Tag Archives: Marine stratocumulus

Stratocumulus deck out of place

Where in the world is this stratocumulus deck?

SCP_Sc_Deck

The subtropical deck extends 2000 km from east to west, It looks like it should be lying over the southeastern Pacific (SEP), but this deck extends from 130-160 W, where the average daytime stratocumulus cover is 25% (see below), compared with 50% over the SEP.

https://i0.wp.com/www.atmos.washington.edu/CloudMap/WebO/Maps/13_1_0_DY_O5.obs.jpg

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Animation: modeled ship tracks

This video depicts the albedo field from two large eddy simulations (LES) of a ship track. In both cases, background aerosol concentration is 10-15 /cm3. Because the boundary layer in the case modeled is quite shallow (300m) and strongly sheared (wind speed of 20 m/s in downdrafts and 15 m/s in updrafts due to surface drag), the boundary layer organizes into roll cells. This organization results in highly anisotropic bulk turbulent diffusivity of scalars. This can be appreciated by inserting a ship track perturbation aligned with or perpendicular to the roll circulation, and noting that the cloud response is quite different, reflecting the more rapid lateral diffusion of the cloud condensation nuclei when the track is inserted perpendicular to the rolls.

Animation courtesy of Andy Berner

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).