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Harold N. Murrow and Robert M. Henry

Abstract

Rigid spherical balloons were released in still air to assess magnitudes of possible self-induced motion. They were found to exhibit oscillations similar to those previously reported for rubber balloons. These oscillations are not simple sinusoidal motion, and are not limited to a single plane. The root-mean-square horizontal velocity is proportional to the vertical terminal velocity, and for the ROSE sphere appears to he one-half the terminal velocity. Slight roughening (roughness ratio = 0.0016) had no apparent effect. Greater roughness (roughness ratio = 0.03) appears to reduce the amplitude and predominant wavelength of the oscillation.

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Robert F. Adler, Robert A. Mack, N. Prasad, Ida M. Hakkarinen, and H-Y. M. Yeh

Abstract

Aircraft passive microwave observations of deep atmospheric convection at frequencies between 18 and 183 GHz are presented in conjunction with visible and infrared satellite and aircraft observations and ground-based radar observations. Deep convective cores are indicated in the microwave data by negative brightness temperature (TB) deviations from the land background (270 K) to extreme TB values below 100 K at 37, 92, and 183 GHz and below 200 K at 18 GHz. These TB minima, due to scattering by ice held aloft by the intense updrafts, are well correlated with areas of high radar reflectivity. For this land background case, TB is inversely correlated with rain rate at all frequencies due to TB-ice-rain correlations. Mean ΔT between vertically polarized and horizontally polarized radiance in precipitation areas is approximately 6 K at both 18 GHz and 37 GHz, indicating nonspherical precipitation size ice particles with a preferred horizontal orientation. Convective cores not observed in the visible and infrared data are clearly defined in the microwave observations and borders of convective rain areas are well defined using the high-frequency (90 GHz and greater) microwave observations.

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Alexandre O. Fierro, Stephanie N. Stevenson, and Robert M. Rabin

Abstract

Total lightning data obtained from the Geostationary Lightning Mapper (GLM) were analyzed to present a first glimpse of relationships with intensity variations and convective evolution in Hurricane Maria (2017). The GLM has made it possible, for the first time, to analyze total lightning within a major hurricane for a long period, far from ground-based detection networks. It is hoped that these observations could enlighten some of the complex relationships existing between intensity fluctuations and the distribution of electrified convection in these systems.

Prior to rapidly intensifying from a category 1 to category 5 storm, Maria produced few inner-core flashes. Increases in total lightning in the inner core (r ≤ 100 km) occurred during both the beginning and end of an intensification cycle, while lightning increases in the outer region (100 < r ≤ 500 km) occurred earlier in the intensification cycle and during weakening. Throughout the analysis period, the largest lightning rates in the outer region were consistently located in the southeastern quadrant, a pattern consistent with modeling studies of electrification within hurricanes. Lightning in the inner core was generally tightly clustered within a 50-km radius from the center and most often found in the southeastern portion of the eyewall, which is atypical. Bootstrapped correlation statistics revealed that the most robust and systematic relationship with storm intensity was obtained for inner-core lightning and maximum surface wind speed. A brief comparison between flash rates from GLM and a very low-frequency ground-based network revealed that not all lightning peaks are seen equally, with hourly flash-rate ratios between both systems sometimes exceeding two orders of magnitude.

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Hwa-Young M. Yeh, N. Prasad, Robert A. Mack, and Robert F. Adler

Abstract

In Part II of the 29 June 1986 case study, a radiative transfer model is used to simulate the aircraft multichannel microwave brightness temperatures presented in Part I and to study the convective storm structure. Ground-based radar data are used to derive hydrometeor profiles of the storm, based on which the microwave upwelling brightness temperatures are calculated. Various vertical hydrometeor phase profiles and the Marshall and Palmer (M-P) and Sekhon and Srivastava (S-S) ice particle size distributions are experimented in the model. The results are compared with the aircraft radiometric data. The comparison reveals that 1) the M-P distribution well represents the ice particle size distribution, especially in the upper tropospheric portion of the cloud; 2) the S-S distribution appears to better simulate the ice particle size at the lower portion of the cloud, which has a greater effect on the low frequency microwave upwelling brightness temperatures; and 3) in deep convective regions, significant supercooled liquid water (∼0.5 g m−3) may be present up to the −30°C layer, while in less convective areas, frozen hydrometeors are predominant above −10°C level.

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Robert M. Cox, John Sontowski, Richard N. Fry Jr., Catherine M. Dougherty, and Thomas J. Smith

Abstract

Atmospheric transport and dispersion over complex terrain were investigated. Meteorological and sulfur hexafluoride (SF6) concentration data were collected and used to evaluate the performance of a transport and diffusion model coupled with a mass consistency wind field model. Meteorological data were collected throughout April 1995. Both meteorological and plume location and concentration data were measured in December 1995. The meteorological data included measurements taken at 11–15 surface stations, one to three upper-air stations, and one mobile profiler. A range of conditions was encountered, including inversion and postinversion breakup, light to strong winds, and a broad distribution of wind directions.

The models used were the MINERVE mass consistency wind model and the SCIPUFF (Second-Order Closure Integrated Puff) transport and diffusion model. These models were expected to provide and use high-resolution three-dimensional wind fields. An objective of the experiment was to determine if these models could provide emergency personnel with high-resolution hazardous plume information for quick response operations.

Evaluation of the models focused primarily on their effectiveness as a short-term (1–4 h) predictive tool. These studies showed how they could be used to help direct emergency response following a hazardous material release. For purposes of the experiments, the models were used to direct the deployment of mobile sensors intended to intercept and measure tracer clouds.

The April test was conducted to evaluate the performance of the MINERVE wind field generation model. It was evaluated during the early morning radiation inversion, inversion dissipation, and afternoon mixed atmosphere. The average deviations in wind speed and wind direction as compared to observations were within 0.4 m s−1 and less than 10° for up to 2 h after data time. These deviations increased as time from data time increased. It was also found that deviations were greatest during inversion dissipation.

The December test included the release and tracking of atmospheric tracers. The MINERVE–SCIPUFF modeling system was used to direct remote sensing equipment. Posttest analyses were performed to determine model reliability. It was found that plume centroid position as determined by the models was within 10% of the observed plume centroid.

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Erik N. Rasmussen, Jerry M. Straka, Matthew S. Gilmore, and Robert Davies-Jones

Abstract

This paper develops a definition of a supercell reflectivity feature called the descending reflectivity core (DRC). This is a reflectivity maximum pendant from the rear side of an echo overhang above a supercell weak-echo region. Examples of supercells with and without DRCs are presented from two days during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX), as well as one day with tornadic high-precipitation supercell storms in central Kansas. It was found that in all cases, tornado formation was preceded by the descent of a DRC. However, the sample reported herein is much too small to allow conclusions regarding the overall frequency of DRC occurrence in supercells, or the frequency with which DRCs precede tornado formation. Although further research needs to be done to establish climatological frequencies, the apparent relationship observed between DRCs and impending tornado formation in several supercells is important enough to warrant publication of preliminary findings.

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C. D. Roberts, D. Calvert, N. Dunstone, L. Hermanson, M. D. Palmer, and D. Smith

Abstract

Observations and eddy-permitting ocean model simulations are used to evaluate the drivers of sea level variability associated with 15 modes of climate variability covering the Atlantic, Pacific, Indian, and Southern Oceans. Sea level signals are decomposed into barotropic, steric, and inverted barometer contributions. Forcings are decomposed into surface winds, buoyancy fluxes, and Ekman pumping. Seasonal-to-interannual sea level variability in the low latitudes is governed almost entirely by the thermosteric response to wind forcing associated with tropical modes of climate variability. In the extratropics, changes to dynamic sea level associated with atmospheric modes of variability include a substantial barotropic response to wind forcing, particularly over the continental shelf seas. However, wind-driven steric changes are also important in some locations. On interannual time scales, wind-forced steric changes dominate, although heat and freshwater fluxes are important in the northwest Atlantic, where low-frequency sea level variations are associated with changes in the Atlantic meridional overturning circulation. Using the version 3 of the Met Office Decadal Prediction System (DePreSys3), the predictability of large-scale dynamic sea level anomalies on seasonal-to-interannual time scales is evaluated. For the first year of the hindcast simulations, DePreSys3 exhibits skill exceeding persistence over large regions of the Pacific, Atlantic, and Indian Oceans. Skill is particularly high in the tropical Indo-Pacific because of the accurate initialization and propagation of thermocline depth anomalies associated with baroclinic adjustments to remote wind forcing. Skill in the extratropics is hindered by the limited predictability of wind anomalies associated with modes of atmospheric variability that dominate local and/or barotropic responses.

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K. E. Hanley, D. J. Kirshbaum, N. M. Roberts, and G. Leoncini

Abstract

Convective-scale ensemble simulations with perturbed initial and lateral boundary conditions are performed to investigate the mechanisms and sensitivities of a central European convection event from the Convective and Orographically Induced Precipitation Study (COPS). In this event, a “primary” squall line developed ahead of a decaying mesoscale convective system (MCS) upstream of the Vosges Mountains (France), weakened over the Rhine valley, then regenerated as a “secondary” squall line over the Black Forest Mountains (Germany). All ensemble members captured the squall-line evolution, but most suffered from a delay in the onset of convection and positional errors of 50–150 km over the COPS region. These errors in the secondary initiation were linked to errors in the primary initiation. Detailed analysis revealed a similar primary initiation mechanism in all members: in the ascending branch of a midlevel frontal circulation ahead of the MCS, convection initiated within a mesoscale moisture anomaly embedded within the prefrontal flow. The differences in the skill of the ensemble members were related to subtle differences in their initial upper-level representation of potential vorticity (PV). Members that verified well possessed a stronger PV gradient at the leading edge of an upper-level trough. This led to more rapid cyclogenesis over northern France and the United Kingdom and faster development and propagation of the midlevel front and the prefrontal moisture anomaly. As a consequence, the squall lines in these members developed earlier and closer to the COPS region. This case study provides an example of the subtle mechanisms by which errors on the larger scales may transfer to the convective scale and lead to errors in quantitative precipitation forecasts.

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

Abstract

A three-dimensional cloud model, radiative transfer model-based simulation system is tested and validated against the aircraft-based radiance observations of an intense convective system in southeastern Virginia on 29 June 1986 during the Cooperative Huntsville Meteorological Experiment. NASA's ER-2, a high-altitude researchaircraft with a complement of radiometers operating at 1 I-pm infrared channel and IS-, 37-, 92-, and 183-GHz microwave channels provided data for this study. The cloud model successfully simulated the cloud systemwith regard to aircraft- and radar-observed cloud-top heights and diameters and with regard to radar-observed reflectivity structure. For the simulation time found to correspond best with the aircraft- and radar-observed structure, brightness temperatures Tb are simulated and compared with observations for all the microwave frequencies along with the 1 1 -pm infrared channel. Radiance calculations at the various frequencies correspond well with the aircraft observations in the areas of deep convection. The clustering of 37-174-GHz Tb observationsand the isolation of the 18-GHz values over the convective cores are well simulated by the model. The radiative transfer model, in general, is able to simulate the observations reasonably well from 18 GHz through 174 GHz within all convective areas of the cloud system. When the aircraft-observed 18- and 37-GHz, and 90- and 174-GHz 7's are plotted against each other, the relationships have a gradual difference in the slope due to the differences in the ice particle size in the convective and more stratiform areas of the cloud. The model is ableto capture these differences observed by the aircraft. Brightness temperature-rain rate relationships compare reasonably well with the aircraft observations in terms of the slope of the relationship.The model calculations are also extended to select high-frequency channels at 220, 340, and 400 GHz to simulate the Millimeter-wave Imaging Radiometer aircraft instrument to be flown in the near future. All three of these frequencies are able to discriminate the convective and anvil portions of the system, providing useful information similar to that from the frequencies below 183 GHz but with potentially enhanced spatial resolution from a satellite platform. In thin clouds, the dominant effect of water vapor is seen at 174, 340, and 400 GHz.In thick cloudy areas, the scattering effect is dominant at 90 and 220 GHz, while the overlying water vapor can attenuate at 174, 340, and 400 GHz. All frequencies (90-400 GHz) show strong signatures in the core.

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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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