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

Abstract

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

Abstract

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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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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Margaret A. LeMone, Gary M. Barnes, and Edward J. Zipser

Abstract

Examination of aircraft and rawinsonde data gathered in nine tropical mesoscale convective line cases indicates that all but two lines systematically increased front-to-rear momentum at heights greater than about 4 km, and rear-to-front momentum at lower levels, where “front” is defined as the direction toward which the line is moving. The convective lines were characterized by a leading 10–20 km wide band of convective clouds, and a trailing region of stratiform cloudiness. Most wore “propagating” lines, moving into the wind at all levels. Consistent with mixing-length theory, the vertical transport of the horizontal wind component parallel to the lines was down the vertical gradient of the component, resulting in a decrease of its vertical shear. Smaller, more random cloud groups and the upper portions of a convective line with isolated towers transported both components of horizontal momentum downgradient.

Normalization of the vertical flux of horizontal momentum normal to the line (u′¯w′¯) suggests that it is achieved mainly by updraft cores which could be traced to the undisturbed mixed layer ahead of the line. The air in the cores is accelerated upward and backward into a mesoscale area of low pressure located in the rear portion of the line's leading convective region. The low pressure is primarily hydrostatic, its intensity proportional to the depth and average buoyancy of the cloudy air overhead. However, dynamic pressure effects are important where convective cores are particularly concentrated. From the aircraft data, the momentum transport by the trailing, stratiform region appears small, but this conclusion needs confirmation by sensing platforms more suited to gathering mesoscale wind field data. The failure to account for the momentum transport properties of two-dimensional convective lines might explain the lack of success in parameterizing the effects of cumulus clouds on the mean wind profile.

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Chin-Hoh Moeng, Gregory S. Poulos, and Margaret A. LeMone
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Edward J. Zipser, Rebecca J. Meitín, and Margaret A. LeMone

Abstract

The structure of the convective band of 14 September in the dense GATE observing array is determined using wind and thermodynamic data primarily from multiple aircraft penetrations, which are well distributed in the vertical and in time.

The well-defined mesoscale features in the line, which are 10–40 km in scale, quasi-two-dimensional, and persist for several hours, determine the distribution of the convective-scale features, which are 5 km or less in size, three-dimensional, not generally detectable for more than one flight leg. At the leading edge, a 30 km zone of strong ascent is computed from two-dimensional continuity. Here, lifting of the ambient air creates a favorable environment—not found elsewhere—for deep cumulonimbus clouds to develop. Their updrafts are weak, 2–4 m s−1 on the average. Behind the updraft zone, below 3–4 km, is a broad descent zone. It corresponds to the stratiform rain area, and has little convection, and some drying at lower levels. On the average, the mass flux by the mesoscale and convective-scale drafts of the updraft zone is about twice as much as that of the descent zone. The rainfall rate in the updraft zone is generally in excess of 8 mm h−1, while that in the downdraft region is less. The horizontal winds normal to the line are strongly modified by pressure forces, while those parallel to the line are changed mainly through mixing. Strong vertical vorticity is created in the line by tilting of the mean shear of the parallel component.

As the system matures, the downdraft mass flux increases relative to the updraft mass flux, so that the net mass flux becomes negative during the decay phase. The fraction of the total rain falling in the stratiform zone increases with time. However, considerable rain still falls from intense convective cells as well as the stratiform “anvil” even when the net mass flux goes to zero in the lowest kilometer.

The structure and evolution of the line is similar to that of tropical squall lines, but it is less spectacular. Winds are weaker, there is less mass flow through the system, movement is slower, and there is less drying in the rain area. The line is aligned with the wind and shear, rather than across it, as is the case for many squall lines.

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David P. Jorgensen, Edward J. Zipser, and Margaret A. LeMone

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Hurricane vertical motion properties are studied using aircraft-measured 1 Hz time series of vertical velocity obtained during radial penetrations of four mature hurricanes. A total of 115 penetrations from nine flight sorties at altitudes from 0.5 to 6.1 km are included in the data set. Convective vertical motion events are classified as updrafts (or downdrafts) if the vertical velocity was continuously positive (or negative) for at least 500 m and exceed an absolute value of 0.5 m s−1. Over 3000 updrafts and nearly 2000 downdrafts are included in the data set. A second criteria was used to define stronger events, called cores. This criteria required that upward (or downward) vertical velocity be continuously greater than an absolute value of 1 m s−1 for at least 500 m.

The draft and core properties are summarized as distributions of average and maximum vertical velocity, diameter, and vertical mass transport in two regions: eyewall and rainband. In both regions updrafts dominated over downdrafts, both in number and mass transport. In the eyewall region, the draft and core strength distributions were similar to data collected by aircraft in GATE cumulonimbus clouds. Unlike GATE clouds, however, the largest updraft cores (larger than 90% of the distribution) were over twice as large and transported twice as much mass as did the corresponding GATE updraft cores. Eyewall ascent was highly organized in a channel several kilometers wide located a few kilometers radically inward from the radius of maximum tangential wind.

As in GATE, the strongest hurricane updraft cores were weak in comparison with the strongest updrafts observed in typical midlatitude thunderstorms. Mean eyewall profiles of radar reflectivity and cloud water content are discussed to illustrate the microphysical implications of the low updraft rates.

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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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