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摘要
This work presents a unique combination of aerosol, cloud microphysical, thermodynamic and turbulence variables to characterize supersaturation fluctuations in a turbulent marine stratocumulus (SC) layer. The analysis is based on observations with the helicopter-borne measurement platform ACTOS and a detailed cloud microphysical parcel model following three different approaches: (1) From the comparison of aerosol number size distributions inside and below the SC layer, the number of activated particles is calculated as 435 +/- 87 cm(-3) and compares well with the observed median droplet number concentration of %26lt; span style=%26apos;border-top: 1px solid #000; color: #000;%26apos;%26gt; N %26lt;/span %26gt;(d) = 464 cm(-3). Furthermore, a 50% activation diameter of D(p50)a parts per thousand 115 nm was derived, which was linked to a critical supersaturation S-crit of 0.16% via Kohler theory. From the shape of the fraction of activated particles, we estimated a standard deviation of supersaturation fluctuations of Sigma(S%26apos;) = 0.09%. (2) These estimates are compared to more direct thermodynamic observations at cloud base. Therefore, supersaturation fluctuations (S%26apos;) are calculated based on highly-resolved thermodynamic data showing a standard deviation of S%26apos; ranging within 0.1%a parts per thousand currency sign Sigma(S%26apos;)a parts per thousand currency sign0.3 %. (3) The sensitivity of the supersaturation on observed vertical wind velocity fluctuations is investigated with the help of a detailed cloud microphysical model. These results show highest fluctuations of S%26apos; with Sigma(S%26apos;)=0.1% at cloud base and a decreasing Sigma(S%26apos;) with increasing liquid water content and droplet number concentration. All three approaches are independent of each other and vary only within a factor of about two.
- 出版日期2012
