Graham Feingold

The Relationship between Drop In-Cloud Residence Time and Drizzle Production in Numerically Simulated Stratocumulus Clouds

Abstract This paper considers the production of drizzle in statocumulus clouds in relation to the boundary-layer turbulent kinetic energy and in-cloud residence times. It is shown that drizzle production in statocumulus of the order of 400 m in depth is intimately related to the vertical velocity structure of the cloud eddies. In a series of two dimensional numerical experiments with fixed cloud condensation nucleus concentrations, the effect on drizzle production of enhanced or diminished vertical velocities is simulated. Rather than do this by simulating clouds exhibiting more or less energy, we modify drop terminal velocities in a manner that conserves the fall velocity relative to the air motions and allows droplet growth to occur in a similar dynamical environment. The results suggest that more vigorous clouds produce more drizzle because they enable longer in-cloud dwell times and therefore prolonged collision-coalescence. In weaker clouds, droplets tend to fall out of the cloud before they have ach...

Spurious production of cloud-edge supersaturations by Eulerian models

Abstract The production of anomalous supersaturations at cloud edges other than cloud base has presented a vexing challenge for modelers attempting to represent the evolution of a droplet spectrum across an Eulerian grid. Although the problem manifests itself most dramatically for models that explicitly predict on the supersaturation field, it is also present in models with bulk condensation schemes in which condensation happens implicitly. Although the problem has been discussed in the context of truncation errors associated with finite difference approximations to advection, this note demonstrates more generally that the cloud-edge supersaturation problem is a fundamental problem associated with the ubiquitous assumption that the forcings on the droplet spectra are well represented by the mean thermodynamic fields. In certain respects, this assumption is equivalent to failing to represent fractional cloudiness within a grid. Although well-known consequences of this problem are the underprediction of tem...

Efficient computation of vapor and heat diffusion between hydrometeors in a numerical model

We present an algorithm for representing diffusion of sensible heat and vapor between hydrometeors and air that is formulated from implicit numerical equations and is therefore stable for long numerical model timesteps, but is solved explicitly without requiring iteration. Liquid, ice, and mixed-phase hydrometeor categories are incorporated into the system, and latent heat of fusion released or absorbed during the diffusion process is accounted for. The method has been . incorporated into the Regional Atmospheric Modeling System RAMS which prognoses ice-liquid potential temperature as its principal thermodynamic variable, but may be adapted to other models as well. Simulations with RAMS used as a parcel model are presented and demonstrate the performance of the new algorithm. q 2000 Elsevier Science B.V. All rights reserved.

National Institute for Global Environmental Change. Final report

Over the past decade or so the evolution and equilibria of persistent decks of stratocumulus climatologically clinging to the edge of summertime subtropical highs has been an issue of increased scientific inquiry. The particular interest in the microphysical structure of these clouds stems from a variety of hypotheses which suggest that anthropogenic influences or biogenic feedbacks may alter the structure of these clouds in a manner which may be climatically significant. Most hypotheses regarding boundary layer influences on climate have been formulated by an examination of the solution space of simple models. The earliest hypothesis of this sort (and the one on the most solid footing) is due to Twomey (1974), who posited that enhanced concentrations of CCN could lead to enhanced droplet reflectivity and enhanced albedos in clouds of modest optical depths. In low lying clouds where the albedo effect dominates, the climate sensitivity to a robust perturbation in cloud albedo may be significant. One of the primary objectives of this current research has been to explore the hypothesis of Twomey. The basic approach was to couple radiative calculations with detailed representations of the droplet spectra. The detailed representation of the droplet spectra was generated by the Large Eddy Simulation-Explicit Microphysics (LES-EM) model coupled to a simple mixed emissivity radiation scheme in order to drive the dynamics. Several simulations were carried out and the resultant microphysical fields were taken from the stationary regime of the turbulent simulation and used to drive a two dimensional radiative model. By comparing the radiative properties of the simulated clouds formed in environments with different CCN concentrations we were able to more accurately quantify the albedo susceptibility of stratocumulus taken to be typical of the FIRE experimental area.