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Joanne Simpson

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Wei-Kuo Tao and Joanne Simpson

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A multidimensional and time-dependent cloud scale model is used to investigate the dynamic and micro-physical processes associated with convective and stratiform regions within a tropical squall-type convective line. The evolution of the total convective and stratiform portions of rainfall is also estimated by using model output. A three-dimensional version of the model covers a horizontal domain about 96 × 96 km2. Frequently, the horizontal extent of an observed stratiform region is over a few hundred kilometers. Therefore, a two-dimensional version of the model with a 512 km horizontal length is also used to incorporate a complete stratiform region.

Two-dimensional model result recapture many interesting features as observed. In particular, the fractional portion of stratiform rain as well as its fractional area coverage are in good agreement with observations. A significant amount of ice particles melted to rain near the freezing level in the trailing part of the modeled squall system during its mature and dissipating stages. The mesoscale circulations above and beneath the freezing level in the stratiform region are also well simulated. Three-dimensional model results could not recapture these features associated with the stratiform region. But explosive growth and a convex-leading edge associated with the convective region are well simulated. The orientation of the three-dimensional simulated convective line is perpendicular to the environmental wind shear as observed. Both of the modeled propagation speeds for the squall systems are in fair agreement with observational case studies.

Sensitivity tests on ice-phase microphysical processes and mesoscale middle and upper level ascent are made to investigate their roles on the formation and structure of tropical squall-type convective lines. Parcel trajectory analyses are also performed to understand the dynamics of simulated squall-type convective lines. Specifically, the origins of air circulation in the convective and stratiform region are investigated using the model generated wind fields. The heat budgets and their associated microphysical processes within the convective and stratiform region are also examined using the model results.

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Alan K. Betts and Joanne Simpson

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We reexamine the idealized hurricane boundary layer budget from Malkus and Riehl using vector diagrams for the thermodynamic budgets in the light of recent observations studies. We conclude that a large air-sea temperature difference can only be maintained with both large fluxes through cloud-base level and a large evaporative cooling of the subcloud layer. The high θE values observed in hurricane eyewalls can be reached if these cloud-base and evaporative fluxes are reduced and the subcloud layer moves toward the sea surface virtual potential temperature.

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Wei-Kuo Tao and Joanne Simpson

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A total of nine three-dimensional experiments are made to study cloud interaction and merging under the influence of different imposed conditions. Large-scale lifting forcing, environmental wind shear and cloud microphysical processes are the three parameters to be varied. The basic design of the study is to generate several convective clouds randomly inside the model domain and, then, to observe and analyze the interactions and merging between the simulated clouds. The locations as well as the intensities of simulated clouds while they interact with each other are not predetermined. A two-dimensional version of the model has been used to investigate the effects upon merging produced by varying large-scale conditions with a GATE dataset. In this study, we continue studying the cloud interactions and merging problems through using a fully three-dimensional model and the same dataset.

Ten merged systems involved precipitating clouds are identified in this numerical study. Eight mergers involve two previously separated clouds; seven of them generally lie along a line parallel to the initial environmental wind shear vector (called parallel cells). Only one merger lies along a line rather perpendicular to the wind shear vector prior to the merging (called perpendicular cells). A significant difference between the parallel and the perpendicular cells is that the latter cells are usually situated closer to each other prior to merging than the former cells. The distance between the perpendicular cells prior to merging is usually about 5 to 6 km. The distance between the parallel cells prior to merging can be 10 km or more. The remaining two merged systems involve three clouds and they are a combination of parallel and perpendicular cells.

The merging mechanism associated with three cloud merging cases is studied through examining the temperature, pressure and wind fields prior to, during and following the merging of clouds. The first case involves a pair of precipitating clouds with differential propagation speeds. Both clouds propagate along the direction of the vertical wind shear. The second case is a perpendicular cell and the third case involves three clouds. A cloud bridge, which consists of a few low-level clouds which develop and connect the merging clouds prior to or during the merging process, occurs in all three cases. Trajectory analyses indicate that the high rising air parcels at the bridge area are strongly affected by either one or two interacting cold outflows. This specific study suggests that the primary initiating mechanism for the occurrence of a precipitating cloud merger is the cloud downdrafts and their associated cold outflows.

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Wei-Kuo Tao and Joanne Simpson

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A total of 48 numerical experiments have been performed to study cloud interactions and merging by means of a two-dimensional multi-cell model. Two soundings of deep convection during GATE and two different magnitudes of large-scale lifting.have been used as the initial conditions and as the main forcing on the model.

Over two hundred groups of cloud systems with a life history of over sixty minutes have been generated under the influence of different combinations of the stratification and large-scale lifting. The results demonstrate the increase in convective activity and in amount of precipitation with increased intensity of large-scale lifting. The results also show increased occurrence of cloud merger with increased intensity of large-scale lifting. The most unfavorable environmental conditions for cloud merging are 1) less unstable stratification of the atmosphere and 2) weaker large-scale lifting.

A total of fourteen cloud systems qualify as mergers. Two selected cases will be described dynamically and thermodynamically in this paper. Although these cloud mergers have been simulated under the influence of different synoptic-scale conditions, the major physical mechanism related to the cloud merging process is the same as that proposed by Simpson. Cumulus downdrafts and associated cold outflows play a dominant role in the merging process in all cases studied.

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Joanne Simpson, Gary Van Helvoirt, and Michael McCumber

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Schlesinger's (1978) three-dimensional cumulus model is applied to showering congestus clouds on day 261 of GATE. Adjustments are made to the microphysical and turbulent parameterizations, the former to be consistent with coalescence growth of warm rain.

A cylindrical initial perturbation, in approximate agreement with GATE observations, was run with a characteristic thermodynamic sounding and four different wind profiles. The four wind profiles were 1) the observed three-dimensional flow, 2) uniplanar winds, 3) unidirectional winds, and 4) zero synoptic flow. A fifth run was made with the observed three-dimensional wind and a slightly moistened destabilized sounding characteristic of a cumulonimbus environment.

Model results are compared with each other and with observations to analyze the effects of varying shear and altered sounding. Relationships between shear, mesovortices and dynamic entrainment are examined, as well as the model clouds’ impact on the environment as a function of shear. The simulations appear to resemble reality in many important aspects. Altostratus layers observed on day 261 are found to be a by-product of convection in three-dimensional shear. Rapid erosion of cloud base to 3.6 km is related to the ambient thermal structure, with wind shear and initial perturbation playing a secondary role.

Some of the apparent conflict regarding lateral versus cloud-top entrainment is clarified, as well as some factors governing convective downdraft structure and intensity. Finally, recommendations are made for further observations and model improvements.

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Xiping Zeng, Wei-Kuo Tao, and Joanne Simpson

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This paper addresses an equation for moist entropy in the framework of cloud-resolving models. After rewriting the energy equation with moist entropy in the place of temperature, an equation for moist entropy is obtained. The equation expresses the internal and external sources of moist entropy explicitly, providing a basis for the use of moist entropy as a prognostic variable in long-term cloud-resolving modeling. In addition, a precise formula for the surface flux of moist entropy from the underlying surface into the air above is derived.

The equation for moist entropy is used to express the Neelin–Held model for the diagnosis of large-scale vertical velocity. After applying the model to a tropical oceanic atmosphere with mean annual soundings, the paper shows the sensitivity of large-scale vertical circulations to the radiative cooling rate and the surface flux of moist entropy, which demonstrates the necessity for a precise equation for moist entropy in the analysis and modeling of large-scale tropical circulations.

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Joanne Simpson, Glenn W. Brier, and R. H. Simpson

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A randomized seeding experiment was carried out on 23 tropical oceanic cumulus clouds on 9 days in the summer of 1965 as part of the joint Navy-ESSA Project Stormfury. Following instructions in sealed envelopes, an aircraft seeded 14 of the clouds with 8-16 pyrotechnic silver iodide generators called Alecto units. Each unit releases about 1.2 kg of silver-iodide smoke. The nine remaining clouds were studied in an identical manner as controls, using the same stack of four instrumented aircraft to penetrate the cloud before and after the seeding run. Cloud growth was documented by aircraft, radar and photogrammetry. The seeded clouds grew vertically an average of 1.6 km more following the seeding run than did the control clouds; the difference is significant at the 0.01 level.

A numerical model of cumulus dynamics was specified in advance of the field program. This model integrates the equation for the vertical acceleration of an entraining cumulus tower, predicting top heights of unseeded and seeded clouds as a function of ambient sounding and horizontal tower dimension. Seedability is defined as the predicted difference between the seeded and unseeded top of the same cloud. Effect of seeding is defined as the difference between the observed top and the predicted unseeded top of the same cloud. Both parameters are computed and graphed for all 23 clouds. Seeded and unseeded clouds separate into distinct populations. This statistical analysis demonstrates that 1) seeding has a clear effect on cumulus growth under specifiable conditions and 2) the model has considerable skill in predicting the amount of growth and in specifying the conditions.

Sources of subjectivity and bias are shown to be small and not to affect the results. The sensitivity of the model predictions to variations in input data is investigated with two examples, one each of large and of negligible cloud growth following seeding. Some possible effects of natural glaciation are examined with the model and future phases of the program are described.

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Wei-kuo Tao, Joanne Simpson, and Su-Tzai Soong

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Two- and three-dimensional configurations of a cloud ensemble model are used to study the statistical properties of cloud ensembles under an observed large-scale condition. The basic design of the model has been presented in papers by Soong, Ogura, and Tao. An observed large-scale lifting and small amplitude random perturbations in the form of temperature fluctuations are imposed continuously in the model. The model then allows many clouds of different sizes to develop simultaneously. A 6-hour time integration is made to allow a large number of convective clouds to develop. After the model integration, horizontal and time averages of various relevant variables are computed to elucidate the statistical properties of clouds. The model is applied to the case of a well-organized intertropical convergence zone (ITCZ) rainband that occurred on 12 August 1974, during the Global Atmospheric Research Program's Atlantic Tropical Experiment.

The statistical properties of clouds, such as mass flux by cloud drafts and vertical velocity as well as condensation and evaporation associated with these cloud drafts are examined in this study. The cloud drafts are further subclassified as inactive and active. Separate contributions to cloud statistics in areas of different cloud activity are then evaluated. The model results compared well with those obtained from aircraft measurements. Some implications of model results to the cumulus parameterization problem are briefly discussed. A comparison between the two- and three-dimensional model simulations is also made.

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Brad Schoenberg Ferrier, Wei-Kuo Tao, and Joanne Simpson

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Part I of this study described a detailed four-class bulk ice scheme (4ICE) developed to simulate the hydro-meteor profiles of convective and stratiform precipitation associated with mesoscale convective systems. In Part II, the 4ICE scheme is incorporated into the Goddard Cumulus Ensemble (GCE) model and applied without any “tuning” to two squall lines occurring in widely different environments, namely, one over the “Pica) ocean in the Global Atmospheric Research Program's (GARP) Atlantic Tropical Experiment (GATE) and the other over a midlatitude continent in the Cooperative Huntsville Meteorological Experiment (COHMEX). Comparisons were made both with earlier three-class ice formulations and with observations. In both cases, the 4ICE scheme interacted with the dynamics so as to resemble the observations much more closely than did the model runs with either of the three-class ice parameterizations. The following features were well simulated in the COHMEX case: a lack of stratiform rain at the surface ahead of the storm, reflectivity maxima near 60 dBZ in the vicinity of the melting level, and intense radar echoes up to near the tropopause. These features were in strong contrast with the GATE simulation, which showed extensive trailing stratiform precipitation containing a horizontally oriented radar bright band. Peak reflectivities were below the melting level, rarely exceeding 50 dBz, with a steady decrease in reflectivity with height above. With the other bulk formulations, the large stratiform rain areas were not reproduced in the GATE conditions.

The microphysical structure of the model clouds in both environments were more realistic than that of earlier modeling efforts. Number concentrations of ice of O(100 L−1) occurred above 6 km in the GATE model clouds as a result of ice enhancement and rime splintering in the 4ICE runs. These processes were more effective in the GATE simulation, because near the freezing level the weaker updrafts were comparable in magnitude to the fall speeds of newly frozen drops. Many of the ice crystals initiated at relatively warm temperatures (above −15°C) grew rapidly by deposition into sizes large enough to be converted to snow. In contrast, in the more intense COHMEX updrafts, very large numbers of small ice crystals were initiated at colder temperatures (below −15°C) by nucleation and stochastic freezing of droplets, such that relatively few ice crystals grew by deposition to sizes large enough to be converted to snow. In addition, the large number of frozen drops of O(5 L−1) in the 4ICE run am consistent with airborne microphysical data in intense COHMEX updrafts.

Numerous sensitivity experiments were made with the four-class and three-class ice schemes, varying fall speed relationships, particle characteristics, and ice collection efficiencies. These tests provide strong support to the conclusion that the 4ICE scheme gives improved resemblance to observations despite present uncertainties in a number of important microphysical parameters.

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