Final Storm Retrospective

Greetings from Bodega Head!

BBACPAX only has two measurement days left, and yesterday evening we received what is likely to be our final storm. Just about a half an inch of rain fell between 3 pm and 11 pm.

In the rest of this post I will review the storm from yesterday and preview our IN measurement strategies for today and Friday, which should sample some interesting flow regimes we have not yet seen.

Yesterday’s Storm:

Yesterday’s system was fairly small, shallow and fast moving. It really rained hard for a short period in the evening, and without the heavy burst between 8 pm and 9 pm, PST this would have been a light event:


Timesereies of surface meteorology at BBY for the 48-hr period ending at 18 UTC on March 6, 2014. Credit: NOAA-HMT.

Note that this corresponds to the period of maximum westerly winds at the surface.

In the lower levels, southwesterly winds built toward the surface in advance of the rain – a now-familiar pattern which happens just before AR conditions are met at BBY (see this post). This can be seen in the timeseries from the 449 MHz profiler:


Time-series of low-level wind profile and IWV flux from the 449 MHz wind profiler at BBY for the period ending at 18 UTC on March 6, 2014. Credit: NOAA-HMT.

Some things to note in the above figure: 1) The mid-level ( > 4 km AMSL) winds became really strong when the mid-level trough arrived. 2) The southwesterly flow caused upslope IWV flux to Cazadero to spike for a short period, which is when it rained at CZC. 3) the wind does not suggest a cold front at the surface, however the wind at 0.8 km height near 9 pm PST does. As we will see later, this system was a developing ET cyclone, and it may be that a frontal boundary was developing over our area last night.

An interesting aerosol note:

During most of the day yesterday, we had light westerly winds and trajectories were coming from the very clean open ocean to our WSW:


Back Trajectories based on 24-hr NAM 4 km forecast ending at 12 UTC on March 6, 2014. Credit: NOAA-HYSPLIT.

Source regions identified by HYSPLIT suggest that we had southwestern airmass exposure yesterday morning, and that this shifted to western exposure as the afternoon wore on. This corresponds to the westerly winds arriving before the rain. During the morning, we noted higher than normal concentrations of coarse-mode particles on the APS. Gavin and Christina also noted a large number – 150 particles – that were sampled by the CFDC-pCVI-ATOFMS system in only a couple hours. These were mostly sea-spray (SSA) like, but did support ice nucleation at -32 degrees Celsius.

I mentioned the above because the storm arrived without the flow regime changing much – i.e. we should have still had numerous course mode aerosol at the start of the rainfall. It will be interesting to evaluate the time-resolved precipitation sample we have to see if the SSA played a large role in cloud nucleation, or were simply scavenged at the beginning of precip.


The storm was the result of deep layer southwesterlies arriving along with AR conditions ahead of a very small upper level trough. Here is the 850 hPa analysis from NAM near 4 pm PST yesterday:


850 hPa wind and temperature from NAM analysis valid at 00 UTC on March 6, 2014. Credit: NCAR (model), Weathernerds (figure).

Note the AR-like southwesterly flow. The IWV in the surface met trace (bottom panel) confirms that AR conditions were with us for a short time.

The 500 hPa analysis:


500 hPa temperature and wind from the NAM analysis valid at 00 UTC on March 6, 2014. Credit: NCAR (model), Weathernerds (figure).

The small developing trough is prominent west of the Rogue River, OR area. The synoptic setup is indicative of a very young, developing ET cyclone. The filamentary TPW plot from satellite composite confirms that we did have elevated moisture over us:


MIMIC-TPW valid at 00 UTC on March 6, 2014. Credit: UW-CIMMS.

This is now the 3rd AR we have had in the last week-plus. Northern California should see another on Sunday!

Airmass exposure and measurement strategies for Thursday (3/6) -Friday (3/7):

We are eyeing an interesting shift in the airmass we will be sampling. This transition should happen this evening near 7 pm. Winds will shift from westerly to Northwesterly as a high pressure area builds. In addition, we should get descending air which may be dust laden. This can be seen in the forecast back trajectories which end at BBY for today:


Back Trajectories based on 24-hr NAM 4 km forecast ending at 12 UTC on March 6, 2014. Credit: NOAA-HYSPLIT.

After 7 pm, winds here at BML are forecast to become northwesterly. At the same time, they will be potentially flowing across a baroclinic zone associated with the rapidly broadening low-pressure center of the storm which just left us. This will cause air parcels to sink along their Lagrangian path.

We also expect that there are elevated dust concentrations to our Northwest. This partially comes from yesterday’s Aqua overpass:


Aerosol optical thickness (AOT) overlaid on true color image from MODIS-Aqua near 2230 UTC on March 5, 2014. Credit: NASA-EOSDIS.

Aqua’s afternoon overflight of the storm system shows an intrusion of dry air into the rear northwest sector. Aerosol Optical depths from the same swath are greater than 0.6 in some areas. It is this dusty area that we expect some of our evening trajectories to pass through.

CFDC and ATOFMS will be coordinated to catch the transition from clean-marine (4 pm to 6 pm) to dusty (7 pm and later) source regions. Comparing the ice nucleation, coarse mode particle counts and mass spectra from the two regimes will be very interesting.



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