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| Titel |
Revisiting Twomey's approximation for peak supersaturation |
| VerfasserIn |
B. J. Shipway |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 7 ; Nr. 15, no. 7 (2015-04-09), S.3803-3814 |
| Datensatznummer |
250119620
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| Publikation (Nr.) |
 copernicus.org/acp-15-3803-2015.pdf |
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| Zusammenfassung |
| Twomey's seminal 1959 paper provided lower and upper bound
approximations to the estimation of peak supersaturation within an
updraft and thus provides the first closed expression for the number
of nucleated cloud droplets. The form of this approximation is
simple, but provides a surprisingly good estimate and has subsequently
been employed in more sophisticated treatments of nucleation
parametrization. In the current paper, we revisit the lower bound
approximation of Twomey and make a small adjustment that can be used
to obtain a more accurate calculation of peak supersaturation under
all potential aerosol loadings and thermodynamic conditions. In order
to make full use of this improved approximation, the underlying
integro-differential equation for supersaturation evolution and the
condition for calculating peak supersaturation are examined. A simple
rearrangement of the algebra allows for an expression to be written
down that can then be solved with a single lookup table with only one
independent variable for an underlying lognormal aerosol
population. While multimodal aerosol with N different dispersion
characteristics requires 2N+1 inputs to calculate the activation
fraction, only N of these one-dimensional lookup tables are
needed. No additional information is required in the lookup table to
deal with additional chemical, physical or thermodynamic
properties. The resulting implementation provides a relatively simple,
yet computationally cheap, physically based
parametrization of droplet nucleation for use in climate and
Numerical Weather Prediction models. |
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