J. Michael Fritsch

A One-Dimensional Entraining/Detraining Plume Model and Its Application in Convective Parameterization

Abstract A new one-dimensional cloud model, specifically designed for application in mesoscale convective parameterization schemes (CPSs), is introduced. The model is unique in its representation of environmental entrainment and updraft detrainment rates. In particular, the two-way exchange of mass between clouds and their environment is modulated at each vertical level by a buoyancy sorting mechanism at the interface of clear and cloudy air. The new entrainment/detrainment scheme allows vertical profiles of both updraft moisture detrainment and updraft vertical mass flux to vary in a physically realistic way as a function of the cloud-scale environment. These performance characteristics allow the parameterized vertical distribution of convective heating and drying to be much more responsive to environmental conditions than is possible with a traditional one-dimensional entraining plume model. The sensitivities of the new model to variations in environmental convective available potential energy and verti...

Convective parameterization for mesoscale models : The Kain-Fritsch Scheme

The Kain-Fritsch (KF) convective parameterization scheme (CPS) is based on the same fundamental closure assumption as the Fritsch-Chappell (FC) (1980) scheme—convective effects are assumed to remove convective available potential energy in a grid element within an advective time period. Its development was motivated by ongoing observational and numerical investigations of mesoscale convective systems that have revealed the potentially significant impact of certain physical processes that were not represented in the FC scheme. For example, in the FC scheme, detrainment from convective clouds to their environment occurs over a limited vertical depth near cloud top. Yet, it has become evident from diagnostic studies (e.g., Leary and Houze 1980; Gamache and Houze 1983) that midlevel detrainment of mass and moisture from deep convective clouds plays an important role in the development of some mesoscale convective systems.

Resolution Requirements for the Simulation of Deep Moist Convection

The spatial resolution appropriate for the simulation of deep moist convection is addressed from a turbulence perspective. To provide a clear theoretical framework for the problem, techniques for simulating turbulent flows are reviewed, and the source of the subgrid terms in the Navier‐Stokes equation is clarified. For decades, cloud-resolving models have used large-eddy simulation (LES) techniques to parameterize the subgrid terms. A literature review suggests that the appropriateness of using traditional LES closures for this purpose has never been established. Furthermore, examination of the assumptions inherent in these closures suggests that grid spacing on the order of 100 m may be required for the performance of cloud models to be consistent with their design. Based on these arguments, numerical simulations of squall lines were conducted with grid spacings between 1 km and 125 m. The results reveal that simulations with 1-km grid spacing do not produce equivalent squallline structure and evolution as compared to the higher-resolution simulations. Details of the simulated squall lines that change as resolution is increased include precipitation amount, system phase speed, cloud depth, static stability values, the size of thunderstorm cells, and the organizational mode of convective overturning (e.g., upright towers versus sloped plumes). It is argued that the ability of the higher-resolution runs to become turbulent leads directly to the differences in evolution. There appear to be no systematic trends in specific fields as resolution is increased. For example, mean vertical velocity and rainwater values increase in magnitude with increasing resolution in some environments, but decrease with increasing resolution in other environments. The statistical properties of the simulated squall lines are still not converged between the 250- and 125-m runs. Several possible explanations for the lack of convergence are offered. Nevertheless, it is clear that simulations with O(1 km) grid spacing should not be used as benchmark or control solutions for resolution sensitivity studies. The simulations also support the contention that a minimum grid spacing of O(100 m) is required for traditional LES closures to perform appropriately for their design. Specifically, only simulations with 250- and 125-m grid spacing resolve an inertial subrange. In contrast, the 1-km simulations do not even reproduce the correct magnitude or scale of the spectral kinetic energy maximum. Furthermore, the 1-km simulations contain an unacceptably large amount of subgrid turbulence kinetic energy, and do not adequately resolve turbulent fluxes of total water. A guide to resolution requirements for the operational and research communities is proposed. The proposal is based primarily on the intended use of the model output. Even though simulations with O(1 km) grid spacing display behavior that is unacceptable for the model design, it is argued that these simulations can still provide valuable information to operational forecasters. For the research community, O(100 m) grid spacing is recommended for most applications, because a modeling system that is well founded should be desired for most purposes.

A benchmark simulation for moist nonhydrostatic numerical models

A benchmark solution that facilitates testing the accuracy, efficiency, and efficacy of moist nonhydrostatic numerical model formulations and assumptions is presented. The solution is created from a special configuration of moist model processes and a specific set of initial conditions. The configuration and initial conditions include: reversible phase changes, no hydrometeor fallout, a neutrally stable base-state environment, and an initial buoyancy perturbation that is identical to the one used to test nonlinearly evolving dry thermals. The results of the moist simulation exhibit many of the properties found in its dry counterpart. Given the similar results, and acceptably small total mass and total energy errors, it is argued that this new moist simulation design can be used as a benchmark to evaluate moist numerical model formulations. The utility of the benchmark simulation is highlighted by running the case with approximate forms of the governing equations found in the literature. Results of these tests have implications for the formulation of numerical models. For example, it is shown that an equation set that conserves both mass and energy is crucial for obtaining the benchmark solution. Results also suggest that the extra effort required to conserve mass in a numerical model may not lead to significant improvements in results unless energy is also conserved.

Improving quantitative precipitation forecasts in the warm season: A USWRP research and development strategy

Warm-season quantitative precipitation forecasts (QPFs) are the poorest performance area of forecast systems worldwide. They stubbornly fall further behind while other aspects of weather prediction...

Mesoscale Convective Complexes in Africa

Abstract Digitized full-disk infrared satellite imagery from the European geostationary satellite (Meteosat) for 1986 and 1987 was used to construct a climatology of mesoscale convective complexes (MCCs) in Africa One hundred ninety-five systems formed over Africa and its near vicinity during the two-year study period. From this database, characteristics of African MCCs were calculated. The results indicate that these MCCs display many of the same characteristics as those found in the Americas, the Indian subcontinent, and the western Pacific region. The systems are predominantly nocturnal and tend to form over or in the immediate vicinity of land. Much of the activity occurs over the African Sahel. while comparatively little occurs over the equatorial rain forest. The average lifetime of African MCCs is about 11.5 h, whereas systems in the western Pacific region and the Americas last about 11 and 10 h, respectively. The size distributions of the African systems are also extremely similar to those of the ...

A Two-Way Interactive Nesting Procedure with Variable Terrain Resolution

Abstract A two-way interactive, nested-grid system tested with The Pennsylvania Slate University/INCAR three-dimensional mesoscale model is described. A mesh structure, designed to minimize numerical noise, together with a procedure for obtaining compatible coarse grid mesh (COM) and fine grid mesh (FOM) terrain conditions, is presented. Also, a method to initialize the nested-grid meshes is proposed. The nested-grid system has been tested with real data and raw terrain under different severe conditions. A 12-h simulation of a propagating jet streak over complex terrain is presented; the results indicate relatively noise-free solutions on both the OGM and FGM domains.

The Large-Scale Environments of the Global Populations of Mesoscale Convective Complexes

Abstract The mean genesis environment was constructed for each of five mesoscale convective complex (MCC) population centers around the world: Africa, Australia, China, South America, and the United States. It is found that the environments are very similar and exhibit many of the same dynamic and thermodynamic structures that are present with systems in the United States. In particular, MCCs initiate within prominent baroclinic zones characterized by locally large values of lower-tropospheric vertical wind shear and convective available potential energy (CAPE). Typically, a low-level jet of air with low static stability, high equivalent potential temperature, oriented nearly perpendicular to the baroclinic zone, intrudes into the genesis region and is forced to ascend over a relatively shallow, surface-based layer of relatively cool air. Pronounced warm advection accompanied by strong lower-tropospheric veering overlays the surface-based cool layer. A local maximum in absolute humidity and a local minimu...

Moist Absolute Instability: The Sixth Static Stability State

It is argued that a sixth static stability state, moist absolute instability, can be created and maintained over mesoscale areas of the atmosphere. Examination of over 130 000 soundings and a numerical simulation of an observed event are employed to support the arguments in favor of the existence of moist absolutely unstable layers (MAULs). Although MAULs were found in many different synoptic environments, of particular interest in the present study are the deep (≥ 100 mb) layers that occur in conjunction with mesoscale convective systems (MCSs). A conceptual model is proposed to explain how moist absolute instability is created and maintained as MCSs develop. The conceptual model states that strong, mesoscale, nonbuoyancy-driven ascent brings a conditionally unstable environmental layer to saturation faster than small-scale, buoyancy-driven convective elements are able to overturn and remove the unstable state. Moreover, since lifting of a moist absolutely unstable layer warms the environment, the temper...

Numerical Simulation of the Meso-β Scale Structure and Evolution of the 1977 Johnstown Flood. Part I: Model Description and Verification

Abstract The Pennsylvania State University/NCAR mesoscale model, originally developed by Anthes and Warner, is modified to simulate the meso-β scale structure and evolution of convectively driven weather systems. The modifications include: (i) two-way interactive nested-grid procedures, (ii) the Fritsch-Chappell convective parameterization scheme, and (iii) the Blackadar boundary layer package. An 18-h simulation of the Johnstown flood of July 1977 is conducted. Compared to the documentation of Hoxit et al. and Bosart and Sanders, the simulation reproduced many of the different aspect of the mesoscale convective complex and squall line that were responsible for the heavy rain over western Pennsylvania. In particular, the model predicts the size, propagation rate and orientation of the mesoscale convective components that were observed in the mid-Atlantic states. The simulated evolution of the planetary boundary layer, cool outflow boundaries and surface pressure perturbations, such as meso-β scale lows, h...