Search Results
You are looking at 1 - 10 of 71 items for
- Author or Editor: Margaret A. Lemone x
- Refine by Access: All Content x
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
No abstract available.
Abstract
No abstract available.
Abstract
Based on personal experience and input from colleagues, the natural history of a field program is discussed, from conception through data analysis and synthesis of results. For convenience, the life cycle of a field program is divided into three phases: the prefield phase, the field phase, and the aftermath. As described here, the prefield phase involves conceiving the idea, developing the scientific objectives, naming the program, obtaining support, and arranging the logistics. The field phase discussion highlights the decision making process, balancing input from data and numerical models, and human interactions. The data are merged, analyzed, and synthesized into knowledge mainly after the field effort.
Three major conclusions are drawn. First, it is the people most of all who make a field program successful, and cooperation and collegial consensus building are vital during all phases; good health and a sense of humor both help make this possible. Second, although numerical models are now playing a central role in all phases of a field program, not paying adequate attention to the observations can lead to problems. And finally, it cannot be overemphasized that both funding agencies and participants must recognize that it takes several years to fully exploit the datasets collected, with the corollary that high-quality datasets should be available long term.
Abstract
Based on personal experience and input from colleagues, the natural history of a field program is discussed, from conception through data analysis and synthesis of results. For convenience, the life cycle of a field program is divided into three phases: the prefield phase, the field phase, and the aftermath. As described here, the prefield phase involves conceiving the idea, developing the scientific objectives, naming the program, obtaining support, and arranging the logistics. The field phase discussion highlights the decision making process, balancing input from data and numerical models, and human interactions. The data are merged, analyzed, and synthesized into knowledge mainly after the field effort.
Three major conclusions are drawn. First, it is the people most of all who make a field program successful, and cooperation and collegial consensus building are vital during all phases; good health and a sense of humor both help make this possible. Second, although numerical models are now playing a central role in all phases of a field program, not paying adequate attention to the observations can lead to problems. And finally, it cannot be overemphasized that both funding agencies and participants must recognize that it takes several years to fully exploit the datasets collected, with the corollary that high-quality datasets should be available long term.
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
Previously published profiles of vertical velocity (w) skewness observed in the convective atmospheric boundary layer show deficits in the upper part of the layer, relative to large eddy simulations designed to apply to highly convective cloudless planetary boundary layers. Thus, we examine w-skewness profiles from data collected in other experiments. We find that skewness profiles in the three highly convective cases with the fewest and smallest clouds agree better with the large eddy simulation results than other profiles presented here and previously; however the deficit at the top of the boundary layer—though smaller—remains.
We hypothesize that the remaining deficit for these three cases results from the presence of ∼10-km wavelength quasi two-dimensional sinusoidal structures, which have near-zero skewness. The small domain and periodic boundary conditions of a large eddy simulation may not allow such structures to develop fully. Removal of the effects of these structures by counting only flight legs nearly parallel to their axes, for two of the cases, improves agreement between the simulation and observations. We speculate that these structures result from gravity waves interacting with the boundary layer.
Abstract
Previously published profiles of vertical velocity (w) skewness observed in the convective atmospheric boundary layer show deficits in the upper part of the layer, relative to large eddy simulations designed to apply to highly convective cloudless planetary boundary layers. Thus, we examine w-skewness profiles from data collected in other experiments. We find that skewness profiles in the three highly convective cases with the fewest and smallest clouds agree better with the large eddy simulation results than other profiles presented here and previously; however the deficit at the top of the boundary layer—though smaller—remains.
We hypothesize that the remaining deficit for these three cases results from the presence of ∼10-km wavelength quasi two-dimensional sinusoidal structures, which have near-zero skewness. The small domain and periodic boundary conditions of a large eddy simulation may not allow such structures to develop fully. Removal of the effects of these structures by counting only flight legs nearly parallel to their axes, for two of the cases, improves agreement between the simulation and observations. We speculate that these structures result from gravity waves interacting with the boundary 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.
A survey of the Journal of the Atmospheric Sciences, the Journal of Applied Meteorology, and the Monthly Weather Review shows that the number of publications per year resulting from GATE (GARP Atlantic Tropical Experiment) peaked in 1980, six years after the experiment's field phase.
A survey of the Journal of the Atmospheric Sciences, the Journal of Applied Meteorology, and the Monthly Weather Review shows that the number of publications per year resulting from GATE (GARP Atlantic Tropical Experiment) peaked in 1980, six years after the experiment's field phase.
Abstract
Evidence indicates that fair-weather to towering cumulus clouds over the East Atlantic Ocean during GATE were frequently organized into mesoscale structures. Three examples of such structures are examined, using gust-probe aircraft data collected in parallel straight-and-level flight tracks at 150 m, and covering an area greater than 30×30 km. The aircraft (two cases) or rawinsonde (one case) data provide vertical profiles of mean wind, temperature and mixing ratio. Cloud patterns are revealed from an upward-looking infrared sensor on the aircraft and radar and satellite pictures.
The data show that the cumulus were organized into bands with horizontal wavelengths of 15–25 km. The circulations appear to extend through the subcloud layer, with all the fields at 150 m well related to the cloudiness overhead. Since the circulations are aligned with the subcloud-layer shear and travel in a direction parallel to the subcloud-layer wind (in the two cases for which band movement is documented), it is believed that they are primarily subcloud-layer phenomena. The subcloud-layer depth is about 600 m, giving aspect ratios of the bands from 25 to 50, in the range of mesoscale cellular convection observed in midlatitudes.
Several physical mechanisms which might explain the bands are discussed.
Abstract
Evidence indicates that fair-weather to towering cumulus clouds over the East Atlantic Ocean during GATE were frequently organized into mesoscale structures. Three examples of such structures are examined, using gust-probe aircraft data collected in parallel straight-and-level flight tracks at 150 m, and covering an area greater than 30×30 km. The aircraft (two cases) or rawinsonde (one case) data provide vertical profiles of mean wind, temperature and mixing ratio. Cloud patterns are revealed from an upward-looking infrared sensor on the aircraft and radar and satellite pictures.
The data show that the cumulus were organized into bands with horizontal wavelengths of 15–25 km. The circulations appear to extend through the subcloud layer, with all the fields at 150 m well related to the cloudiness overhead. Since the circulations are aligned with the subcloud-layer shear and travel in a direction parallel to the subcloud-layer wind (in the two cases for which band movement is documented), it is believed that they are primarily subcloud-layer phenomena. The subcloud-layer depth is about 600 m, giving aspect ratios of the bands from 25 to 50, in the range of mesoscale cellular convection observed in midlatitudes.
Several physical mechanisms which might explain the bands are discussed.
Abstract
This is the second paper of a two-part series documenting the structure of and momentum transport by a subtropical mesoscale convective system near Taiwan, using Doppler radar data and in situ data from the NOAA P-3. Part I defines the basic system structure and evolution. In Part II, the momentum transport by the system is estimated and related to system structure, and the momentum budget for a portion of the embedded convective band is evaluated.
Profiles of the vertical flux of horizontal momentum are constructed from in situ data, Doppler radar data, and both combined, in a coordinate system with u normal to the line and positive eastward, since the low-level air is feeding the line from the east. Differences in the fluxes from the two sources appear to be mainly due to an underestimation of the mean vertical velocity from the Doppler radar data. The discrepancy results partially from the concentration of convergence in the boundary layer—precisely where the Doppler cannot adequately sample the convergence—and partially from Doppler problems above 5 km. However, the momentum-flux profile generated from both data sources has features consistent with the structure of the line: p̄
The momentum budget reveals some behavior that differs from that of earlier systems such as that studied by Lafore et al. For example, above 7 km the momentum transport and pressure gradient reinforce to produce substantial acceleration of air exiting the band at high levels toward the front (east), although the vertical transport contributes only a small amount to the observed acceleration. The u positive acceleration at higher levels, being larger than the Doppler estimates of dŪ/dt at lower levels, increases the overall u shear within the convective band. Estimation of the vertical momentum-flux divergence and pressure-gradient term at low levels from the in situ data supports this results. In previously observed tropical systems, u shear was increased by convective bands only when the u shear was negative. At midlevels, the vertical transport of line-parallel wind (v) by the line acts to increase and slightly elevate the southerly jet maximum in the environmental wind profile usually seen in this region. As in previously documented systems, dV̄/dz decreases with time within the band.
Abstract
This is the second paper of a two-part series documenting the structure of and momentum transport by a subtropical mesoscale convective system near Taiwan, using Doppler radar data and in situ data from the NOAA P-3. Part I defines the basic system structure and evolution. In Part II, the momentum transport by the system is estimated and related to system structure, and the momentum budget for a portion of the embedded convective band is evaluated.
Profiles of the vertical flux of horizontal momentum are constructed from in situ data, Doppler radar data, and both combined, in a coordinate system with u normal to the line and positive eastward, since the low-level air is feeding the line from the east. Differences in the fluxes from the two sources appear to be mainly due to an underestimation of the mean vertical velocity from the Doppler radar data. The discrepancy results partially from the concentration of convergence in the boundary layer—precisely where the Doppler cannot adequately sample the convergence—and partially from Doppler problems above 5 km. However, the momentum-flux profile generated from both data sources has features consistent with the structure of the line: p̄
The momentum budget reveals some behavior that differs from that of earlier systems such as that studied by Lafore et al. For example, above 7 km the momentum transport and pressure gradient reinforce to produce substantial acceleration of air exiting the band at high levels toward the front (east), although the vertical transport contributes only a small amount to the observed acceleration. The u positive acceleration at higher levels, being larger than the Doppler estimates of dŪ/dt at lower levels, increases the overall u shear within the convective band. Estimation of the vertical momentum-flux divergence and pressure-gradient term at low levels from the in situ data supports this results. In previously observed tropical systems, u shear was increased by convective bands only when the u shear was negative. At midlevels, the vertical transport of line-parallel wind (v) by the line acts to increase and slightly elevate the southerly jet maximum in the environmental wind profile usually seen in this region. As in previously documented systems, dV̄/dz decreases with time within the band.
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.
