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Thomas M. Rickenbach

Abstract

Precipitation estimation over the tropical oceans is commonly performed using passive infrared (IR) measurements of cloud-top brightness temperature from geostationary satellites to infer the location of deep convection. It has been recognized in recent years that the majority of tropical precipitation is produced by mesoscale convective systems (MCSs). However, the relationship between the IR cloud-top patterns associated with MCSs and the underlying precipitation is not well understood. The assumption that the coldest cloud tops are associated with deep, active convection has been central to the characterization of cloud system motion and organization, and to many IR-based rainfall retrievals. Previous studies suggested that this view may be oversimplified when applied to propagating convective systems, such as squall lines. The goal of this study was to understand the evolution of the cold cloud associated with tropical oceanic squall line MCSs, and to discuss the implications for the retrieval of precipitation organization and rainfall rate from satellite IR data.

Shipboard radar reflectivity and Geostationary Meteorological Satellite (Japan) brightness temperature data collected during the Tropical Oceans Global Atmospheres Coupled Ocean–Atmosphere Response Experiment have been used to study the evolution of two tropical oceanic squall line MCSs. Results suggested a complex, evolution-dependent relationship between the radar-derived precipitation pattern associated with mesoscale convective systems and the overlying cloud tops observed by the satellite. The coldest clouds formed in the wake of the leading edge of the propagating lines, following the intensification of deep convection on the leading edge. In an environment of deep tropospheric directional wind shear (e.g., during westerly wind bursts), the cold cloud shield became spatially decoupled from the source convection to form swaths of cold cloud. These cloud swaths were generally normal to the squall line orientation. As convection on the leading edge of the squall line weakened, the cold cloud shield expanded. As a result the coldest clouds were more closely associated with weaker surface precipitation.

These results have implications for the interpretation of the location, shape, and motion of cold cloud features in the tropical western Pacific region. They may also help to explain the poor performance of IR rain retrieval algorithm when applied to instantaneous images. Results from this study may aid in the interpretation of twice-per-day “snapshots ” of MCSs (from IR, microwave, and radar sensors) from the recently launched Tropical Rainfall Measuring Mission satellite.

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Thomas M. Rickenbach

Abstract

This paper examines the origins of a secondary nocturnal maximum in cloudiness and precipitation in southwestern Amazonia, a diurnal feature observed previously by many investigators. Analysis is based on satellite, radar, sounding, and profiler observations of precipitating systems and cloudiness from the Tropical Rainfall Measuring Mission Large-Scale Biosphere–Atmosphere (TRMM-LBA) and the coincident Wet-Season Atmospheric Mesoscale Campaign (WETAMC) field programs during the early 1999 wet season. The general finding is that following the collapse of the nearly ubiquitous and locally generated afternoon (“noon balloon”) convection, organized deep convection contributes to a postmidnight maximum in raining area and high cloudiness, and to a lesser extent rainfall. Nocturnal convective systems have the effect of weakening and delaying the onset of the following afternoon's convection. Many of these nocturnal convective events are traced to large- scale squall lines, which propagate westward thousands of kilometers from their point of origin along the northeast coast of Brazil. In addition, a previously undescribed nocturnal stratiform drizzle phenomenon, generated above the melting layer independently from deep convection, contributes significantly to nocturnal cloud cover. Results from this study underscore the complex influence of propagating large-scale organized convection in locally modulating the diurnal variation in clouds and rain. The greatest significance of the nocturnal drizzle may be the potential effect on the diurnal radiation budget by the extensive midlevel nocturnal clouds rather than their marginal contribution to nocturnal rainfall.

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Thomas M. Rickenbach and Steven A. Rutledge

Abstract

The occurrence frequency and rainfall production of mesoscale convective systems (MCSs) relative to smaller groups of convective clouds over the tropical oceans is not well known. Eighty days of shipboard radar data collected during the recent Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) were used to provide a detailed view of convection in the western Pacific warm pool, a region of global climatological significance. The aim of this study was to document the frequency of occurrence, rainfall production, and depth of convection observed during TOGA COARE within a simple and meaningful framework of convective horizontal organization. Organization was characterized in terms of the horizontal scale and morphology of convective systems. Precipitation events were defined based on whether they attained the length scale of an MCS, and on whether convection was organized into lines.

About four-fifths of rainfall during COARE was associated with MCS-scale squall lines. These occurred in a variety of wind regimes but tended to be most common prior to low-level westerly wind maxima. Systems of isolated cells produced 12% of all COARE rainfall and were observed during periods of both very weak and very strong low-level winds. These two modes occurred about equally as often, and together they accounted for about 90% of observed convection. Cloud height populations associated with MCS organization were distinct from sub-MCS-scale cloud systems, with more rainfall from shallower clouds for sub-MCS convection. The distribution of total rainfall by cloud height for COARE was interpreted as a superposition of rainfall-cloud height distributions from each mode of organization. These results raise the possibility that isolated cell periods may represent a distinct, nonnegligible heat source in the large-scale heat budget when compared to the dominant MCS-scale systems.

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Richard H. Johnson, Paul E. Ciesielski, and Thomas M. Rickenbach

Abstract

Two features of Yanai et al.’s profiles of Q 1 and Q 2—the commonly observed double-peak structure to Q 2 and an inflection in the Q 1 profile below the melting level—are explored using estimates of convective and stratiform rainfall partitioning based on Massachusetts Institute of Technology (MIT) radar reflectivity data collected during TOGA COARE. The MIT radar data allow the Q 1 and Q 2 profiles to be classified according to stratiform rain fraction within the radar domain and, within the limitations of the datasets, allow interpretations to be made about the relative contributions of convective and stratiform precipitation to the mean profiles. The sorting of Q 2 by stratiform rain fraction leads to the confirmation of previous findings that the double-peak structure in the mean profile is a result of a combination of separate contributions of convective and stratiform precipitation. The convective contribution, which has a drying peak in the lower troposphere, combines with a stratiform drying peak aloft and low-level moistening peak to yield a double-peak structure. With respect to the inflection in the Q 1 profile below the 0°C level, this feature appears to be a manifestation of melting. It is the significant horizontal dimension of the stratiform components of tropical convective systems that yields a small but measurable imprint on the large-scale temperature and moisture stratification upon which the computations of Q 1 and Q 2 are based. The authors conclude, then, that the rather subtle features in the Q 1/Q 2 profiles of Yanai et al. are directly linked to the prominence of stratiform precipitation within tropical precipitation systems.

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Thomas M. Rickenbach, Rosana Nieto Ferreira, and Hannah Wells

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This study examines the geographic and temporal characteristics of the springtime transition to the summer precipitation regime of isolated convection in the southeastern (SE) United States during 2009–12, using a high-resolution surface radar-based precipitation dataset. Isolated convection refers herein to isolated elements or small clusters of precipitation in radar imagery less than 100 km in horizontal dimension. Though the SE United States does not have a monsoon climate, it is useful to apply the established framework of monsoon onset to study the timing and regional variation of the onset of the summer isolated convection regime. Overall, isolated convection rain onset in the SE U.S. domain occurs in late May. Onset begins in south Florida in mid-April, continuing nearly simultaneously across the southeastern coastal plain in early to mid-May. In the northern domain, from Virginia to the Ohio Valley, onset generally occurs much later (mid-June to early July) with more variable onset timing. The sharpness of onset timing is most evident in the coastal plain and Florida. Results suggest the hypothesis, to be examined in a forthcoming study, that the timing of isolated convection onset in the spring may be triggered by specific synoptic-scale events within gradual seasonal changes in atmospheric conditions including extratropical cyclone tracks, convective instability, and the westward migration of the North Atlantic subtropical high. This approach may offer a useful framework for evaluating long-term changes in precipitation for subtropical regimes in an observational and modeling context.

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Rosana Nieto Ferreira, Linwood Hall, and Thomas M. Rickenbach

Abstract

The seasonal and interannual variability of the structure, evolution, and propagation of midlatitude cyclones in the southeast United States are studied using a composite analysis. In the upper levels, the composites show that the axis of the wintertime upper-level trough remains north–south oriented and propagates eastward along 40°N, while the summertime upper-level trough has a much slower propagation at a farther north latitude and an axis that is tilted in the northeast–southwest direction. Upper-level circulation changes are consistent with a shift from wintertime “cyclonic behavior” to summertime “anticyclonic behavior” midlatitude cyclones. Significant changes in the low-level structure and precipitation patterns of midlatitude cyclones ensue from these upper-level changes. While the winter composite is characterized by eastward-propagating midlatitude cyclones that extend deep into the subtropics, the summer composite is characterized by semistationary midlatitude troughs that only briefly skirt the subtropics. Wintertime precipitation occurs only in and ahead of the surface low pressure center, whereas summertime precipitation occurs in all days of the composite. As a result, over 70% (30%) of wintertime (summertime) precipitation in the Carolinas occurs on days when midlatitude cyclones are present. The wintertime composites also show that midlatitude cyclones produce more precipitation on the windward side of the Appalachians than over the Carolinas, suggesting a rain shadow effect of the mountains.

The ENSO-related variability of the structure, evolution, and propagation of midlatitude cyclones shows the presence of a more intense and southward-displaced upper-level jet, stronger midlatitude cyclones, and more intense precipitation over a larger area during El Niño than La Niña or normal years.

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Rosana Nieto Ferreira, Thomas M. Rickenbach, Dirceu L. Herdies, and Leila M. V. Carvalho

Abstract

A comparison of the submonthly variability of atmospheric circulation and organization of convection in South America during January–February–March of 1998 (JFM98) and January–February–March of 1999 (JFM99) is presented. According to the National Centers for Environmental Prediction reanalysis, the South American low-level jet (SALLJ) was about twice as strong during JFM of the 1998 El Niño episode than during JFM of the 1999 La Niña episode. The difference in SALLJ strength between these two years translated into stronger transport of moist tropical air into the subtropics during JFM98 than during JFM99. An objective tracking technique was used to identify large, long-lived convective cloud systems in infrared imagery. The stronger SALLJ was accompanied by larger and more numerous long-lived convective cloud systems and nearly twice as much rainfall in subtropical South America (parts of southern Brazil, Uruguay, and Argentina) during JFM98 than during JFM99.

The difference between JFM98 and JFM99 SALLJ strength in Bolivia is in part explained by submonthly variability associated with the South Atlantic convergence zone (SACZ). Periods when the SACZ is present are marked by southerly or weak northerly winds in Bolivia. The South Atlantic convergence zone was more prominent during JFM99 than during JFM98 contributing to a weaker SALLJ during JFM99. Large, long-lived convective cloud systems in subtropical South America tended to occur during times when the SACZ was absent and the SALLJ was strong over Bolivia. Interannual variability associated with the El Niño–Southern Oscillation also contributed to the observed interannual variability of the SALLJ in Bolivia.

In the tropical portions of South America nearly 6 times more large, long-lived convective cloud systems were observed during JFM99 than during JFM98. This was accompanied by more plentiful precipitation in portions of the Amazon basin and in the Bolivian Altiplano during JFM99 than during JFM98. Interannual variability associated with the El Niño–Southern Oscillation was an important contributor to the observed convective cloud system and precipitation differences in tropical South America.

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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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Minh D. Phan, Burrell E. Montz, Scott Curtis, and Thomas M. Rickenbach

Abstract

Millions of people in the United States regularly acquire information from weather forecasts for a wide variety of reasons. The rapid growth in mobile device technology has created a convenient means for people to retrieve this data, and in recent years, mobile weather applications (MWAs) have quickly gained popularity. Research on weather sources, however, has been unable to sufficiently capture the importance of this form of information gathering. As use of these apps continues to grow, it is important to gain insight on the usefulness of MWAs to consumers. To better examine MWA preferences and behaviors relating to acquired weather information, a survey of 308 undergraduate students from three different universities throughout the southeast United States was undertaken. Analyses of the survey showed that smartphone MWAs are the primary weather forecast source among college students. Additionally, MWA users tend to seek short-term forecast information, like the hourly forecast, from their apps. Results also provide insight into daily MWA use by college students as well as perceptions of and preferential choices for specific MWA features and designs. The information gathered from this study will allow other researchers to better evaluate and understand the changing landscape of weather information acquisition and how this relates to the uses, perceptions, and values people garner from forecasts. Organizations that provide weather forecasts have an ever-growing arsenal of resources to disseminate information, making research of this topic extremely valuable for future development of weather communication technology.

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Patrick T. Haertel, George N. Kiladis, Andrew Denno, and Thomas M. Rickenbach

Abstract

Vertical structures of 2-day waves and the Madden–Julian oscillation (MJO) are projected onto vertical normal modes for a quiescent tropical troposphere. Three modes capture the gross tropospheric structure of 2-day waves, while only two modes are needed to represent most of the baroclinic structure of the MJO. Deep circulations that project onto the first baroclinic mode are associated with deep cumulonimbus and stratiform rainfall. Shallow circulations that project onto higher wavenumber modes are associated with precipitating shallow cumulus and congestus and stratiform rainfall. For both disturbances the horizontal divergence contributed by shallow modes is an important factor in the column-integrated moist enthalpy budget. These modes converge moist static energy for a time prior to when deep circulations export moist static energy. These results highlight the importance of properly representing the effects of shallow cumulus, congestus, and stratiform precipitation in theories of convectively coupled waves and in atmospheric models.

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