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Margaret A. Lemone

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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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Xiaoqing Wu and Margaret A. LeMone

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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 David P. Jorgensen

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This is the second paper of a two-part series documenting the structure of and momentum transport by a subtropical mesoscale convective system near Taiwan, using Doppler radar data and in situ data from the NOAA P-3. Part I defines the basic system structure and evolution. In Part II, the momentum transport by the system is estimated and related to system structure, and the momentum budget for a portion of the embedded convective band is evaluated.

Profiles of the vertical flux of horizontal momentum are constructed from in situ data, Doppler radar data, and both combined, in a coordinate system with u normal to the line and positive eastward, since the low-level air is feeding the line from the east. Differences in the fluxes from the two sources appear to be mainly due to an underestimation of the mean vertical velocity from the Doppler radar data. The discrepancy results partially from the concentration of convergence in the boundary layer—precisely where the Doppler cannot adequately sample the convergence—and partially from Doppler problems above 5 km. However, the momentum-flux profile generated from both data sources has features consistent with the structure of the line: p̄uw is negative at lower levels, consistent with the westward tilt of most updrafts at those levels, and positive at upper levels, consistent with the updrafts' eastward tilt. This positive flux is countergradient and not consistent with previous observations, but is suggested in numerical simulations of systems in an environment similar to that for this system, with relatively low convective available potential energy(CAPE), high relative humidity aloft, and positive u shear through the depth of the system. The simulated systems have relatively weak updrafts and gust fronts, also matching this case. The flux p̄vw is downgradient above ∼5 km and countergradient below, but is consistent with the average positive vertical velocity carrying southerlies (V̄>0) upward.

The momentum budget reveals some behavior that differs from that of earlier systems such as that studied by Lafore et al. For example, above 7 km the momentum transport and pressure gradient reinforce to produce substantial acceleration of air exiting the band at high levels toward the front (east), although the vertical transport contributes only a small amount to the observed acceleration. The u positive acceleration at higher levels, being larger than the Doppler estimates of dŪ/dt at lower levels, increases the overall u shear within the convective band. Estimation of the vertical momentum-flux divergence and pressure-gradient term at low levels from the in situ data supports this results. In previously observed tropical systems, u shear was increased by convective bands only when the u shear was negative. At midlevels, the vertical transport of line-parallel wind (v) by the line acts to increase and slightly elevate the southerly jet maximum in the environmental wind profile usually seen in this region. As in previously documented systems, dV̄/dz decreases with time within the band.

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Margaret A. LeMone and Rebecca J. Meitin

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Evidence indicates that fair-weather to towering cumulus clouds over the East Atlantic Ocean during GATE were frequently organized into mesoscale structures. Three examples of such structures are examined, using gust-probe aircraft data collected in parallel straight-and-level flight tracks at 150 m, and covering an area greater than 30×30 km. The aircraft (two cases) or rawinsonde (one case) data provide vertical profiles of mean wind, temperature and mixing ratio. Cloud patterns are revealed from an upward-looking infrared sensor on the aircraft and radar and satellite pictures.

The data show that the cumulus were organized into bands with horizontal wavelengths of 15–25 km. The circulations appear to extend through the subcloud layer, with all the fields at 150 m well related to the cloudiness overhead. Since the circulations are aligned with the subcloud-layer shear and travel in a direction parallel to the subcloud-layer wind (in the two cases for which band movement is documented), it is believed that they are primarily subcloud-layer phenomena. The subcloud-layer depth is about 600 m, giving aspect ratios of the bands from 25 to 50, in the range of mesoscale cellular convection observed in midlatitudes.

Several physical mechanisms which might explain the bands are discussed.

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Margaret A. LeMone and William T. Pennell

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A definite relationship between cloud distribution and sub-cloud layer structure and fluxes in fair weather is documented using measurements of wind, temperature, humidity and overhead cloud occurrence from the NCAR DeHasvilland aircraft. Three cases are used. These were extracted from data taken to the north of Puerto Rico on 14 and 15 December 1972 in mesoscale regions of reasonably uniform convection ranging from suppressed with very little shallow cloudiness to slightly enhanced with active (but non-precipitating) trade cumulus having tops to 2000 m. On both days synoptic conditions were suppressed and the surface winds were from the cast at 10 to 15 m s−1.

In the highly suppressed cases, there is evidence that cloud distribution was determined by subcloud layer circulations—roll vortices which persisted throughout the flight patterns. In the more enhanced case, the predominant coupling was by well-defined cloud scale updrafts which were traceable to at least 100 m below cloud base.

As a consequence of these interactions, the fluxes of moisture and momentum in the upper subcloud layer were found to be strongly coupled to cloud distribution. A comparison of direct measurements from the aircraft and the results of budget computations by other workers for several suppressed situations in the trades suggests that almost all of the fluxes out of the mixed layer are concentrated in mesoscale cloud patches and that a large function of the transport is due to motions on the scale of the individual cumulus.

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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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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, Lesley F. Tarleton, and Gary M. Barnes

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We examine the pressure fields wound the cloud-base updraft of three cumulus clouds observed in environments with low vertical shear of the horizontal wind near cloud base. These fields are compared to the corresponding pressure fields beneath convective clouds embedded in moderate to large shear. All of the pressure fields are derived from aircraft measurements taken during the 1981 Cooperative Convective Experiment, CCOPE.

The pressure fields associated with these low-shear clouds are weaker than those for the clouds in higher shear. Furthermore, the low-shear fields are not consistently dominated by the dynamic pressure created by the interaction of the cloud-base updraft with the vertical shear of the horizontal wind. The weaker dynamic pressure is due to the smaller size and intensity of the cloud-base updraft as well as the smaller vertical shear of the horizontal wind. The reduction of the dynamic Pressure allows buoyancy effects on the pressure field to become more apparent.

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