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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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Joanne Simpson, Jane C. Eden, and Anthony R. Olsen

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Combination of numerical simulation, many simultaneous measurements, and a large assortment of statistical tools, employed at all stages, have been found useful in design and evaluation of modification experiments on cumulus clouds. A randomized sample is essential, although non-random controls have supplemented it by providing necessary information on natural distributions.Obstacles to definitive estimates of treatment effects are huge natural variability compounded by the expense and labor involved in obtaining an adequately large data sample. A 26 pair data set from a dynamic seeding experiment on isolated Florida cumuli is used here to illustrate both the problems and the combined approach used to overcome them. In this data set, rain volumes from unmodified single cumuli varied by three orders of magnitude on days screened as suitable. The field phase of the experiment cost above $250,000, requiring instrumented aircraft, calibrated radar, and several radiosondes daily.Numerical simulation of seeded and unseeded cumulus towers defined the key screening variable “seedability,” namely the predicted height difference between seeded and unseeded towers, so that only days on which the physical seeding hypothesis would be expected to work are selected for experimentation. On those days, randomization is between clouds selected by the experimenters as suitable.Classical and Bayesian statistics are used together in the evaluation, with both univariate and multivariate analyses. Various well-known probability density distributions fitted the seeded and unseeded rainfalls. Among the best were gamma, log-normal, beta-K and beta-P. Seed-control differences were examined with nonparametric and parametric tests (some of the latter after data transformation) and effects of random and systematic measurement errors were considered. In all tests, the seed-control rainfall difference was significant at better than 5%. A multiplicative seeding factor of 2–3 was estimated in several ways (allowing for or getting around the bias problem with ratio estimators related to long-tailed distributions).

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Joanne Simpson, Michael Garstang, Edward J. Zipser, and Gordon A. Dean

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Data for a 3-day period from an experimental region formed by 6 islands of the Windward Group in the West Indies, a research vessel in the tropical Atlantic, an instrumented aircraft, and TIROS VI and VII form the basis for the study of a synoptic-scale tropical disturbance. Detailed analysis coupled with careful integration of all the observations, make it possible to describe the structure of the system, and show it to be distinct from classically modelled travelling disturbances. Instead, the disturbance is shown to be uncorrelated with any low-level perturbation of wind direction; its evolution and its translation depend upon small in situ development and decay in the upper troposphere.

Examples of similar disturbances are shown as evidence that this is not an isolated system. Emphasis is placed upon the necessity of establishing dynamic and physical models of the flow field and their attendant cloud and rainfall patterns, as well as the role played by convective and mesoscale processes in the formation, maintenance and decay of such systems. To do this, carefully designed experiments based upon a selected island network and incorporating joint aircraft, oceanographic, and satellite programs are necessary.

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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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Edward Rodgers, William Olson, Jeff Halverson, Joanne Simpson, and Harold Pierce

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The distribution and intensity of total (i.e., combined stratified and convective processes) rain rate/latent heat release (LHR) were derived for Tropical Cyclone Paka during the period 9–21 December 1997 from the F-10, F-11, F-13, and F-14 Defense Meteorological Satellite Special Sensor Microwave Imager and the Tropical Rainfall Measuring Mission Microwave Imager observations. These observations were frequent enough to capture three episodes of inner-core convective bursts and a convective rainband cycle that preceded periods of rapid intensification. During these periods of convective bursts, satellite sensors revealed that the rain rates/LHR 1) increased within the inner-core region, 2) were mainly convectively generated (nearly a 65% contribution), 3) propagated inward, 4) extended upward within the mid- and upper troposphere, and 5) became electrically charged. These factors may have increased the areal mean ascending motion in the mid- and upper-troposphere eyewall region, creating greater cyclonic angular momentum, and, thereby, warming the center and intensifying the system.

Radiosonde measurements from Kwajalein Atoll and Guam, sea surface temperature observations, and the European Centre for Medium-Range Forecasts analyses were used to examine the necessary and sufficient conditions for initiating and maintaining these inner-core convective bursts. For example, the necessary conditions such as the atmospheric thermodynamics [i.e., cold tropopause temperatures, moist troposphere, and warm SSTs (>26°C)] fulfill the necessary conditions and suggested that the atmosphere was ideally suited for Paka’s maximum potential intensity to approach supertyphoon strength. Further, Paka encountered moderate vertical wind shear (<15 m s−1) before interacting with the westerlies on 21 December. The sufficient conditions that include horizontal moisture and the upper-tropospheric eddy relative angular momentum fluxes, on the other hand, appeared to have some influence on Paka’s convective burst. However, the horizontal moisture flux convergence values in the outer core were weaker than some of the previously examined tropical cyclones. Also, the upper-tropospheric outflow generation of eddy relative angular momentum flux convergence was much less than that found during moderate tropical cyclone–trough interaction. These results indicated how important the external necessary condition and the internal forcing (i.e., convective rainband cycle) were in generating Paka’s convective bursts as compared with the external sufficient forcing mechanisms found in higher-latitude tropical cyclones. Later, as Paka began to interact with the westerlies, both the necessary (i.e., strong vertical wind shear and colder SSTs) and sufficient (i.e., dry air intrusion) external forcing mechanisms helped to decrease Paka’s rain rate.

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Joanne Simpson, Jeffrey Halverson, Harold Pierce, Carlos Morales, and T. Iguchi
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Brad Schoenberg Ferrier, Joanne Simpson, and Wei-Kuo Tao

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Different definitions of storm precipitation efficiency were investigated from numerical simulators of convective systems in widely varying ambient conditions using a two-dimensional cloud model with sophisticated ice microphysics. The model results indicate that the vertical orientation of the updrafts, which is controlled by the vertical wind shear, and the ambient moisture content are important in determining storm efficiency.

In terms of rainfall divided by condensation, simulated efficiencies ranged from 20%–35% for convective systems that tilted strongly against the low-level shear (upshear), to 40%–50% for erect storms. Changes in environmental moisture produced smaller variations in efficiency that were less than 10%. Upright convection allows for effective collection of cloud condensate by precipitation, whereas lower efficiencies in upshear storms are due to greater evaporation of cloud at middle levels and evaporation of rain at lower levels. Development of trailing stratiform precipitation is promoted by the rearward transport of moisture and condensate in upshear-tilted updrafts with evaporation moistening the ambient air as it passes through the convection. The fraction of rainfall from stratiform processes increases with upshear tilt of the convection and is inefficient. Rainfall from convection tilting downshear is efficient in terms of the total condensation, but is inefficient in terms of the flux of vapor into the storm because the gust fronts are too weak to completely block the low-level inflow.

Different closure assumptions in cumulus parameterization schemes that use functional relationships for precipitation efficiency were evaluated. None of them showed consistent agreement with the efficiency parameters diagnosed from the simulations.

Detailed diagnostics over various temporal and spatial scales indicate that storm efficiency determined by total condensation varied much less than that obtained from moisture convergence. The former definition should be more useful in cumulus parameterizations. Spatial variations in moisture convergence were dominated by changes in net condensation within the area of the storm, while variability at larger scales resulted from the advection of dry air in downdraft wakes.

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Christian Kummerow, William Barnes, Toshiaki Kozu, James Shiue, and Joanne Simpson

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This note is intended to serve primarily as a reference guide to users wishing to make use of the Tropical Rainfall Measuring Mission data. It covers each of the three primary rainfall instruments: the passive microwave radiometer, the precipitation radar, and the Visible and Infrared Radiometer System on board the spacecraft. Radiometric characteristics, scanning geometry, calibration procedures, and data products are described for each of these three sensors.

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Joanne Simpson, Michael Garstang, Edward J. Zipser, and Gordon Dean

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