Abstract

The evolution of the turbulent structure of an intense, quasi-steady thunderstorm is examined using Doppler radar estimates of turbulent kinetic energy dissipation rates (ε) and radial shears of raw radial velocity (ΔV _r/ΔR). A comparison of turbulent patterns with mean storm airflow is made.

Observations taken during the quasi-steady mature stage reveal that turbulent intensity and activity peaked at mid to upper storm levels. The primary storm updraft was nearly turbulence-free at low levels, but exhibited increasingly turbulent activity at higher levels indicating a transition from quasi-laminar flow to bubble-like flow. Comparison of ε and ΔV _r/ΔR patterns with environmental parameters such as equivalent potential temperature and momentum suggests that buoyancy and wind shear acted together to generate turbulent eddies, some greater than 500 m in size, at middle storm levels. At mid to upper storm levels, patterns of ε and ΔV _r/ΔR exhibited considerable spatial and temporal variability, and maximum estimated dissipation rate estimates approached 0.15 m² s⁻³. During one particular time period, 11 local ε maxima were estimated, some with magnitudes exceeding 0.07 m² s⁻³.

Abstract

Vertical divergence of the mountain wave's momentum flux has recently been hypothesized to be an important contribution to the global momentum budget. Wavebreaking theories and envelope orography have been employed to explain the divergence of the momentum flux. Here, cloud-top radiational cooling is shown to locally destabilize the environment and disrupt the propagation of the mountain wave in idealized two-dimensional simulations, thus drastically altering the expected momentum flux profile. Also, simulations of two-dimensional mountain waves indicate that nonlinearities can increase the wave response if the lower layer is decoupled from the flow aloft or decrease the wave response by providing multiple reflection levels for the incident mountain wave. The onset of wavebreaking and the level at which the wave breaks can be influenced by the ambient thermodynamic profile.

Abstract

The interaction of topographically induced thermally and mechanically driven diurnal flow regimes in the lee of the Rockies is shown to lead to the growth of a mesoscale convective system (MCS). An organic MCS observed during the 1977 combined South Park Area Cumulus Experiment and High Plains Experiment is numerically simulated with a two-dimensional nonhydrostatic cloud model covering spatial scales of 1000 km. In this numerical investigation,mesoγ-, mesoβ- and mesoα-scales of motion are represented simultaneously. As a result, interesting features of cloud-mesoscale interaction are predicted that cannot be represented in cloud parameterization frameworks. Based on the results of this simulation, a six-stage conceptual model of orogenic development is given.

Abstract

A detailed analysis of the dynamics and thermodynamics responsible for the structure, growth and propagation of an orogenic mesoscale convective system simulated in two dimensions is made. The process of scale interaction is addressed through Fourier analysis and Reynolds averaging analysis of representative predicted variables, diabatic forcing and momentum acceleration terms. Additional dynamical analysis is accomplished through sensitivity experiments in which Coriolis, diabatic heating and ambient airflow are varied.

The general conclusion is that the simulated orogenic development is a geostrophic adjustment process to convective heating which is itself modulated and maintained by orographically induced flow systems. The heating scales range over a nearly continuous spectrum ranging from 10–250 km. The heating occurs in response to primary advective gravity modes. The larger-scale gravity-wave disturbances modulate the smaller scales by organizing mean upward vertical motion patterns. The largest gravity-wave modes are modulated by constraints of the slope flow circulation, namely a phasing of an advective mode with a localized break in the plains inversion.

The simulated growth to mesoα-scale proportions occurs from the horizontal expansion of the disturbance through interaction with the mountain-plains scale slope flow circulation. Similar to upscale two-dimensional turbulence cascade, the mountain plains solenoid deforms thermal patterns, increasing their scale. As the scale reaches mesoα-scale proportions, geostrophic adjustment frequencies are sufficient to allow the thermal fields to persist. Implications to the problem of cumulus parameterization and limitations of the two-dimensional framework of this numerical study are discussed.

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 MCC's 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 at an average depth of 10.8 mm. Relative to a normalized, IR-defined life cycle, the averaged trends of hourly rainfall area, intensity, and volume all have well-defined growth/ decay cycles, but with significantly staggered maxima. Average rainfall intensity (R), and the proportion of measurable reports due to convective intensifies, attain maxima early in the life cycle. Hourly rainfall volumes (ν) are more symmetrically distributed in time, with the maximum occurring near the largest anvil size (based on −54°C IR threshold). Active rainfall area (A) continues to increase until ∼1 h after maximum anvil size. The IR-defined, intense mesoconvective stage corresponds to that portion of the life cycle from maximum R to maximum A, and is so termed because of the large areal extent and volumetric rate of convective precipitation intensities. A large area of stratiform precipitation is generated during this stage; it persists and becomes increasingly dominant as convective activity subsides during the latter stages of the life cycle. Averaged mappings of the precipitation data show that throughout the MCC life cycle, the heaviest rainfall tends to be displaced 50–100 km south of the cloud-shield centroid, while the stratiform pattern tends to be more MCC-centered.

A statistical analysis of these precipitation characteristics, derived individually for each case, provides an estimate of the natural interstorm variability for typical summertime MCCS. A comparison of various composite subsets of the sample reveals several interesting tendencies: 1) smaller, less-organized systems tended to be “drier” than similar-sized but better-organized MCCS; 2) large systems were "rainier” than smaller ones through much of the life cycle, not only in terms of A and V, as expected, but also in terms of R; 3) large systems tended to be “rdnice” in the eastern part of the sample domain than in the western part, but this was not so for small systems; and 4) the eastern systems: both large and small, had a more coherent and intense core of heavy precipitation through their life cycle than the western systems.

Abstract

A variety of meso-β-scale (20–200 km, <6 h) temporal and spatial characteristics associated with the life-cycle of the meso-α-scale (200–2000 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 stratiform anvil precipitation develop. The decay of the system is marked by the weakening and difluent propagation of its meso-β convective components. Hourly precipitation data reveal a characteristic precipitation life-cycle in relation to the MCC's satellite appearance. These typical meso-β-scale characteristics offer potential tools for the short-range forecasting of MCCs and their hydrological consequences.

Abstract

Rapid advances in the quality and quantity of atmospheric observations have placed a demand for the development of techniques to assimilate these data sources into numerical forecasting models. Four-dimensional variational assimilation is a promising technique that has been applied to atmospheric and oceanic dynamical models, and to the retrieval of three-dimensional wind fields from single-Doppler radar observations.

This study investigates the feasibility of using space–time variational assimilation for a complex discontinuous numerical model including cloud physics. Two test models were developed: a one-dimensional and a two-dimensional liquid physics kinematic microphysical model. These models were used in identical-twin experiments, with observations taken intermittently. Small random errors were introduced into the observations. The retrieval runs were initialized with a large perturbation of the observation run initial conditions.

The models were able to retrieve the original initial conditions to a satisfactory degree when observations of all the model prognostic variables were used. Greater overdetermination of the degrees of freedom (the initial condition being retrieved) resulted in greater improvement of the errors in the observations of the initial conditions but at a rapid increase in computational cost. Experiments where only some of the prognostic variables were observed also improved the initial conditions, but at a greater cost. To substantially improve the first guess of the field not observed, some spot observations are needed.

The proper scaling of the variables was found to be important for the rate of convergence. This study suggests that scaling factors related to the error variance of the observations give good convergence rates.

To show how this technique can be used when observations are general functions of the prognostic variables of the model (e.g., reflectivity or liquid water path), a form is derived that shows that this can be accomplished. This is considered to be an advantage of this technique over other assimilation techniques, since it is particularly suitable to remote-sensing systems where only integral parameters or derivatives of model prognostic variables are observed.

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 system's 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 MCC's stratiform precipitation production.

While these conclusions are based on only a few cases, their dynamical implications on the poorly understood processes accompanying MCC growth are significant. Is this meso-β-scale convective cycle a generalized, fundamental feature accompanying the development of many MCCs? Could the cycle be a “fingerprint” of a dynamic, upscale transition of the incipient convective system from its convective-β-scale stage into a nascent meso-α-scale system? This possibility is discussed by relating the results to numerical simulations and other studies of mesoscale convection in the literature.

Abstract

The 28 October 1986 First ISCCP (International Satellite Cloud Climatology Program) Regional Experiment (FIRE) case was simulated using the Regional Atmospheric Modeling System developed at Colorado State University. This three-dimensional mesoscale model was applied in nonhydrostatic and nested-grid mode, using explicit, bulk microphysics and radiation. The simulation resulted in very good agreement between observed and model-predicted dynamic and cloud fields. Cloud height, thickness, areal extent, and microphysical composition were verified against GOES satellite imagery, lidar, and aircraft measurements taken during the FIRE cirrus intensive field observation. Cloud-top generation zones and layering were simulated. Sensitivity simulations were run to determine long- and shortwave radiative forcing. Also, a simulation was run with no condensate to examine cloud feedbacks on the environment. Longwave radiation appeared to be instrumental in developing weak convective-like activity, thereby increasing the cloud's optical depth.

Abstract

This study uses observed data and a numerical simulation to examine the generation of thermally driven flows across the Colorado mountain barrier on meso-β to meso-α scales. The observations were collected from remote surface observing systems at exposed mountaintop locations throughout the state of Colorado, over the summers of 1984–88, as part of the Rocky Mountain Peaks Experiment (ROMPEX). The data show the development of a recurrent circulation system across the Colorado mountain barrier, operating on a diurnal timescale. From the observations, the basic structure of the flow system appears as a daytime inflow toward the highest terrain, and a nocturnal outflow away from it. However, when examined in detail, the flow system exhibits more unusual behavior, especially west of the barrier crest. Here, winds in the early evening are occasionally observed to onset abruptly from an easterly direction, generally counter to the upper-level winds. Observations from ROMPEX for 26 August 1985 are used to provide comparison data for a numerical simulation with the Regional Atmospheric Modeling System (RAMS). This three-dimensional case study experiment is initialized with data from the National Meteorological Center and incorporates two-way interactive grid nesting.

From the observed data and case study simulation, four distinct phases of the regional-scale circulation system have been identified. In the development phase, a deep mountain-plains solenoid is generated through terrain heating along the Front Range. This circulation system transforms in the late afternoon transition phase into a westward-propagating density current (WPDC). The third phase, called the “density-current propagation phase,” occurs as the WPDC moves westward across the mountains, leaving in its wake strong southeasterly flow at the mountaintop level. This current appears to be the cause of the peculiar easterly component winds found in the ROMPEX mountaintop observations along the western slope. In the final late-night adjustment phase, the WPDC dissipates near the western edge of the Colorado mountains and a steady southerly flow evolves over the high mountain terrain. This southerly flow is the steady response to the differential heating that develops between the low-lying plains and the intermountain region.