Ray L. McAnelly

A Composite Model of Mesoscale Convective Complexes

Abstract A composite analysis technique is used to investigate the evolution of mesoscale features of mesoscale convective complexes (MCCs). The early stage of the MCC lifecycle is characterized by convergence, vertical motion and heating being centered in the lower troposphere. As the MCC matures the level of peak upward motion and heating shifts to the upper troposphere. The system achieves and maintains its maximum divergence, upward motion, and anticyclonic vorticity in the upper troposphere during the latter half of the life cycle. This is in contrast to GATE tropical clusters where the maximum divergence, upward motion, and anticyclonic vorticity occurred at the mature stage of the cluster and then weakened. This difference might be explained by an MCC being an inertially stable form of mesoscale convective system whose radius exceeds the Rossby radius of deformation. The MCC is shown to be an efficient rain producer, exhibiting a precipitation efficiency exceeding 100% at the mature stage due to th...

The Precipitation Life Cycle of Mesoscale Convective Complexes over the Central United States

Abstract The mesoscale convective complex (MCC) is a common and particularly well-organized class of meso-&α scale storm systems over the central United States. As observed by infrared (IR) satellite, the typical MCCs 10–12 h evolution displays a fairly consistent sequence of events, including the monotonic areal expansion of its anvil from its formation to its maximum size, followed by the monotonic shrinkage of the colder cloud top areas as the system weakens and dissipates. Primarily within the growth phase of this cycle, a characteristic IR signature reflects the MCC in its most intense, mesoconvective stage, which lasts ∼4 h and during which the coldest cloud top area reaches its largest extent. Hourly precipitation data have been analyzed for 122 MCC cases that were selected from June–August 1977–83 and screened to insure a reasonable conformity with the typical IR life cycle. On average. these systems produced a rainfall volume of 3.46 km 3 during their life cycle, over an area of 3.20×105km2and a...

Cloud venting - A review and some new global annual estimates

Abstract In this paper we review observational and modeling studies of cloud venting by a wide variety of cloud types ranging from ordinary cumuli, to ordinary cumulonimbi, mesoscale convective systems and tropical and extratropical cyclones. We have used explicit cloud-resolving simulations with RAMS to illustrate the nature of the process of venting of boundary layer air by several cloud system types and to provide quantitative estimates of the transport rates for different storms. In order to help global modelers prioritize their efforts in developing and refining cloud transport parametrization schemes, we have also attempted to make global estimates of the contributions of the various storm types to venting of boundary layer air. We find that on a global-annual basis, the extratropical cyclone has the highest boundary layer mass flux of all cloud venting systems, followed by the general class of MCSs (excluding MCCs), ordinary thunderstorms, tropical cylcones, and MCCs. We estimate an annual flux of 4.95 × 10 19 kg of boundary layer air by these cloud systems, which represents a venting of the entire boundary layer about 90 times a year.

Satellite and radar survey of mesoscale convective system development

Abstract An investigation of several hundred mesoscale convective systems (MCSs) during the warm seasons (April–August) of 1996–98 is presented. Circular and elongated MCSs on both the large and small scales were classified and analyzed in this study using satellite and radar data. The satellite classification scheme used for this study includes two previously defined categories and two new categories: mesoscale convective complexes (MCCs), persistent elongated convective systems (PECSs), meso-β circular convective systems (MβCCSs), and meso-β elongated convective systems (MβECSs). Around two-thirds of the MCSs in the study fell into the larger satellite-defined categories (MCCs and PECSs). These larger systems produced more severe weather, generated much more precipitation, and reached a peak frequency earlier in the convective season than the smaller, meso-β systems. Overall, PECSs were found to be the dominant satellite-defined MCS, as they were the largest, most common, most severe, and most prolific ...

A long-lived mesoscale convective complex. I - The mountain-generated component

Abstract Using data collected during Colorado State Universitys South Park Area Cumulus Experiment in 1977, a sequence of multi-scale convective events leading to the formation of a mesoscale convective complex is described. In the first phase, surface-based cool advection in the elevated mountain basin delayed the full transition of the morning boundary layer into a deep mixed layer until well after convective instability was reached over the adjacent ridges. The second phase was earmarked by the formation of convective precipitation echoes at “hot spots” over the high mountain terrain. Two groups of cells then propagated. eastward across the mountain basin, forming a line of discrete cells which moved across the foothills toward the High Plains. The cells further intensified at the. foothills/High Plains interface and formed a still larger, north-south line of thunderstorms. In the third phase, this north–south line of thunderstorms evolved into an expanding meso-β-scale convective cluster as it contin...

A long-lived mesoscale convective complex. II - Evolution and structure of the mature complex

Abstract An eight-day episode in August 1977 is described, wherein 14 mesoscale convective complexes (MCCs) developed in the central United States, including one to the immediate Ice of the Rocky Mountains on each day of the episode. In Part I of this article, the daytime genesis of one of these systems was traced from its pre-convective roots in the mountains of central Colorado to its incipient MCC stage on the plains of eastern Colorado. In this paper, its continued nocturnal development into a large MCC over Kansas is followed. Satellite imagery shows that this system remained coherent for at least three days as it passed off the east coast and across the western Atlantic Ocean. Analysis is focused on the mature stage of this and a second MCC in the episode in order to compare their major dynamic features to those of similar midlatitude systems reported in the literature, and also to previous studies of tropical mesoscale convective systems. Many of the important characteristics of midlatitude MCCs fo...

Upscale Evolution of MCSs: Doppler Radar Analysis and Analytical Investigation

The development of two small mesoscale convective systems (MCSs) in northeastern Colorado is investigated via dual-Doppler radar analysis. The first system developed from several initially isolated cumulonimbi, which gradually coalesced into a minimal MCS with relatively little stratiform precipitation. The second system developed more rapidly along an axis of convection and generated a more extensive and persistent stratiform echo and MCS cloud shield. In both systems, the volumetric precipitation rate exhibited an early meso-b-scale convective cycle (a maximum and subsequent minimum), followed by reintensification into a modest mature stage. This sequence is similar to that noted previously in the developing stage of larger MCSs by McAnelly and Cotton. They speculated that the early meso-b convective cycle is a characteristic feature of development in many MCSs that is dynamically linked to a rather abrupt transition toward mature stage structure. This study presents kinematic evidence in support of this hypothesis for these cases, as derived from dual-Doppler radar analyses over several-hour periods. Mature stage MCS characteristics such as deepened low- to midlevel convergence and mesoscale descent developed fairly rapidly, about 1 h after the early meso-b convective maximum. The dynamic linkage between the meso-b convective cycle and evolution toward mature structure is examined with a simple analytical model of the linearized atmospheric response to prescribed heating. Heating functions that approximate the temporal and spatial characteristics of the meso-b convective cycle are prescribed. The solutions show that the cycle forces a response within and near the thermally forced region that is consistent with the observed kinematic evolution in the MCSs. The initial response to an intensifying convective ensemble is a self-suppressing mechanism that partially explains the weakening after a meso-b convective maximum. A lagged response then favors reintensification and areal growth of the weakened ensemble. A conceptual model of MCS development is proposed whereby the early meso-b convective cycle and the response to it are hypothesized to act as a generalized forcing‐feedback mechanism that helps explain the upscale growth of a convective ensemble into an organized MCS.

Meso-β-scale Characteristics of an Episode of Meso-α-scale Convective Complexes

Abstract A variety of meso-β-scale (20–200 km, 6 h) convective complex (MCC) are described. The analysis is based on a typical episode of MCCs in the central United States. Thunderstorms in the MCC are generally well-organized into meso-β-scale convective features. The larger MCCs are typically preceded by several of these meso-β convective clusters or bands, which tend to be aligned along linear meso-α-scale features such as the eastern slope of the Rockies or thermodynamic discontinuities evident in hourly surface or satellite data. The intense development of these larger systems involves the growth, merger and interaction of those meso-β convective feature located nearest the intersection of the meso-α axes along which they are aligned. Throughout the mature phase of the MCC, multiple meso-β convective components may persist within the more uniform meso-α cloud shield as expanding regions of str...

Mesoscale Snowfall Prediction and Verification in Mountainous Terrain

Short-term forecasting of precipitation often relies on meteorological radar coverage to provide information on the intensity, extent, and motion of approaching mesoscale features. However, in significant portions of mountainous regions, radar coverage is lacking because of topographic blocking, and the absence of radar signatures in sections of the radar scan produces uncertain or even misleading information to the public and operational forecasters. In addition, echo characteristics within the radar volume scan are often influenced by the vertical extent and type of precipitation. Each of these conditions limits the opportunity for accurate snowfall prediction and studies of precipitation climatology. To improve both short-term forecasting and postevent verification studies, much greater use can be made of specifically sited surface observations, tailored graphical output from mesoscale models, satellite remote sensing, and case study knowledge of local topographic influences. In this paper, methods to support snowfall forecasts and verification in radar-limited mountainous terrain are demonstrated that include matching the output parameters and graphics from high-resolution mesoscale models to surface mesonets and snowfall observations, analysis of continuous and event-based measurements of snow density, application of multispectral satellite data for verification and trend analysis, and characterization of orographic influences in different winter storm scenarios. The advantages of improved wintertime quantitative precipitation forecasting (QPF) in mountain regions include public safety responsibilities that are critical to National Weather Service (NWS) operations, and are relevant to any mountainous region with radar scan limitations or during periods of radar data outages.

Early growth of mesoscale convective complexes : a meso-β-scale cycle of convective precipitation?

Abstract The evolution of precipitation fields associated with several mesoscale convective complexes (MCCs) has been inferred from radar reflectivity data. In almost all cases examined, including those described in the literature, the systems convective ensemble undergoes a marked modulation in intensity, on a meso-β time scale, during the early growth stage of its meso-α-scale life cycle. This β-scale convective cycle is most evident in time series of volumetric precipitation rate due to convective echo, and is characterized by a temporal maximum, followed by a minimum ∼0.5-1 h later, superimposed on an otherwise several-hour, monotonically increasing trend. While the latter is due to a concurrent, several-hour increase in the areal extent of convective echo, the β-scale perturbation is dominated by a modulation in intensity of the existing convective entities that the system constitutes at that stage of growth. The β-scale cycle also marks the onset of a sustained, increased growth rate of the MCCs s...