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Margaret A. LeMone

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.

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Margaret A. Lemone

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.

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Margaret A. LeMone

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.

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Gilles Sommeria and Margaret A. LeMone

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.

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Stephen Nicholls and Margaret A. Lemone

Abstract

The structure of the convective atmospheric boundary layer and the characteristics of the associated turbulent mixing processes in undisturbed conditions over the tropical ocean are investigated using data collected during the GARP Atlantic Tropical Experiment (GATE). The data were obtained by a number of aircraft equipped with turbulence measuring instrumentation. The fluxes of momentum, sensible and latent heat throughout the subcloud layer are presented for four cases considered in detail. It is shown that the sensible and latent heat fluxes at the top of the mixed layer (and therefore the distribution of heating and moistening in the boundary layer) are strongly affected by the presence or absence of cumulus convection while the virtual heat flux remains unaffected. Features of this cloud-subcloud interaction are discussed in the light of Betts (1976) coupled cloud-subcloud layer model.

An examination of the spectra (and cospectra) of subcloud-layer variables shows that with the exception of vertical velocity, the spectra are generally dominated by low-frequency fluctuations. This behavior is attributed to the effects of entrainment which may produce relatively large, long-lived excursions from the mixed-layer average in the weakly mixed GATE boundary layer and to the existence of mesoscale (∼10 km) variability.

Wherever possible comparisons are drawn with previous measurements and corresponding situations over land, where mixing processes are usually much more energetic. Aircraft measurements of the sensible and latent heat fluxes are compared with those derived from tethered balloon probes and budget calculations which were employed concurrently during GATE. Aircraft and tethered balloon fluxes showed good agreement; however, the budget results differ, probably due to a different sampling strategy.

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Margaret A. LeMone and Mitchell W. Moncrieff

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.

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Margaret A. LeMone and Edward J. Zipser

Abstract

This is the first part of a two-part paper defining the nature of the vertical air motion in and around GATE cumulonimbus clouds. The statistics are from a total of 104 km of flight legs, flown on six days in GATE, at altitudes from near the surface to 8100 m. The basic data sets analyzed are time series of vertical velocity at a frequency of 1 Hz. For the purpose of study, convective events are divided into two categories: drafts, requiring only that vertical velocity be continuously positive (negative) for 500 m and exceed an absolute value of 0.5 m s−1 for 1 s; and cores, the stronger portions of the stronger drafts, requiring that upward (downward) vertical velocity be continuously greater than an absolute value of 1 m s−1 for 500 m. The distributions of average vertical velocity, maximum vertical velocity, diameter and mass flux are given for drafts and cores at five altitude intervals between 150 m and 8 km. In all cases, the distributions are approximately log-normal.

Above cloud base, updrafts tend to be smaller but more intense than downdrafts. Updrafts and down-drafts near cloud base are comparable in size and intensity. Downdraft cores are smaller than updraft cores at all attitudes. They also are weaker, except near cloud base, where updraft and downdraft cores have comparable intensity. In the middle troposphere, only 10% of the updraft cores have mean vertical velocities greater than 5 m s−1, and only 10% have diameters in excess of 2 km.

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David P. Jorgensen and Margaret A. LeMone

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.

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Christopher Lucas, Edward J. Zipser, and Margaret A. LeMone

Abstract

No abstract available.

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David P. Jorgensen, Margaret A. LeMone, and Stanley B. Trier

Abstract

This study documents the precipitation and kinematic structure of a mature, eastward propagating, oceanic squall line system observed by instrumented aircraft during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). Doppler radar and low-level in situ observations are used to show the evolution of the convection from an initially linear NNW–SSE-oriented convective line to a highly bow-shaped structure with an embedded low- to midlevel counterclockwise rotating vortex on its northern flank. In addition to previously documented features of squall lines such as highly upshear-tilted convection on its leading edge, a channel of strong front-to-rear flow that ascended with height over a “rear-inflow” that descended toward the convective line, and a pronounced low-level cold pool apparently fed from convective and mesoscale downdrafts from the convective line; rearward, the observations of this system showed distinct multiple maxima in updraft strength with height and reflectivity bands extending rearward transverse to the principal convective line. Vertical motions within the active convective region of the squall line system were determined using a new approach that utilized near-simultaneous observations by the Doppler radars on two aircraft with up to four Doppler radial velocity estimates at echo top. Echo-top vertical motion can then be derived directly, which obviates the traditional dual-Doppler assumption of no vertical velocity at the top boundary and results in a more accurate estimate of tropospheric vertical velocity through downward integration of horizontal divergence.

Low-level flight-level observations of temperature, wind speed, and dew point collected rearward of the squall line are used to estimate bulk fluxes of dry and moist static energy. The strong near-surface fluxes, due to the warm sea and high winds, combined with estimates of mesoscale advection, are used to estimate boundary layer recovery time; they indicate that the boundary layer could recover from the effects of the cold dome within about 3 h of first cold air injection if the observed near-surface winds were maintained. However, the injection and spreading of air from above leads to cooling at a fixed spot ∼20 km rearward of the convective line (surface θ e minimum point), suggesting that the cold pool could be still intensifying at the time of observation. Recovery time at a point is probably similar to that measured in previous studies.

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