Donald J. Perkey

A Time-Dependent Lateral Boundary Scheme for Limited-Area Primitive Equation Models

Abstract Before high-resolution numerical models can be of use operationally, they must be restricted to a limited domain, thus necessitating lateral boundary conditions which allow the changes outside the limited domain to influence the results while not contaminating the forecast with spurious boundary-reflected energy. Such a set of time-dependent lateral boundary conditions are presented in this paper. This boundary condition set is investigated using the linear analytic and finite-difference advection equations, the non-linear finite-difference shallow-water equations, and the hydrostatic primitive equations. The results illustrate how the boundary condition transforms long- and medium-length interior advective and gravity waves into short waves which can then be removed by a low pass filter, thereby giving the appearance that the exiting wave simply passed through the boundary. The results also indicate that large-scale advective and gravity waves enter the forecast domain with little degradation. T...

Release of Potential Instability: Part I. A Sequential Plume Model within a Hydrostatic Primitive Equation Model

Abstract The release of potential instability by large-scale lifting and the subsequent interaction of cumulus convection and the hydrostatic mesoscale flow in a most complex scale-interaction process. This process is an essential part of tropical weather but it is also important in extratropical cyclones through the formation of mesoscale rainbands that contribute much of the precipitation. The purpose of this paper is to qualitatively and quantitatively clarify the potential instability release process within a framework that will permit calculation of convective/mesoscale interactions. The approach is to use an extension of the Lagrangian form of the one-dimensional cumulus model to provide values of convective-scale changes to a hydrostatic primitive equation model. This cumulus sub-routine locates the base of the convection, computes the cumulus plume that will build, accounts for the environmental subsidence, and mixes the subsided environment with the cumulus plume after rainout. These plumes build...

Sensitivity of Trajectory Calculations to the Temporal Frequency of Wind Data

Abstract A mesoscale primitive equation model is used to create a 36-h simulation of the three-dimensional wind field of an intense maritime extratropical cyclone. The control experiment uses the simulated wind field every 15 min in a trajectory model to calculate back trajectories from various horizontal and vertical positions of interest relative to synoptic features of the storm. The latter trajectories are compared to trajectories that were calculated with the simulated wind data degraded in time to 30 min, 1 h, 3 h, 6 h, and 12 h. Various error statistics reveal significant deterioration in trajectory accuracy between trajectories calculated with 1- and 3-h data frequencies. Trajectories calculated with 15-min, 30-min, and 1-h data frequencies yielded similar results, while trajectories calculated with data time frequencies 3 h and greater yielded results with unacceptably large errors.

A Description and Preliminary Results from a Fine-Mesh Model for Forecasting Quantitative Precipitation

Abstract The purpose of this paper is two-fold; first, to document and present a limited-area, moist primitive equation model and, second, to give some preliminary results of experiments testing the sensitivity of the models quantitative precipitation forecasts to the initial horizontal and vertical relative humidity distribution. Three experiments were performed. The first case used a 1.5 km vertical grid and a Gandin humidity analysis based on standard rawinsonde observations. The results indicate that the model has some skill at forecasting precipitation amounts and location in regions of predominately stable rain and in regions of convective rain. However, some glaring defects in both the initialization of the mass-flow fields and the initialization of the moisture field were evident. The second experiment attempted to enhance the initial moisture field to reflect a narrow band of moisture which was suggested by satellite cloud observations. The inclusion of this moisture band increased the precipita...

Release of Potential Instability: Part II. The Mechanism of Convective/Mesoscale Interaction

Abstract In Part I the convective processes important during the release of potential instability were described qualitatively and evaluated quantitatively in a parameterized cumulus model within a primitive equation model. Part II includes a more detailed examination of convective/mesoscale interactions through a basic simulation experiment and tests under different physical conditions and with different computational grids. The cumulus model was documented in Part I and the primitive equation model is documented herein. The example, for which detailed dynamical fields are shown, began with 6 h of convective activity that developed a saturated neutrally buoyant mesoscale updraft which produced the bulk of the precipitation by 12 h into the integration. The potential instability process is readily understandable and verifiable in general terms by numerical simulation. Increasing moisture bandwidth or large-scale ascent results in a wider precipitation band. Permitting evaporation of convective precipitati...

A Numerical Case Study of the Effects of Latent Heating on a Developing Wave Cyclone

Abstract The effects of latent heating on the development of a wave cyclone are investigated using a multi-level primitive equation model to simulate the cyclone system with (wet) and without (dry) latent heating. While the dry simulation failed to properly predict either the formation of the closed circulation which developed throughout the depth of the troposphere or the pronounced northwest-to-southeast horizontal tilt of the upper-level trough shown by observations, the wet simulation successfully reproduced both these features. The mechanisms for the generation of the regional-scale closed system are examined and the influence of latent heating on large-scale dynamics and energetics is discussed. Results indicate that latent heat release stabilized the troposphere and reduced the large-scale horizontal temperature gradient. Also, through the enhancement of ageostrophic flow, the addition of latent heat generated kinetic energy in both the lower and upper troposphere at the expense of the available po...

A Numerical Investigation of a Mesoscale Convective System

Abstract On 25 April 1975, as part of the National Aeronautics and Space Administrations Atmospheric Variability Experiment IV, frequent upper-air soundings were taken at eastern United States synoptic sounding sites. An intense, long-lived mesoscale convective weather system developed late in the AVE IV period and moved eastward during the remainder of the experiment. With the use of dry and moist numerical simulations, performed with Drexel Universitys Limited Area and Mesoscale Prediction System (LAMPS), interaction between the widespread, long-lived convective complex and its large-scale environment are examined. Dissecting the differences between moist and dry simulations reveals that, within the moist numerical simulation, significant up-scale feedbacks occur between the convective system and its large-scale meteorological setting. Pronounced differences in temperature, divergence, vorticity, and height develop between the two simulations. Physical reasons for these differences are discussed. Comp...

A Numerical Case Study of the Squall Line of 6 May 1975

Abstract Results of moist and dry fine-mesh (∼140 km) numerical simulations of the 6 May 1975 Omaha squall line are presented. The moist fine-mesh simulation reproduced several observable features of the squall system and was then used to study other unobservable features. Differential thermal advection was responsible for the creation of potential instability. Low-level horizontal thermal advection contributed to pressure falls which, in turn, enhanced the convergence into the warm tongue. These processes initiated a band of convection. Results of the dry simulation suggested that the location and orientation of the initial low-level convergence was determined by dry mechanisms, that the convective latent heat release increased the rate of cyclonic-scale occlusion and that the occlusion process was followed by the dissipation of the convection. In addition to the fine-mesh simulations, a meso-mesh (∼35 km) simulation was conducted. Because of the increased resolution, this simulation was able to reproduc...

The 1980–81 Drought in Eastern Pennsylvania

Newspapers, television, and newsweeklies contained numerous articles proclaiming drought conditions during 1980 and 1981. This study investigates the causes and consequences of the drought as it affected eastern Pennsylvania. Precipitation data indicate below-average amounts during this period, while temperature records show above-average values. These values show that a meteorological drought did occur in this region during 1980 and 1981. However, meteorological factors were only part of the cause of the regions water shortage. In addition to analyzing the droughts meteorological origin, this study probes the anthropogenic and regional social-political causes and impacts of the water shortage. Although regional water storage facilities are adequate when below-average precipitation amounts occur in very local areas, they are not adequate when below-average amounts occur over larger regions. This inadequacy is compounded when demands such as the needs of other political regions and the river-basin ecolog...

A Method of Parameterizing Cumulus Transports in a Mesoscale Primitive Equation Model: The Sequential Plume Scheme

During the 1960s and early 1970s, parameterization of cumulus activity for use in cyclonic-scale forecast and extended-range climate numerical models was the topic of a great deal of research. Parameterization efforts for cyclonic-scale forecast models concentrated on the convective transport of temperature and moisture in the conditionally unstable atmosphere of the tropics (Lilly 1960), especially emphasizing the role of latent heat release in tropical cyclone development (Charney and Eliassen 1964; Kuo 1965; Ooyama 1969). In the global circulation and extended-range climate models, researchers focused their efforts on determining a quick, acceptable method of convective adjustment (Manabe and Strickler 1964; Arakawa et al. 1968; Oliger et al. 1970; Krishnamurti and Moxim 1971).