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Margaret A. LeMone

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.

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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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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

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.

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Margaret A. Lemone

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.

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Margaret A. Lemone

Abstract

No abstract available.

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Xiaoqing Wu
and
Margaret A. LeMone

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.

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Margaret A. Lemone
and
William T. Pennell

Abstract

A performance analysis of the three turbulence-measuring aircraft which participated in the GATE is presented. These aircraft were a Lockheed C-130 operated by the Meteorological Research Flight Centre of the U.K. Meteorological Office, a Douglas DC-6 operated by the Research Flight Facility of the National Oceanographic and Atmospheric Administration, and a Lockheed L-188 operated by the Research Aviation Facility of the National Center for Atmospheric Research.

The results are based on formal intercomparison flights and analysis of fair weather days on which two or more of the aircraft were flying. In the formal intercomparison flights, two or more of the aircraft flew side by side in the fair weather atmospheric mixed layer. In both cases, the aircraft flew L-shaped patterns, consisting of 30 km legs along and normal to the mixed layer wind direction.

Quantities compared include the variances of three wind components, potential temperature, moisture, and the vertical fluxes of horizontal momentum, temperature, and moisture. The analysis shows that when all components of the gust probe system are working properly, interaircraft biases are less than the expected atmospheric variability. Quirks of the three data sets are pointed out for the benefit of future GATE data users.

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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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