Long-resident droplets at the stratocumulus top
Turbulence models predict low droplet-collision rates in stratocumulus clouds, which should imply a narrow droplet size distribution and little rain. Contrary to this expectation, rain is often observed in stratocumuli. In this paper, we explore the hypothesis that some droplets can grow well ab...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-05-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/16/6563/2016/acp-16-6563-2016.pdf |
| Summary: | Turbulence models predict low droplet-collision rates in stratocumulus
clouds, which should imply a narrow droplet size distribution and little
rain. Contrary to this expectation, rain is often observed in stratocumuli.
In this paper, we explore the hypothesis that some droplets can grow well
above the average because small-scale turbulence allows them to reside at
cloud top for a time longer than the convective-eddy time <i>t</i>*.
Long-resident droplets can grow larger because condensation due to longwave
radiative cooling, and collisions have more time to enhance droplet growth.
We investigate the trajectories of 1 billion Lagrangian droplets in direct
numerical simulations of a cloudy mixed-layer configuration that is based on
observations from the flight 11 from the VERDI campaign. High resolution is
employed to represent a well-developed turbulent state at cloud top. Only
one-way coupling is considered. We observe that 70 % of the droplets spend
less than 0.6<i>t</i>* at cloud top before leaving the cloud, while 15 % of
the droplets remain at least 0.9<i>t</i>* at cloud top. In addition, 0.2 % of
the droplets spend more than 2.5<i>t</i>* at cloud top and decouple from the
large-scale convective eddies that brought them to the top, with the result
that they become memoryless. Modeling collisions like a Poisson process leads
to the conclusion that most rain droplets originate from those memoryless
droplets. Furthermore, most long-resident droplets accumulate at the
downdraft regions of the flow, which could be related to the closed-cell
stratocumulus pattern. Finally, we see that condensation due to longwave
radiative cooling considerably broadens the cloud-top droplet size
distribution: 6.5 % of the droplets double their mass due to radiation in
their time at cloud top. This simulated droplet size distribution matches the
flight measurements, confirming that condensation due to longwave radiation
can be an important mechanism for broadening the droplet size distribution in
radiatively driven stratocumuli. |
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| ISSN: | 1680-7316 1680-7324 |