Jon M. Reisner

Thermally Forced Low Froude Number Flow past Three-Dimensional Obstacles

Abstract The present study extends the discussion of the flow of a density-stratified fluid past three-dimensional obstacles for Froude number O(1) to flows past an isolated obstacle with heated/cooled surface. The study focuses on a response of thermally forced stratified flows representative of mesoscale flows past mountainous islands such as Hawaii or Taiwan. In order to minimize the span of the parameter space, uniform ambient wind and stratification, axially symmetric bell-shaped hills with moderate slopes representative of mesoscale mountains, and a simple thermal forcing function that mimics natural effects over mountainous islands are assumed. The earths rotation, surface friction, viscosity, dissipation, and moisture are neglected. With these simplifications flows can be characterized with two parameters: the Froude number and a characteristic scale of thermal forcing (defined later in this paper). The principal question addressed in this study is under what circumstances will a transition occur...

High-Resolution Simulation of the Electrification and Lightning of Hurricane Rita during the Period of Rapid Intensification

Abstract In this paper, a high-resolution simulation establishing relationships between lightning and eyewall convection during the rapid intensification phase of Rita will be highlighted. The simulation is an attempt to relate simulated lightning activity within strong convective events (CEs) found within the eyewall and general storm properties for a case from which high-fidelity lightning observations are available. Specifically, the analysis focuses on two electrically active eyewall CEs that had properties similar to events observed by the Los Alamos Sferic Array. The numerically simulated CEs were characterized by updraft speeds exceeding 10 m s−1, a relatively more frequent flash rate confined in a layer between 10 and 14 km, and a propagation speed that was about 10 m s−1 less than of the local azimuthal flow in the eyewall. Within an hour of the first CE, the simulated minimum surface pressure dropped by approximately 5 mb. Concurrent with the pulse of vertical motions was a large uptake in light...

A Study of Cloud Mixing and Evolution Using PDF Methods. Part I: Cloud Front Propagation and Evaporation

Abstract The evolution of mean relative humidity (RH) is studied in an isobaric system of clear and cloudy air mixed by an incompressible velocity field. A constant droplet radius assumption is employed that implies a simple dependence of the mixing time scale, τeddy, and the reaction (evaporation) time scale, τreact, on the specifics of the droplet size spectrum. A dilemma is found in the RH e-folding time, τefold, predicted by two common microphysical schemes: models that resolve supersaturation and ignore subgrid correlations, which gives τefold ∼ τreact, and PDF schemes that assume instantaneous evaporation and predict τefold ∼ τeddy. The resolution of this dilemma, Magnussen and Hjertager’s eddy dissipation concept (EDC) model τefold ∼ max(τeddy, τreact), is revealed in the results of 1D eddy diffusivity simulations and a new probability density function (PDF) approach to cloud mixing and evolution in which evaporation is explicitly resolved and the shape of the PDF is not specified a priori. The EDC...

Another Look at Stochastic Condensation for Subgrid Cloud Modeling: Adiabatic Evolution and Effects

The theory of stochastic condensation, which models the impact of an ensemble of unresolved supersaturation fluctuations S on the volume-averaged droplet-size distribution f (r), is revisited in the modern context of subgrid cloud parameterization. The exact transition probability density for droplet radius driven by independent, Gaussian S fluctuations that are periodically renewed is derived and shown to be continuous but not smooth. The Fokker–Planck model follows naturally as the smooth-in-time approximation to this discrete-in-time process. Evolution equations for the moments of f (r) that include a contribution from subgrid S fluctuations are presented; these new terms are easily implemented in moment-based cloud schemes that resolve supersaturation. New, self-consistent expressions for the evolution of f (r) and mean supersaturation S in a closed, adiabatic volume are derived without approximation; quite appropriately, these coupled equations exactly conserve total water mass. The behavior of this adiabatic system, which serves as a surrogate for a closed model grid column, is analyzed in detail. In particular, a new nondimensional number is derived that determines the relative impact of S fluctuations on droplet spectral evolution, and the contribution of fluctuations to S is shown to be negative definite and maximal near the accommodation length and has a direct correspondence to the analysis of Cooper. Observational support for the theory of stochastic condensation is found in cloud droplet spectra from cumulus cloud fields measured during the Rain in the Cumulus over the Ocean (RICO) and Small Cumulus Microphysics Study (SCMS) campaigns. Increasing spectral broadening with increasing spatial scale is discovered and compares well with theoretical predictions. However, the observed spectra show evidence of non-Gaussian S fluctuations and inhomogeneous mixing, processes neglected in the current theory.