Monthly Archives: March 2014

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.


The Surprise AR

Greetings from Bodega Head!

What was supposed to be an unremarkable series of evening rain showers turned into nearly an inch of rain here yesterday (Monday, March 3, 2014)!

The official forecasts were slow to pick up on this rain. in fact, as late as Sunday afternoon, the national weather service was predicting 50% PoP and was not assigning a quantitative precipitation forecast. There was much about this storm that was unexpected. Let’s investigate a bit to see why it was so hard to predict in advance.

Local conditions:

We are thoroughly soaked here! It was a damp chilly morning, but winds are light out of the SSE and things are slowly warming up. Surface weather timeseries:


Timeseries of surface met conditions at BBY for the 48 hour period ending at 0800 PST on March 4, 2014. Credit: NOAA-HMT.

Notice two things in the above plot: 1) rain started falling just after 2 pm yesterday and by 2 am an inch had fallen at BML. 2) AR conditions existed for 24 hours over BML, from roughly midnight Monday to midnight Tuesday. This second point can be validated by the integrated water vapor (IWV) in the bottom panel. IWV greater than 2 cm corresponds to AR conditions. Interestingly, the NE Pacific did contain a long filamentary feature of enhanced moisture over the weekend and leading into Monday:


MIMIC PW analysis for the NE Pacific valid 2000 UTC on March 3, 2014.

We had an inch of rain, over 2 cm of IWV at BML for more than 24 hours, and a long filament of enhanced water vapor which connected to the tropical Pacific and made “landfall” on the Northern California coast.

Yet, as I mentioned above, the rainfall was somewhat a surprise. The reason is that there was not an obvious dynamic forcing mechanism to provide lift – and convert water vapor into water condensate. This can be seen in the GCM forecasts for Monday afternoon.

Synoptic Overview:

The upper level flow over the Northeastern Pacific Ocean has been strictly zonal for several days. At the surface, the nearest large-scale (thousands of kilometers or greater) storm has been slowly making it’s way east from the Central Pacific. As of 00 UTC Sunday, the ECMWF deterministic forecast for 4 pm PST Monday looked like:


ECMWF 48 hour forecast MSLP (left) and 500 hPa geopotential (right) valid at 00 UTC on March 4, 2014. Credit: ECMWF.

There isnt much in the synoptic scale surface (left panel) or upper level (right panel) fields that suggest rain on the west coast. If we look at the drift in this solution for the following 2 days, nothing changes:


Same as above, except 24 hour forecast.


Same as above, except analysis.

The surface and 500 hPa analysis from ECMWF above make it clear that yesterdays AR conditions were not typical, in that the AR did not arrive in the warm sector of a pacific extratropical cyclone.

Nature of the March 3 storm:

So how did so much rain fall at BML? We are not on a steep mountain ridge, so orographic lift will not provide rising motion – and condensation, clouds, autoconversion and rain – unless there is a coastal jet. There was no coastal jet yesterday, as can be seen in the 449 MHz wind profiles:


Timeseries of wind vectors measured by the 449 MHz wind profiling radar at BML and upslope IWV flux for the 48 hours ending at 16 UTC on March 4, 2014. Credit: NOAA-HMT.

The plot above does elucidate two features, however. The first is a mid-level trough which propagates above the profiling radar. Winds at 3 km turn cyclonically with time – from southwesterly to westerly to northwesterly – during the time period starting at noon yesterday and ending at 4 am.

The second feature is the delay in BML rain compared to the rain at Cazadero. Note that the coastal rain rate peaks while the mid-level trough passes above, while the CZC rain rate peaks when the wind projected along 230 degrees (SW) is strongest.

Coastal rain at BML was primarily due to lift and pressure dropping due to low-level thermal vorticity advection. Wind at the surface was southerly to south-southeasterly thoughout the past 48 hours. The wind speed trace on the surface meteorology timeseries was remarkably constant (see first figure, this post). When the trough approached above 3 km, positive vorticity advection increased with height. From quasi-geostrophic theory, lift should be forced in the lowest levels as a result. Even though there was not a strong upper trough approaching the area, there was local dynamically forced ascent. Its signature can be seen in the NAM analysis valid at 12 UTC on March 3, 2014:


Analysis wind shear in the layer bounded by the surface and 850 hPa, valid at 12 UTC on March 3, 2014. Credit: NCAR (model), Weathernerds (visualization).

The low (surface to 850 hPa) shear vectors clearly show the trough approaching the Northern California coast. Because this mid-level feature was not very widespread, significant precipitation was confined to the Northern California coastal ranges:


  24-hr rain estimate from composite of WSR-88D radar network for March 3, 2014. Credit: NOAA-NWS.

Lift and rain at CZC were forced both by thermal vorticity advection and by orographic uplift. In the IWV plot above, rain at CZC (green bars) becomes heavy as soon as moderate southwesterly winds are established in the layer from 500 m to 2 km AMSL. Because these southwesterly winds flow directly uphill from the coast to the Cazadero site, there is forced ascent. Additionally, the timing of the strong low-level southwesterlies coincided with the arrival of extended AR conditions at BBY. In fact, the Southwesterly winds can be thought of as an extension of the AR itself:


GFS Analysis water vapor mixing ratio (g/kg) at 850 hPa valid at 00 UTC on March 4, 2014. Credit: NCAR (model), Weathernerds (figure).

The GFS analysis of water vapor mixing ratio at 850 hPa from 00 UTC yeseterday picked up on a channel of moist Southwesterlies intersecting the CA coast. This is the same flow that can be seen in the 449 MHz profiling radar plot, and likely extends SW well into the tropical Pacific. This flow is the AR.


The information presented above confirms that yesterday’s rain storm was an AR without an extratropical cyclone. Or, an AR without upper-level dynamic forcing. The final piece of interesting data I will share is the S-Band vertical profiling radar timeseries from Cazadero. Rain was heavy throughout the afternoon, and clouds were deep – reaching to 7 km at echo top height. However, there is a clear lack of seeder-feeder development in this storm:


S-Band signal-to-noise ratio (with brightband) timeseries from CZC ending at 16 UTC on March 4, 2014. Credit: NOAA-HMT.

Long-range transported aerosol:

A precursor seeder (upper) cloud, along with satellite and aircraft aerosol measurements, is something we use to indicate that upper-level long-range transported ice nuclei were participating in the precipitation process above CZC. In this case, the upper level cloud was, and the upper level ice nuclei may have been, missing.

In this post, I showed a GEOS-5 forecast which indicated dust may arrive just before and during Monday’s rain event. We took time-resolved precipitation samples during the rainy periods yesterday. It will be interesting to see if the rain samples contain significant dust from this storm; and whether that dust is present from time periods when clouds were likely shallow, or whether it appears only when echo tops are well above the freezing level.


After the Storm

Greetings from Bodega Head!

Skies have been mostly overcast the last several days, however plenty of color can be found nearby if you know where to look!


Anenomes in a tide pool near Mussel Point, Bodega Marine Reserve, CA.

We are expecting a quiet weather day today, with overcast skies and maybe passing drizzle as the storm which soaked much of CA the last 3 days has stalled and continues to dissipate near the Central coast. The OPC surface analysis:


Surface analysis for the Northeast Pacific Valid at 12 UTC on March 2, 2014. Credit: NOAA-OPC.

A broad area of low pressure is in control over the Northern Pacific. This will continue over the next several days. However, as we will see in coming posts, the storms which come eastward toward the Pacific coast are not expected to intensify or dig south, due to a bland zonal upper level pattern.

Local Conditions:

It has been a couple days since I posted a weather update from BBACPAX, so I will review what happened here for the past 2 days.

We picked up an additional inch of rain here on Friday evening and early morning Saturday. Most of this came near a mesoscale front which passed us at 3 am PST on Friday.


Surface met variables at BML for the 48 hours ending at 00 UTC on March 1, 2014. Credit: NOAA-HMT.

The rain early Friday morning and late Friday night pushed our 3-day rain total to about 3.25 inches. Through most of yesterday, pressures rose and winds shifted from Easterly to Westerly and Northerly, then Southerly as of this morning. The strongest sustained winds we have measured at the site during BBACPAX, 31 knots, occurred mid-morning yesterday:


Surface met variables at BML for the 48 hours ending at 17 UTC on March 2, 2014. Credit: NOAA-HMT.

New Particle Formation?

During the strong Easterly flow period yesterday, the sun came out, visibility was extremely good and we had very low particulate mass. However, total particle counts peaked over 20,000 per cc. There was a single size mode measured by SMPS near 30 nm, but larger particles were nearly non-existent. We also noticed a slow increase then decrease in the SO2 concentration. Taken together, these measurements after a major rain storm are indicative of a new particle formation (NPF) event. During NPF, organic gases (VOC) participate in heterogeneous reactions and condense to form secondary organic particles in the Aitken size mode. NPF must occur in the absence of larger organic aerosols, as the pre-existing larger particles will grow at the expense of the VOC gas before it can nucleate. Thus, NPF is often observed after rain has scavenged existing particles from the planetary boundary layer.   Trajectories over BML also passed over the Central Valley and North Bay Area, which are places that could provide the VOC and oxidizers necessary for the reactions. It is yet unsure whether NPF occurred locally or whether it occurred upstream of our location and the Aitken mode particles were simply transported here. We will have more analysis of the chemical data associated with this event soon!

Local aerosol transport:

Pressures are increasing at BML as a high pressure center builds to our southeast. As this continues, winds will turn ever more Westerly. However, late tomorrow, we expect a weak surface low pressure system to traverse the northwest corner of California. As a result, the transport source regions we are exposed to here at BML will perform a slow walk from South of Point Reyes, to Southwest of us over the open water, and return to our south once more. I have included a back trajectory analysis from the North American Model forecast below:


Forecast back trajectories from BML for the next 24 hours based on NAM 4 km forecast. Credit: NOAA-HYSPLIT.

We are interested in characterizing the ice nuclei activity of naturally emitted marine biological aerosols and measuring their chemical mixing state in-line using our CFDC-pCVI-ATOFMS system. We prefer West to Southwest trajectories in order to perform this analysis, since the source regions associated with W or SW trajectories are over the open ocean and therefore less likely to experience shipping traffic or continental air influences. Because of these trajectories, we have identified this afternoon and tomorrow morning as targets for CFDC-pCVI-ATOFMS measurements.

Outlook and long-range aerosol transport:

As the surface low approaches and trajectories (and onshore winds) return to southerly tomorrow evening, we may see some rain. Most of this rain will occur in the late evening near 6 – 8 pm:


3-hr precipitation forecast from MM5 4km valid at 1900 PST on March 3, 2014.

GEOS-5 also expects us to see an increase in dust AOT over CA during this same period.


Dust AOT forecast by GEOS-5 valid at 1900 PST on March 3, 2014. Credit: NASA-GMAO.

It is uncertain whether cloud tops will be high enough to intercept this next dust plume, as both the surface system associated with the rain, and the large scale trough associated with the dust are fairly weak. We will have to watch the S-Band radars at CZC and STR, our ATOFMS LVN and our precipitation samples to learn whether dust participates in the precipitation forming process for this small storm.

Next chance for rain:

After Monday night, the next storm system to impact our area will arrive late Wednesday to early Thursday. We will examine that system in the next weather post.