hindrances to precipitation symmetry that were more prevalent in Cristobal than in Bertha: (i) convective downdrafts that transported low entropy air downward into the boundary layer, cooling and stabilizing the lower troposphere downstream in the left-of-shear and upshear quadrants; (ii) subsidence in the upshear quadrants, which acted to increase the temperature and decrease the relative humidity of the midtroposphere, resulting in capping of the boundary layer; and (iii) lateral advection of midtropospheric dry air from the environment, which dried the TC’s upshear quadrants.
Abstract
This study evaluates the impact of the modification of the vertical eddy diffusivity (K m ) in the boundary layer parameterization of the Hurricane Weather Research and Forecasting (HWRF) Model on forecasts of tropical cyclone (TC) rapid intensification (RI). Composites of HWRF forecasts of Hurricanes Earl (2010) and Karl (2010) were compared for two versions of the planetary boundary layer (PBL) scheme in HWRF. The results show that using a smaller value of K m , in better agreement with observations, improves RI forecasts. The composite-mean, inner-core structures for the two sets of runs at the time of RI onset are compared with observational, theoretical, and modeling studies of RI to determine why the runs with reduced K m are more likely to undergo RI. It is found that the forecasts with reduced K m at the RI onset have a shallower boundary layer with stronger inflow, more unstable near-surface air outside the eyewall, stronger and deeper updrafts in regions farther inward from the radius of maximum wind (RMW), and stronger boundary layer convergence closer to the storm center, although the mean storm intensity (as measured by the 10-m winds) is similar for the two groups. Finally, it is found that the departure of the maximum tangential wind from the gradient wind at the eyewall, and the inward advection of angular momentum outside the eyewall, is much larger in the forecasts with reduced K m . This study emphasizes the important role of the boundary layer structure and dynamics in TC intensity change, supporting recent studies emphasizing boundary layer spinup mechanism, and recommends further improvement to the HWRF PBL physics.
Abstract
This study evaluates the impact of the modification of the vertical eddy diffusivity (K m ) in the boundary layer parameterization of the Hurricane Weather Research and Forecasting (HWRF) Model on forecasts of tropical cyclone (TC) rapid intensification (RI). Composites of HWRF forecasts of Hurricanes Earl (2010) and Karl (2010) were compared for two versions of the planetary boundary layer (PBL) scheme in HWRF. The results show that using a smaller value of K m , in better agreement with observations, improves RI forecasts. The composite-mean, inner-core structures for the two sets of runs at the time of RI onset are compared with observational, theoretical, and modeling studies of RI to determine why the runs with reduced K m are more likely to undergo RI. It is found that the forecasts with reduced K m at the RI onset have a shallower boundary layer with stronger inflow, more unstable near-surface air outside the eyewall, stronger and deeper updrafts in regions farther inward from the radius of maximum wind (RMW), and stronger boundary layer convergence closer to the storm center, although the mean storm intensity (as measured by the 10-m winds) is similar for the two groups. Finally, it is found that the departure of the maximum tangential wind from the gradient wind at the eyewall, and the inward advection of angular momentum outside the eyewall, is much larger in the forecasts with reduced K m . This study emphasizes the important role of the boundary layer structure and dynamics in TC intensity change, supporting recent studies emphasizing boundary layer spinup mechanism, and recommends further improvement to the HWRF PBL physics.
