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Abstract
The vertical transport of horizontal momentum normal to a line of cumulonimbus observed during GATE on 14 September 1974 is against the vertical momentum gradient, contrary to the predictions of mixing-length theory. Data from repeated aircraft passes normal to the line's axis at heights from 0.15 to 5.5 km are used to document the flux and determine its source. The flux is concentrated in roughly a 25 km wide “active zone” just behind the leading edge of the line, in kilometer-scale convective updrafts accelerated upward by buoyancy and toward the rear of the line by mesoscale pressure forces. The fall in mesoscale pressure from the leading edge to the rear of the active zone is mainly hydrostatic, resulting from relatively high virtual temperatures and the 60 degree tilt of the leading edge from the vertical, with the clouds at the surface well ahead of those aloft.
Evaluation of the terms in the momentum-flux generation equation confirms that the above process, reflected by the velocity-buoyancy correlation term, is responsible for generating momentum flux of the observed sign. The component of momentum flux parallel to the axis of the convective band is generated much like “down-gradient” momentum flux within the fair-weather subcloud layer.
Abstract
The vertical transport of horizontal momentum normal to a line of cumulonimbus observed during GATE on 14 September 1974 is against the vertical momentum gradient, contrary to the predictions of mixing-length theory. Data from repeated aircraft passes normal to the line's axis at heights from 0.15 to 5.5 km are used to document the flux and determine its source. The flux is concentrated in roughly a 25 km wide “active zone” just behind the leading edge of the line, in kilometer-scale convective updrafts accelerated upward by buoyancy and toward the rear of the line by mesoscale pressure forces. The fall in mesoscale pressure from the leading edge to the rear of the active zone is mainly hydrostatic, resulting from relatively high virtual temperatures and the 60 degree tilt of the leading edge from the vertical, with the clouds at the surface well ahead of those aloft.
Evaluation of the terms in the momentum-flux generation equation confirms that the above process, reflected by the velocity-buoyancy correlation term, is responsible for generating momentum flux of the observed sign. The component of momentum flux parallel to the axis of the convective band is generated much like “down-gradient” momentum flux within the fair-weather subcloud layer.
Abstract
Horizontal roll vortices influence the distribution of turbulence, with turbulence variances and fluxes concentrated in regions of positive roll vertical velocity ωr. This “modulation” of turbulence can be explained simply in terms of the advection of turbulence-generating elements by rolls.
A budget equation is derived for the roll-modulated turbulence energy. Evaluations of various terms in the equation shows that the modulation of turbulence variance is accounted for primarily by a similar modulation in mechanical and buoyancy production near the surface and by vertical transport at higher levels (∼100 m). Energy exchange between rolls and turbulence is relatively unimportant. That is, the rolls modulate, turbulence energy mainly by redistributing turbulence and turbulence-producing elements, rather than by exchanging energy.
Similarly, it is shown that the exchange of energy between rolls and roll-modulated turbulence contributes considerably less to the energy equation of rolls than does the major term, buoyancy.
Abstract
Horizontal roll vortices influence the distribution of turbulence, with turbulence variances and fluxes concentrated in regions of positive roll vertical velocity ωr. This “modulation” of turbulence can be explained simply in terms of the advection of turbulence-generating elements by rolls.
A budget equation is derived for the roll-modulated turbulence energy. Evaluations of various terms in the equation shows that the modulation of turbulence variance is accounted for primarily by a similar modulation in mechanical and buoyancy production near the surface and by vertical transport at higher levels (∼100 m). Energy exchange between rolls and turbulence is relatively unimportant. That is, the rolls modulate, turbulence energy mainly by redistributing turbulence and turbulence-producing elements, rather than by exchanging energy.
Similarly, it is shown that the exchange of energy between rolls and roll-modulated turbulence contributes considerably less to the energy equation of rolls than does the major term, buoyancy.
Abstract
The wind and temperature fields of the Planetary boundary layer (PBL) are investigated during periods in which horizontal roll vortices are present. Measurements from a 444 m tower and from inertially-stabilized aircraft indicate the rolls are maintained primarily by 1) production of energy from the cross-roll component of the mean PBL wind spiral (inflectional instability and 2) buoyancy. Complicating a simple picture of two-dimensional rolls are other kilometer-scale eddies whose energy exchanges with the tolls may be important.
The importance of inflectional instability is indicated by the similarity of roll structure to that predicted by models based on the formation of the rolls as a result of instability in the cross-wind (V component of the Ekman spiral. Rolls observed are generally oriented from 10° to 20° to the left of the wind at inversion base, with maximum roll vertical velocity at 0.33zi(where zi is inversion height) and maximum lateral velocity at 0.07zi Atmospheric roll magnitude compare favorably to those predicted by Brown, but predictions are consistently low with a maximum underestimate of 40%.
Both tower and aircraft measurements indicate substantial heat flux by rolls. It is shown that including positive roll heat flux into Brown's neutral equilibrium energy budget will lead to rolls of larger magnitude.
Abstract
The wind and temperature fields of the Planetary boundary layer (PBL) are investigated during periods in which horizontal roll vortices are present. Measurements from a 444 m tower and from inertially-stabilized aircraft indicate the rolls are maintained primarily by 1) production of energy from the cross-roll component of the mean PBL wind spiral (inflectional instability and 2) buoyancy. Complicating a simple picture of two-dimensional rolls are other kilometer-scale eddies whose energy exchanges with the tolls may be important.
The importance of inflectional instability is indicated by the similarity of roll structure to that predicted by models based on the formation of the rolls as a result of instability in the cross-wind (V component of the Ekman spiral. Rolls observed are generally oriented from 10° to 20° to the left of the wind at inversion base, with maximum roll vertical velocity at 0.33zi(where zi is inversion height) and maximum lateral velocity at 0.07zi Atmospheric roll magnitude compare favorably to those predicted by Brown, but predictions are consistently low with a maximum underestimate of 40%.
Both tower and aircraft measurements indicate substantial heat flux by rolls. It is shown that including positive roll heat flux into Brown's neutral equilibrium energy budget will lead to rolls of larger magnitude.
Abstract
The cloud-base diameters of 40 cumulus clouds traversed by aircraft on 14 days of the Cooperative Convective Precipitation Experiment (CCOPE) are shown to increase with the vertical shear of the horizontal wind through cloud base. The relationship is stronger when only the largest clouds sampled in each of the 16 populations are considered. The relationship is strongest when the cloud diameter is normalized by the maximum achievable cloud height, as estimated by the parcel equilibrium height. Assuming a cloud diameter—height ratio of around 1, this implies that larger shear enables clouds to reach a larger fraction of their maximum possible size given the thermodynamic conditions. Alternatively, larger shear may lead to clouds with larger diameter-height ratios. The correct interpretation is probably a combination of the two.
The physical mechanisms for the growth of these largest clouds seem to involve interaction among clouds and the interaction of the clouds with cloud—and boundary layer—induced tropospheric gravity waves, as discussed by Clerk et al. (1986), since these interactions are stronger with stronger vertical shear of the horizontal wind through cloud base. Once produced, the larger clouds that produce outflows have a greater chance to enlarge or to produce new clouds in situations with stronger shear, enhancing the chance of sampling larger clouds.
Abstract
The cloud-base diameters of 40 cumulus clouds traversed by aircraft on 14 days of the Cooperative Convective Precipitation Experiment (CCOPE) are shown to increase with the vertical shear of the horizontal wind through cloud base. The relationship is stronger when only the largest clouds sampled in each of the 16 populations are considered. The relationship is strongest when the cloud diameter is normalized by the maximum achievable cloud height, as estimated by the parcel equilibrium height. Assuming a cloud diameter—height ratio of around 1, this implies that larger shear enables clouds to reach a larger fraction of their maximum possible size given the thermodynamic conditions. Alternatively, larger shear may lead to clouds with larger diameter-height ratios. The correct interpretation is probably a combination of the two.
The physical mechanisms for the growth of these largest clouds seem to involve interaction among clouds and the interaction of the clouds with cloud—and boundary layer—induced tropospheric gravity waves, as discussed by Clerk et al. (1986), since these interactions are stronger with stronger vertical shear of the horizontal wind through cloud base. Once produced, the larger clouds that produce outflows have a greater chance to enlarge or to produce new clouds in situations with stronger shear, enhancing the chance of sampling larger clouds.
Abstract
The relationship of satellite-derived cloud motions to actual convective systems within a convectively active phase of the intraseasonal oscillation is examined by using both cloud-scale properties produced by a cloud-resolving model and field observations to clarify what is going on at shorter time- and space scales. Each convective system has a life cycle of up to 1–2 days. Described in terms of active convection, the system consists of successive precipitation cells generated ahead of the gust front. Described in terms of its cloud shield, the system is more continuous. When easterly winds prevail above 2 km, both precipitating clouds and upper-tropospheric anvil clouds move westward with about the same phase speed (∼10 m s−1). However, during the westerly wind period, precipitating clouds move eastward with a phase speed of ∼10 m s−1, which is better represented by the radar observations and surface precipitation. The westward movement of cloud patterns viewed from the satellite images is mostly due to the horizontal advection of the anvil by the mean flow and the creation of new convective cells to the west of the old convective clouds.
Abstract
The relationship of satellite-derived cloud motions to actual convective systems within a convectively active phase of the intraseasonal oscillation is examined by using both cloud-scale properties produced by a cloud-resolving model and field observations to clarify what is going on at shorter time- and space scales. Each convective system has a life cycle of up to 1–2 days. Described in terms of active convection, the system consists of successive precipitation cells generated ahead of the gust front. Described in terms of its cloud shield, the system is more continuous. When easterly winds prevail above 2 km, both precipitating clouds and upper-tropospheric anvil clouds move westward with about the same phase speed (∼10 m s−1). However, during the westerly wind period, precipitating clouds move eastward with a phase speed of ∼10 m s−1, which is better represented by the radar observations and surface precipitation. The westward movement of cloud patterns viewed from the satellite images is mostly due to the horizontal advection of the anvil by the mean flow and the creation of new convective cells to the west of the old convective clouds.
Abstract
A definite relationship between cloud distribution and sub-cloud layer structure and fluxes in fair weather is documented using measurements of wind, temperature, humidity and overhead cloud occurrence from the NCAR DeHasvilland aircraft. Three cases are used. These were extracted from data taken to the north of Puerto Rico on 14 and 15 December 1972 in mesoscale regions of reasonably uniform convection ranging from suppressed with very little shallow cloudiness to slightly enhanced with active (but non-precipitating) trade cumulus having tops to 2000 m. On both days synoptic conditions were suppressed and the surface winds were from the cast at 10 to 15 m s−1.
In the highly suppressed cases, there is evidence that cloud distribution was determined by subcloud layer circulations—roll vortices which persisted throughout the flight patterns. In the more enhanced case, the predominant coupling was by well-defined cloud scale updrafts which were traceable to at least 100 m below cloud base.
As a consequence of these interactions, the fluxes of moisture and momentum in the upper subcloud layer were found to be strongly coupled to cloud distribution. A comparison of direct measurements from the aircraft and the results of budget computations by other workers for several suppressed situations in the trades suggests that almost all of the fluxes out of the mixed layer are concentrated in mesoscale cloud patches and that a large function of the transport is due to motions on the scale of the individual cumulus.
Abstract
A definite relationship between cloud distribution and sub-cloud layer structure and fluxes in fair weather is documented using measurements of wind, temperature, humidity and overhead cloud occurrence from the NCAR DeHasvilland aircraft. Three cases are used. These were extracted from data taken to the north of Puerto Rico on 14 and 15 December 1972 in mesoscale regions of reasonably uniform convection ranging from suppressed with very little shallow cloudiness to slightly enhanced with active (but non-precipitating) trade cumulus having tops to 2000 m. On both days synoptic conditions were suppressed and the surface winds were from the cast at 10 to 15 m s−1.
In the highly suppressed cases, there is evidence that cloud distribution was determined by subcloud layer circulations—roll vortices which persisted throughout the flight patterns. In the more enhanced case, the predominant coupling was by well-defined cloud scale updrafts which were traceable to at least 100 m below cloud base.
As a consequence of these interactions, the fluxes of moisture and momentum in the upper subcloud layer were found to be strongly coupled to cloud distribution. A comparison of direct measurements from the aircraft and the results of budget computations by other workers for several suppressed situations in the trades suggests that almost all of the fluxes out of the mixed layer are concentrated in mesoscale cloud patches and that a large function of the transport is due to motions on the scale of the individual cumulus.
Abstract
Pressure perturbations are measured from an aircraft by subtracting its pressure altitude from its actual altitude. The pressure perturbation is equal to the resulting “D-value” multiplied by the acceleration of gravity and density of air. Normally, the actual altitude is measured using a radar altimeter, but this becomes increasingly difficult over increasingly complex terrain.
Here, we document a technique in which inertial altitude is used instead of radar altitude, eliminating the need for extremely accurate navigation or simple terrain, and apply it to document the pressure field at the base of an evolving cumulus congestus in CCOPE. Analysis of both this case study and aircraft self-calibration maneuvers in clear, undisturbed air suggests that a D-value (pressure) accuracy of 2 m (20 Pa) is achievable at cumulus-congestus scales. This accuracy is degraded, however, if the phenomenon of interest is large compared to the flight track.
Abstract
Pressure perturbations are measured from an aircraft by subtracting its pressure altitude from its actual altitude. The pressure perturbation is equal to the resulting “D-value” multiplied by the acceleration of gravity and density of air. Normally, the actual altitude is measured using a radar altimeter, but this becomes increasingly difficult over increasingly complex terrain.
Here, we document a technique in which inertial altitude is used instead of radar altitude, eliminating the need for extremely accurate navigation or simple terrain, and apply it to document the pressure field at the base of an evolving cumulus congestus in CCOPE. Analysis of both this case study and aircraft self-calibration maneuvers in clear, undisturbed air suggests that a D-value (pressure) accuracy of 2 m (20 Pa) is achievable at cumulus-congestus scales. This accuracy is degraded, however, if the phenomenon of interest is large compared to the flight track.
Abstract
The effects of quasi-two-dimensional convective bands on the environmental flow are investigated by comparing the observed mass and momentum fluxes and horizontal pressure changes to those predicted by the Moncrieff archetypal model (M92). The model idealizes the organized convection as two-dimensional and steady state, with three flow branches—a front-to-rear jump updraft, a front overturning updraft, and a rear overturning current, which can be an updraft or a downdraft. Flow through the branches satisfies mass continuity and Bernoulli's equation. The vertical divergence of line-normal momentum flux averaged over the volume is constrained to be zero. Coriolis and buoyancy effects are neglected. The model predicts the vertical mass flux, the vertical divergence of the vertical flux of line-normal momentum, and the pressure change across the line (independent of height). A simple equation for the vertical transport of line-parallel momentum follows from the model assumptions.
Case studies show a systematic linkage of fluxes and structure and a relationship of some of these changes to differences in the environmental sounding. The M92 successfully replicates the general shapes of the vertical mass flux and line-normal momentum flux profiles, and to some degree how they change with environment. The M92 correctly predicts both the magnitude and shape of the curves in cases occurring in near-neutral environments (low buoyancy or high shear) and with system width-to-depth ratios close to the dynamically based value of 4:1. The model is less successful for systems in more unstable environments or those with large horizontal extent, probably due to the neglect of the generation of horizontal momentum by the buoyancy distribution. The observed sign of the average pressure changes across the line is consistent with that predicted by the model in the upper half of the system, where some case studies suggest that buoyancy effects should be minimized. Letting the model (4:1 aspect ratio) represent the dynamically active part of a mesoscale system, the rearward advective broadening of the inert anvil region is simply related to the (rearward) outflow speed of the jump updraft, U 1. Since U 1 increases as tropospheric shear decreases, the model correctly associates broad mesoscale systems with small tropospheric shear. Success in predicting the vertical flux of line-parallel momentum was fair.
Abstract
The effects of quasi-two-dimensional convective bands on the environmental flow are investigated by comparing the observed mass and momentum fluxes and horizontal pressure changes to those predicted by the Moncrieff archetypal model (M92). The model idealizes the organized convection as two-dimensional and steady state, with three flow branches—a front-to-rear jump updraft, a front overturning updraft, and a rear overturning current, which can be an updraft or a downdraft. Flow through the branches satisfies mass continuity and Bernoulli's equation. The vertical divergence of line-normal momentum flux averaged over the volume is constrained to be zero. Coriolis and buoyancy effects are neglected. The model predicts the vertical mass flux, the vertical divergence of the vertical flux of line-normal momentum, and the pressure change across the line (independent of height). A simple equation for the vertical transport of line-parallel momentum follows from the model assumptions.
Case studies show a systematic linkage of fluxes and structure and a relationship of some of these changes to differences in the environmental sounding. The M92 successfully replicates the general shapes of the vertical mass flux and line-normal momentum flux profiles, and to some degree how they change with environment. The M92 correctly predicts both the magnitude and shape of the curves in cases occurring in near-neutral environments (low buoyancy or high shear) and with system width-to-depth ratios close to the dynamically based value of 4:1. The model is less successful for systems in more unstable environments or those with large horizontal extent, probably due to the neglect of the generation of horizontal momentum by the buoyancy distribution. The observed sign of the average pressure changes across the line is consistent with that predicted by the model in the upper half of the system, where some case studies suggest that buoyancy effects should be minimized. Letting the model (4:1 aspect ratio) represent the dynamically active part of a mesoscale system, the rearward advective broadening of the inert anvil region is simply related to the (rearward) outflow speed of the jump updraft, U 1. Since U 1 increases as tropospheric shear decreases, the model correctly associates broad mesoscale systems with small tropospheric shear. Success in predicting the vertical flux of line-parallel momentum was fair.
Abstract
Oceanic cumulonimbus updraft and downdraft events observed in the Western Pacific during the TAMEX program by NOAA P-3 research aircraft are analyzed and discussed. The basic dataset consists of flight-level data from 10 missions in the Taiwan region during May and June 1987. The 1 Hz time series of vertical velocity is used to define convective updrafts using the criteria that the velocity must be continuously positive for at least 0.5 km and exceed 0.5 m s−1 for 1 s. A subset of the strongest drafts, termed cores, are defined as events that exceed 1 m s−1 for 0.5 km. Downdrafts and downdraft cores are defined analogously. The statistics are from a total of 12 841 km of flight legs and consist of 359 updrafts and 466 downdrafts at altitudes from 150 m to 6.8 km MSL. The populations of average vertical velocity, maximum vertical velocity, diameter, and mass transport for both drafts and cores are approximately log-normally distributed, consistent with the results of previous studies of convective characteristics in other locations. TAMEX drafts and cores are comparable in size and strength with those measured in GATE and hurricanes but much weaker than those measured in continental thunderstorms.
The median core updraft was less than 3 m s−1, implying a time scale for ascent from cloud base to the freezing level of about 35 min. The microphysical implications of the low updraft rates are illustrated by comparing vertical profiles of radar reflectivity for TAMEX with those in other regions. The data are consistent with the hypothesis that the oceanic convection that was studied in GATE, hurricanes, and TAMEX is dominated by warm rain coalescence processes and that a large fractional rainout occurs below the freezing level. The rapid reduction of cloud water and radar reflectivity above the freezing level, as well as observations of abundant ice particles in all but the strongest updraft cores at temperatures just below 0°C, implies a rapid conversion of cloud water and rain to ice and graupel as the air ascends through the freezing level. The, lack of reports of hail and other forms of severe weather in these oceanic regions is consistent with the aircraft and radar observations.
The data from the “best” organized weather system investigated by the P-3 during TAMEX are used to examine the relationship of cloud buoyancy and vertical motion. Water loading and entrainment has a significant role in reducing both the core virtual temperature excess over the environment and the updraft velocity from what would be expected from the convective available potential energy of the environmental air. The majority of the strongest downdrafts possess positive temperature perturbations (probably as a result of mixing with nearby updraft air) with the negative buoyancy being sustained by large amounts of rainwater.
Abstract
Oceanic cumulonimbus updraft and downdraft events observed in the Western Pacific during the TAMEX program by NOAA P-3 research aircraft are analyzed and discussed. The basic dataset consists of flight-level data from 10 missions in the Taiwan region during May and June 1987. The 1 Hz time series of vertical velocity is used to define convective updrafts using the criteria that the velocity must be continuously positive for at least 0.5 km and exceed 0.5 m s−1 for 1 s. A subset of the strongest drafts, termed cores, are defined as events that exceed 1 m s−1 for 0.5 km. Downdrafts and downdraft cores are defined analogously. The statistics are from a total of 12 841 km of flight legs and consist of 359 updrafts and 466 downdrafts at altitudes from 150 m to 6.8 km MSL. The populations of average vertical velocity, maximum vertical velocity, diameter, and mass transport for both drafts and cores are approximately log-normally distributed, consistent with the results of previous studies of convective characteristics in other locations. TAMEX drafts and cores are comparable in size and strength with those measured in GATE and hurricanes but much weaker than those measured in continental thunderstorms.
The median core updraft was less than 3 m s−1, implying a time scale for ascent from cloud base to the freezing level of about 35 min. The microphysical implications of the low updraft rates are illustrated by comparing vertical profiles of radar reflectivity for TAMEX with those in other regions. The data are consistent with the hypothesis that the oceanic convection that was studied in GATE, hurricanes, and TAMEX is dominated by warm rain coalescence processes and that a large fractional rainout occurs below the freezing level. The rapid reduction of cloud water and radar reflectivity above the freezing level, as well as observations of abundant ice particles in all but the strongest updraft cores at temperatures just below 0°C, implies a rapid conversion of cloud water and rain to ice and graupel as the air ascends through the freezing level. The, lack of reports of hail and other forms of severe weather in these oceanic regions is consistent with the aircraft and radar observations.
The data from the “best” organized weather system investigated by the P-3 during TAMEX are used to examine the relationship of cloud buoyancy and vertical motion. Water loading and entrainment has a significant role in reducing both the core virtual temperature excess over the environment and the updraft velocity from what would be expected from the convective available potential energy of the environmental air. The majority of the strongest downdrafts possess positive temperature perturbations (probably as a result of mixing with nearby updraft air) with the negative buoyancy being sustained by large amounts of rainwater.
Abstract
Results from a detailed three-dimensional model of the atmospheric boundary layer are compared with observational data in a case of nonprecipitating convection in a tropical boundary layer. The model is a slightly improved version of the one developed by Sommeria (1976) in collaboration with J.W. Deardorff. The experimental data come from the NCAR 1972 Puerto Rico experiment which provided a good set of aircraft turbulence measurements in the fair weather mixed layer over the tropical ocean. The comparison involves statistical properties of the turbulent field as well as some structural features in the presence of small clouds.
Abstract
Results from a detailed three-dimensional model of the atmospheric boundary layer are compared with observational data in a case of nonprecipitating convection in a tropical boundary layer. The model is a slightly improved version of the one developed by Sommeria (1976) in collaboration with J.W. Deardorff. The experimental data come from the NCAR 1972 Puerto Rico experiment which provided a good set of aircraft turbulence measurements in the fair weather mixed layer over the tropical ocean. The comparison involves statistical properties of the turbulent field as well as some structural features in the presence of small clouds.
