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

Abstract

High-resolution 24-h runs of the Advanced Research version of the Weather Research and Forecasting Model are used to test eight objective methods for estimating convective boundary layer (CBL) depth h, using four planetary boundary layer schemes: Yonsei University (YSU), Mellor–Yamada–Janjic (MYJ), Bougeault–LaCarrere (BouLac), and quasi-normal scale elimination (QNSE). The methods use thresholds of virtual potential temperature Θυ, turbulence kinetic energy (TKE), Θυ,z, or Richardson number. Those that identify h consistent with values found subjectively from modeled Θυ profiles are used for comparisons to fair-weather observations from the 1997 Cooperative Atmosphere–Surface Exchange Study (CASES-97).

The best method defines h as the lowest level at which Θυ,z = 2 K km−1, working for all four schemes, with little sensitivity to horizontal grid spacing. For BouLac, MYJ, and QNSE, TKE thresholds did poorly for runs with 1- and 3-km grid spacing, producing irregular h growth not consistent with Θυ-profile evolution. This resulted from the vertical velocity W associated with resolved CBL eddies: for W > 0, TKE profiles were deeper and Θυ profiles more unstable than for W < 0. For the 1-km runs, 25-point spatial averaging was needed for reliable TKE-based h estimates, but thresholds greater than free-atmosphere values were sensitive to horizontal grid spacing. Matching Θυ(h) to Θυ(0.05h) or Θυ at the first model level were often successful, but the absence of eddies for 9-km grids led to more unstable Θυ profiles and often deeper h.

Values of h for BouLac, MYJ, and QNSE, are mostly smaller than observed, with YSU values close to slightly high, consistent with earlier results.

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

Abstract

Time series of 1-Hz vertical velocity data collected during aircraft penetrations of oceanic cumulonimbus clouds over the western Pacific warm pool as part of the Equatorial Mesoscale Experiment (EMEX) are analyzed for updraft and downdraft events called cores. An updraft core is defined as occurring whenever the vertical velocity exceeds 1 m s−1 for at least 500 m. A downdraft core is defined analogously. Over 19 000 km of straight and level flight legs are used in the analysis. Five hundred eleven updraft cores and 253 downdraft cores are included in the dataset.

Core properties are summarized as distributions of average and maximum vertical velocity, diameter, and mass flux in four altitude intervals between 0.2 and 5.8 km. Distributions are approximately lognormal at all levels. Examination of the variation of the statistics with height suggests a maximum in vertical velocity between 2 and 3 km; slightly lower or equal vertical velocity is indicated at 5 km. Near the freezing level, virtual temperature deviations are found to be slightly positive for both updraft and downdraft cores. The excess in updraft cores is much smaller than that predicted by parcel theory.

Comparisons with other studies that use the same analysis technique reveal that EMEX cores have approximately the same strength as cores of other oceanic areas, despite warmer sea surface temperatures. Diameter and mass flux are greater than those in GATE but smaller than those in hurricane rainbands. Oceanic cores are much weaker and appear to be slightly smaller than those observed over land during the Thunderstorm Project.

The markedly weaker oceanic vertical velocities below 5.8 km (compared to the continental cores) cannot be attributed to smaller total convective available potential energy or to very high water loading. Rather, the authors suggest that water loading, although less than adiabatic, is more effective in reducing buoyancy of oceanic cores because of the smaller potential buoyancy below 5.8 km. Entrainment appears to be more effective in reducing buoyancy to well below adiabatic values in oceanic cores, a result consistent with the smaller oceanic core diameters in the lower cloud layer. It is speculated further that core diameters are related to boundary layer depth, which is clearly smaller over the oceans.

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

Abstract

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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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

Abstract

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

Abstract

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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David P. Jorgensen, Margaret A. LeMone, and Ben Jong-Dao Jou

Abstract

The precipitation, thermodynamic, and kinematic structure of an oceanic mesoscale convective system is studied using airborne Doppler and in situ (flight-level) data collected by the NOAA P-3 aircraft. The system, a well-organized, stationary, north-south convective line, was located near the east coast of Taiwan. In Part I, the basic structure of the line is documented with both datasets, a procedure revealing the strengths and weakness of both approaches.

The Doppler data reveal that the warm, moist air feeding the line enters from the east side. Most updrafts associated with the leading edge of the convective line tilt westward below 5 km and then eastward above 5 km. This change of tilt corresponds to a change in the sign of the vertical flux of east-west momentum. To the east of the leading edge, a 10-km-wide zone of strong mesoscale descent is seen. The band is not a complete barrier to the low-level southeasterly flow, and at times and places along the line the inflowing air can move through the band with little or no upward acceleration. The minimum pressures at low levels lie east of the highest reflectivity and also underneath the tilted updraft at upper levels, in agreement with the tilt of the updraft, the buoyancy distribution, and the interaction of the updraft with the vertical shear of the horizontal wind. The Doppler data show very few convective-scale downdrafts and no low-level gust front that would organize the convection as in propagating squall lines, although lack of resolution in the pseudo-dual-Doppler data at the lowest levels may mask features with horizontal scales <5 km. Vertical incidence Doppler observations show only a few relatively weak convective-scale downdrafts within the heavy rainfall region of the convective line.

The in situ data confirm that warm, moist air feeds the convective line from the east side, but they show a larger fraction of air coming into the convection from the boundary layer than do the Doppler data. They confirm that the line is not an effective barrier to the flow: some air from the east of the line, including boundary-layer air, passes through the line without joining the updrafts. Again, some weak convective-scale downdrafts are evident, but a gust front was not detected. However, at low levels, a pool of low-θe, air lies 10–20 km to the west of the line, outside the dual-Doppler domain. This cool air apparently originated to the north (beneath an extensive stratiform area, but preexisting baroclinicity associated with a front may have also contributed to the cool air) and advected southward. Vertically incident Doppler data confirm the upper-level downdraft zone to the east of the updraft. Above 2 km, the pressure and vertical velocity fields are consistent, with low pressure lying beneath the tilting updrafts in both datasets. Below 2 km, the in situ data reveal a mesolow beneath the westward-tilting updraft that was not captured by the Doppler data, apparently because of contamination of the very lowest levels by ground clutter.

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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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Margaret A. LeMone, Thomas W. Schlatter, and Robert T. Henson

Scientific investigation is supposed to be objective and strictly logical, but this is not always the case: the process that leads to a good conclusion can be messy. This narrative describes interactions among a group of scientists trying to solve a simple problem that had scientific implications. It started with the observation of a cloud exhibiting behavior associated with supercooled water and temperatures around −20°C. However, other aspects of the cloud suggested an altitude where the temperature was around −40°C. For several months following the appearance of the cloud on 23 March 2011, the people involved searched for evidence, formed strong opinions, argued, examined evidence more carefully, changed their minds, and searched for more evidence until they could reach agreement. While they concluded that the cloud was at the higher and colder altitude, evidence for supercooled liquid water at that altitude is not conclusive.

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