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This paper discusses common modes of mesoscale convective organization. Using 2-km national composite reflectivity data, the authors investigated linear mesoscale convective systems (MCSs) that occurred in the central United States during May 1996 and May 1997. Based upon the radar-observed characteristics of 88 linear MCSs, the authors propose a new taxonomy comprising convective lines with trailing (TS), leading (LS), and parallel (PS) stratiform precipitation. While the TS archetype was found to be the dominant mode of linear MCS organization, the LS and PS archetypes composed nearly 40% of the studied population. In this paper, the authors document the characteristics of each linear MCS class and use operational surface and upper air data to describe their different environments. In particular, wind profiler data reveal that the stratiform precipitation arrangement associated with each class was roughly consistent with the advection of hydrometeors implied by the mean middle-and upper-tropospheric storm-relative winds, which were significantly different among the three MCS modes. Case study examples are presented for both the LS and PS classes, which have received relatively little attention to this point. As well, the authors give a general overview of the synoptic-scale meteorology accompanying linear MCSs in this study, which was generally similar to that observed by previous investigators.

This study examines the radar-indicated structures and other features of extreme rain events in the United States over a 3-yr period. A rainfall event is defined as "extreme" when the 24-h precipitation total at one or more stations surpasses the 50-yr recurrence interval amount for that location. This definition yields 116 such cases from 1999 to 2001 in the area east of the Rocky Mountains, excluding Florida. Two-kilometer national composite radar reflectivity data are then used to examine the structure and evolution of each extreme rain event. Sixty-five percent of the total number of events are associated with mesoscale convective systems (MCSs). While a wide variety of organizational structures (as indicated by radar reflectivity data) are seen among the MCS cases, two patterns of organization are observed most frequently. The first type has a line, often oriented east-west, with "training" convective elements. It also has a region of adjoining stratiform rain that is displaced to the north of the line. The second type has a back-building or quasi-stationary area of convection that produces a region of stratiform rain downstream. Surface observations and composite analysis of Rapid Update Cycle Version 2 (RUC-2) model data reveal that training line/adjoining stratiform (TL/AS) systems typically form in a very moist, unstable environment on the cool side of a preexisting slow-moving surface boundary. On the other hand, back-building/quasistationary (BB) MCSs are more dependent on mesoscale and storm-scale processes, particularly lifting provided by storm-generated cold pools, than on preexisting synoptic boundaries.

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The kinematic and thermodynamic characteristics of the October and November 2011 Madden–Julian oscillations (MJOs) that occurred over the Indian Ocean during Dynamics of the MJO (DYNAMO) are investigated. Analyses are presented 1) for two primary sounding arrays, where results are independent of model parameterizations, and 2) on larger scales, including the Indian Ocean, using operational and reanalysis data. Mean precipitation during DYNAMO was characterized by maxima in two east–west bands north and south of the equator. This pattern alternated between two bands during the inactive phase of the MJOs and a single rainfall maximum on the equator during the active phases. Precipitation over the northern sounding array (NSA), where the MJO signal was strongest, was significantly modulated by the MJOs, while the southern array experienced more frequent, briefer episodes of rainfall mostly related to ITCZ convection. Over the NSA the MJOs were characterized by gradual moistening of the low to midtroposphere over approximately 2-week periods. The October MJO featured multiple westward-moving, 2-day disturbances whereas the November MJO principally comprised two prominent Kelvin waves. Patterns of moistening, divergence, and vertical motion suggest a stepwise progression of convection, from shallow cumulus to congestus to deep convection. Tilted thermal anomalies in the upper troposphere–lower stratosphere reveal gravity or Kelvin waves excited by the MJO convective envelopes, which modulate the tropopause and contribute to preactive-phase upper-tropospheric moistening. While there is a number of similarities in the characteristics of the two MJOs, there are sufficient differences to warrant caution in generalizing results from these two events.

During the second week of September 2013, a seasonally uncharacteristic weather pattern stalled over the Rocky Mountain Front Range region of northern Colorado bringing with it copious amounts of moisture from the Gulf of Mexico, Caribbean Sea, and the tropical eastern Pacific Ocean. This feed of moisture was funneled toward the east-facing mountain slopes through a series of mesoscale circulation features, resulting in several days of unusually widespread heavy rainfall over steep mountainous terrain. Catastrophic flooding ensued within several Front Range river systems that washed away highways, destroyed towns, isolated communities, necessitated days of airborne evacuations, and resulted in eight fatalities. The impacts from heavy rainfall and flooding were felt over a broad region of northern Colorado leading to 18 counties being designated as federal disaster areas and resulting in damages exceeding $2 billion (U.S. dollars). This study explores the meteorological and hydrological ingredients that led to this extreme event. After providing a basic timeline of events, synoptic and mesoscale circulation features of the event are discussed. Particular focus is placed on documenting how circulation features, embedded within the larger synoptic flow, served to funnel moist inflow into the mountain front driving several days of sustained orographic precipitation. Operational and research networks of polarimetric radar and surface instrumentation were used to evaluate the cloud structures and dominant hydrometeor characteristics. The performance of several quantitative precipitation estimates, quantitative precipitation forecasts, and hydrological forecast products are also analyzed with the intention of identifying what monitoring and prediction tools worked and where further improvements are needed.

Observations from 1 km beneath to 25 km above the sea surface reveal the complex interactions in Indian Ocean westerly wind bursts associated with the Madden-Julian oscillation.

Atmospheric soundings, radar, and air-sea flux measurements collected during Dynamics of the Madden-Julian Oscillation (DYNAMO) are employed to study MJO convective onset (i.e., the transition from shallow to deep convection) in the tropical Indian Ocean. The findings indicate that moistening of the low-midtroposphere during the preonset stage of the MJO is achieved by simultaneous changes in the convective cloud population and large-scale circulation. Namely, cumuliform clouds deepen and grow in areal coverage as the drying by large-scale subsidence and horizontal (westerly) advection wane. The reduction of large-scale subsidence is tied to the reduction of column radiative cooling during the preonset stage, which ultimately links back to the evolving cloud population. While net column moistening in the preonset stage is tied to large-scale circulation changes, a new finding of this study is the high degree to which the locally driven diurnal cycle invigorates convective clouds and cumulus moistening each day. This diurnal cycle is manifest in a daytime growth of cumulus clouds (in both depth and areal coverage) in response to oceanic diurnal warm layers, which drive a daytime increase of the air-sea fluxes of heat and moisture. This diurnally modulated convective cloud field exhibits prominent mesoscale organization in the form of open cells and horizontal convective rolls. It is hypothesized that the diurnal cycle and mesoscale cloud organization characteristic of the preonset stage of the MJO represent two manners in which local processes promote more vigorous daily-mean column moistening than would otherwise occur.