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Wei-Kuo Tao and Joanne Simpson

Abstract

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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Joanne Simpson and William L. Woodley

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After four summer periods of randomized experimentation with dynamic cumulus seeding in a 1.3 × 104km2 target area in south Florida, 14 seed and 23 control cases are available, with increased documentation of radar measurement accuracy.Seed-control rainfall comparisons are made for “floating” and total target for the 6 h period following the first seeding. On days screened as suitable for the experiments, natural rain volume varied by a factor of 62 for floating target and by a factor of 25 for total target. Area seed-control rainfall differences are not significant with six classical tests, nor is the difference between random and non-random controls.Analysis of isolated experimental clouds obtained on days of multiple cloud seeding produced significant findings. Results were stratified depending on whether the single clouds dissipated in the target area without merger or whether they merged with a neighbor. With the former stratification, the mean seeded rainfall exceeded the mean control rainfall by a factor of 2, a result (one-tailed significance of 3%) that is consistent with earlier single cloud studies. No meaningful rainfall comparison was possible with the latter stratification because, on the average, the seeded clouds merged (and were dropped) 13 min earlier than the controls. This disparity in mean lifetimes before merger (two-tailed significance level of 0.5%) suggests that seeding is promoting merger in FACE as intended.Several Bayesian approaches are used to estimate a probability distribution of a multiplicative seeding factor, based on gamma rainfall distributions, with the same shape parameter for seeded and control populations. The most general treatment assigns prior probabilities to three variables, the common shape parameters, the mean of the control distribution, and the multiplicative seeding factor. With existing data, 95% of the area under the marginal density of the seeding factor lies between about 0.7 and 1.7, with a mean just above and a mode just below 1.After extensive search for physically meaningful covariates or predictors, radar echo motions in or near the target related to two distinct rainfall populations. Category 1 comprised those cases where echoes were “marching” across the area. Category 2 comprised those cases with growth and dissipation virtually without motion. Echo motion is shown to be a statistically significant covariate, accounting for 30% of the variation in the total rainfall. For the afternoon measurement period, the mean target rainfall in Category 2 cases exceeded that in Category 1 cases by a factor of 2.5.Separate seed-control comparisons in the two categories indicate that different effects of seeding might be sought in continued experimentation. Although the existing sample is small, there is evidence that in Category 1 (marching) the seeding effect is probably not multiplicative.Attempts are in progress to estimate the number of further cases required to resolve a range of postulated seeding effects in this experimental context.

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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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Ronald Biondini, Joanne Simpson, and William Woodley

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The data obtained from the Florida Area Cumulus Experiment in the years 1970–75 are analyzed statistically. Specifically a set of empirically derived predictors for both seeded and unseeded rainfall is identified. First the experiment is briefly described and the data given. The concept of echo motion categories is presented. The responses to be predicted and the variables used as predictors are listed and described and the methods for obtaining the prediction models are given. Next comes a listing of the model equations obtained by those methods, along with some commentary on their possible physical meaning. Examples illustrate the use of some of these prediction models for estimating seeding effects and possible bias in selection of experimental days. A discussion of the echo motion covariate and the basic predictor variables, their histories, rationales and some theoretical indications of their importance completes the main body of the paper.

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Joanne S. Malkus and Robert H. Simpson

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Yansen Wang, Wei-Kuo Tao, and Joanne Simpson

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A two-dimensional cloud-resolving model is linked with a TOGA COARE flux algorithm to examine the impact of the ocean surface fluxes on the development of a tropical squall line and its associated precipitation processes. The model results show that the 12-h total surface rainfall amount in the run excluding the surface fluxes is about 80% of that for the run including surface fluxes (domain-averaged rainfall, 3.4 mm). The model results also indicate that latent heat flux or evaporation from the ocean is the most influential factor among the three fluxes (latent heat, sensible heat, and momentum) for the development of the squall system. The average latent and sensible heat fluxes in the convective (disturbed) region are 60 and 11 W m−2 larger, respectively, than those of the nonconvective (clear) region due to the gust wind speed, a cool pool near the surface, and drier air from downdrafts associated with the convective activity. These results are in good agreement with observations.

In addition, sensitivity tests using a simple bulk aerodynamic approximation as well as a Blackadar-type surface flux formulation have predicted much larger latent and sensible heat fluxes than those obtained using the TOGA COARE flux algorithm. Consequently, much more surface rainfall was simulated using a simple aerodynamic approximation or a Blackadar-type surface flux formulation. The results presented here also suggest that a fine vertical resolution (at least in the lowest model grid point) is needed in order to study the interactive processes between the ocean and convection using a cloud-resolving model.

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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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Haiyan Jiang, Jeffrey B. Halverson, and Joanne Simpson

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It has been well known for years that the heavy rain and flooding of tropical cyclones over land bear a weak relationship to the maximum wind intensity. The rainfall accumulation history and rainfall potential history of two North Atlantic hurricanes during 2002 (Isidore and Lili) are examined using a multisatellite algorithm developed for use with the Tropical Rainfall Measuring Mission (TRMM) dataset. This algorithm uses many channel microwave data sources together with high-resolution infrared data from geosynchronous satellites and is called the real-time Multisatellite Precipitation Analysis (MPA-RT). MPA-RT rainfall estimates during the landfalls of these two storms are compared with the combined U.S. Next-Generation Doppler Radar (NEXRAD) and gauge dataset: the National Centers for Environmental Prediction (NCEP) hourly stage IV multisensor precipitation estimate analysis. Isidore produced a much larger storm total volumetric rainfall as a greatly weakened tropical storm than did category 1 Hurricane Lili during landfall over the same area. However, Isidore had a history of producing a large amount of volumetric rain over the open gulf. Average rainfall potential during the 4 days before landfall for Isidore was over a factor of 2.5 higher than that for Lili. When using the TRMM-based MPA-RT rainfall estimate, results are consistent with a previous study, which analyzed just the infrared-based rain estimation; that is, the rain potential history could be used as a predictor for the storm’s potential for inland flooding 3–4 days in advance of landfall.

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Joanne Simpson and Cecilia Griffith
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