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Edward J. Zipser

Abstract

The thunderstorm frequency over the oceans during the Global Atmospheric Research Program Atlantic Tropical Experiment is quantified by examination of over 20 000 surface hourly observations from research ships. The overall thunderstorm frequency is one thunderstorm day per ship per month. There were many examples of intense mesoscale systems, such as squall lines, passing over the ships, extending to 13–17 km in altitude, but that nevertheless produce few reports of lightning. This reinforces the idea, based on data from other tropical ocean regions and from global satellite data, that in spite of the ubiquitous “hot towers” over tropical oceans, marine cumulonimbus product little lightning.

Climatological data from the monsoon regions of the Tropics are analyzed to reveal that during periods of onshore flow and heavy rainfall the oceanic regime of high rainfall but little lightning moves onshore. A rain-thunderstorm ratio is defined and used to characterize convective rainfall regimes as continental (relatively little) or maritime (relatively great) rainfall compared to the number of thunderstorm days. In regions such as West Africa and south Asia, the seasonal rainfall peak is actually accompanied by a thunderstorm minimum.

It is further suggested that the data support the idea, not original here, that vertical velocities in oceanic cumulonimbus clouds tend to be low compared with continental clouds. Radar data from the companion paper in this issue are also consistent with this idea. It is hypothesized that most oceanic storms have updrafts weaker than a possible threshold value, below which the supercooled liquid water, large ice particles, and ice-ice collisions are not present in the mixed-phase region in sufficient concentrations for electrification leading to lightning.

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Edward J. Zipser

Abstract

The “hot tower” hypothesis requires the existence of deep cumulonimbus clouds in the deep Tropics as essential agents, which accomplish the mass and energy transport essential for the maintenance of the general circulation. As the role of the deep convective clouds has been generally accepted, the popularity of referring to these deep “hot” towers as undilute towers also has gained acceptance. This paper examines the consequences of assuming that the deep convective clouds over tropical oceans consist of undilute ascent from the subcloud layer.

Using simple applications of parcel theory, it is concluded that observed properties of typical cumulonimbus updrafts in low- to midtroposphere over tropical oceans are inconsistent with the presence of undilute updrafts. Such undilute updrafts are far more consistent with observations in severe storms of midlatitudes. The observations over tropical oceans can be hypothetically explained by assuming large dilution of updrafts by entrainment below about 500 hPa, followed by freezing of condensate. This freezing and subsequent ascent along an ice adiabat reinvigorates the updrafts and permits them to reach the tropical tropopause with the necessary high values of moist static energy, as the hot tower hypothesis requires. The large difference observed between ocean and land clouds can be explained by assuming slightly smaller entrainment rates for clouds over land. These small entrainment differences have a very large effect on updrafts in the middle and upper troposphere and can presumably account for the large differences in convective vigor, ice scattering, and lightning flash rates that are observed. It follows that convective available potential energy (CAPE) is not a particularly good predictor of the behavior of deep convection.

Using the Tropical Rainfall Measuring Mission (TRMM) to map a proxy for the most intense storms on earth between 36°S and 36°N, they are found mostly outside the deep Tropics, with the notable exception of tropical Africa.

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Edward J. Zipser

Abstract

The Line Islands Experiment, conducted on and near Palmyra, Fanning and Christmas Islands during February-April 1967, produced extensive data on disturbances of the equatorial trough zone. One disturbance which passed through the heart of the data network is analyzed in detail. This disturbance intensified rapidly just east of Fanning Island during the night of 31 March–1 April, but satellite observations show that it dissipated rapidly during the daylight hours of 1 April. The convergence-divergence patterns associated with the growth and decay of the disturbance are most intense in the lowest 500 m. Data from serial rawinsonde releases on the islands, combined with research aircraft data, are presented which demonstrate that highly unsaturated downdrafts are produced, first on the convective scale and the mesoscale, and finally becoming organized over the entire 600-km extent of the system. Cumulus development is effectively suppressed in the downdraft air, only being restored after 6–12 hr by the greatly enhanced energy flux from sea to atmosphere, and through the boundary layer. In order to produce the observed downdrafts, it is shown that the three-dimensional circulation patterns and thermodynamic processes within regions of intense convection are closely analogous to those in typical mid-latitude squall lines.

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Edward J. Zipser

The Line Islands Experiment has resulted in unique and comprehensive data for studies of the meteorology of the equatorial Pacific. It is one of several recent field programs in tropical meteorology designed to attack the central problem of scale interactions, especially the role of convective and mesoscale systems. Some of the recent evidence is reviewed that indicates the importance of these interactions in understanding the non-steady state aspects of tropical disturbances. A variety of results from the Line Islands Experiment are summarized, with emphasis on their relevance to the planning of GARP tropical experiments.

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Edward J. Szoke
and
Edward J. Zipser

Abstract

This is Part II of a two-part paper describing the vertical profile of radar reflectivity in GATE convective cells. Time-height radar life histories for 42 cells over three GATE days are examined, using data from the Quadra radar with 5-minute resolution. Mean profiles and plots of cell characteristics are generated, and confirm that the mean profiles in Part I are representative of the active portion of the cell lifetime. There are marked differences between the cell life histories of isolated cells and the longer-lived cells associated with mesoscale systems. In contrast to cells sampled in organized systems, the isolated cells are often of very limited vertical extent and must be dominated by the warm rain process. When forcing features exist such as gust fronts and intersecting lines of convection, they appear to dominate the generation of new convection, and isolated strong echoes are not observed.

Composite life histories for typical GATE cells are constructed. The typical radar echo forms first at an altitude of 2.5 km and reaches the surface about 5 minutes later, strongly suggesting early domination by the warm rain process. At the same time the echo top rises and the mid-to-late stages of cell lifetime involve both warm rain and ice processes. The reflectivity profiles of the longer-lived echoes change relatively little in the middle 50% of the life cycle.

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Daniel J. Cecil
and
Edward J. Zipser

Abstract

A key component in the maintenance and intensification of tropical cyclones is the transverse circulation, which transports mass and momentum and provides latent heat release via inner core convective updrafts. This study examines these updrafts indirectly, using satellite-borne observations of the scattering of upwelling microwave radiation by precipitation-sized ice particles and satellite-borne observations of lightning. The observations are then compared to tropical cyclone intensity (defined here as maximum sustained wind speed) and the resulting relationships are assessed. Substantial updrafts produce large ice particles aloft, which in turn produce microwave ice-scattering signatures. The large ice, together with supercooled liquid water also generated by substantial updrafts, is a necessary ingredient in charge separation, which leads to lightning. Various parameters derived from the inner core ice-scattering signature are computed for regions encircling hurricanes and typhoons, and observations of lightning activity or inactivity are analyzed.

High correlations with future tropical cyclone intensity result from the ice-scattering signature parameters most closely associated with the areal extent of at least moderate precipitation rates. As expected, the relationship reveals increasing intensity with increasing ice-scattering signature. Indicators of more intense convection yield less information concerning tropical cyclone intensity. Correlations tend to be of the same sign for both present cyclone intensity at the time of the satellite overpass and subsequent intensity change. Correlations are higher for future cyclone intensity than for either of these. The lightning observations are much more limited than the microwave observations, because the short amount of time in which lightning can be detected may not adequately represent a particular storm’s electrical activity. The inner core lightning observations show no clear relationship to tropical cyclone intensification. However, the lightning observations do suggest an increased likelihood of inner core lightning in weak tropical storms and strong hurricanes/typhoons. In the examination of case studies, the paradoxical situation of much greater lightning frequency in rainbands than in eyewalls is noted.

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Daniel J. Cecil
and
Edward J. Zipser

Abstract

Part I of this two-part paper treats Tropical Rainfall Measuring Mission (TRMM) radar, passive microwave, and lightning observations in hurricanes individually. This paper (Part II) examines relationships between these parameters (and implications of the relationships). Quantitative relationships between lightning occurrence and 85-GHz brightness temperature, 37-GHz brightness temperature, and radar reflectivity in the mixed phase region are established separately for hurricane eyewall regions, inner rainband regions, and outer rainband regions; other tropical oceanic regions; and tropical continental regions. When any of the brightness temperature or radar parameters are held constant as controls, lightning is more frequent in hurricane outer rainbands than elsewhere over tropical oceans, and more frequent over continents than even in the outer rainbands. Reflectivity profiles associated with specific brightness temperatures are presented, demonstrating a link between high-altitude ice phase precipitation and 85-GHz scattering and a link between lower-altitude precipitation and 37-GHz scattering. Based on the combination of radar, passive microwave, and lightning observations, it is proposed that supercooled cloud water occurs preferentially in outer rainbands compared to other tropical oceanic precipitation. The suspected microphysical differences produce only subtle differences in the remote sensing parameters other than lightning.

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Karen I. Mohr
and
Edward J. Zipser

Abstract

This study used the 85-GHz ice scattering signature to describe the size, intensity, and geographic distribution of mesoscale convective systems (MCSs) between 35°N and 35°S for January, April, July, and October 1993. An MCS was defined as an area below 250 K of at least 2000 km2, with an enclosed minimum brightness temperature below 225 K. The geographic distribution of MCSs identified by these criteria was consistent with large-scale seasonal dynamics. There was no significant relationship (R2 ≈ 0.05) between the size and intensity for the MCSs in the study database. Tropical South America, tropical Africa, and the oceanic warm pool had the greatest number of MCSs. Equatorial regions such as tropical Africa had the smallest median areas. The subtropical oceans had the largest median areas, about 20% greater than other regions. MCSs in the continental regions tended to have colder minimum brightness temperatures than MCSs in the oceanic regions. The sub-tropical oceans had the warmest median minimum brightness temperatures, and tropical Africa had the coldest. Sunrise/sunset stratification of the data provided additional insight into land–water differences. MCSs were 35% more frequent over the oceans at sunrise than at sunset and 60% more frequent over tropical continents at sunset than at sunrise. Except over the subtropical oceans, MCSs tended to be larger at sunrise than at sunset. Continental MCSs tended to be colder at sunset than sunrise and colder than oceanic MCSs, particularly at sunset. The minimum brightness temperatures of oceanic MCSs tended to be only marginally colder at sunrise than at sunset. In general, continental MCSs appeared to be smaller and more intense than oceanic MCSs, and the largest and the most intense MCSs occurred more frequently in the subtropics.

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Edward J. Zipser
and
Joseph H. Golden

Abstract

On 14 August 1977, there was a mini-outbreak of three tornadoes about 40 km cut of Denver, Colorado. There were no significant synoptic-scale disturbances affecting Colorado on that day. Mesoscale analysis is used to establish several smaller scale systems that influenced storm development. The most notable feature of the mesoscale band of parent thunderstorms was the active growth along their northwest flank, in spite of cell movement toward the east. On the convective scale, the situation can be described as discrete propagation of multi-cell storms by new cell development on the left rear flank. Two of the three tornadoes were documented photographically, and post-analysis shows that they were of large size and long duration, but slow moving. Structural features of the largest tornado are analyzed in different portions of the life cycle, and compared with other cases in the literature. This tornado moved on a track curving toward the north-northwest, remaining at least 5–10 km distant from any significant precipitation. A dust band believed to represent an inflow jet was observed, which was in a different quadrant from similar features in other cases. Aspects of the tornadoes which could cause public confusion are noted, such as the disproportionately short condensation funnel from high-based cumulus clouds.

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Christopher E. Samsury
and
Edward J. Zipser

Abstract

Aircraft flight-level data from 787 radial legs in 20 hurricanes are analyzed to identify the composite kinematic structure in the hurricane eyewall, and especially with secondary horizontal wind maxima (SHWM) that occur outside the eyewall. Similar to previous studies, analysis of the flight-level wind data in the eyewall reveals radial convergence near the radius of maximum wind (RMW), and the highest frequency of updraft and the largest upward mass transport radially inward of the RMW.

More than 20% of the flight legs contain substantial secondary horizontal wind maxima of specified strength and length. The kinematic structure associated with SHWM is similar to that of the hurricane eyewall with radial convergence near the radius of maximum wind and a preferred location for maximum upward motions and upward mass transport just inside the RMW. Statistical analysis confirms the similarity in characteristics between radial and vertical velocities of the eyewall and near the SHWM. In addition, for both the eyewalls and SHWM, the radial velocity composite results show that the radial mass transport in the planetary boundary layer must be largely confined to the lowest 1000 m.

Lower fuselage radar reflectivity data from 13 of the hurricanes are used to assess whether the outer wind maxima are associated with rainbands, and vice versa. In the radial legs with SHWM for which radar data were available, the secondary horizontal wind maximum was frequently associated with a mesoscale reflectivity feature (rainband). In contrast, many rainbands, more than 70%, were without wind maxima. The results from this study show that to some extent an outer eyewall or rainband with SHWM can act as a barrier to inflow to the inner eyewall. Additionally, it is possible that thermodynamic modification of inflow air may occur as a result of convective-scale vertical motions associated with a rainband. In those cases when an outer rainband encircles the eyewall, it is possible that these factors act together with subsidence to weaken the inner eyewall.

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