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Abstract
A Queen Air, instrumented to make 1-Hz measurements of the kinematic, dynamic, and thermodynamic fields, and radar, mesonet, and soundings from the Cooperative Convective Precipitation Experiment 1981 is used to monitor the evolution of the updraft at cloud base of a large cumulus congestus over the High Plains. The environment is characterized by modest instability and strong horizontal wind shear. Twelve passes completed by the Queen Air just below cloud base from the late growth to the dissipation stage reveal that the main updraft splits into two with the south updraft rotating cyclonically and moving to the right of the mean winds. This cell is associated with a pressure perturbation in excess of 1 mb that is most likely caused by the interaction of the updraft with the shear of the horizontal wind. Saturation-point analyses of the updraft and the subcloud layer demonstrate that in the early stages air from near the surface ascended into the cloud, but as the cloud ages, air from the upper subcloud and transition layers contributes to the updraft. This air has little or no buoyancy, which loads to cloud collapse. Mass flux and saturation-point analyses predict the cloud's demise adequately, in contrast to the trends of vertical velocity, virtual potential temperature, or moisture at cloud base. A pressure perturbation caused by updraft–shear interaction is an important mechanism for cloud intensification, but it must act in concert with another forcing mechanism, typically a gust front, to tap the most unstable air found in the lower subcloud layer in the High Plains.
The observations support the numerical simulations of cumulonimbi in the presence of strong shear, albeit for a much smaller cloud. Congestus clouds merit attention as they are suitable targets for a variety of platforms and will lead to a more complete understanding of the convective cloud spectrum.
Abstract
A Queen Air, instrumented to make 1-Hz measurements of the kinematic, dynamic, and thermodynamic fields, and radar, mesonet, and soundings from the Cooperative Convective Precipitation Experiment 1981 is used to monitor the evolution of the updraft at cloud base of a large cumulus congestus over the High Plains. The environment is characterized by modest instability and strong horizontal wind shear. Twelve passes completed by the Queen Air just below cloud base from the late growth to the dissipation stage reveal that the main updraft splits into two with the south updraft rotating cyclonically and moving to the right of the mean winds. This cell is associated with a pressure perturbation in excess of 1 mb that is most likely caused by the interaction of the updraft with the shear of the horizontal wind. Saturation-point analyses of the updraft and the subcloud layer demonstrate that in the early stages air from near the surface ascended into the cloud, but as the cloud ages, air from the upper subcloud and transition layers contributes to the updraft. This air has little or no buoyancy, which loads to cloud collapse. Mass flux and saturation-point analyses predict the cloud's demise adequately, in contrast to the trends of vertical velocity, virtual potential temperature, or moisture at cloud base. A pressure perturbation caused by updraft–shear interaction is an important mechanism for cloud intensification, but it must act in concert with another forcing mechanism, typically a gust front, to tap the most unstable air found in the lower subcloud layer in the High Plains.
The observations support the numerical simulations of cumulonimbi in the presence of strong shear, albeit for a much smaller cloud. Congestus clouds merit attention as they are suitable targets for a variety of platforms and will lead to a more complete understanding of the convective cloud spectrum.
Abstract
The global positioning system dropwindsondes deployed in Hurricane Bonnie on 26 August 1998 with supporting deployments in Hurricanes Mitch (1998) and Humberto (2001) are used to identify three unusual thermodynamic structures in the lower-cloud and subcloud layers. Two of these structures impact the energy content of the inflow and therefore the intensity of the hurricane. First, positive lapse rates of equivalent potential temperature are found near the top of the inflow. These layers insulate the inflow from the negative impacts of entrainment mixing and promote rapid energy increases, especially near the eyewall. The second structure is a rapid decrease of equivalent potential temperature adjacent to the sea surface. This is essentially a prominent surface layer that owes its existence to both higher moisture content and a superadiabatic lapse rate. The steep lapse rate most often occurs under and near the eyewall where wind speeds at the surface exceed hurricane force. The author speculates that water loading from spray increases the residence time of air parcels in the surface layer, contributing to the creation of this structure. The third feature is a moist absolutely unstable layer previously identified by Bryan and Fritsch for the midlatitudes. These layers are found adjacent to the eyewall, in rainbands, and in the hub cloud within the eye and are evidence of mesoscale or vortex-scale convergence and the very modest instabilities often found in the core of a hurricane.
Abstract
The global positioning system dropwindsondes deployed in Hurricane Bonnie on 26 August 1998 with supporting deployments in Hurricanes Mitch (1998) and Humberto (2001) are used to identify three unusual thermodynamic structures in the lower-cloud and subcloud layers. Two of these structures impact the energy content of the inflow and therefore the intensity of the hurricane. First, positive lapse rates of equivalent potential temperature are found near the top of the inflow. These layers insulate the inflow from the negative impacts of entrainment mixing and promote rapid energy increases, especially near the eyewall. The second structure is a rapid decrease of equivalent potential temperature adjacent to the sea surface. This is essentially a prominent surface layer that owes its existence to both higher moisture content and a superadiabatic lapse rate. The steep lapse rate most often occurs under and near the eyewall where wind speeds at the surface exceed hurricane force. The author speculates that water loading from spray increases the residence time of air parcels in the surface layer, contributing to the creation of this structure. The third feature is a moist absolutely unstable layer previously identified by Bryan and Fritsch for the midlatitudes. These layers are found adjacent to the eyewall, in rainbands, and in the hub cloud within the eye and are evidence of mesoscale or vortex-scale convergence and the very modest instabilities often found in the core of a hurricane.
Abstract
Eye and eyewall traits were ascertained for 209 images from 37 tropical cyclones (TCs) using the lower-fuselage 5.6-cm radar, aboard the two National Oceanic and Atmospheric Administration WP-3Ds. These TCs were almost entirely from the Atlantic basin and were sampled from 1997 to 2012. For the eye these traits included area, maximum diameter, and roundness; for the eyewall the traits included area, completeness, maximum width, maximum reflectivity value and location, number of local reflectivity maxima, and mean rain rate. These variables were compared to TC intensity and motion characteristics from the best-track dataset, and environmental characteristics from the Statistical Hurricane Intensity Prediction Scheme.
Interrelationships between eyewall features revealed that eyewall reflectivity features became more homogeneous as eye and eyewall areas shrank, and maximum reflectivity and rain rate increased as the eyewall became wider and more complete. As the TC intensified, the eye area decreased, while the eyewall area increased due to increasing completeness and width. Rain rate was also found to be higher for faster-moving TCs. Stronger vertical shear of the horizontal wind was found to be associated with more asymmetric eyewall reflectivity. The maximum reflectivity value occurred most often on the downshear side of the eyewall, and to the right of the storm motion, verifying prior research. There were no relationships found between the reflectivity and sea surface temperature or environmental relative humidity. A schematic incorporating typical eye and eyewall traits is presented.
Abstract
Eye and eyewall traits were ascertained for 209 images from 37 tropical cyclones (TCs) using the lower-fuselage 5.6-cm radar, aboard the two National Oceanic and Atmospheric Administration WP-3Ds. These TCs were almost entirely from the Atlantic basin and were sampled from 1997 to 2012. For the eye these traits included area, maximum diameter, and roundness; for the eyewall the traits included area, completeness, maximum width, maximum reflectivity value and location, number of local reflectivity maxima, and mean rain rate. These variables were compared to TC intensity and motion characteristics from the best-track dataset, and environmental characteristics from the Statistical Hurricane Intensity Prediction Scheme.
Interrelationships between eyewall features revealed that eyewall reflectivity features became more homogeneous as eye and eyewall areas shrank, and maximum reflectivity and rain rate increased as the eyewall became wider and more complete. As the TC intensified, the eye area decreased, while the eyewall area increased due to increasing completeness and width. Rain rate was also found to be higher for faster-moving TCs. Stronger vertical shear of the horizontal wind was found to be associated with more asymmetric eyewall reflectivity. The maximum reflectivity value occurred most often on the downshear side of the eyewall, and to the right of the storm motion, verifying prior research. There were no relationships found between the reflectivity and sea surface temperature or environmental relative humidity. A schematic incorporating typical eye and eyewall traits is presented.
Abstract
On 10 February 1993, during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment, the two NOAA WP-3Ds and the NCAR Electra flew under the anvil region of a decaying mesoscale convective system (MCS). Satellite and radar observations show that the MCS had a lifetime of 16 h and deep convection had essentially ceased along the leading edge at the time of sampling. The two NOAA aircraft flew at 35-m altitude over a 150 km × 300 km area to map conditions in the wake of this MCS and estimate the fluxes at the air–sea interface using the bulk aerodynamic approximation. A 20000-km2 area of divergence greater than 5 × 10−5 s−1 characterizes the wake, which is 2°C cooler and 0.5 g kg−1 drier than the environment. Sensible heat fluxes are three times greater and latent fluxes are double that found in the nearby undisturbed environment. These higher fluxes are to the east of the divergence center as a result of the superposition of the MCS fields on the westerly flow. Mixed-layer heights are suppressed and a surface mesolow (0.5 mb) is found more than 100 km behind the leading edge. SST is depressed nearly 0.4°C and is coincident with a divergent stress field and the coolest air in the wake. These fields demonstrate that MCSs stabilize the atmosphere at a given location long after the convective cells have either decayed or moved away. Highest surface fluxes occur in response to convective and mesoscale modification.
Abstract
On 10 February 1993, during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment, the two NOAA WP-3Ds and the NCAR Electra flew under the anvil region of a decaying mesoscale convective system (MCS). Satellite and radar observations show that the MCS had a lifetime of 16 h and deep convection had essentially ceased along the leading edge at the time of sampling. The two NOAA aircraft flew at 35-m altitude over a 150 km × 300 km area to map conditions in the wake of this MCS and estimate the fluxes at the air–sea interface using the bulk aerodynamic approximation. A 20000-km2 area of divergence greater than 5 × 10−5 s−1 characterizes the wake, which is 2°C cooler and 0.5 g kg−1 drier than the environment. Sensible heat fluxes are three times greater and latent fluxes are double that found in the nearby undisturbed environment. These higher fluxes are to the east of the divergence center as a result of the superposition of the MCS fields on the westerly flow. Mixed-layer heights are suppressed and a surface mesolow (0.5 mb) is found more than 100 km behind the leading edge. SST is depressed nearly 0.4°C and is coincident with a divergent stress field and the coolest air in the wake. These fields demonstrate that MCSs stabilize the atmosphere at a given location long after the convective cells have either decayed or moved away. Highest surface fluxes occur in response to convective and mesoscale modification.
Abstract
On 12 September 1988 the two NOAA WP-3D aircraft conducted an experiment in and around an intense, outer rainband located 175 km southeast of the center of Hurricane Gilbert. Radial-height cross sections along a constant azimuth reveal a rapid and an exceptionally large increase of the equivalent potential temperature θ e of the inflow but in a region radially outward from the rainband. Kinematic analyses that incorporate both in situ and pseudo-dual-Doppler data illustrate that the inflow is only 2 km deep and strongly divergent prior to reaching the convective core of the band. The Doppler-derived wind fields, which compare favorably with the in situ wind fields, demonstrate that there is a radially outward or return flow directly above the inflow. Soundings show that this return flow is unusually moist despite being dominated by mesoscale descent, which contrasts the dry conditions found under the anvil of virtually all tropical mesoscale convective systems.
A one-dimensional general structure entrainment model of the inflow layer, initialized with a wind field derived from the pseudo-dual-Doppler analysis, demonstrates that the overlying return flow adds substantial energy to the inflow via entrainment. The placement of this high-θ e layer directly above the inflow is due to the circulation associated with the rainband. Low convective available potential energy, high shear of the radial wind, and a weak cold outflow at the surface are factors that help produce the shallow return flow. The analyses demonstrate that significant spatial variations of the flux divergence of heat and moisture exist in the inflow to a tropical cyclone, the variations are closely related to the secondary circulations produced by convectively active rainbands, and these variations produce significant asymmetric of θ e within the inflow. Rainbands of this type have thermodynamic characteristics similar to an eyewall and may be the type of rainband that evolves into a convective ring.
Abstract
On 12 September 1988 the two NOAA WP-3D aircraft conducted an experiment in and around an intense, outer rainband located 175 km southeast of the center of Hurricane Gilbert. Radial-height cross sections along a constant azimuth reveal a rapid and an exceptionally large increase of the equivalent potential temperature θ e of the inflow but in a region radially outward from the rainband. Kinematic analyses that incorporate both in situ and pseudo-dual-Doppler data illustrate that the inflow is only 2 km deep and strongly divergent prior to reaching the convective core of the band. The Doppler-derived wind fields, which compare favorably with the in situ wind fields, demonstrate that there is a radially outward or return flow directly above the inflow. Soundings show that this return flow is unusually moist despite being dominated by mesoscale descent, which contrasts the dry conditions found under the anvil of virtually all tropical mesoscale convective systems.
A one-dimensional general structure entrainment model of the inflow layer, initialized with a wind field derived from the pseudo-dual-Doppler analysis, demonstrates that the overlying return flow adds substantial energy to the inflow via entrainment. The placement of this high-θ e layer directly above the inflow is due to the circulation associated with the rainband. Low convective available potential energy, high shear of the radial wind, and a weak cold outflow at the surface are factors that help produce the shallow return flow. The analyses demonstrate that significant spatial variations of the flux divergence of heat and moisture exist in the inflow to a tropical cyclone, the variations are closely related to the secondary circulations produced by convectively active rainbands, and these variations produce significant asymmetric of θ e within the inflow. Rainbands of this type have thermodynamic characteristics similar to an eyewall and may be the type of rainband that evolves into a convective ring.
Abstract
Observations in the boundary layer by the NOAA AOC WP-3D aircraft from 8 to 10 October 1985 document the development of a second vortex, which evolves into the circulation center for Tropical Storm Isabel. The new circulation develops just outside the radius of maximum winds and is associated with intensifying convection 90 km from the original center. The original center loses its identity as convection dissipates around it.
Low surface pressure, warm, dry air, and low equivalent potential temperature are found in the new center near its formation time. The new center is found beneath the downwind anvil of the intense convection in the rainband and appears to form over a period of 3–6 h, although significant changes in the storm-scale airflow north of the original center are occurring over the proceeding 24 h. The new center moves with a speed and direction similar to that of the original center. The observations of Isabel are compared to beat bursts, subsidence, and midlevel mesovortices that have been observed in tropical and midlatitude mesoscale convective systems. It is hypothesized that subsidence warming beneath the anvil, in the appropriate environment, could lower the pressure by several millibars and serve as an incipient perturbation for a tropical cyclone.
Abstract
Observations in the boundary layer by the NOAA AOC WP-3D aircraft from 8 to 10 October 1985 document the development of a second vortex, which evolves into the circulation center for Tropical Storm Isabel. The new circulation develops just outside the radius of maximum winds and is associated with intensifying convection 90 km from the original center. The original center loses its identity as convection dissipates around it.
Low surface pressure, warm, dry air, and low equivalent potential temperature are found in the new center near its formation time. The new center is found beneath the downwind anvil of the intense convection in the rainband and appears to form over a period of 3–6 h, although significant changes in the storm-scale airflow north of the original center are occurring over the proceeding 24 h. The new center moves with a speed and direction similar to that of the original center. The observations of Isabel are compared to beat bursts, subsidence, and midlevel mesovortices that have been observed in tropical and midlatitude mesoscale convective systems. It is hypothesized that subsidence warming beneath the anvil, in the appropriate environment, could lower the pressure by several millibars and serve as an incipient perturbation for a tropical cyclone.
Abstract
The thermodynamic modification of the subcloud layer in the GATE area is shown to be a function of precipitating convection. A critical rate of 2 mm h−1, based on the Z – R relationship, in conjunction with 4 km × 4 km scale 15 min mean radar maps, distinguishes between evaporation of precipitation in the subcloud layer (no change in moist static energy h) and vertical mass transport associated with penetrative downdrafts (decreases in h) into this layer from near and above cloud base. The spatial extent of the outflow of the active downdrafts is limited to a convective-mesoscale area directly under and as much as 15 km downwind of the precipitation causing the change. A more extensive wake region occurs on the upwind side of the precipitating region.
The initial thermodynamic environment directly affects energy transport per unit mass by moist convection. Precipitating cells which operate upon an initially undisturbed atmosphere cause a net transfer of 60% more energy per unit mass than those convective clouds which operate upon regions previously modified by precipitation and downdrafts. Results suggest that large, linearly shaped, moving cloud lines are the centers of the most efficient energy transfer per unit mass.
Abstract
The thermodynamic modification of the subcloud layer in the GATE area is shown to be a function of precipitating convection. A critical rate of 2 mm h−1, based on the Z – R relationship, in conjunction with 4 km × 4 km scale 15 min mean radar maps, distinguishes between evaporation of precipitation in the subcloud layer (no change in moist static energy h) and vertical mass transport associated with penetrative downdrafts (decreases in h) into this layer from near and above cloud base. The spatial extent of the outflow of the active downdrafts is limited to a convective-mesoscale area directly under and as much as 15 km downwind of the precipitation causing the change. A more extensive wake region occurs on the upwind side of the precipitating region.
The initial thermodynamic environment directly affects energy transport per unit mass by moist convection. Precipitating cells which operate upon an initially undisturbed atmosphere cause a net transfer of 60% more energy per unit mass than those convective clouds which operate upon regions previously modified by precipitation and downdrafts. Results suggest that large, linearly shaped, moving cloud lines are the centers of the most efficient energy transfer per unit mass.
Abstract
From 0600 UTC 2 August to 1200 UTC 3 August Hurricane Guillermo (1997) deepened by 54 hPa over the eastern North Pacific Ocean, easily exceeding the thresholds that define rapid intensification (RI). The NOAA WP-3Ds observed a portion of this RI with similar two-aircraft missions on consecutive days. The aircraft jettisoned 70 successful global positioning system (GPS) dropwindsondes (or GPS sondes), which reveal how conditions in the lower troposphere on the octant to quadrant scale evolved within 250 km of the eye. Reflectivity fields demonstrate that the deepening is correlated with a spiraling in of the northern eyewall that reduces the eye diameter by 10 km. This behavior contrasts the more uniform contraction witnessed during eyewall replacement cycles. Mixing between the lower eye and eyewall, as detailed by other investigators, appears to have triggered the reduction in the eye diameter. After RI the eyewall remains asymmetrical with the tallest echo tops and heaviest rain rates located on the east or trailing side of the hurricane and to the left of the deep-layer shear vector. Net latent heat release within 60 km of the circulation center increases 21% from 2 to 3 August and is matched by a 30% increase in the inflow below 2 km at the 100-km radius. The GPS sondes, combined with aircraft in situ data for the eyewall region, reveal that the tropical cyclone (TC) establishes an annulus adjacent to and under the eyewall where the tangential wind component and equivalent potential temperature increase substantially. The radial extent of this annulus is constrained by the rainbands that remain robust throughout RI. The results support the argument that RI is controlled by processes within 100 km of the circulation center, and in particular within the eyewall.
Abstract
From 0600 UTC 2 August to 1200 UTC 3 August Hurricane Guillermo (1997) deepened by 54 hPa over the eastern North Pacific Ocean, easily exceeding the thresholds that define rapid intensification (RI). The NOAA WP-3Ds observed a portion of this RI with similar two-aircraft missions on consecutive days. The aircraft jettisoned 70 successful global positioning system (GPS) dropwindsondes (or GPS sondes), which reveal how conditions in the lower troposphere on the octant to quadrant scale evolved within 250 km of the eye. Reflectivity fields demonstrate that the deepening is correlated with a spiraling in of the northern eyewall that reduces the eye diameter by 10 km. This behavior contrasts the more uniform contraction witnessed during eyewall replacement cycles. Mixing between the lower eye and eyewall, as detailed by other investigators, appears to have triggered the reduction in the eye diameter. After RI the eyewall remains asymmetrical with the tallest echo tops and heaviest rain rates located on the east or trailing side of the hurricane and to the left of the deep-layer shear vector. Net latent heat release within 60 km of the circulation center increases 21% from 2 to 3 August and is matched by a 30% increase in the inflow below 2 km at the 100-km radius. The GPS sondes, combined with aircraft in situ data for the eyewall region, reveal that the tropical cyclone (TC) establishes an annulus adjacent to and under the eyewall where the tangential wind component and equivalent potential temperature increase substantially. The radial extent of this annulus is constrained by the rainbands that remain robust throughout RI. The results support the argument that RI is controlled by processes within 100 km of the circulation center, and in particular within the eyewall.
Abstract
During 11 h on 26 August 1998, two NOAA WP-3D aircraft deployed 85 Global Positioning System (GPS) dropwindsondes within 2° of latitude of the circulation center of Hurricane Bonnie as it made landfall in North Carolina. About 75% of the sondes successfully collected data, which were used to create a series of storm-relative horizontal maps of kinematic and thermodynamic variables from 10 m to 2 km. Reflectivity fields were analyzed from the Weather Surveillance Radar-1988 Dopplers (WSR-88Ds) located at Wilmington and Morehead City, North Carolina, and the tail and lower fuselage radars aboard the WP-3Ds.
GPS sonde performance and deployment spacing is adequate to identify several aspects of the vortex. These include 1) warm, dry, stable air in the offshore flow that results in reduced equivalent potential temperatures entering the southern portion of the eyewall, 2) cooler air collocated with the upwelled water in the right-rear quadrant and under the eyewall, and 3) an atypical radial wind pattern with strong inflow southwest of the circulation center and outflow northeast of the center. The strongly asymmetric structure found at 10 m becomes much more homogeneous by 2-km altitude.
No intense rainbands developed over land in the onshore flow nor did the bands in the onshore flow undergo any significant changes once they made landfall. Beyond the eyewall the offshore flow contained much less precipitation than the onshore portion of the storm.
Characteristics beyond the eyewall appear to have been modulated by the proximity to land but hurricane intensity did not vary. The authors infer that the lower energy content of the inflow was offset by the contraction of the eyewall.
Abstract
During 11 h on 26 August 1998, two NOAA WP-3D aircraft deployed 85 Global Positioning System (GPS) dropwindsondes within 2° of latitude of the circulation center of Hurricane Bonnie as it made landfall in North Carolina. About 75% of the sondes successfully collected data, which were used to create a series of storm-relative horizontal maps of kinematic and thermodynamic variables from 10 m to 2 km. Reflectivity fields were analyzed from the Weather Surveillance Radar-1988 Dopplers (WSR-88Ds) located at Wilmington and Morehead City, North Carolina, and the tail and lower fuselage radars aboard the WP-3Ds.
GPS sonde performance and deployment spacing is adequate to identify several aspects of the vortex. These include 1) warm, dry, stable air in the offshore flow that results in reduced equivalent potential temperatures entering the southern portion of the eyewall, 2) cooler air collocated with the upwelled water in the right-rear quadrant and under the eyewall, and 3) an atypical radial wind pattern with strong inflow southwest of the circulation center and outflow northeast of the center. The strongly asymmetric structure found at 10 m becomes much more homogeneous by 2-km altitude.
No intense rainbands developed over land in the onshore flow nor did the bands in the onshore flow undergo any significant changes once they made landfall. Beyond the eyewall the offshore flow contained much less precipitation than the onshore portion of the storm.
Characteristics beyond the eyewall appear to have been modulated by the proximity to land but hurricane intensity did not vary. The authors infer that the lower energy content of the inflow was offset by the contraction of the eyewall.
