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JOANNE SIMPSON

Abstract

This study is based on the radar-evaluated rainfall data from 52 south Florida cumulus clouds, 26 seeded and 26 control clouds, selected by a randomization procedure. The fourth root of the rainfall for both seeded and control populations was well fitted by a gamma distribution for probability density. The gamma distribution is prescribed by two parameters, one for scale and one for shape. Since the coefficient of variation of seeded and control cloud populations was the same, the shape parameters for the two gamma distributions were the same, while the seeded population's scale parameter was such as to shift the distribution to higher rainfall values than the control distribution. The best-fit gamma functions were found by application of the principle of maximum entropy.

Specification of tractable distributions for natural and modified rainfall populations provides an important prerequisite for the evaluation of seeding effects by Bayesian statistics, a continuing objective in the Experimental Meteorology Laboratory cumulus seeding programs.

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JOANNE SIMPSON and VICTOR WIGGERT

Abstract

A one-dimensional numerical cumulus model was tested against data from a randomized seeding experiment made in South Florida in 1968. Fourteen GO clouds were studied. Nine were seeded by pyrotechnics with 1 kg of silver iodide each, while five were studied identically as controls.

Various seeding subroutines and assumptions regarding the ice phase are compared. The experimental aircraft data are used to guide the modeling assumptions and to select the most realistic ones. Seedability and seeding effect correlate to 0.96 for seeded clouds in the three best models. A high correlation is found between seedability and radar-measured rainfall increase from seeding. Also, a high correlation is found between model predictions of the difference in precipitation fallout between seeded and control clouds and the measured rainfall differences, although the model predictions are much smaller in magnitude. A calculation is undertaken showing that coalescence within the cloud body on descent of the raindrops easily accounts for the discrepancy.

The model predictions for each GO cloud are discussed in comparison with actual measurements on the cloud.

The 1968 experiment was found to subdivide into two periods, one fair and one disturbed, with quite different effects of seeding. The two periods and corresponding cloud behavior are compared. It is concluded that the disturbed period was less favorable for seeding because of higher unseeded cloud growth and strong wind shear. Implications of this result for future modeling efforts are discussed.

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JOANNE SIMPSON and VICTOR WIGGERT

Abstract

This paper presents a model of the growth of cumulus clouds. The water content and maximum height of rising towers are calculated using a buoyancy equation with consideration of effects of entrainment and water load. The latter is subject to effects of modeled microphysical effects. Precipitation growth is parameterized in terms of an autoconversion equation and a collection equation. A precipitation fallout scheme is devised that depends on water content, drop spectrum, and the vertical rise rate of the tower.

Then “freezing subroutines” are devised to model the effects of silver-iodide seeding. A hierarchy of seeding routines, using different ice collection efficiencies and terminal velocities, is partially tested against the data of the Stormfury 1965 tropical cumulus-seeding experiment.

Some preliminary numerical experiments on warm clouds are performed, assuming changes in drop spectra from hygroscopic seeding.

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Yansen Wang, Wei-Kuo Tao, and Joanne Simpson

Abstract

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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Wei-Kuo Tao, Joanne Simpson, and Michael McCumber

Abstract

A reasonably accurate and noniterative saturation adjustment scheme is proposed to calculate the amount of condensation (and/or deposition) necessary to remove any supersaturated vapor, or amount of evaporation (and/or sublimation) necessary to remove any subsaturation in the presence of cloud droplets (and/or cloud ice). This proposed scheme can be implemented for a nonhydrostatic cloud model. The derivation of the scheme, evaluation of its performance and tests for sensitivity to variations in a few key parameters will be presented in this note.

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Ronald L. Holle, Joanne Simpson, and Steven W. Leavitt

Abstract

The largest network of surface cameras ever established in the tropics for studies of cloud cover was deployed during the. Global Atmospheric Research Programe's Atlantic Tropical Experiment (GATE) in 1974.

Analysis was made of 2572 hourly whole-sky photographs taken aboard four U.S. ships during the daytime hours of nearly every day covering the three phases of GATE. The cloud-cover analyses were made on a grid divided into 100 squares covering most of the overhead sky, much as previously made for Barbados and adjacent Atlantic Ocean cloudiness. Analyzed cloud types include low, middle, high, two kinds of total, and two kinds of combined upper cloudiness. Rainfall duration was obtained from drops impinging an the glass dome covering the whole-sky lens.

Average total cloudiness was between 70 and 85% for different ships and individual phases of GATE. From 22 to 56% were low clouds, and the rest were upper clouds. Large standard deviations of means are attributable mainly to frequent values near 0 and 100% cloudiness.

Maps of cloudiness amount show a zonal region of active convection, containing maxima in low clouds and rainfall, to be located south of 7°N in Phases I, and near 8°N in Phases II and III.

The convective code observed aboard ship shows a consistent positive relationship to low and total cloudiness and precipitation, and an inverse relation to net radiation. There is not a good correlation of upper cloudiness to code because of obscuration by lower clouds.

Variations of mean cloudiness at the four U.S. ships, for daylight hours only, show a weak midday minimum in low clouds and a rather strong sunset peak. Total and upper clouds, and rainfall, have consistent afternoon maxima. Statistical analyses of rainfall data indicate significant afternoon maxima in Phases II and III.

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

Abstract

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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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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Jeffrey B. Halverson, Brad S. Ferrier, Thomas M. Rickenbach, Joanne Simpson, and Wei-Kuo Tao

Abstract

An active day during the Coupled Ocean–Atmosphere Response Experiment (COARE) Intensive Observation Period (IOP) is examined in which nine convective systems evolved and moved eastward across the region of shipboard radar coverage in the Intensive Flux Array (IFA) within westerly wind burst conditions. The detailed genesis, morphology, and interactions between these cloud systems are documented from a radar and satellite perspective. One of these systems was a large and complex elliptical cluster, among the largest observed during the Tropical Ocean Global Atmosphere COARE. Multiple, parallel deep convective lines spaced 20–30 km apart and embedded within this system were initially oriented from north-northwest to south-southeast, oblique to the storm motion. Furthermore, the lines underwent counterclockwise realignment as the system moved eastward. The influence of strong lower-tropospheric directional and speed shear on these convective system properties is examined in the context of a dynamic, large-scale near-equatorial trough/transequatorial flow regime. A daily analysis of flow conditions during the 119-day IOP revealed that this type of synoptic regime was present in the IFA at least 40% of the time.

Radar-derived rainfall statistics are examined throughout the life cycles of each individual convective system. Spatial mapping of accumulated rainfall reveals long, linear swaths produced by the most intense cells embedded within convective lines. The evolution of rainfall properties includes an increase in the stratiform rainfall fraction and areal coverage in later generations of systems, with a peak in total rainfall production after local midnight. These trends can be explained by anvil cloud interactions originating within the sequence of closely spaced disturbances, including the effects of both enhanced midtropospheric moisture and also strong reversing (easterly) shear. The issue of boundary layer recovery between the frequent, intense convective systems on this day is also examined.

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Joanne Simpson, G. Roff, B. R. Morton, K. Labas, G. Dietachmayer, M. McCumber, and R. Penc

Abstract

A waterspout funnel and spray ring were observed under a cumulus line over the Great Salt Lake for about 5 min shortly after sunrise on 26 June 1985. Videotaped features strongly suggested that the funnel rotation was anticyclonic, These observations have been used as the basis for a study of the initiation and evolution of waterspouts through a series of numerical experiments at two scales, that of a cloud and a waterspout.

The cloud scale has been simulated using an improved Goddard-Schlesinger model with nearby Salt Lake City soundings. The main model improvements have been 1) a parameterized, three-class ice phase and 2) a line initialization in addition to the more common axisymmetric buoyant bubble. Cloud-scale vortex pairs developed for each mode of initiation, but a much stronger, more upright, low-level anticyclonic vortex grew from the line initiation than from the bubble. However, cumulus-scale vortices are common while waterspouts are rare, and the real test of a model is whether a waterspout can develop in the limited cumulus lifetime.

The 600-m horizontal grid of the cloud model cannot resolve waterspouts, and a modified Monash high-resolution axisymmetric vortex model with vertical domain and small section has been “embedded” at selected positions and initiated at selected times in the computed flow field of the cloud. Many experiments have been carried out with the vortex model. In the most important series, the boundary conditions were changed with the fields of the model cumulus as it evolved, and the time at which the vortex was started was varied through the lifetime of the parent cloud. Results showed that for each mode of cloud initiation, the vortex that started at the anticyclonic center grew faster than those started at other centers. This result fits with the observed anticyclonic rotation of the waterspout, strongly suggesting that the cloud vorticity was important in its initiation. The greatest azimuthal speed for the bubble-initiated cloud was 11 ms−1 when the vortex model was started at 28 min cloud time with time-varying boundary conditions, whereas it was 21 m s−1 when started at 12 min in the line-initiated cloud. Speeds were comparable when the inner domain moved with the anticyclonic cloud center. These speeds are close to the spray-ring threshold azimuthal velocity of roughly 22 m s−1 estimated by Golden from photographs.

Together, these model results support the hypothesis that, at least in some circumstances, cloud processes alone can produce waterspouts in the absence of external vorticity sources such as surface convergence lines or other shear features.

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