Greetings from a rain-soaked Bodega Head!
We have picked up nearly an inch of rain since 7 am PST.
I missed a post last night due to an internet outage, so I’m going to leave two posts here this afternoon. Ill get to the weather, our rain outlook for the rest of the day, Petaluma Gap flow and maybe a Coastal Jet in post two. For this post, I want to revisit the cross-Pacific transported dust we have been anticipating.
Yesterday evening’s GEOS-5 run was still bullish on our chances of seeing dust pass overhead this morning near 10 am:
Dust AOT over North America valid at 1800UTC on Feb 26, 2014. Credit: NASA-GMAO.
Lo and behold, we started picking up many dust-like spectra from our ATOFMS, Laverne, around mid-morning. We also saw ice-nuclei activity in the CFDC increase. If you animate the following loop:
GEOS-5 dust global loop Credit: NASA-GMAO.
You can watch the nature of this dust as it enters the eastern Pacific. Dust travels around the south flank of the storm as it heads East, and in doing so, eventually intercepts the Pacific coast. Dust traveling this pathway certainly would have had to cross the baroclinic zone associated with a trailing cold front from West to East, meaning the air parcels would have experienced forced descent along isentropic surfaces. Thus, if the forecast was reality, any dust traveling around the South flank of the storm would have descended toward the surface.
Our instruments here at the surface are seeing dust, so score one for the model.
We are also expecting that dust may still be present at high levels after the cold front with this storm. That feature is not well captured in the model forecast, so why are we anticipating it?
The storm became occluded on Monday, Feb. 24 while it was still in the Central Pacific. Here is a surface analysis for 1800 UTC on 02/24:
Surface analysis for Gameboy Color (R) valid at 1800 UTC on Feb 24, 2014. Credit: NOAA-OPC.
During this time, dust from a large plume leaving NE Asia was making it’s way across the Pacific Ocean too. Before we try to find it in relation to the storm, lets look at the MODIS Level 2 cloud top temperature for this approximate time:
Cloud top temperature over the Central Pacific on Feb 24. Credit: NASA-GSFC.
The MODIS (Terra) swath over the storm is from approximately 20 UTC. The deep purple colors indicating very cold cloud tops form a comma shape, which is classically associated with an occluded extratropical cyclone. The left inside of the comma is near the low pressure center and is an area where dry air is intruding into the storm circulation. It is relatively free of clouds.
MODIS-Terra observed aerosol optical depth in this relatively cloud free region (the aerosol products rely on visible wavelengths and cannot function over cloudy regions.):
Aerosol Optical Thickness at 550 nm over the North Central Pacific on Feb 24, 2014. Credit: NASA-GSFC.
The cloud free region shows elevated aerosol optical thickness, suggesting that the dust has made it into this part of the storm circulation, which is behind the cold front, along with the dry air.
For us, this means we are expecting more dust to be included in the rain samples and ambient air near the time the cold front passes. Observing the shift in insoluble residue chemistry from the rain samples may tell us something interesting about the chemical history of the dust as it was processed during its journey.