Mid-February Wind Review

Greetings from Bodega Head! Today will be a short post. We are still anticipating quite a storm to arrive later in the week, and today should be our last day in the bright sunshine, northerly afternoon winds and relatively high pressure until the beginning of next week.

In anticipation, some of the crew got out for some R&R yesterday afternoon. After so much time in the coastal grassland, its easy to forget that whole communities of giants live just minutes away:


BBACPAX at the Armstrong Redwoods State Nature Reserve.

Local Conditions:


Timeseries of local weather conditions at the Bodega Marine Reserve. Credit: DRI-WRCC.

We have had southeasterly winds for most of the morning, leading to elevated accumulation mode particle counts at the trailer (3000 particles per cc) and few course mode particles. Unlike over the weekend, we did not see any NOx, or CO spikes from the campground to our East.  We are nearing the common time for the sea-breeze to come onshore. This has happened between 10 am and 11 am PST every morning during episodes of high pressure, and we are preparing to sample elevated coarse mode (primarily sea salt) aerosols throughout this afternoon. This may be the last day we see a diurnal shift in ambient aerosol conditions for quite some time. The animation I have linked below shows the high-resolution MM5 forecast 10 m winds from this morning through late Tuesday.

MM5 surface wind loop Credit: DRI-CEFA.

As time passes, the elongated clockwise wind pattern offshore dissipates in favor of strengthening southerlies at the coast, followed by strong southerlies throughout the domain and eventually, it is possible that we get a coastal jet windward of the Sonoma County coastal range. There are also some slight hints that we may see Petaluma Gap Flow – something we will be watching with great interest.

Review of wind conditions for the first half of our study:

We are approaching the halfway point of BBACPAX, and with a prolonged period of stormy weather approaching I thought it prudent to review the primary flow regimes of the past 13 days here at the site.

In this post, I presented an analysis of the typical quiescent day here at BML. The flow we were seeing at our sampling site was dominated by a diurnal cycle related to the large-scale northerlies plus the sea-breeze circulation. For the most part, we saw strong (20 kts or more) Northwesterlies in the late afternoon hours, light south-southeast winds in the early morning, and transition periods in-between.

A wind-rose diagram of the data from the nearby DRI-WRCC station shows these preferred flow regimes very clearly:


Wind Rose generated from 10-m tower anemometer at the DRI-WRCC Bodega Marine Reserve site. Credit: WRI-WRCC.

This data has been compiled from the period Feb 12 – Feb 24, 2014. This period included one 24-30 hour period of large-scale southerlies associated with a landfalling pacific wintertime cyclone (PWC), but for the most part it was dominated by high pressure and quiescent conditions.

It is interesting to compare the Bodega Marine Reserve wind rose to that from an inland site:


The wind data in this rose comes from the Blue Oak Ranch Reserve site of the same network. The site is located windward of the Central CA coastal range but inland. Instead of two primary flow regimes here, we see simply one prevailing wind flow direction, WSW.

The diurnal flow reversal we experienced during our first two weeks at Bodega Marine Lab had a profound effect on the aerosol number, size and chemistry we observed. That data is being analyzed now, and I hope to include some relevant blog posts about aerosol and trace gas concentration (with cool figures!) soon.

Next chance for rain:

We are still watching the storm which should arrive Wednesday. Ill post tomorrow about our chances to see unique flow features such as the Coastal Jet and Petaluma Gap Flow.


Previewing the Late-Week Storm

Greetings from Bodega Head!

Local Conditions:

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.

For Friday:


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.

Cross-Pacific Dust

Greetings from Bodega Head!


Its a pleasant sunny day with light WNW winds. We are still under the control of the NE Pacific ridge and as a consequence, we are stuck with miserably nice weather.

Yesterdays conditions here at Bodega Marine Lab were considerably less windy than in days past, due to slight offshore flow dominating the local scale. See the figure below for a reference point:


West Coast surface analysis for 18 UTC on Feb 20, 2014. Credit: NOAA-OPC

This flow pattern took some of the bite out of the sea-breeze. The timeseries of our nearby anemometer  shows the calm conditions, but with a shift to offshore flow during the overnight hours (i.e. during the sea-breeze reversal).


48 hour timeseries of surface weather variables at BBY. Credit: NOAA-HMT.

There isnt much else to report in local weather news. We are still anticipating a storm reaching the Northern California coast next Friday and Saturday. Ill update the forecast of that storm tomorrow.

Long-Range Transported Aerosol:

Yesterday we were excited by the possibility that a small plume of enhanced dust concentration may cross our area. According to global circulation model (GCM) / chemical transport model (CTM) forecasts This plume originated in Asia several days ago, was cut off from it’s parent plume by a mid-Pacific storm, and then would have been steered clockwise around the ridge to our location. We were hoping to draw some of this dust through our sampling manifold, cause it to participate in heterogeneous immersion ice nucleation in our CFDC and then investigate the ice crystal residue chemistry through a combination of a pumped counterflow virtual impactor and an aerosol time-of-flight mass spectrometer.

The impetus for believing the dust would reach us came from a forecast made by the NASA global modeling and assimilation office (GMAO) GEOS-5.


Tropospheric dust AOT forecast valid at 3 pm PST on Feb 20, 2014. Credit: NASA GMAO.

The GEOS-5 is a global circulation model which contains some simplified aspects of a chemical transport model. Dust mass mixing ratio is forecast based on emission from a saltation-imapction type emission model and the GEOS-5 3 dimensional winds. The model shortwave emission model is used to calculate the aerosol optical thickness (AOT) which would result from the dust mass and vertical distribution in each atmospheric column.

Unfortunately, we did not detect elevated dust levels at the surface yesterday afternoon in BBY, meaning that the point forecast for our area of the GEOS-5 domain may have busted. The caveat here is that we are only privy to the total column dust AOT through the web portal. It could be that dust in this plume simply never reached the surface.

Let’s examine further cross-pacific dust transport and how we might verify global model (such as GEOS-5) forecasts.

Recall that in a previous post I posted an image from MODIS-Aqua which indicated a region of high total aerosol optical thickness leaving the Asian continent in a large plume. That image was a composite of all “swaths” (the footprint of a satellite instrument) of MODIS level 2 aerosol optical thickness (AOT) at 550 nm from February 17. Here is the corresponding image from 02-18:


North Pacific swaths of MODIS level 2 AOT from Feb 18, 2014. Credit NASA-GSFC.

There is still a large area (the size of the entire western US) of elevated AOT east of Japan. The satellite carrying MODIS, Aqua, crossed this aerosol plume near 16:30 UTC. Lets see what the GEOS-5 dust forecast looked like for this time and date:


Tropospheric dust AOT forecast valid at 17 UTC on Feb 18, 2014. Credit: NASA GMAO.

The model does predict a plume of moderately optically thick dust extending east from Asia over Japan at the correct date and time. For now, we will not worry about whether the model gets the magnitude of AOT correct, but rather simply worry about a “qualitative plume” arriving at the correct location in space and time. We should note that the meridional extent of the plume in GEOS-5 is smaller than it appears in MODIS AOT.

The next question we should ask is “Is the dust confined to a discrete vertical layer?” followed by “If so, how elevated is the dust layer?” To partially answer this question we can turn to CALIOP. CALIOP is an active, polarized, visible light emitting/receiving interferometer. It flies on CALIPSO in the same orbit and slightly behind Aqua. When the atmosphere is optically thin (no thick clouds) CALIOP can estimate the vertical profile of aerosol optical extinction (related to AOT) at nadir. Lets look at what CALIOP saw along this path:


One orbital path of the CALIPSO satellite on Feb 18, 2014. Credit: NASA-LARC.

The pink section of the above swath passed over the approximate area of our plume. The measurement along the pink section looked like:


532 nm attenuated backscatter (top) and target classification (bottom) from CALIOP along a path which crossed the northeast Pacific on Feb 18, 2014.

The top plot is the time-altitude cross section of 532 nm attenuated backscatter. The plume we are interested can be seen between 134.5 and 139 E longitude as a shallow near-surface layer of yellow shading. The bottom plot is generated based on backscatter and the 532 nm depolarization ratio. After data is collected, regions of significant backscatter signals are classified based on their likely size and shape relative to Mie spheres. According to the classification algorithm, this near-surface layer is full of aerosol.

So far, so good. We identified a layer of aerosol in MODIS data, its timing and position was forecast by GEOS-5, we saw an aerosol layer with CALIOP, and are now confident that this aerosol layer is confined to the lowest 3 km.

What about the aerosol plume that was supposed to cross the California coast yesterday? Unfortunately, the data from the CALIPSO swath which passed over this area is not yet available to view online. However we can check MODIS:


North Pacific swaths of MODIS level 2 AOT from Feb 20, 2014. Credit NASA-GSFC.

MODIS identified very low AOT in the cloud-free pixels near the north-central California coast during its overpass on Feb 20, 2014. This area was crossed by MODIS near 2230 UTC. It is near the same area forecast by GEOS-5 to contain elevated dust aerosol optical depth at 2300 UTC (see third figure, this post).

In this case, the GEOS-5 model and MODIS imagery
do not agree on the timing and location of an aerosol (possibly dust) plume. Our instruments also did not encounter dust spectra at a rate above background level, so it may be that the GEOS-5 forecast lead us on a goose chase yesterday.

Going forward, we will continue to use global model dust forecasts to guide our measurement strategies, but they must be taken with a large remote-sensing grain of salt.

Tomorrow’s post: Medium range convergence of global model forecasts.


The Return of the RRR?

Those familiar with boating in shallow harbors will recognize the term Red, Right, Return.


Channel markers near Westside Marina, Bodega Bay, CA.

Unfortunately I am talking about a more sinister RRR, the ridiculously resilient ridge. See the Beast’s press clippings here, and here. Here at Bodega Head we are eyeing a prolonged period of quiescent conditions. The weather should be beautiful for at least a week, but we are here to identify aerosol impacts on precipitation.. which means we need some raindrops! How long will we be waiting? I’ll get to that after reviewing the scant precip. we picked up last night.

Review of yesterday’s storm:

In yesterday‘s post I looked ahead to our near-hit with a landfalling cold front. It looked like we may get some periods of light drizzle, but would not get enough precipitation at Bodega Marine Laboratory to muddy the hiking trails. This was indeed the case. The NOAA-HMT rainguage picked up somewhere near 0.05 inches (about 2 tips of the tipping bucket) and we collected about 20 ml of rain sample in our collection beakers.


Timeseries of surface weather conditions at BBY for the 48 hours ending at 1400 PST on Feb 19, 2014. Credit: NOAA-HMT.

Cazadero, CA fared slightly better. CZC, which is NOAA’s “coastal mountain Atmospheric River Observatory”, received a little over a tenth of an inch of rain.

Local conditions:

At 2 pm, skies are cloud-free, temps have increased to the upper fifties, and the wind is out of the NW, having increased to 17 kts from near calm this morning. Pressures have increased since last night as well.

Synoptic Overview:

A Ridge has built over the Northeast Pacific. The current 850 hPa geopotential height analysis from the GFS shows this poignantly:


850 hPa geopotential height and wind analysis for 1200 UTC on Feb 19, 2014. Credit: NOAA (model), NCAR (visualization).

This means we expect a “groundhog day” experience for the foreseeable future. Tomorrow: sunny skies, NNW to NW winds, pleasant high temperature, winds gusting in the afternoon. Friday: same, Saturday: same. See this post for a description of the local sunny day, its diurnal cycle and sea-breeze. The good news? Long range model forecasts see an end in sight.

I will examine the GFS first:

By this weekend, the ridge at upper (600 hPa) levels is still preventing any storms from developing near our area.


600 hPa height and wind speed from GFS valid at 1200 UTC on Feb 22, 2014. Credit: NOAA (model), U Hawaii (visualization)

Note the sharp left turn in the Jet Stream near 150 degrees west latitude as it leaves a deep trough and encounters the ridge. This is reminiscent of the RRR pattern that was in control for most of December 2013 and January 2014.

However, by middle of next weak, the ridge has weakened over the NE Pacific and by Friday, February 28 there is a robust upper level trough approaching California.


600 hPa height and wind speed from GFS valid at 1200 UTC on Feb 28, 2014. Credit: NOAA (model), U Hawaii (visualization)

A stronger upper level system is in tow just behind, with a more active pattern established across the Pacific mid-latitudes.

Since these forecasts have been integrated for 240 hours to arrive at this solution, it may be far from the truth 10 days hence. lets examine whether the similar solutions are arrived at by other models. Do we see “convergence” toward this future storm?

The European centre for medium range weather forecasting (ECMWF) produces the world’s most respected global circulation model. ECMWF runs an ensemble of forecasts which are started from different initial states. This approach simulates adding stochastic variability to the model system of equations. The mean and standard deviation of the model state variables (such as 500 hPa geopotential height) give one an idea of the likelihood that a certain solution will appear. More simply: If we see the same storm in the ECMWF ensemble prediction system at 240 hours forecast along with low standard deviation, we can be more confident that the February 28 storm may come to pass.


Ensemble mean and standard deviation 500 hPa geopotential height from ECMWF EPS valid at 1200 UTC on Feb 28, 2014. Credit: ECMWF

In the above, we see that two troughs, with normalized standard deviation less than one do appear at 240 hours in the ECMWF ensemble. One near the coast as in the GFS forecast for this day and one further into the Pacific. Note that the geopotential contours do not seem as dramatic as in the GFS forecast. This is because they have been calculated as the mean of all ensemble members, which smooths “transient features” such as this type of trough.

Exciting! One global model found the storm, and an ensemble of other global model simulations agree that it will come. Will the next system have access to enough water vapor? Recall that while yesterday’s storm encountered a lot of low-level moisture the deep layer integrated water vapor characteristic of an AR was missing. Once again we will turn to the University of Hawaii’s GFS products to try and predict.


Forecast total precipitable water (surface to tropopause) from GFS valid 00 UTC on Feb 28, 2014. Credit: NOAA (model), U Hawaii (visualization).

Here we see a plume of elevated (3.5 cm or more) integrated water vapor reaching the Central California coast on Thursday afternoon. If the GFS (and ECMWF) forecasts are correct on all counts, we should have the correct dynamic and thermodynamic environments for rainy weather at the end of next week.

Long Range aerosol transport:

In tomorrow’s post I will revisit the pollutant plume we identified in the Northwest Pacific yesterday, talk about it’s timing relative to the Feb 28th storm, and discuss some more aerosol remote sensing data.


Holding Out for What Rain May Come

Greetings from Bodega Head!

Skies are overcast this morning, which partially obscured this spectacular sunrise.


Photo near sunrise looking SE from the “crows nest” of our trailer. Photo taken at Bodega Marine Reserve.

Today’s weather at a glance:

Winds have remained northwesterly overnight, and just like the last 2 nights, wind speeds dropped below 6 kts. We are nearly saturated again this morning, however the overnight low did not dip much below 50 degrees F (10 degrees C).


Time-series of surface weather variables near our trailer at BBY ending at 1500 UTC on Feb 18, 2014. Credit: NOAA-HMT.

The warmth is a change from the previous 2 nights, and owes to the cloud cover and deeper moist near-surface layer. The “big” story weather-wise is that integrated water vapor (IWV) over the site has nearly doubled, from 0.7 cm to 1.4 cm as of 8 am PST. This means we have more water vapor but are still well short of AR conditions (4-5 cm IWV or more).


An upper level trough is approaching the US Pacific Northwest. It is not a particularly deep trough, and it will stay primarily to the north of our site. Ive posted two snapshots of the GFS 500 hPa geopotential height forecast below:


GFS forecast geopotential height and absolute vorticity at 500 hPa. Valid 1800 UTC on February 18, 2014. Credit: NCAR/NCEP and Antonio Riggi of Weatherbell, inc.

This first snapshot is valid at 10 am PST and shows the trough just offshore.


GFS forecast geopotential height and absolute vorticity at 500 hPa. Valid 0600 UTC on February 19, 2014. Credit: NCAR/NCEP and Antonio Riggi of Weatherbell, inc.

The second snapshot shows the trough just onshore 12 hours later. Notice the bottom of the trough is well north of the CA/Oregon border.

This weak upper system is associated with a surface cold front, which is currently just offshore.


Surface Analysis for the Northeast Pacific valid at 1200 UTC on Feb 18, 2014. Credit: NOAA-OPC.

The nearby occluded cyclone is lacking a deep moist layer, however. A MIMIC TPW snapshot suggests that we should not expect integrated water vapor to rise much further over BBY.


Merged total precipitable water over the North Pacific at 1300 UTC on Feb 18, 2014. Credit UW-CIMSS.

Local Outlook:

Will the cold front come onshore at BBY? Will we get any rain? Local high-resolution mesoscale model forecasts suggests that this is going to be a nearly dry event. The snapshot below shows the 925 hPa winds and temperature from the The Desert Research Institute Program for Climate Ecosystem and Fire Applications (DRI-CEFA) MM5 forecasts.


925 hPa wind vectors and Temperature (C) from MM5 4 km forecast valid 0600 UTC on Feb 19, 2014. Credit DRI-CEFA.

At 10 pm PST, the wind shift and temperature gradient in the NW corner of California seems indicative of a weak front, however the winds south of this area will slowly shift from NW to N, nothing more. Without much forced ascent, either from dynamics or a coastal jet, we do not expect much rain. The same model forecast for precipitation shows scattered drizzle in our area from 2 am to 6am tonight:


3-hr accumulated precipitation from MM5 4 km valid at 4 am PST on Feb 19, 2014. Credit: DRI-CEFA.

Local aerosol and reactive gas transport:

We will not have a wind reversal in advance of this storm. Winds have been steady out of the NW for 48 hours and will become northerly after the storm passes to our North. The more northerly winds may bring us some potential for terrestrial bio-IN aerosol. Possibly from the uncovered soil of wintering vineyards?

Long range transported aerosol:

The aerosol optical depths over the Northeast Pacific have been clean since Saturday’s system came ashore. The MODIS (Aqua) image from yesterday suggests that a plume of pollution has just moved east of Japan, and we will watch the progress of this area of high AOT as it has the potential to move across the Pacific in the coming days.


MODIS Level 2 Aerosol Optical Thickness at 550 nm for February 17, 2014.


The lack of regional dynamic forcing, combined with steady northwest winds means that we do not expect much rain here at BBY. We are not lacking for low-level moisture and have been watching some mid-level clouds overhead today, so it may be that we have a low layer of drizzle develop in the early morning hours. Winds which shift to northerly for the next several days should expose our instruments to terrestrial and even anthropogenic aerosol and gas sources. We will be monitoring a pollutant plume which is currently over the Northwest Pacific as it tracks eastward.

Next chance of rain:

No significant chance through Saturday.


The Wintertime Diurnal Cycle in Coastal Sonoma

Greetings from Bodega Head!

We are having another quiet, sunny weather day here and with no precipitation or polluted airmass to discuss I am going to do something a little different.

Yesterday we noticed that our NW to WNW winds really picked up in the afternoon. As I write this post at 2 pm PST, the same has begun to happen today. Lets look at the surface met station output for the last 24+ hours from Bodega Bay (BBY) and Santa Rosa (SRS):


BBY Surface met variables for the 2-day period ending at 1 pm PST on Feb 17, 2014. Credit: NOAA-HMT

This first figure is our local met station at BBY. Notice that the wind direction was steady from 300 degrees or so (WNW wind) yesterday afternoon, while the wind speed steadily rose. Overnight the wind calmed and the wind became almost entirely northerly. Note that the maximum wind occurred a couple hours after the daily maximum temperature was reached. Stronger, WNW wind conditions returned this afternoon at about the same time.

Now compare to the surface met timeseries from SRS:


SRS Surface met variables for the 2-day period ending at 1 pm PST on Feb 17, 2014. Credit: NOAA-HMT

Since yesterday morning in Santa Rosa, the winds have been very weak near 2 m/s. Wind direction has meandered near northwesterly.

The difference between two relatively close sites under high pressure and clear skies is due to the sea-breeze circulation. Santa Rosa is approx. 20 miles inland from the coast. Santa Rosa has reached a warmer high temperature each of the last 2 afternoons. Currently it is 56 degrees Fahrenheit on Bodega Head, while the Santa Rosa regional airport is reporting 64 degrees. The regional temperature differences are summed up nicely in this short range 4-km WRF forecast valid at  1 pm PST today.


WRF-4km 10 meter winds and temperature valid at 2100 UTC on Feb 17, 2014. Credit: NOAA-NWS Sacramento WFO.

The thermal contrast between the coastal waters and the warm inland of Sonoma County causes a shallow overturning circulation to form, with relatively low pressure over the warm land and high pressure over the cool sea during the daytime. Winds at the daytime land-sea boundary (our site) are onshore at the surface and offshore overhead. This circulation reverses itself overnight.

The entire sea-breeze circulation may only extend 50 km horizontally, however this is still large enough for the coriolis effect to become important. Because of the coriolis effect, the sea-breeze circulation turns away from coast-normal, and the direction of this turning reverses itself in a predictable diurnal fashion as well. To see how, I have included this figure from Haurwitz (1946!):


Theoretical sea breeze diurnal hodograph. Credit: Haurwitz, B., 1946, Comments on the Sea Breeze Circulation, J. Meteorology, Vol. 4, No. 1, 1-8.

The above is a hodograph, or a trace of the path that the head of a wind vector centered at the origin will take over time. In this case, the graph depicts the relative wind direction and strength over a 24 hour period indexed to the time of maximum thermal contrast between the land and ocean. The point labelled “0” corresponds to the time of maximum temperature gradient.

Haurwitz created this plot while working at MIT, so his frame of reference has the ocean to the East (the positive “x” axis in this plot). It takes a bit of mental gymnastics, but one can superpose this diurnally varying sea-breeze vector onto a NW wind vector like the one that exists for our broad local area. If we reverse the x-axis to account for land being to the east on the West Coast and also reverse the y-axis to account for the fact that coriolis acts to the right of the flow, we see that at the time of maximum thermal contrast between land and sea (late afternoon) we should expect a strong wind which is more westerly than the prevailing flow. On the other hand, overnight we should expect a much weaker and more northerly wind. This is precisely what we have seen in our local wind timeseries over the past 36 hours.

What does this mean for aerosol-cloud interaction? Not much, since we do not expect to have any clouds under these weather conditions. However, we are measuring the ambient aerosol chemistry, sizes and the trace reactive gas concentrations. We should expect that overnight when the flow comes straight out of the north we are more likely to measure terrestrial and anthropogenic aerosols and gases. Conversely, during the late afternoon when the wind is stronger from the northwest, we should only expect marine influences.

Next chance for rain:

We are still expecting a slight chance of rain tomorrow (Tuesday, February 18) evening. Chances are not great, but we may get some periods of light drizzle after 9 pm PST. Ill have a more thorough write-up of the coming weather system here tomorrow.


The Sun is Shining and the Weather is Sweet

Greetings from Bodega Head! After 0.12 inches of rainfall last night, we woke up to beautiful sunny skies and calm conditions. So did the local Harbor Seals.Seals_n_Horseshoe

Seals in Horseshoe Cove this morning. Photo taken at the Bodega Marine Reserve.

Pressures rose overnight, winds dropped to near nil, and this morning was the first since we arrived that we did not have a surface stratocumulus layer.

Update since yesterday’s discussion:

The NOAA-HMT surface met station is still down, however there is a nearby station (500 m from our trailer) with similar data operated by The University of Nevada’s Desert Research Institute (DRI). Conditions as of 9 am are in this chart:


Selected surface Wx parameters from Feb 15 and early Feb 16. Credit: DRI-WRCC.

We picked up some intermittent spurts of rain that added up to a little over a tenth of an inch. Just after the rain the wind direction shifted to WNW and NW, signalling that the cold front had passed, and we later hit an overnight low of 41 degrees. By far our chilliest night of the young trip.

At the top of the Coastal Range at CZC, the NOAA-HMT station reported just over half an inch of rain. Yesterday’s post discussed the possible coupling of the orographic cloud with the upper level “seeder” cloud near the time of cold front passage at CZC. It appears that something like this did happen, although the upper level clouds lowered considerably as the front approached and the orographic rain echoes deepened so that the entire cloud system over CZC was more or less homogeneous, contained warm-rain only, and extended only to approximately 3.5 AMSL. These altitudes are typically lower than most dust and long-range transported aerosols in the post-frontal region of a North Pacific Cyclone, so it is unlikely that these aerosols nucleated in the CZC precipitation. Additionally, the cloud tops were very likely warmer than 0 °C, thus any long-range dust or aerosols containing biological matter would not have aided ice nucleation.


S-Band vertical precipitation radar from CZC. SNR during Feb 15 and Feb 16, 2014. Credit: NOAA-HMT.

Todays Weather at a Glance:

Its currently 55 degrees with NW wind @ 4 kts here at Bodega Head. We are subsaturated for the first time this trip, and high pressure has returned to the Sonoma coast.


We are fully in the post-frontal regime, with localized high pressure and NW winds bringing clean maritime air to our location.


Surface analysis valid at 1200 UTC on Feb 16, 2014. Credit: NOAA-OPC.

The upper levels show slight ridging overhead, which is likely to dissipate in the next couple days.


GFS Analysis valid at 1200 UTC on Feb 16, 2014. Credit: NCAR.

Local aerosol and reactive gas transport:

We are likely to receive primarily marine air for the next 2 days. Yesterday evening after the rain started, total aerosol concentrations crashed to 200 per cubic centimeter, a very low value which is likely due to rain scavenging most of the particulates in the boundary layer. This morning, the condensation particle counter (CPC) has been showing a slight rebound in concentration to near 800 per cubic centimeter. Most of this rebound has been detected by the aerodynamic particle sizer (APS), while the concentrations reported by the scanning mobility particle sizer (SMPS) have remained quite low.  This is quite indicative of an aerosol load which is mostly large particles and probably fresh sea salt.


We are expecting clear skies, mild temperatures near 60 degrees and light winds from the northwest for the rest of the day.

Next rain event:

We expect the next rain event to come late Tuesday into Wednesday morning. It may be similar in nature to Saturday’s event, with a “near-miss” of an upper-level trough providing the large-scale forcing. National Weather Service forecasts only put the chance of rain at 40% for our area, so we may not receive more than a trace here.