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David P. Jorgensen and Margaret A. LeMone

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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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Sharon A. Lewis, Margaret A. LeMone, and David P. Jorgensen

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Airborne Doppler and flight-level data are used to document the structure and evolution of portions of a late-stage horseshoe-shaped squall line system and its effect on vertical momentum and mass transports. This system, which occurred on 20 February 1993 during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment, was similar to many previously studied, but had some unique features. First, a slow-moving transverse band, which formed the southern leg of the horseshoe, drew most of its low-level updraft air from the squall-line stratiform region on its north side rather than the “environment” to the south. Second, a long-lived cell with many properties similar to a midlatitude supercell, formed 150 km to the rear of the squall line. This cell was tracked for 4 h, as it propagated into and then through the cold pool, and finally dissipated as it encountered the convection forming the northern edge of the horseshoe. Finally, as the squall line was dissipating, a new convective band formed well to its rear.

The transverse band and the long-lived cell are discussed in this paper. Quadruple-Doppler radar data, made possible by tightly coordinated flights by the two NOAA P3s, are used to document the flow with unprecedented accuracy. At lower levels, the transverse band flow structure is that of a two-dimensional convective band feeding on its north side, with vertical fluxes of mass and horizontal momentum a good match to the predictions of the Moncrieff archetype model. At upper levels, the transverse band flow is strongly influenced by the squall line, whose westward-tilting updraft leads to much larger vertical velocities than predicted by the model. The long-lived cell, though weak, has supercell-like properties in addition to its longevity, including an updraft rotating in the sense expected from the environmental hodograph and an origin in an environment whose Richardson number falls within the Weisman–Klemp “supercell” regime.

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

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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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Stanley B. Trier, William C. Skamarock, and Margaret A. LeMone

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Mechanisms responsible for meso- and convective-scale organization within a large tropical squall line that occurred on 22 February 1993 during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment are investigated using a three-dimensional numerical cloud model. The squall line occurred in an environment typical of fast-moving tropical squall lines, characterized by moderate convective available potential energy and moderate-to-strong vertical shear beneath a low-level jet with weak reverse vertical shear above.

A well-simulated aspect of the observed squall line is the evolution of a portion of its leading convective zone from a quasi-linear to a three-dimensional bow-shaped structure over a 2-h period. This transition is accompanied by the development of both a prominent mesoscale vortex along the northern edge of the 40–60-km long bow-shaped feature and elongated bands of weaker reflectivity situated rearward and oriented transverse to the leading edge, within enhanced front-to-rear system relative midlevel flow, near the southern end of the bow. The vertical wind shear that arises from the convectively induced mesoscale flow within the squall-line system is found to be a critical factor influencing 1) the development of the vortex and 2) through its associated vertical pressure gradients, the pronounced along-line variability of the convective updraft and precipitation structure. The environmental wind profile is also critical to system organization since the orientation of its vertical shear (in layers both above and below the environmental jet height) relative to the local orientation of the incipient storm-induced subcloud cold pool directly influences the onset of the convectively induced mesoscale flow.

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Margaret A. LeMone, Mukul Tewari, Fei Chen, and Jimy Dudhia

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Heights of nocturnal boundary layer (NBL) features are determined using vertical profiles from the Advanced Research Weather Research and Forecasting Model (ARW-WRF), and then compared to data for three moderately windy fair-weather nights during the April–May 1997 Kansas-based Cooperative Atmosphere–Surface Exchange Study (CASES-97) to evaluate the success of four PBL schemes in replicating observations. The schemes are Bougeault–LaCarrere (BouLac), Mellor–Yamada–Janjić (MYJ), quasi-normal scale elimination (QNSE), and Yonsei University (YSU) versions 3.2 and 3.4.1. This study’s chosen objectively determined model NBL height h estimate uses a turbulence kinetic energy (TKE) threshold equal to 5% , where TKE′ is relative to its background (free atmosphere) value. The YSU- and MYJ-determined h could not be improved upon. Observed heights of the virtual temperature maximum h Tvmax and wind speed maximum h Smax, and the heights h 1wsonde and h 2wsonde, between which the radiosonde slows from ~5 to ~3 m s−1 as it rises from turbulent to nonturbulent air, and thus brackets h, were used for comparison to model results. The observations revealed a general pattern: h Tvmax increased through the night, and h Tvmax and h Smax converged with time, and the two mostly lay between h 1wsonde and h 2wsonde after several hours. Clear failure to adhere to this pattern and large excursions from observations or other PBL schemes revealed excess mixing for BouLac and YSU version 3.2 (but not version 3.4.1) and excess thermal mixing for QNSE under windy conditions. Observed friction velocity was much smaller than model values, with differences consistent with the observations reflecting local skin drag and the model reflecting regional form drag + skin drag.

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Stanley B. Trier, Margaret A. LeMone, and William C. Skamarock

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Past studies of the effects of mesoscale convective systems (MCSs) on the environmental flow have been limited by data coverage and resolution. In the current study the MCS-scale (stormwide) horizontal accelerations and momentum budget associated with an oceanic MCS are analyzed using output from a high-resolution three-dimensional numerical model integrated over a large domain. The simulation is based on an observed MCS that occurred on 22 February 1993 during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. An important aspect of both the observed and simulated MCS is its evolution from a quasi-two-dimensional to an asymmetric three-dimensional morphology, which was demonstrated in companion studies to result from the finite length of the MCS interacting with environmental vertical shear that varies in direction with height. Herein, the authors focus on the effects of the three-dimensional structure on MCS-scale horizontal accelerations.

The horizontal accelerations over the central portion of the MCS, where its leading edge is perpendicular to the low-level environmental vertical shear, resemble those from available observations and two-dimensional models of linear squall-type MCSs. However, the vertical structure of horizontal accelerations is quite different on the MCS scale. Zonal accelerations, which are aligned along the environmental low-level vertical shear, generally exceed meridional accelerations in the lower and upper troposphere, and are dominated by the vertical flux convergence term at low levels, and by the horizontal flux convergence term at upper levels. In contrast, zonal accelerations are weaker than meridional accelerations at midlevels, owing to strong cancellation of zonal accelerations in the central portion with those along the northern periphery of the MCS, where both the alignment of the convective band relative to the environmental vertical shear and its mesoscale organization are different. This compensation between different regions of the MCS results in modifications to the environmental vertical shear by mesoscale convection that differ substantially from those typically reported in idealized studies of two-dimensional squall lines. Since three-dimensional organization often occurs in MCSs that lack persistent external linear forcing, the current findings may have implications for the parameterization of the momentum effects of mesoscale deep convection in large-scale models.

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Margaret A. LeMone, Edward J. Zipser, and Stanley B. Trier

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A collection of case studies is used to elucidate the influence of environmental soundings on the structure and evolution of the convection in the mesoscale convective systems sampled by the turboprop aircraft in the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean–Atmosphere Response Experiment (COARE). The soundings were constructed primarily from aircraft data below 5–6 km and primarily from radiosonde data at higher altitudes.

The well-documented role of the vertical shear of the horizontal wind in determining the mesoscale structure of tropical convection is confirmed and extended. As noted by earlier investigators, nearly all convective bands occurring in environments with appreciable shear below a low-level wind maximum are oriented nearly normal to the shear beneath the wind maximum and propagate in the direction of the low-level shear at a speed close to the wind maximum; when there is appreciable shear at middle levels (800–400 mb), convective bands form parallel to the shear. With appreciable shear at both levels, the lower-level shear determines the orientation of the primary convective bands. If the midlevel shear is opposite the low-level shear, secondary bands parallel to the midlevel shear will extend rearward from the primary band in later stages of its evolution; if the midlevel shear is 90 degrees to the low-level shear, the primary band will retain its two-dimensional mesoscale structure. Convection has no obvious mesoscale organization on days with little shear or days with widespread convection.

Environmental temperatures and humidities have no obvious effect on the mesoscale convective pattern, but they affect COARE convection in other ways. The high tops of COARE convection are related to high parcel equilibrium levels, which approach 100 mb in some cases. Convective available potential energies are larger than those in the GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment (GATE) mainly because of the higher equilibrium levels. The buoyancy integrated over the lowest 500 mb is similar for the two experiments. Convective inihibitions are small, enabling convection to propagate with only weak forcing. Comparison of slow-moving shear-parallel bands in COARE and GATE suggests that lower relative humidities between the top of the mixed layer and 500 mb can shorten their lifetimes significantly.

COARE mesoscale organization and evolution differs from what was observed in GATE. Less-organized convection is more common in COARE. Of the convective bands observed, a greater fraction in COARE are faster-moving, shear-perpendicular squall lines. GATE slow-moving lines tend to be longer lived than those for COARE. The differences are probably traceable to differences in environmental shear and relative humidity, respectively.

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

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No abstract available

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Margaret A. Lemone, Tae Y. Chang, and Christopher Lucas

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No abstract available.

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