Tag Archives: Aerosol Cloud Interactions

Cold air outbreak over the North Atlantic (animation)

Here, we can see a cold air outbreak in this GOES-13 animation, using visible (day) and near-IR (night). This particular event shows offshore flow emerging from the NE US and Eastern Canada on the 6th March. The cold air floods over almost the entire North Atlantic region, eventually passing over the Azores (ARM ENA site, 39N, 28W) on the 8th March. The cold air outbreak is mared by open cellular convection in the low clouds. During the time that the cold air outbreak is over the Azores, the concentration of cloud condensation nuclei drops to lower than 50 /cm3, after the precipitation associated with the low clouds removes particles by coalescence scavenging. The event can also be seen on NASA Worldview.

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Animation: LES simulations showing ship track sensitivity to background aerosol

This video depicts the albedo field from two LES simulations of a ship track. In the upper panel, the background cloud condensation nuclei concentration is approximately 10-15 /cm3, while in the lower panel, it is 100-150 /cm3. The ship track aerosol perturbation applied to the high background aerosol case is 10x as large as the low background aerosol case in order to make a visible signature, as the weaker perturbation used in the low aerosol case has a much weaker impact under conditions of high background aerosol.

Animation courtesy of Andy Berner

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

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.

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).