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Su-Tzai Soong

Abstract

An axisymmetric warm cloud model including 36 classes of droplets from 1 to 4096 μm is developed. A reference spectrum is prescribed for the formation of droplets around condensation nuclei. Other microphysical processes incorporated in the model are condensation/evaporation, stochastic coalescence, sedimentation and drop breakup. Accurate computation for the condensation/evaporation process and for the stochastic coalescence equation can be achieved by the methods used in this study. The differences in the life cycle and the precipitation process between maritime and continental cumuli with cloud tops around 3 km are investigated in this model by taking into account only the differences in microphysical processes. The production of large drops in the continental cloud is confined to the cloud top region and a distinct bimodal distribution in the drop spectrum is formed as the large drops fall into the lower parts of the cloud. The failing rain has little effect on the cloud life cycle because the amount is very small. Cloud decay is mainly caused by mixing and evaporation in the cloud top and in levels just above the cloud base. In the maritime cloud, large drops are produced throughout the depth of the cloud. The drop spectrum also has a bimodal distribution and this bimodal distribution is produced by the coalescence process. There is also a period of intensive rain. The evaporation of some of this rainwater in the subcloud layer is the main reason for production of a downdraft, which in turn caused the decay of the cloud.

A cloud model with parameterized microphysical processes is also used to examine the adequacy of the parameterization scheme. Rain formed in the parameterized cloud earlier than in the maritime cloud, but its rainfall pattern and life span are close to those of the maritime cloud. On the other hand, the continental cloud and the parameterized cloud both exhibit a second updraft pulsation. However, the overall structure and important factors such as the cloud efficiency of the parameterized cloud are different from either the continental or the maritime clouds.

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Su-Tzai Soong and Yoshimitsu Ogura

Abstract

Axisymmetric and slab-symmetric cumulus cloud models with Kessler's parameterizations for microphysical processes are developed. By using a staggered grid arrangement and applying a modified upstream difference scheme, erroneous behavior in the center of a simulated cloud, which would result with the use of the ordinary upstream difference scheme, is eliminated. A comparison between the present two models of different geometries confirms in general the conclusions reached in previous studies: the updraft in an aixsymmetric model grows more vigorously than in a slab-symmetric model. However, the ratio of the maximum updraft in the slab-symmetric model to that in the axisymmetric model is 0.53 in this study, notably larger than Murray's 0.12. An analysis of the pressure gradient force associated with could motions reveals that the vertical pressure gradient force due to perturbed pressure is: 1) of the same order of magnitudes as that of the thermal buoyancy force in the core region of the cloud; is 2) acting in the opposite direction of the net force due to excess heat, moisture, and the weight of liquid water; and 3) is larger in absolute magnitude in the slab-symmetric model than in the axisymmetric one.

Also included are differences in the evolution of the modeled clouds in relation to different intensities of initial buoyant elements used in initiating convection in a conditionally unstable atmosphere and in relation to differences in the size of integration domains.

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Su-Tzai Soong and Yoshimitsu Ogura

Abstract

A cumulus cloud population is determined over the trade-wind region under an undisturbed weather condition from the large-scale heat and moisture budgets. The BOMEX data during the period 22–23 June 1969 are used. Six classes of clouds (A–F) are considered. Each class of clouds changes the large-scale mean temperature and humidity through condensation, evaporation and convective transport processes. The change due to each class of clouds is determined using the time-dependent, two-dimensional, axisymmetric cloud model developed by Soong and Ogura (1973).

It is found that the cloud population, consisting of 66.4 A clouds, 8.9 D clouds and 1.6 E clouds km−2 DAY−1, satisfies approximately the large-scale heat and moisture budget requirements. The cloud top heights of the three classes of clouds are 0.8, 1.6 and 2.0 km, respectively, and the cloud radii at the cloud base level during the mature stage are 62.5, 150 and 225 m, respectively. Consequently, the fractional area coverage of the predicted cloud population is 4.5%. Among the three classes of clouds, only E clouds penetrate into the trade-wind inversion layer. Approximately 18% of the total convective mass flux at the cloud base level is associated with the E clouds. The estimated Bowen ratio from the cloud population is less than 0.1. The estimated evaporation rate is 7.4 mm day−1 and the precipitation rate 0.29 mm day−1. These values are in good agreement with those observed during the period considered.

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Su-Tzai Soong and Ernest C. Kung

Abstract

Short-period cycles in the large-scale atmospheric circulation were investigated with time series of kinetic energy, its generation and outflow, which were computed twice a day for a 5-year period over the North American Continent. The spectra were computed from the autocorrelation curves, and were compared with the red noise spectra to evaluate the statistical significance of the energy cycles.

The maxima of time spectra of different energy parameters show frequent agreement. The commonly reported kinetic energy cycles with periods of one to two weeks are observed; however, they are not statistically significant and also show very high year-to-year irregularity. Significant cycles with periods around 40 days and 2–4 days are also noted.

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Su-Tzai Soong and Wei-Kuo Tao

Abstract

The vertical transport of horizontal momentum in a convective tropical rainband is studied using a two-dimensional cloud ensemble model. Twelve simulations are made under the same large-scale conditions. The vertical transports of v momentum (parallel to the rainband) are essentially the same in all of the simulations, even though the structure of the clouds is different in each of the runs. The magnitude of the v-memomentum transport by clouds is fairly large. It takes only half of a day to smooth out the tropical low-level easterly jet parallel to the rainband if no other processes am operating. The vertical transports of u momentum (perpendicular to the rainband) are quite different in all of the simulations. This difference can be explained by the dissimilarities in the distributions of horizontal momentum associated with various cloud configurations.

The simulated vertical transports of horizontal momentum are compared with those computed with the Schneider and Lindzen scheme. The results suggest that their scheme is basically correct and usable if some improvements are made.

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Wei-Kuo Tao and Su-Tzai Soong

Abstract

A three-dimensional numerical cloud ensemble model has been developed to investigate the collective feedback effects of cloud systems on the large-scale environment. An observed large-scale lifting is imposed continuously in the model. Small amplitude random perturbations in the form of temperature fluctuations are also continuously fed into the model at low levels to simulate random thermals over the tropical ocean. The model allows several clouds of various sizes to develop simultaneously inside the domain. An integration of the model is made for six hours of simulated time in order to allow large numbers of convective clouds to develop. Following each simulation, the collective feedbacks of cloud systems on the large-scale temperature, moisture and horizontal momentum fields are computed. Horizontal and time averages of various relevant variables are also computed to elucidate the statistical properties of the clouds. The model was applied to a case of a well-defined ITCZ rainband over the eastern tropical Atlantic ocean.

Nine simulations are made under the same large-scale conditions. The location, number and configuration of the clouds that form in the model are usually different in each of the nine simulations, but after an hour or two all simulated clouds assume a band structure instead of being randomly distributed. The orientations of the bands resemble the observations, and the bands are aligned along the direction of the lower tropospheric wind shear. The differences among these simulations on the cloud heating and drying effects are small. The model results for the total heating and moistening effects are also in fairly good agreement with those estimated from observations. The vertical transports of v-momentum (parallel to the simulated rainband) are essentially the same in all of the simulations, but the vertical transports of u-momentum (normal to the rainband) are quite different in some of these simulations. The physical process involved is the generation of horizontal momentum by the pressure gradient force in the momentum equation. This generated horizontal momentum can be selectively transported vertically by clouds.

In order to examine whether different large-scale forcing, environmental wind shear or microphysical processes result in different cloud ensembles or different cloud heating and moistening profiles, three additional experiments are studied. The simulated clouds developed randomly instead of assuming a preferred elongation in a case of no vertical wind shear. No collective vertical transports of horizontal momentum by clouds occur in this case. It is also found that the collective cloud feedback effects on temperature and moisture are sensitive to both magnitude of lifting and cloud microphysical processes. A comparison of the three-dimensional model simulation with a two-dimensional simulation is also made.

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

Abstract

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

Abstract

A two-dimensional, time-dependent, and nonhydrostatic numerical cloud model is used to study the development and structure of a subtropical squall line that occurred during TAMEX (Taiwan Area Mesoscale Experiment). The model includes a parameterized ice-phase microphysical scheme and long- and shortwave radiative transfer processes, as well as heat and moisture fluxes from the ocean surface. It was found that dynamic and kinematic structures of this simulated subtropical squall line are quite similar to its counterparts observed in the tropics and midlatitudes. For example, the squall line has a quasi-steady structure with a successive generation of cells at the gust front that propagate rearward relative to the front, the precipitation, and an evaporatively cooled downdraft at low and midlevels. This particular subtropical squall line is also shown to have a distinct midtropospheric rear inflow and a moderate anvil component of the total precipitation. The vertical transport of horizontal momentum, as well as latent heat release by the simulated subtropical squall system and by squall systems that occur in other geographic locations (both simulated and observed), are compared and presented.

We also investigate the roles of 1) heat and moisture fluxes from the ocean, 2) longwave radiative cooling, 3) microphysical processes, and 4) presumed mesoscale convergence lifting on the structure and propagation of this subtropical squall line. Among the seven two-dimensional simulations considered, the general structure of the squall system, such as its propagation speed and its “weak-evolution”-type multicell characteristics, do not change significantly in most of the cases. It was found that each process has a different impact on the total surface precipitation over an 8-h simulation time. The order of importance of each process to the total surface precipitation, beginning with the most important, is microphysics, longwave radiative transfer, heat and moisture input from the ocean, and prestorm mesoscale convergence lifting.

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Yoshi Ogura, Hann-Ming Juang, Ke-Su Zhang, and Su-Tzai Soong

The SESAME-AVE IV (9–10 May 1979) rawinsonde data were analyzed to uncover possible triggering mechanisms for severe storms that developed over western Oklahoma and Texas. The high frequency of observations (at 3 h intervals) and high vertical resolution of reported data (at 25 mb intervals) at all stations permitted investigation of the diurnal variation of the planetary boundary layer on the synoptic scale. Thunderstorms developed first just ahead of a stationary front over the Texas panhandle on the afternoon of 9 May. This area was characterized by the absence of a strong inversion (or “lid”) that represented an interface between very warm and dry air aloft, and warm moist tropical air below. Apparently, mesoscale low-level ascending motion associated with frontal lifting and/or the inland sea breeze effect led to the removal of the lid. Another noteworthy feature in this storm event was the strong vertical wind shear at low and middle levels over the storm area. When combined with the development of a deep boundary layer with weak stratification during the daytime, the Richardson number became less than one in the boundary layer in the prestorm situation. The results of our numerical linear stability analysis indicate that the observed basic states were indeed symmetrically unstable. This may suggest that the triggering processes were argumented by symmetric instability. Although a well defined dry line was present, it does not seem to have contributed directly to initiation of storms in this case. It also was observed that, as the thermal low began to weaken in the early evening, the cold air behind a stationary front started advancing eastward and helped to extend the line of thunderstorms deep into central Texas. This may be another process whereby some storms prefer to develop in the late afternoon or early evening.

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Michale McCumber, Wei-Kuo Tao, Joanne Simpson, Richard Penc, and Su-Tzai Soong

Abstract

A numerical cloud model is used to evaluate the performance of several ice parameterizations. Results from simulations using these schemes are contrasted with each other, with an ice-free control simulation, and with observations to determine to what extent ice physics affect the realism of these results. Two different types of tropical convection are simulated. Tropical squall-type systems are simulated in two dimensions so that a large domain can be used to incorporate a complete anvil. Nonsquall-type convective lines are simulated in three dimensions owing to their smaller horizontal scale.

The inclusion of ice processes enhances the agreement of the simulated convection with some features of observed convection, including the proportion of surface rainfall in the anvil region, and the intensity and structure of the radar brightband near the melting level in the anvil. In the context of our experimental design, the use of three ice classes produces better results than two ice classes or ice-free conditions, and for the tropical cumuli, the optimal mix of the bulk ice hydrometeors is cloud ice-snow-graupel. We infer from our modeling results that application of bulk ice microphysics in cloud models might be case specific, which is a significant limitation. This can have serious ramifications for microwave interpretation of cloud microphysical properties. Generalization of ice processes may require a larger number of ice categories than we have evaluated and/or the prediction of hydrometeor concentrations or particle-size spectra.

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