Greetings from Bodega Head!
We woke up to shallow marine stratocumulus this morning, the first time we saw something other than sunny skies since Tuesday. We currently have lightly offshore flow, which is bringing air from the Westside Park Campground over our instrument trailer. The weekenders burning campfires there have given us some fine mode particulates and reactive gases to sample. Currently we are measuring:
1500 particles per cubic centimeter (cc) on the CPC
An accumulation size mode near 65 nanometers (nm) on the SMPS
A nearly flat APS trace, with total “course mode” particle count of 6 per cc.
Elevated NOx at 7 parts per billion (ppb)
Elevated CO at 180 ppb
These numbers are a dramatic shift from our typical afternoon measurements. Under the influence of strong onshore flow, we typically see NOx near 0 ppb, CO near 140 ppb, around 500 total particles per cc, no accumulation mode and a course mode near 800 nm with total course mode count from 12 – 15 particles per cc.
We are expecting to encounter similar conditions after the winds come onshore again this afternoon.
Medium Range Forecast Drift:
For the last few posts I have been pointing to the end of this week as our next chance of rain. The models were converging (GFS and ECMWF agreed) on a storm hitting California on Friday 2/28 and Saturday 3/1. This is still the favored solution in the models. In fact, this storm has only gotten more robust given new forecast initial conditions. The tendency of a solution at a particular time to change given a change in initial model conditions is referred to as model drift. You can think of it like this:
I am interested in a storm on Friday, February 28 at 1200 UTC.
The first hint of this storm came in the 240 hr forecast from the ECMWF model initialized at 12 UTC on February 18. The hint looked like:
500 hPa geopotential height over the Northern Hemisphere from the ECMWF Ensemble Prediction System. Initialized at 12 UTC on Feb 18. Valid at 12 UTC on Feb 28. Credit: ECMWF.
In this scenario, February 28 at 12 UTC is the forecast “valid time” and February 18 at 12 UTC is the forecast “initial time”.
If I keep the valid time the same, but look at the model forecast intialized on the next day, I will start to get an idea of how updated information about the state of the real atmosphere on February 19 changed the forecast for February 28:
Same as last figure, but initialized at 12 UTC on February 19.
Model drift can be a qualitative way to examine whether new information adds confidence that the forecast event (our storm) will occur or whether this new information makes the event forecast less confident.
Lets go through the progression or new initial times for our Friday storm, ending with yesterdays initial time:
Same as last figure, but initialized at 12 UTC on February 20.
Same as last figure, but initialized at 12 UTC on February 21.
Same as last figure, but initialized at 12 UTC on February 22.
We see that after each new day’s initialization, the upper level trough nearing the West Coast on Feb 28 becomes deeper, until on the last intital day (Feb 22) there is a dramatic deepening, and the upper level feature in the ensemble forecast now looks like a storm in a deterministic model, i.e. it looks like a real world event.
This is good news for our storm. Over time, the ensemble of simulations became more confident that the event (a strong upper level trough, supporting a winter storm on Friday, February 28) would occur.
There is something else of note that has happened to our storm, however.
To see it, examine this animation of upper level wind flow forecast by the GFS:
GFS 600 hPa loop Credit: U Hawaii.
Start at the valid time 18 UTC on February 24 (30 hours into the forecast). There is a closed upper level low in the geopotential height field near 4oN; 145 W. Stepping through the forecast from here, we can see this closed low “retrograde” back toward the Aleutian Islands. As the low moves out of the Jet Stream (the belt of high wind speeds), two shortwave eddies in the height field move into its place. As these rotate counter-clockwise around the larger scale trough, they intensify. The first spins up a storm which should hit our area late on Wednesday, Feb 26. The second becomes a deep trough, then a cut-off low which becomes our Friday storm. We may be in for a prolonged period of wet weather this Wednesday through Saturday!
Other storm characteristics:
Our partners from CalWater-2 just finished a few exploratory flights into Pacific atmospheric rivers (AR). AR commonly reach California in association with storms like the one on tap for later in the week. If the broad scale circulation can pick up enough water vapor from the tropics and advect it Northeast, a filament of integrated water vapor (IWV) or precipitable water (PW) is visible in satellite imagery. We can look at a model mock-up of the same metrics courtesy of U Hawaii:
Tropospheric Precipitable Water from GFS valid at 00 UTC on Feb 27, 2014. Credit: U Hawaii.
The first image is PW over the Pacific ocean on Wednesday afternoon (4 pm PST). There is a tongue of elevated moisture which is expected to hit the central California coast in concert with the arrival of the Wednesday storm. This filamentary object in the PW field is not as impressive as a classic AR, but it is good news for the prospect for rain.
Same as above figure, valid at 12 UTC on Feb 28, 2014. Credit: U Hawaii.
For Friday, we see a plume of elevated moisture also arriving along with the second storm. This happens early in the morning on Feb 28. As before, the maximum PW content is not quite up to AR standards, but it is better than none.
Long-Range Transported Dust:
Our BBACPAX crew is anxious to observe some dust particles and measure their ice nucleating abilities. We may get our wish during the next two storms. GEOS-5 predicts a very large dust plume to leave the Asian continent today, travel across the Pacific near 30 N and wrap around the circulation of our developing storm systems by sometime Tuesday. This mid-Pacific dust belt will continue to feed into the storm’s southern flank through Friday, which may elevated dust aerosol optical depths over California to levels we have not observed during our trip. See the animation of the GEOS-5 dust AOT forecast at the link below:
GEOS-5 Global Dust Forecast Credit: NASA-GMAO.
The questions are: Will the dust behave as forecast, and will we observe it at the surface?
We cannot yet answer the first question. We may have validation information from satellite data tomorrow after the first A-TRAIN flights over the Plume region east of Japan. However, unlike the weak plume forecast on Feb 20 discussed in this post, the dust plume forecast by GEOS-5 in this case is massive, meaning there is less chance that the model misses some rainout, surface deposition, or flow perturbation by half a degree that may cause the plume to disappear, dissipate, or dive south before its forecast target.
For the second question: The dust in the model most likely makes the first part of its journey at higher levels where winds are fast. It must do this in order to cross nearly half the Pacific ocean in 2 days. However, on our side is the fact that it must travel from west to east through a cold front on the south side of the developing storm. In doing so, the air carrying the storm must descend in order to stay on isentropic surfaces. The fact that there is a new baroclinic zone associated with the wednesday storm means that air parcels which follow the southern pathway will likely have to descend twice before crossing the coast. This improves our odds at seeing some dust at the surface.
Working against us is that during these descents, the dusty air will also likely encounter a lot of rain, which will scavenge most of it from the atmosphere. However, this is precisely the phenomena we want to catch using our rain samplers: aerosol interaction with cloud microphysics.
If the three-phases of our developing storms: dynamics, water vapor, and long-range dust line up as the models are predicting, this could be an epic event both for our scientific goals and for the badly strained water resources of California. This blog will be keeping track of the developing storm for the rest of the week.