Joseph B. Klemp

A Description of the Advanced Research WRF Version 2

The Technical Note series provides an outlet for a variety of NCAR manuscripts that contribute in specialized ways to the body of scientific knowledge but which are not suitable for journal, monograph, or book publication. Reports in this series are issued by the NCAR Scientific Divisions ; copies may be obtained on request from the Publications Office of NCAR. Designation symbols for the series include: Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the National Science Foundation.

The Simulation of Three-Dimensional Convective Storm Dynamics

Abstract A new three-dimensional cloud model has been developed for investigating the dynamic character of convective storms. This model solves the compressible equations of motion using a splitting procedure which provides numerical efficiency by treating the sound wave modes separately. For the subgrid turbulence processes, a time-dependent turbulence energy equation is solved which depends on local buoyancy, shear and dissipation. First-order closure is applied to nearly conservative variables with eddy coefficients based on the computed turbulence energy. Open lateral boundaries are incorporated in the model that respond to internal forcing and permit gravity waves to propagate out of the integration domain with little apparent reflection. Microphysical processes are included in the model using a Kessler-type parameterization. Simulations conducted for an unsheared environment reveal that the updraft temperatures follow a moist adiabatic lapse rate and that the convection is dissipated by water loadin...

A time-split nonhydrostatic atmospheric model for weather research and forecasting applications

The sub-grid-scale parameterization of clouds is one of the weakest aspects of weather and climate modeling today, and the explicit simulation of clouds will be one of the next major achievements in numerical weather prediction. Research cloud models have been in development over the last 45 years and they continue to be an important tool for investigating clouds, cloud-systems, and other small-scale atmospheric dynamics. The latest generation are now being used for weather prediction. The Advanced Research WRF (ARW) model, representative of this generation and of a class of models using explicit time-splitting integration techniques to efficiently integrate the Euler equations, is described in this paper. It is the first fully compressible conservative-form nonhydrostatic atmospheric model suitable for both research and weather prediction applications. Results are presented demonstrating its ability to resolve strongly nonlinear small-scale phenomena, clouds, and cloud systems. Kinetic energy spectra and other statistics show that the model is simulating small scales in numerical weather prediction applications, while necessarily removing energy at the gridscale but minimizing artificial dissipation at the resolved scales. Filtering requirements for atmospheric models and filters used in the ARW model are discussed.

A Theory for Strong, Long-Lived Squall Lines

Abstract We study herein the mechanics of long-lived, line-oriented, precipitating cumulus convection (squall lines) using two- and three-dimensional numerical models of moist convection. These models, used in juxtaposition, enable us to address the important theoretical issue of whether a squall line is a system of special, long-lived cells, or whether it is a long-lived system of ordinary, short-lived cells. Our review of the observational literature indicates that the latter is the most consistent paradigm for the vast majority of cases but, on occasion, a squall line may be composed of essentially steady, supercell thunderstorms. The numerical experiments presented herein show that either type of squall line may develop from an initial line-like disturbance depending on the magnitude and orientation of the environmental shear with respect to the line. With shallow shear, oriented perpendicular to the line, a long-lived line evolves containing individually short-lived cells. Our analysis of this type o...

The dependence of numerically simulated convective storms on vertical wind shear and buoyancy

Abstract The effects of vertical wind shear and buoyancy on convective storm structure and evolution are investigated with the use of a three-dimensional numerical cloud model. By varying the magnitude of buoyant energy and one-directional vertical shear over a wide range of environmental conditions associated with severe storms, the model is able to produce a spectrum of storm types qualitatively similar to those observed in nature. These include short-lived single cells, certain types of multicells and rotating supercells. The relationship between wind shear and buoyancy is expressed in terms of a nondimensional convective parameter which delineates various regimes of storm structure and, in particular, suggests optimal conditions for the development of supercell type storms. Applications of this parameter to well-documented severe storm cases agree favorably with the model results, suggesting both the value of the model in studying these modes of convection as well as the value of this representation i...

The Resolution Dependence of Explicitly Modeled Convective Systems

Abstract The representation of convective processes within mesoscale models with horizontal grid sizes smaller than 20 km has become a major concern for the simulation of mesoscale weather systems. In this paper, the authors investigate the effects of grid resolution on convective processes using a nonhydrostatic cloud model to help clarify the capabilities and limitations of using explicit physics to resolve convection in mesoscale models. By varying the horizontal grid interval between 1 and 12 km, the degradation in model response as the resolution is decreased is documented and the processes that are not properly represented with the coarser resolutions are identified. Results from quasi-three-dimensional squall-line simulations for midlatitude-type environments suggest that resolutions of 4 km are sufficient to reproduce much of the mesoscale structure and evolution of the squall-line-type convective systems produced in 1-km simulations. The evolution at coarser resolutions is characteristically slow...

An Upper Boundary Condition Permitting Internal Gravity Wave Radiation in Numerical Mesoscale Models

Abstract A radiative upper boundary condition is proposed for numerical mesoscale models which allows vertically propagating internal gravity waves to pass out of the computational domain with minimal reflection. In this formulation, the pressure along the upper boundary is determined from the Fourier transform of the vertical velocity at that boundary. This boundary condition can easily be incorporated in a wide variety of models and requires little additional computation. The radiation boundary condition is derived from the linear, hydrostatic, Boussinesq equations of motion, neglecting Coriolis effects. However, tests of this radiation boundary condition in the presence of nonhydrostatic, Coriolis, nonlinear and non-Boussinesq effects suggest that it would be effective in many mesoscale modeling applications.

The structure and classification of numerically simulated convective storms in directionally varying wind shears

Abstract Using a three-dimensional numerical cloud model, we investigate the effects of directionally varying wind shear on convective storm structure and evolution over a wide range of shear magnitudes. As with a previous series of experiments using unidirectional wind shear profiles (Weisman and Klemp), the current results evince a spectrum of storm types ranging from short lived single cells at low shears, multicells at intermediate shears, to supercells at high shears. With a clockwise curved hodograph, the supercellular growth is confined to the right flank of the storm system while multicellular growth is favored on the left flank. An analysis of the dynamic structure of the various cells reveals that the quasi-steady supercell updrafts are strongly enhanced by dynamically induced pressure gradients on the right flank of the storm system. We use this feature along with other related storm characteristics (such as updraft rotation) to propose a dynamically based storm classification scheme. Following...

The Influence of the Shear-Induced Pressure Gradient on Thunderstorm Motion

Abstract In the present investigation we propose a simple theory to explain how a veering environmental wind shear vector can cause an initially symmetric updraft to grow preferentially to the right of the shear vector and acquire cyclonic rotation. The explanation offered is based on linear theory which predicts that interaction of the mean shear with the updraft produces favorable vertical pressure gradients along its right flank. To asses the validity of linear theory for large-amplitude updrafts, the three-dimensional, shallow, anelastic equations are numerically integrated using a simple parameterization for latent heating within a cloud and the linear and nonlinear forcing terms are separately analyzed. These results suggest that although the nonlinear effects strongly promote splitting of the updraft, the linear forcing remains the dominant factor in preferentially enhancing updraft growth on the right flank. We believe this differential forcing is a major contributor to the observed predominance o...

Prediction of Landfalling Hurricanes with the Advanced Hurricane WRF Model

Abstract Real-time forecasts of five landfalling Atlantic hurricanes during 2005 using the Advanced Research Weather Research and Forecasting (WRF) (ARW) Model at grid spacings of 12 and 4 km revealed performance generally competitive with, and occasionally superior to, other operational forecasts for storm position and intensity. Recurring errors include 1) excessive intensification prior to landfall, 2) insufficient momentum exchange with the surface, and 3) inability to capture rapid intensification when observed. To address these errors several augmentations of the basic community model have been designed and tested as part of what is termed the Advanced Hurricane WRF (AHW) model. Based on sensitivity simulations of Katrina, the inner-core structure, particularly the size of the eye, was found to be sensitive to model resolution and surface momentum exchange. The forecast of rapid intensification and the structure of convective bands in Katrina were not significantly improved until the grid spacing ap...