Category Archives: Southern Ocean

POC formation SSW of Tasmania

We have a nice case of pocket of open cell formation today SSW of Tasmania. Helpfully (and somewhat atypically), this transition occurs during daylight hours, meaning we should be able to get quality microphysical retrievals for this case. The morning Terra pass has no sign of a disturbance, while the afternoon Aqua pass has a small but well defined POC. It should be possible to use cases like this to observationally constrain the threshold for transition from closed to open cells.

Distinguishing clouds over ice surfaces

The MODIS channel 3-6-7 blend available on Worldview is very effective at distinguishing clouds over ice surfaces. In this Worldview scene near Antarctica (1st Feb 2013; 73S, 103W), in the MODIS 3-6-7 channel blend (image below), one can see open ocean (black), surface sea/land ice (red), and clouds overlying them both (whiter hues). In visible imagery (image at bottom), it is often difficult to distinguish between clouds and sea ice. It is interesting that some of these clouds are so optically thin that one can see the outline of the ice edge through them.

Image

Image

Clouds looking for a model to host them

Worldview shows the Southern Ocean clouds that climate models struggle and strive to represent. Almost exclusively found poleward of the storm track, these extensive layer clouds are constructed from multiple and single layers, frequently decoupled from the surface. Surprisingly, sometimes convection seems to be present, making one wonder whether the layers may sometimes be fed by cumulus detrainment. Much of their optical thickness appears to be attributable to supercooled liquid water.

Image

 

In Worldview’s 3-6-7 channel blend (primarily indicating microphysical differences), the convective elements can be seen in orange/red, penetrating the yellow/whiter stratiform layers:

HostClouds_zoom_367

Plume to POC in the Southern Ocean: Island effects and ACI?

A plume of thicker cloud, originating near the South Orkney Islands, transitions to a pocket of open cells (located at -54.0 N, -43.386 E), presumably following drizzle-induced aerosol scavenging and dynamical transition. The image was taken around 1230 UTC, May 17, 2014
A plume of thick cloud, originating near the South Orkney Islands, transitions to a pocket of open cells (located at -54.0 N, -43.4 E), presumably following drizzle-induced aerosol scavenging and dynamical transition. The image was taken around 1230 UTC, May 17, 2014

In this case from the Southern Ocean, the 3-6-7 channel image from MODIS on Terra, taken during the day pass on May 17, 2014, clearly shows a plume of thick, closed-cell MSc cloud transitioning to open-cell convection. HYSPLIT trajectories indicate that the flow in the boundary layer during the preceding 24 hours was from the south. What is the source of the mystery plume?

Backing up a day, the Terra pass on May 16 is difficult to interpret, but the Aqua pass is the key to the mystery:

A cloud tail behind Coronation Island under southerly flow on May 16, 2014
A cloud tail behind Coronation Island under southerly flow on May 16, 2014

It seems that under southerly flow, given the background environment, Coronation Island (located at -60.571 N, -45.676 E) generated a cloud tail in the MSc deck with elevated LWP. The trajectory analysis reveals that flow in the boundary layer took roughly 21 hours to advect north to where the distinct change in microphysics and dynamics becomes obvious. This is a unique example of a mechanically forced LWP perturbation in boundary layer cloud driving aerosol-cloud-precipitation feedbacks and triggering a downstream change in boundary layer structure.

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).