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The Spurious Production of Cloud-Edge Supersaturations by Eulerian Models

Bjorn StevensDepartment of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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Robert L. WalkoDepartment of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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William R. CottonDepartment of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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Graham FeingoldCooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, Colorado

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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 temperature and the erroneous representation of the mean buoyancy flux within a grid box, we also demonstrate that the spurious production of droplets can arise in response to the spurious production of supersaturations in models with detailed microphysical representations.

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 temperature and the erroneous representation of the mean buoyancy flux within a grid box, we also demonstrate that the spurious production of droplets can arise in response to the spurious production of supersaturations in models with detailed microphysical representations.

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