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:

HMT-BBYsfc_18UTC030614

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:

HMT-BBYiwvnU_18UTC0306

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:

HYSPLIT-BTraj_NAM-4km_12UTC0305_12UTC0305-24hr

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.

Synoptic-scale:

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:

weathernerds_nam_850_temp_2014030600_F00

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:

weathernerds_nam_500_temp_2014030600_F00

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_0000UTC0306

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:

HYSPLIT-BTraj_NAM-4km_12UTC0306_12UTC0306-24hr

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:

Aqua-TrueColor-n-AOT_NEPac-2230UTC0306

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.

-Andy

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:

HMT-BBYsfc_16UTC0304

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_TPW-NEPac_20UTC0303

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-Deter_500hPaZ_00UTC0302-00UTC0304

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:

ECMWF-Deter_500hPaZ_00UTC0303-00UTC0304

Same as above, except 24 hour forecast.

ECMWF-Deter_500hPaZ_00UTC0304-00UTC0304

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:

HMT-BBYIWVnU_16UTC0304

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:

weathernerds_nam_shr_shear_sfc-850_2014030312_F00

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:

WSR-88D_Comp24hrPrcp_USsw_NCDC

  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-850-Spechum_00UTC0304-f00

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.

Review:

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:

HMT-CZCSNR_16UTC0304

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.

-Andy

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!

IMG_0812

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:

OPC-sfc_NEPac12UTC0302

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.

HMT-BMLsfcmet_00UTC0301

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:

HMT-BMLsfcmet_17UTC0302

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:

HYSPLIT-BTraj_NAM-4km_12UTC0302_0400PST0302

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:

MM5-3hrPrcp_03UTC0304

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.

GEOS5-DustAOTGlob_00UTC0302-03UTC0304

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.

-Andy

Local Meteorology and Air Quality

During certain periods, the air here at Bodega Marine Lab (BML) has been stunningly clean — approaching what might be found in the remote ocean.  We’re finding, somewhat unsurprisingly, that local meteorology heavily affects the composition of particles that we sample at BML.

Here’s a case study from last weekend.  The scene: nothing but Pacific Ocean to our west, with cow pastures, campgrounds, and a popular vacation town to our east and south.  On sunny days under fair weather (high pressure) we experience a diurnal cycle of sea breeze and land breeze.  In aerosol particle terms, we see a very segmented signal: distinct ‘sea breeze’ and ‘land breeze’ aerosol characteristics.  Let’s have a look at the aerosol particle size distributions next to the surface meteorological conditions (from the nearby DRI met station on Bodega Marine Reserve):

SeaBreezeCycle_BBY

Two days of aerosol particle size distributions (top two panels), carbon monoxide and solar radiation (3rd panel from top), ambient temperature and RH, with wind speed and direction on the bottom panel. The correlation of wind direction with the number concentration of small particles (2nd panel from top) is striking.

The sources of particles in air masses that we sample during the sea breeze (wind from 300 degrees) are ideally very limited: particles ejected from the ocean as sea spray, and particles that form through chemical reactions in the atmosphere just above the ocean.  In contrast, the land breeze (wind from about 140 degrees)  carries with it particles from a wide variety of sources: fossil fuel combustion, biomass combustion (fires), food cooking, and agriculture.

If you aren’t familiar with this kind of data, and you have a keen eye, you’ll start to figure out what’s happening just by thinking about what particle sources we’re sampling under the land breeze vs the sea breeze.  Since the sea breeze contains sea spray, you can see that sea spray aerosol particles are typically characterized by larger particle sizes (the colors in the top plot show higher concentrations near the 1 micron diameter mark), compared to the kinds of aerosols observed during the land breeze episodes (smaller proportions of larger particles compared with to the huge number of small particles).  It’s well known to atmospheric chemists that particles that are emitted directly from combustion sources (so-called ‘primary’ combustion aerosol) are very small, and tend to grow as they react with other trace gases in the atmosphere or are incorporated into clouds and fogs.

I can tell you anecdotally, that the composition of the particles is very different between these two episodes.  Sea salt particles (containing mainly sodium chloride) that travel through polluted air quickly react with the nitrogen oxide pollutants to form sodium nitrate particles.  The land breezes also bring with them fresh combustion particles.  You’ll notice some small spikes in carbon monoxide on either end of the sea breeze periods, just before and after the switch — those are actually from fires from local campgrounds directly upwind of us — it was a weekend in a vacation destination town after all!

One really exciting thing for a atmospheric chemist is to see their science in the world around them.  To live in and around the environment; to observe with your own senses what you’re observing with your instruments.  This is why I’m an environmental scientist.

-Doug Collins, Grad Student (UCSD)

Round 2 is on its way

Greetings from Bodega Head!

After a good soaking yesterday, the sun has come out for a brief hello.

 

Recap of Wednesday’s rain (round 1)

We received around 2 and a quarter inches of rain here yesterday, largely between 7 am Wednesday morning and 8 pm Wednesday night. A frontal boundary passed us around 7 pm Wednesday. It wasn’t a classical cold front, which can be seen from the 449 MHz wind profiler and RASS timeseries:

HMT-BBY_UTvFlux_1900UTC0227

Time-vertical cross-section of wind and virtual temperature from 449 MHz radar + RASS at BBY for the period ending at 11 am PST on Feb 27, 2014. Credit: NOAA-HMT.

As can be seen in the wind profiles, winds turned from SSE to SW after the boundary, and the lower atmosphere actually warmed. As we will see, freezing levels did not change much after the boundary either.

The profiling radars at Cazadero (CZC) and Santa Rosa (STR) capture this beautifully:

HMT-CZCSNR_19UTC0227

Time-Vertical cross section of signal-to-noise ratio from the S-Band Radar at CZC for the period ending at 11 am PST on Feb 27, 2014. Credit: NOAA-HMT.

HMT-STRSNR_19UTC0227

Time-vertical cross section of signal-to-noice ratio from the S-Band Radar at STR for the period ending at 11 am PST on Feb 27, 2014. Credit: NOAA-HMT.

For each case, the bright band does not ascend or descend after the time of frontal passage. It is apparent that CZC received a period of very heavy rain between 6 pm and 8 pm, near the frontal boundary. In fact, cloud echo-top heights reached to nearly 9 km AMSL. The wind profiler here at BBY shows that this period coincided with a dramatic increase in upsolpe water vapor flux:

HMT-BBY_UiwvFlux_1900UTC0227

Time-vertical cross section of wind and timeseries of upslope IWV at BBY for the period ending at 11 am PST on Feb 27, 2014. Credit NOAA-HMT.

In this case, the upslope wind direction is defined along the vector which points from BBY to CZC. as Southerly winds really sped up near 6 pm yesterday, upslope IWV also shot up, and for the period where the value exceeded 20 knot-inches, precipitation was enhanced at CZC compared to BBY.

Potential for dust in precip. samples:

Yesterday in this post, I mentioned that we observed dust at the surface for most of the morning, while the storm was coming ashore. We also expected high altitude dust in the storm’s central circulation. There are a few ways that elevated dust could reach our site at the ground. 1) Dust aerosol could be scavenged by falling raindrops, which then make it into our precipitation sampler. 2) Dust aerosol could act as cloud condensation nuclei, which grow into or become incorporated in a rain-drop or precipitating ice and make it into our precipitation sampler. 3) Dust aerosol could act as ice nuclei, which grow into ice-phase precipitation particles, melt on their path to the surface and make it into our precipitation sampler.

We are most interested in the last possibility, which means that we would really like to collect dust during periods when the precipitation profiling radar is telling us that there is cloud ice over our heads (i.e. a brightband is present) in a deep layer.

Even at STR, whose radar likely mimicked the cloud structure that existed over us at BBY, there were long periods of high cloud-top echoes over brightband rain yesterday. Given that we expected a deep tropospheric layer of Asian dust, particularly in the early phases of this storm, that is good news for our precipitation residue analysis!

Looking ahead to Round 2:

Since late yesterday evening we have entered an interlude between storms. Winds were southwesterly for a short time, but will soon become southerly in advance of the next system. We are going to experience a rare event here on the Northern California coast. We are going to see a small but powerful extratropical cyclone whose circulation center passes to our south. We expect the low pressure center to come onshore early Saturday morning near Monterrey Bay. Before this time, we will encounter another frontal boundary here at BBY. According to model forecasts, a mesoscale band of strong SE winds will cross the coast near 4 am PST tomorrow morning:

MM5-10mU_12UTC0227-12UTC0228

10m wind speed and wind barbs forecast by 4 km MM5 valid at 4 am PST on Feb 28, 2014. Credit: DRI-CEFA.

After this time, winds will shift to SE, then rotate to S, then E, then eventually NE and Northerly as the low pressure center comes fully onshore. We expect high wind speeds at several points tomorrow, as the particular series of events mimics a rainband, then near-miss of a tropical storm. Pretty unique for the west coast!

The bottom line is that it should start raining again tonight at around 4 am and we could have periods of rain through 1 pm Saturday. We may pick up 1-2 additional inches of rain here.

-Andy

Storm Day! Part 2

Greetings from rain-soaked Bodega Head!

We have now picked up over an inch and a half of rain since this morning at 7 am. We have had periods of light rain, heavy rain, big ole fat rain, and rain that even seemed to come up from underneath. The short range, high-res MM5 forecast suggested that we would get 1.5 inches for all of wednesday:

DRI-MM5-24hrAccum_1200UTC0226-1200UTC0227

24 hr accumulated precipitation forecast by 4 km MM5 valid at 4 am UTC on Feb 26, 2014. Credit: DRI-CEFA.

We reached Atmospheric River conditions here at the BML HMT station just after midnight and have been under an AR ever since. The surface met time-series over the last 48 hours or so looks like:

HMT-BBYsfc_0100UTC-022714

Sfc met variables collected at the 10m tower at BML for the 48 hours ending at 5pm PST on Feb. 26, 2014. Remeber that the time ordinate increases to the right! Credit: NOAA-HMT.

My favorite panels are the lowermost 2. Winds have been southerly or south-southeasterly all day. This has been the case throughout deep layer overhead:

HMT-BBY449UFlx_0100UTC-022714

Time-vertical profile of low-layer winds, freezing level, and upslope water vapor flux. Credit: NOAA-HMT.

Since the 449 MHz wind profiler is alive and kicking again, I couldnt resist posting a plot of the near-surface winds. For the most part the wind profiles themselves have been uniform and southerly in the lowest 5 km. The approaching storm is a very deep system, and since we are on the east flank, we will continue to get southerly winds until the first cold front passes us.

The radar senses virtual temperature (via density) using RASS. The black dots in the later stages of the picture correspond to the freezing level above BBY, which is currently about 1.9 km high. Notice that after 2 pm, the winds in the lowest 2 km have veered. As a result, the freezing level has raised slightly. Veering winds correspond to warm-air advection (WAA), so this low-level warming should be expected.

We did not have a prolonged period of westerly winds as this storm approached, and therefore did not see a coastal jet. Interestingly, even though we did not, and we have not had terribly stable stratification, Cazadero has recieved less rain this event than BBY. Upslope water vapor flux corresponds to the low-layer integrated component of the wind which points from BBY to CZC multiplied by the water vapor mixing ratio. In the lower panel of the plot above, this value has been increasing along with wind speed. It is expected that as upslope wv flux increases, orographic enhancement will cause more rain to fall at CZC.

CZC has thus-far recieved near 1.4 inches. Often, this rain has formed in a deep layer extending above the freezing level:

HMT-CZCSNR_0100UTC-022714

Timeseries of S-Band radar signal-to-noise ratio above CZC. Credit: NOAA-HMT.

The freezing level can be diagnosed from this plot by the location of the “bright band” of echos near 1.9 km.

Even though we have struck out on it so far, we may yet get a short lived coastal jet. The first front is expected to come ashore at BML tonight near 10 pm. This is depicted in the MM5 forecast below:

DRI-MM5-925hPaUT_1200UTC0226-0600UTC0227

925 hPa winds (barbs and isotachs) from 4 km MM5 valid at 0600 UTC Feb 26, 2014. Credit: DRI-CEFA.

After the front, we expect a short (until 1 pm, PST Thursday) period of westerlies, during which we may have the right conditions for a jet.

Winds will turn southerly again Thursday into Friday, until another front reaches us in advance of Friday’s storm system.

At this point, to call today’s and Friday’s system separate is probably improper. The upper level potential vorticity maximum associated with Fridays storm is so rapidly deepening, that it will engulf the PV max associated with today’s storm soon:

See that here Credit: U Hawaii

At the lower levels, this rapid upper level deepening is going to intensify a small, but intense extratropical cyclone which will hit California early Saturday morning:

See the fun cyclogenesis Credit: U Hawaii

Just for fun, I thought I would post what the high-resolution forecast of this event looks like at 72 hours simulation time.

WCoast_TC

10 m wind and SLP from 4 km MM5 valid at 12 UTC on Mar 1, 2014. Credit: DRI-CEFA.

Is that a west coast TC?

Other curiosities from today:

In this post, I mentioned that we encountered low level dust this morning, and that this system was relatively very dust laden. We also found Petaluma Gap flow for most of the morning. This can be seen in the surface station map from near 10 am PST:

MWest-Obs-USLP_1800UTC0226

Surface station temperature and sea-level pressure from stations in N. California. Credit: Meso-West, U Utah.

High pressure over the CV and East Bay, compared to relative low pressure in the North Bay and coastal Sonoma, are causing flow through the Petaluma Gap to our site in this picture.

As a result, we measured very elevated particle counts, NO, NOx and Ozone for much of today. It’s a strange result, but sometimes when it is raining the air is more polluted at Bodega Bay than when it is sunny and high pressure is in control.

This is especially true in light of the data we have recently worked up for periods of quiet conditions from early in the trip. We now have an idea of just how clean the air can be here when flow is onshore. A post reviewing that information will appear here tomorrow.

-Andy.

which is going to accelerate and intensify Friday’s system, so that it will come right on the heels of today’s rain.

From Mongolia, with Love

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:

GEOS-5_NAmer_dustAOT_00UTC0226-18UTC0226

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:

OPC-NPacSfc_1800UTC0224

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:

Terra_CTT_NCPac_0224

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.):

Terra_AOT550_NCPac_0224

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.

-Andy