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Christopher Lucas and Edward J. Zipser

Abstract

This study provides quantitative estimates of the thermodynamic and kinematic structures of the troposphere during various convective regimes observed during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. The data source is the upper air soundings from six stations in the intensive flux array. A correction algorithm has been applied to the humidity data to remove biases between the stations. The data are analyzed using the nonhierarchical clustering method known as k means. Eleven thermodynamic clusters and 20 kinematic clusters are selected.

The thermodynamic clusters are grouped into four general categories based on their midtropospheric equivalent potential temperature. Deep convective activity varies with the thermodynamic structure of the environment. When the “dry intrusion” group is observed, convection is suppressed. The “fair weather” category corresponds to undisturbed periods with light winds and small mesoscale convective systems (MCSs). The largest MCSs and the majority of the rainfall occur with the “active” and “convective recovery” categories.

The kinematic clusters are also divided into four general categories based on the strength, direction, and depth of the low-level zonal flow. The timing of the clusters is related to the intraseasonal oscillation (ISO). Dry phases of the ISO are characterized by the “low-level easterly” category. During transition periods between the easterly and westerly phases of the ISO, the “calm” category is often seen. The “moderate shear westerly” group is seen just before the strongest westerlies. The majority of the clusters fall into the “strong shear westerly” group, associated with the peak westerly phase of the ISO.

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Christopher Lucas, Edward J. Zipser, and Brad S. Ferrier

Abstract

Two-dimensional experiments using the Goddard Cumulus Ensemble model are performed in order to examine the influence of environmental profiles of wind and humidity on the dynamical and microphysical structure of mesoscale convective systems (MCSs) over the tropical oceans. The initial environments used in this study are derived from the results of a cluster analysis of the TOGA COARE sounding data. The model data are analyzed with methods and measurements similar to those used in observational studies.

Experiments to test the sensitivity of MCSs to the thermodynamic profile focus on the role of humidity in the free troposphere. In the experiments, a constant amount of relative humidity is added to every level above the boundary layer. As humidity is increased, model storms transition from weak, unsteady systems with little precipitation to strong, upshear-tilted systems with copious rainfall. This behavior is hypothesized to be the result of the entrainment of environmental air into the updraft cores.

Experiments to test the sensitivity of MCSs to the kinematic profile focus on the amount of vertical wind shear in the midlevels, between approximately 2 and 10 km. Five kinematic profiles are used. The dynamical and microphysical characteristics of the runs changed dramatically in different shear environments. Shear in the midlevels affects the convective systems by altering the perturbation pressure field. Stronger shear results in a broader and deeper mesolow below the updraft and a more intense dynamic high above the leading edge.

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

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No abstract available.

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

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No abstract available

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

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No abstract available.

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