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Joanne Simpson, Bruce R. Morton, Michael C. McCumber, and Richard S. Penc

Abstract

The GATE data base for days 261 and 186 is used for a combined observational and numerical investigation of interacting cumulus processes that may be important in the generation of waterspouts. The results suggest that the existence of cumulus-scale parent vortices is a necessary condition for the production of waterspouts, but not in itself sufficient. For generation of a visible funnel, the vortices must undergo intensification below cloud base to sea level during the short time span in which the convective updraft is active.

A high-resolution version of Schlesinger's three-dimensional cumulus model with a Kessler-type precipitation scheme is used to analyze the organization of cumulus-scale vorticity on these two days, which had contrasting thermal stratification and cloud features. On day 261, the soundings near but outside the cloud cluster had a relatively deeper cloud layer with weaker conditional instability and vertical wind shear at low levels. In the numerical simulation of the congestus, downdraft under-runs updraft early, so that the strong vortex pair at midcloud levels do not extend to the surface, where the vorticity remains weak even though a wide range of wind profiles was tried in the model. On day 186, the soundings near the waterspouts showed a more unstable subcloud and lower cloud layer capped by a stable dry layer restricting cumulus growth below 4 km. Four wind profiles were used in this case to initiate the numerical model; two with strong low-level shear resulted in strong parent vortices with their maximum intensity at the surface. These vortices strengthened in the convergence between side-by-side updraft and downdraft, which both extended to the surface, a rare configuration for GATE but characteristic of many midwestern tornadic developments.

The observations from both days suggest that the waterspouts formed ahead of the wind shift, resulting from passage of a gust front, both in zones where it may be surmised that two gust fronts were approaching one another. A brief summary is given of results on tropical gust fronts to provide a basis for discussing their role in the generation of tropical waterspouts, and it is shown that 1) they can produce a favorable environment for the parent vortices and 2) they can cause additional vortex intensification. Order-of-magnitude vorticity calculations suggest that small-scale, low-level convergence may have been sufficient, particularly on day 186 when the waterspout signatures were observed at intersecting convergent features to intensify the parent vortex in as little as 5–10 min.

On day 261, additional model experiments simulating pregust front conditions, namely low-level destabilization and increased shear, show stronger parent vortices at low levels. Reasons for the rarity of GATE waterspouts are suggested, and a renewed observational program is proposed for the Florida Keys relating waterspouts to cloud interactions and boundary layer features.

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Joanne Simpson, William L. Woodley, and Robert M. White

A serious drought in the spring of 1971 occurred in South Florida. In view of NOAA's research experience in dynamic cumulus modification (massive silver iodide seeding to invigorate cumulus updrafts by means of release of latent heat of fusion), the state government sought our aid in combating this drought by means of a seeding effort. NOAA responded by an extension of its experimental program with both practical and research objectives.

Two of the NOAA Research Flight Facility aircraft were used for airborne pyrotechnic seeding from 1 April to 31 May 1971. A one-dimensional numercial cumulus model was run in real time on each day, eliminating 38 days in the period as unsuitable. Flights were conducted on 16 days, with actual seeding on 14. Seven suitable days were lost due to routine aircraft maintenance. Radars and raingages were used to calculate rain amounts from all seeded and many unseeded clouds.

Seeding was conducted in an attempt to promote merger by treating clouds close together in space and also the upshear towers of each previously seeded complex. A total of not less than 180,000 acre-ft of water was calculated to have fallen from the seeded clouds in two target areas totaling about 7200 n mi2. A conservative estimate attributed 100,000 acre-ft as due to seeding, although without randomization this evaluation cannot be made firm. Satellite and synoptic studies accompanied rainfall evaluation on all seeding days. Among the important scientific results is that some frontal conditions appeared suitable for dynamic seeding in Florida, offering hope for extension of the technique into dry periods.

Some aspects of NOAA's future policy in the rain enhancement aspects of weather modification are presented.

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Wei-Kuo Tao, Joanne Simpson, Stephen Lang, Michael McCumber, Robert Adler, and Richard Penc

Abstract

A simple algorithm to estimate the latent heating of cloud systems from their vertical hydrometer profiles is proposed. The derivation as well as the validation of the algorithm is based on output generated by a non-hydrostatic cloud model with parameterized microphysical processes. Mature and decaying stages of a GATE squall-type convective system have been tested. The algorithm-derived heating budget is in reasonable agreement with the budget predicted by the cloud model. The input to the proposed algorithm can be obtained from either a rain retrieval technique based on information from multichannel passive microwave signals or a kinematic cloud model based on information from Doppler radar wind fields and radar reflectivity patterns. Such an application would have significant implications for spaceborne remote sensing and the large-scale weather prediction data assimilation problem.

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Robert F. Adler, Hwa-Young M. Yeh, N. Prasad, Wei-Kuo Tao, and Joanne Simpson

Abstract

A three-dimensional cloud model-microwave radiative transfer model combination is used to study the relations among the precipitation and other microphysical characteristics of a tropical oceanic squall line and the upwelling radiance at pertinent microwave frequencies. Complex brightness temperature-rain rate relations are evident at the full horizontal resolution (1.5 km) of the models, with spatial avenging producing smoother, shifted relations, in most cases. Nonprecipitating cloud water is shown to be important in understanding the resulting distribution of brightness temperature. At the mature stage, convective portions of the cloud system are shown to produce different brightness temperature relations than the stratiform portion, primarily related to the distribution of cloud water. The evolution of the convective system from a small convective complex through its mature stage and the beginning of its dissipation also is shown to result in a variation of brightness temperature-rain relations, related to the distribution of cloud water and the evolution of ice in the precipitating system. The results of the study paint to the need to take into account the evolution of nonprecipitating cloud water and precipitation-sized ice in the retrieval of rain team from microwave space observations. This effect is evident for both the life cycle of individual convective elements and the life cycle of the convective system as a whole.

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Xiaowen Li, Wei-Kuo Tao, Alexander P. Khain, Joanne Simpson, and Daniel E. Johnson

Abstract

A two-dimensional cloud-resolving model is used to study the sensitivities of two microphysical schemes, a bulk scheme and an explicit spectral bin scheme, in simulating a midlatitude summertime squall line [Preliminary Regional Experiment for Storm-Scale Operational and Research Meteorology (PRE-STORM), 10–11 June 1985]. In this first part of a two-part paper, the developing and mature stages of simulated storms are compared in detail. Some variables observed during the field campaign are also presented for validation. It is found that both schemes agree well with each other, and also with published observations and retrievals, in terms of storm structures and evolution, average storm flow patterns, pressure and temperature perturbations, and total heating profiles. The bin scheme is able to produce a much more extensive and homogeneous stratiform region, which compares better with observations.

However, instantaneous fields and high temporal resolution analyses show distinct characteristics in the two simulations. During the mature stage, the bulk simulation produces a multicell storm with convective cells embedded in its stratiform region. Its leading convection also shows a distinct life cycle (strong evolution). In contrast, the bin simulation produces a unicell storm with little temporal variation in its leading cell regeneration (weak evolution). More detailed, high-resolution observations are needed to validate and, perhaps, generalize these model results. Interactions between the cloud microphysics and storm dynamics that produce the sensitivities described here are discussed in detail in Part II of this paper.

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Gerald M. Heymsfield, Jeffrey B. Halverson, Joanne Simpson, Lin Tian, and T. Paul Bui

Abstract

A persistent, mesoscale region of intense eyewall convection contained within Hurricane Bonnie on 23 August 1998 is examined from multiple observations synthesized from the National Aeronautics and Space Administration ER-2 and DC-8 aircraft. The intense convection occurred late in the day as Bonnie was attaining its minimum central pressure and during a stage when the inner core featured a markedly asymmetric structure. The internal structure of this convective burst and its relationship to the warm core are presented using a synthesis of high-resolution satellite, aircraft radar, and in situ data. An exceptionally vigorous eyewall tower within the burst and penetrating to nearly 18 km is described. A second intense eyewall tower, adjacent to the eye, is shown to be associated with a mesoscale subsiding current of air, with vertical velocities on the order of several meters per second that descends at least 9 km and extends horizontally nearly 25 km into the eye interior. The subsidence is a much deeper and broader-scale feature than the convectively induced, symmetric overturning that commonly occurs on the upper-level flanks of convective towers in other tropical environments. The air supplying the deep current probably originates both at tropopause height and also from air detrained out of the adjacent updraft at midlevels. Strong downdrafts within the eye could not be associated with every hot tower. Whether this result was due to undersampling by aircraft or whether deep eye downdrafts are indeed sporadic, it is plausible that up to 3°C of midlevel eye warming observed in Bonnie may arise from one or more of these convectively induced episodes rather than as a result of a gradual sinking motion applied uniformly throughout the eye.

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Xiaowen Li, Wei-Kuo Tao, Alexander P. Khain, Joanne Simpson, and Daniel E. Johnson

Abstract

Part I of this paper compares two simulations, one using a bulk and the other a detailed bin microphysical scheme, of a long-lasting, continental mesoscale convective system with leading convection and trailing stratiform region. Diagnostic studies and sensitivity tests are carried out in Part II to explain the simulated contrasts in the spatial and temporal variations by the two microphysical schemes and to understand the interactions between cloud microphysics and storm dynamics. It is found that the fixed raindrop size distribution in the bulk scheme artificially enhances rain evaporation rate and produces a stronger near-surface cool pool compared with the bin simulation. In the bulk simulation, cool pool circulation dominates the near-surface environmental wind shear in contrast to the near-balance between cool pool and wind shear in the bin simulation. This is the main reason for the contrasting quasi-steady states simulated in Part I. Sensitivity tests also show that large amounts of fast-falling hail produced in the original bulk scheme not only result in a narrow trailing stratiform region but also act to further exacerbate the strong cool pool simulated in the bulk parameterization.

An empirical formula for a correction factor, r(q r) = 0.11q r −1.27 + 0.98, is developed to correct the overestimation of rain evaporation in the bulk model, where r is the ratio of the rain evaporation rate between the bulk and bin simulations and qr(g kg−1) is the rain mixing ratio. This formula offers a practical fix for the simple bulk scheme in rain evaporation parameterization.

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Alexandre O. Fierro, Joanne Simpson, Margaret A. LeMone, Jerry M. Straka, and Bradley F. Smull

Abstract

An airflow trajectory analysis was carried out based on an idealized numerical simulation of the nocturnal 9 February 1993 equatorial oceanic squall line observed over the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) ship array. This simulation employed a nonhydrostatic numerical cloud model, which features a sophisticated 12-class bulk microphysics scheme. A second convective system that developed immediately south of the ship array a few hours later under similar environmental conditions was the subject of intensive airborne quad-Doppler radar observations, allowing observed airflow trajectories to be meaningfully compared to those from the model simulation. The results serve to refine the so-called hot tower hypothesis, which postulated the notion of undiluted ascent of boundary layer air to the high troposphere, which has for the first time been tested through coordinated comparisons with both model output and detailed observations.

For parcels originating ahead (north) of the system near or below cloud base in the boundary layer (BL), the model showed that a majority (>62%) of these trajectories were able to surmount the 10-km level in their lifetime, with about 5% exceeding 14-km altitude, which was near the modeled cloud top (15.5 km). These trajectories revealed that during ascent, most air parcels first experienced a quick decrease of equivalent potential temperature (θe) below 5-km MSL as a result of entrainment of lower ambient θe air. Above the freezing level, ascending parcels experienced an increase in θe with height attributable to latent heat release from ice processes consistent with previous hypotheses. Analogous trajectories derived from the evolving observed airflow during the mature stage of the airborne radar–observed system identified far fewer (∼5%) near-BL parcels reaching heights above 10 km than shown by the corresponding simulation. This is attributed to both the idealized nature of the simulation and to the limitations inherent to the radar observations of near-surface convergence in the subcloud layer.

This study shows that latent heat released above the freezing level can compensate for buoyancy reduction by mixing at lower levels, thus enabling air originating in the boundary layer to contribute to the maintenance of both local buoyancy and the large-scale Hadley cell despite acknowledged dilution by mixing along updraft trajectories. A tropical “hot tower” should thus be redefined as any deep convective cloud with a base in the boundary layer and reaching near the upper-tropospheric outflow layer.

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William L. Woodley, Jill Jordan, Anthony Barnston, Joanne Simpson, Ron Biondini, and John Flueck

Abstract

The Florida Area Cumulus Experiment of 1970–76 (FACE-1) is a single-area, randomized, exploratory experiment to determine whether seeding cumuli for dynamic effects (dynamic seeding) can be used to augment convective rainfall over a substantial target area (1.3 × 104 km2) in south Florida. Rainfall is estimated using S-band radar observations after adjustment by raingages. The two primary response variables are rain volumes in the total target (TT) and in the floating target (FT), the most intensely treated portion of the target. The experimental unit is the day and the main observational period is the 6 h after initiation of treatment (silver iodide flares on seed days and either no flares or placebos on control days). Analyses without predictors suggest apparent increases in both the location (means and medians) and the dispersion (standard deviation and interquartile range) characteristics of rainfall due to seeding in the FT and TT variables with substantial statistical support for the FT results and lesser statistical support for the TT results. Analyses of covariance using meteorologically meaningful predictor variables suggest a somewhat larger effect of seeding with stronger statistical support. These results are interpreted in terms of the FACE conceptual model.

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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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