Aerosol Cloud Interactions and Regimes in the Southern Ocean

Closed and open cellular boundary layer organization in the Southern Ocean (-50.5N, -142.5E) on August 26, 2013
Closed and open cellular boundary layer organization in the Southern Ocean (-50.5N, -142.5E) on August 26, 2013

 

In this case from the Southern Ocean, the boundary layer mesoscale organization exhibits intermingled regions of closed and open MSc cells. In Worldview, examining the evolution of this airmass over hours (rocking between Terra and Aqua overpasses) or days shows a slow, quasi-equilibrium evolution in the relative areal coverage of closed and open cells. This mix of boundary layer organization in the cloud field is first observable on  July 18th, when the airmass lies just south of New Zealand at -50.5N, 180.0E. With relatively little disturbance, the boundary layer air advects eastward over the next two weeks, always containing a mixture of both closed and open cell regions.

The persistence of regions of both closed and open cells under what would seem to be relatively homogeneous large-scale meteorological forcing is suggestive of the bi-stability of marine boundary layer organization, first suggested by Baker and Charlson (1990). While still not fully understood, the quasi-equilibrium between adjacent regions of closed and open cells likely results from feedbacks among aerosols, cloud microphysics, precipitation, and cloud-top entrainment, as examined in Berner et al. (2013).

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Strong scattering from CALIPSO in trade Cu environments

July 6th 2013:

Between 5N-13S there is strong lidar scattering (red colors below). Clouds have tops at 1-2 km. The Worldview image from Terra, including a zoom in on the milky regions seen from a distance, shows that the clouds appear to be small Cu organized in lines. The scattering values below cloud (0.005 /km/sr) would give an extinction of roughly 0.125 /km assuming a lidar ratio for coarse aerosols (e.g. Masonis et al. 2003) of 25 sr. These extinction values extend over a height range of about 1km, giving a total AOD at 532 nm of roughly 0.12, which seems quite large. It is possible that some of the scattering is coming from optically thin clouds, which we found to be very prevalent in the trades (Leahy et al. 2012).

Surface wind speeds do not seem particularly high (7-10 m/s), according to AMSR-2.

Link to CALIPSO quick look for this case

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NE Pacific stratocumulus features

Marine stratocumulus and overlying clouds west of Baja California, here seen in Worldview using MODIS channels 3-6-7.

To the NW, and west of California, in this MODIS image, it looks like compensating subsidence from the NW-SE precipitating feature is quite dramatic. Impact on region albedo? I don’t think these systems can be perturbed by anthropogenic aerosol once they get going. The timescale for aerosol recycling is very short in such systems I would guess, but that still remains to be determined.